hypermedia-systems/book/CH06_MorehtmxPatterns.adoc

= More Htmx Patterns
:chapter: 06
:url: ./more-htmx-patterns/

== Active Search

So far so good with Contact.app: we have a nice little web application with some significant improvements over a plain
HTML-based application. We've added a proper "`Delete Contact`" button, done some dynamic validation of input and looked
at different approaches to add paging to the application.  As we have said, many web developers would expect that
a lot of JavaScript-based scripting would be required to get these features, but we've done it all in relatively
pure HTML, using only htmx attributes.

We _will_ eventually add some client-side scripting to our application: hypermedia is powerful, but it isn't _all powerful_ and
sometimes scripting might be the best (or only) way to achieve a given goal.  For now, however, let's see what we can accomplish
with hypermedia.

The first advanced htmx feature we will create is known as the "`Active Search`" pattern.  Active Search is when, as a
user types text into a search box, the results of that search are dynamically shown.  This pattern was made popular
when Google adopted it for search results, and many applications now implement it.

To implement Active Search, we are going to use techniques closely related to the way we did email validation in the
previous chapter.  If you think about it, the two features are similar in many ways: in both cases we want to issue
a request as the user types into an input and then update some other element with a response.  The server-side implementations
will, of course, be very different, but the frontend code will look fairly similar due to htmx's general approach of "`issue
a request on an event and replace something on the screen.`"

=== Our Current Search UI

Let's recall what the search field in our application currently looks like:

.Our search form
[source,html]
----
<form action="/contacts" method="get" class="tool-bar">
    <label for="search">Search Term</label>
    <input id="search" type="search" name="q" value="{{ request.args.get('q') or '' }}"/> <1>
    <input type="submit" value="Search"/>
</form>
----
<1> The `q` or "`query`" parameter our client-side code uses to search.

Recall that we have some server-side code that looks for the `q` parameter and, if it is present, searches the contacts
for that term.

As it stands right now, the user must hit enter when the search input is focused, or click the "`Search`" button.  Both
of these events will trigger a `submit` event on the form, causing it to issue an HTTP `GET` and re-rendering the whole
page.

Currently, thanks to `hx-boost`, the form will use an AJAX request for this `GET`, but we  don't yet get that nice
search-as-you-type behavior we want.

=== Adding Active Search

To add active search behavior, we will attach a few htmx attributes to the search input.  We will leave the current form as it is, with an `action` and `method`, so that the normal
search behavior works even if a user does not have JavaScript enabled.  This will make our "`Active Search`" improvement a nice "`progressive enhancement.`"

So, in addition to the regular form behavior, we _also_ want to issue an HTTP `GET` request when a key up occurs.  We want
to issue this request to the same URL as the normal form submission.  Finally, we only want to do this after a small
pause in typing has occurred.

As we said, this functionality is very similar to what we needed for email validation. We can, in fact copy
the `hx-trigger` attribute directly from our email validation example, with its small 200-millisecond delay, to allow a
user to stop typing before a request is triggered.

This is another example of how common patterns come up again and again when using htmx.

.Adding active search behavior
[source,html]
----
<form action="/contacts" method="get" class="tool-bar">
    <label for="search">Search Term</label>
    <input id="search" type="search" name="q" value="{{ request.args.get('q') or '' }}" <1>
           hx-get="/contacts" <2>
           hx-trigger="search, keyup delay:200ms changed"/> <3>
    <input type="submit" value="Search"/>
</form>
----
<1> Keep the original attributes, so search will work if JavaScript is not available.
<2> Issue a `GET` to the same URL as the form.
<3> Nearly the same `hx-trigger` specification as for the email input validation.

We made a small change to the `hx-trigger` attribute: we switched out the `change` event for the `search` event.
The `search` event is triggered when someone clears the search or hits the enter key.  It is a non-standard event, but
it doesn't hurt to include here.  The main functionality of the feature is provided by the second triggering event, the `keyup`.
As in the email example, this trigger is delayed with the `delay:200ms` modifier to "`debounce`" the input requests and
avoid hammering our server with requests on every keyup.

=== Targeting The Correct Element

What we have is close to what we want, but we need to set up the correct target.  Recall that the default
target for an element is itself.  As things currently stand, an HTTP `GET` request will be issued to the `/contacts` path,
which will, as of now, return an entire HTML document of search results, and then this whole document will be inserted
into the _inner_ HTML of the search input.

This is, in fact, nonsense: `input` elements aren't allowed to have any HTML inside of them. The browser will,
sensibly, just ignore the htmx request to put the response HTML inside the input.  So, at this point, when a user
types anything into our input, a request will be issued (you can see it in your browser development console if you try
it out) but, unfortunately, it will appear to the user as if nothing has happened at all.

To fix this issue, what do we want to target with the update instead?  Ideally we'd like to just target the actual
results: there is no reason to update the header or search input, and that could cause an annoying flash as focus jumps
around.

The `hx-target` attribute allows us to do exactly that.  Let's use it to target the results body, the `tbody` element in
the table of contacts:

.Adding active search behavior
[source,html]
----
<form action="/contacts" method="get" class="tool-bar">
    <label for="search">Search Term</label>
    <input id="search" type="search" name="q" value="{{ request.args.get('q') or '' }}"
           hx-get="/contacts"
           hx-trigger="search, keyup delay:200ms changed"
           hx-target="tbody"/> <1>
    <input type="submit" value="Search"/>
</form>
<table>
    ...
    <tbody>
       ...
    </tbody>
</table>
----
<1> Target the `tbody` tag on the page.

Because there is only one `tbody` on the page, we can use the general CSS selector `tbody` and htmx will target the
body of the table on the page.

Now if you try typing something into the search box, we'll see some results: a request is made and the results are inserted
into the document within the `tbody`.  Unfortunately, the content that is coming back is still an entire HTML document.

Here we end up with a "`double render`" situation, where an entire document has been inserted _inside_ another element, with
all the navigation, headers and footers and so forth re-rendered within that element.  This is an example of one of those mis-targeting issues we mentioned earlier.

Thankfully, it is pretty easy to fix.

=== Paring Down Our Content

Now, we could use the same trick we reached for in the "`Click To Load`" and "`Infinite Scroll`" features: the `hx-select`
attribute.  Recall that the `hx-select` attribute allows us to pick out the part of the response we are interested in using
a CSS selector.

So we could add this to our input:

.Using "`hx-select`" for active search
[source, html]
----
<input id="search" type="search" name="q" value="{{ request.args.get('q') or '' }}"
       hx-get="/contacts"
       hx-trigger="change, keyup delay:200ms changed"
       hx-target="tbody"
       hx-select="tbody tr"/> <1>
----
<1> Adding an `hx-select` that picks out the table rows in the `tbody` of the response.

However, that isn't the only fix for this problem, and, in this case, it isn't the most efficient one.  Instead, let's
change the _server-side_ of our Hypermedia-Driven Application to serve _only the HTML content needed_.

=== HTTP Request Headers In Htmx

In this section, we'll look at another, more advanced technique for dealing with a situation where we only want a _partial
bit_ of HTML, rather than a full document. Currently, we are letting the server create the full HTML document as response
and then, on the client side, we filter the HTML down to the bits that we want.  This is easy to do, and, in fact, might
be necessary if we don't control the server side or can't easily modify responses.

In our application, however, since we are doing "`Full Stack`" development (that is: we control both frontend _and_ backend
code, and can easily modify either) we have another option: we can modify our server responses to return only the content
necessary, and remove the need to do client-side filtering.

This turns out to be more efficient, since we aren't returning all the content surrounding the bit we are interested in,
saving bandwidth as well as CPU and memory on the server side.  So let's explore returning
different HTML content based on the context information that htmx provides with the HTTP requests it makes.

Here's a look again at the current server-side code for our search logic:

.Server-side search
[source,python]
----
@app.route("/contacts")
def contacts():
    search = request.args.get("q")
    if search is not None:
        contacts_set = Contact.search(search) <1>
    else:
        contacts_set = Contact.all()
    return render_template("index.html", contacts=contacts_set) <2>
----
<1> This is where the search logic happens.
<2> We simply re-render the `index.html` template every time, no matter what.

How do we want to change this?  We want to render two different bits of HTML content _conditionally_:

* If this is a "`normal`" request for the entire page, we want to render the `index.html` template in the current
  manner.  In fact, we don't want anything to change if this is a "`normal`" request.
* However, if this is an "`Active Search`" request, we only want to render the content that is within the `tbody`,
  that is, just the table rows of the page.

So we need some way to determine exactly which of these two different types of requests to the `/contact` URL is being
made, in order to know exactly which content we want to render.

It turns out that htmx helps us distinguish between these two cases by including a number of HTTP _Request Headers_ when
it makes requests.  Request Headers are a feature of HTTP, allowing clients (e.g., web browsers) to include name/value pairs
of metadata associated with requests to help the server understand what the client is requesting.

Here is an example of (some of) the headers the FireFox browser issues when requesting `https://manning.com`:

.HTTP headers
[source,http]
----
GET / HTTP/2
Host: www.manning.com
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:103.0) Gecko/20100101 Firefox/103.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
DNT: 1
Connection: keep-alive
Cookie: ...
Upgrade-Insecure-Requests: 1
Sec-Fetch-Dest: document
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: none
Sec-Fetch-User: ?1
Sec-GPC: 1
TE: trailers
----

Htmx takes advantage of this feature of HTTP and adds additional headers and, therefore, additional _context_ to the
HTTP requests that it makes.  This allows you to inspect those headers and choose
what logic to execute on the server, and what sort of HTML response you want to send to the client.

Here is a table of the HTTP headers that htmx includes in HTTP requests:

`HX-Boosted`::
This will be the string "`true`" if the request is made via an element using hx-boost

`HX-Current-URL`::
This will be the current URL of the browser

`HX-History-Restore-Request`::
This will be the string "`true`" if the request is for history restoration after a miss in the local history cache

`HX-Prompt`::
This will contain the user response to an hx-prompt

`HX-Request`::
This value is always "`true`" for htmx-based requests

`HX-Target`::
This value will be the id of the target element if it exists

`HX-Trigger-Name`::
This value will be the name of the triggered element if it exists

`HX-Trigger`::
This value will be the id of the triggered element if it exists

Looking through this list of headers, the last one stands out: we have an id, `search` on our search input.  So the
value of the `HX-Trigger` header should be set to `search` when the request is coming from the search input, which
has the id `search`.

Let's add some conditional logic to our controller to look for that header and, if the value is `search`, we render
only the rows rather than the whole `index.html` template:

.Updating our server-side search
[source,python]
----
@app.route("/contacts")
def contacts():
    search = request.args.get("q")
    if search is not None:
        contacts_set = Contact.search(search)
        if request.headers.get('HX-Trigger') == 'search': <1>
          # TODO: render only the rows here <2>
    else:
        contacts_set = Contact.all()
    return render_template("index.html", contacts=contacts_set) <2>
----
<1> If the request header `HX-Trigger` is equal to "`search`" we want to do something different.
<2> We need to learn how to render just the table rows.

OK, so how do we render only the result rows?

=== Factoring Your Templates

Now we come to a common pattern in htmx: we want to _factor_ our server-side templates.  This means that we want to
break our templates up a bit so that they can be called from multiple contexts.  In this case, we want to break the rows of
the results table out to a separate template we will call `rows.html`. We will include it from
the original `index.html` template, and also use it in our controller to render it by itself when we want to respond with only the
rows for Active Search requests.

Here's what the table in our `index.html` file currently looks like:

.The contacts table
[source, html]
----
    <table>
        <thead>
        <tr>
            <th>First</th> <th>Last</th> <th>Phone</th> <th>Email</th> <th></th>
        </tr>
        </thead>
        <tbody>
        {% for contact in contacts %}
            <tr>
                <td>{{ contact.first }}</td>
                <td>{{ contact.last }}</td>
                <td>{{ contact.phone }}</td>
                <td>{{ contact.email }}</td>
                <td><a href="/contacts/{{ contact.id }}/edit">Edit</a>
                    <a href="/contacts/{{ contact.id }}">View</a></td>
            </tr>
        {% endfor %}
        </tbody>
    </table>
----

The `for` loop in this template is what produces all the rows in the final content generated by `index.html`.
What we want to do is to move the `for` loop and, therefore, the rows it creates out to a _separate template file_ so that
only that small bit of HTML can be rendered independently from `index.html`.

Again, let's call this new template `rows.html`:

.Our new `rows.html` file
[source, html]
----
{% for contact in contacts %} <2>
    <tr>
        <td>{{ contact.first }}</td>
        <td>{{ contact.last }}</td>
        <td>{{ contact.phone }}</td>
        <td>{{ contact.email }}</td>
        <td><a href="/contacts/{{ contact.id }}/edit">Edit</a>
            <a href="/contacts/{{ contact.id }}">View</a></td>
    </tr>
{% endfor %}
----

Using this template we can render only the `tr` elements for a given collection of contacts.

Of course, we still want to include this content in the `index.html` template: we are _sometimes_ going to be
rendering the entire page, and sometimes only rendering the rows.  In order to keep the `index.html` template rendering
properly, we can include the `rows.html` template by using the jinja `include` directive at the position we want the content
from `rows.html` inserted:

.Including the new file
[source, html]
----
    <table>
        <thead>
        <tr>
            <th>First</th>
            <th>Last</th>
            <th>Phone</th>
            <th>Email</th>
            <th></th>
        </tr>
        </thead>
        <tbody>
        {% include 'rows.html' %} <1>
        </tbody>
    </table>
----
<1> This directive "`includes`" the `rows.html` file, inserting its content into the current template.

So far, so good: our `/contacts` page is still rendering properly, just as it did before we split the rows out of the
`index.html` template.

=== Using Our New Template

The last step in factoring our templates is to modify our web controller to take advantage of the new `rows.html` template
file when it responds to an active search request.

Since `rows.html` is just another template, just like `index.html`, all we need to do is call the `render_template`
function with `rows.html` rather than `index.html`. This will render _only_ the row content rather than the entire
page:

.Updating our server-side search
[source,python]
----
@app.route("/contacts")
def contacts():
    search = request.args.get("q")
    if search is not None:
        contacts_set = Contact.search(search)
        if request.headers.get('HX-Trigger') == 'search':
          return render_template("rows.html", contacts=contacts_set) <1>
    else:
        contacts_set = Contact.all()
    return render_template("index.html", contacts=contacts_set)
----
<1> Render the new template in the case of an active search.

Now, when an Active Search request is made, rather than getting an entire HTML document back, we only get a partial
bit of HTML, the table rows for the contacts that match the search.  These rows are then inserted into the `tbody` on
the index page, without any need for `hx-select` or other client-side processing.

And, as a bonus, the old form-based search _still works_. We conditionally render the rows
only when the `search` input issues the HTTP request via htmx.  Again, this is a progressive enhancement to our
application.

.HTTP Headers & Caching
****
One subtle aspect of the approach we are taking here, using headers to determine the content of what we return, is
a feature baked into HTTP: caching.  In our request handler, we are now returning different content depending on the
value of the `HX-Trigger` header.  If we were to use HTTP Caching, we might get into a situation where someone makes
a _non-htmx_ request (e.g., refreshing a page) and yet the _htmx_ content is returned from the HTTP cache, resulting
in a partial page of content for the user.

The solution to this problem is to use the HTTP Response `Vary` header and call out the htmx headers that you are using
to determine what content you are returning.  A full explanation of HTTP Caching is beyond the scope of this book, but the 
https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching[MDN article on the topic] is quite good, and the https://htmx.org/docs/#caching[htmx
documentation] discusses this issue as well.
****

=== Updating the Navigation Bar With "`hx-push-url`"

One shortcoming of our current Active Search implementation, when compared with the normal form submission, is that when
you submit the form version it updates the navigation bar of the browser to include the search term.  So, for example, if
you search for "`joe`" in the search box, you will end up with a url that looks like this in your browser's nav bar:

.The updated location after a form search
----
https://example.com/contacts?q=joe
----

This is a nice feature of browsers: it allows you to bookmark this search or to copy the URL and send it to someone else.
All they have to do is to click on the link, and they will repeat the exact same search.  This is also tied in with
the browser's notion of history: if you click the back button it will take you to the previous URL that you came
from.  If you submit two searches and want to go back to the first one, you can simply hit back and the browser
will "`return`" to that search.

As it stands right now, during our Active Search, we are not updating the browser's navigation bar. So, users aren't getting
nice copy-and-pasteable links and you aren't getting history entries either, which means no back button support.  Fortunately, we've already seen how to fix this: with the `hx-push-url` attribute.

The `hx-push-url` attribute lets you tell htmx "`Please push the URL of this request into the browser's navigation bar.`"
Push might seem like an odd verb to use here, but that's the term that the underlying browser history API uses, which
stems from the fact that it models browser history as a "`stack`" of locations: when you go to a new location, that
location is "`pushed`" onto the stack of history elements, and when you click "`back`", that location is "`popped`" off
the history stack.

So, to get proper history support for our Active Search, all we need to do is to set the `hx-push-url` attribute to
`true`. 

.Updating the URL during active search
[source, html]
----
<input id="search" type="search" name="q" value="{{ request.args.get('q') or '' }}"
       hx-get="/contacts"
       hx-trigger="change, keyup delay:200ms changed"
       hx-target="tbody"
       hx-push-url="true"/> <1>
----
<1> By adding the `hx-push-url` attribute with the value `true`, htmx will update the URL when it makes a request.

Now, as Active Search requests are sent, the URL in the browser's navigation bar is updated to have the proper query in
it, just like when the form is submitted.

You might not _want_ this behavior.  You might feel it would be confusing to users to see the navigation bar updated
and have history entries for every Active Search made, for example.  Which is fine: you can simply omit the `hx-push-url`
attribute and it will go back to the behavior you want.  The goal with htmx is to be flexible enough to achieve the UX
that _you_ want, while staying within the declarative HTML model.

=== Adding A Request Indicator

A final touch for our Active Search pattern is to add a request indicator to let the user know that a search is in
progress.  As it stands the user has no explicit signal that the active search functionality is handling a request. If the search takes a bit, a user may end up thinking that the feature isn't working.  By adding a request indicator we let
the user know that the hypermedia application is busy and they should wait (hopefully not too long!) for the request to
complete.

Htmx provides support for request indicators via the `hx-indicator` attribute.  This attribute takes, you guessed it,
a CSS selector that points to the indicator for a given element.  The indicator can be anything, but it is typically
some sort of animated image, such as a gif or svg file, that spins or otherwise communicates visually that "`something
is happening.`"

Let's add a spinner after our search input:

.Adding a request indicator to search
[source, html]
----
<input id="search" type="search" name="q" value="{{ request.args.get('q') or '' }}"
       hx-get="/contacts"
       hx-trigger="change, keyup delay:200ms changed"
       hx-target="tbody"
       hx-push-url="true"
       hx-indicator="#spinner"/> <1>
<img id="spinner" class="htmx-indicator" src="/static/img/spinning-circles.svg" alt="Request In Flight..."/> <2>
----
<1> The `hx-indicator` attribute points to the indicator image after the input.
<2> The indicator is a spinning circle svg file, and has the `htmx-indicator` class on it.

We have added the spinner right after the input.  This visually co-locates the request indicator with the element
making the request, and makes it easy for a user to see that something is in fact happening.

It just works, but how does htmx make the spinner appear and disappear? Note that the indicator `img` tag has the `htmx-indicator` class on it.  `htmx-indicator` is a CSS class that is
automatically injected into the page by htmx.  This class sets the default `opacity` of an element to `0`, which hides
the element from view, while at the same time not disrupting the layout of the page.

When an htmx request is triggered that points to this indicator, another class, `htmx-request` is added to the indicator
which transitions its opacity to 1.  So you can use just about anything as an indicator, and it will be hidden by default. Then, when a request is in flight, it will be shown.  This is all done via standard CSS classes, allowing you to control
the transitions and even the mechanism by which the indicator is shown (e.g., you might use `display` rather than
`opacity`). 

.Use Request Indicators!
****
Request indicators are an important UX aspect of any distributed application.  It is unfortunate that browsers have
de-emphasized their native request indicators over time, and it is doubly unfortunate that request indicators are not
part of the JavaScript ajax APIs.

Be sure not to neglect this significant aspect of your application.  Requests might seem instant when you are
working on your application locally, but in the real world they can take quite a bit longer due to network latency.  It's
often a good idea to take advantage of browser developer tools that allow you to throttle your local browser's response
times.  This will give you a better idea of what real world users are seeing, and show you where indicators might help
users understand exactly what is going on.
****

With this request indicator, we now have a pretty sophisticated user experience when compared with plain HTML, but
we've built it all as a hypermedia-driven feature.  No JSON or JavaScript to be seen.  And our implementation has the benefit of being a progressive enhancement; the application will continue to work for clients
that don't have JavaScript enabled.

== Lazy Loading

With Active Search behind us, let's move on to a very different sort of enhancement: lazy loading.  Lazy loading is
when the loading of a particular bit of content is deferred until later, when needed.  This is commonly used as a
performance enhancement: you avoid the processing resources necessary to produce some data until that data is actually
needed.

Let's add a count of the total number of contacts to Contact.app, just below the bottom of our contacts table.  This will
give us a potentially expensive operation that we can use to demonstrate how to add lazy loading with htmx.

First let's update our server code in the `/contacts` request handler to get a count of the total number of contacts.
We will pass that count through to the template to render some new HTML.

.Adding a count to the UI
[source,python]
----
@app.route("/contacts")
def contacts():
    search = request.args.get("q")
    page = int(request.args.get("page", 1))
    count = Contact.count() <1>
    if search is not None:
        contacts_set = Contact.search(search)
        if request.headers.get('HX-Trigger') == 'search':
            return render_template("rows.html", contacts=contacts_set, page=page, count=count)
    else:
        contacts_set = Contact.all(page)
    return render_template("index.html", contacts=contacts_set, page=page, count=count) <2>
----
<1> Get the total count of contacts from the Contact model.
<2> Pass the count out to the `index.html` template to use when rendering.

As with the rest of the application, in the interest of staying focused on the _hypermedia_ part of Contact.app, we'll skip over the details of how `Contact.count()` works.  We just need to know that:

* It returns the total count of contacts in the contact database.
* It may be slow (for the sake of our example).

Next lets add some HTML to our `index.html` that takes advantage of this new bit of data, showing a message next
to the "Add Contact" link with the total count of users.  Here is what our HTML looks like:

.Adding a contact count element to the application
[source, html]
----
<p>
    <a href="/contacts/new">Add Contact</a> <span>({{ count }} total Contacts)</span><1>
</p>
----
<1> A simple span with some text showing the total number of contacts.

Well that was easy, wasn't it?  Now our users will see the total number of contacts next to the link to add new
contacts, to give them a sense of how large the contact database is.  This sort of rapid development is one of the
joys of developing web applications the old way.

Here is what the feature looks like in our application:

.Total contact count display
image::screenshot_total_contacts.png[(22 total Contacts)]

Beautiful.

Of course, as you probably suspected, all is not perfect.  Unfortunately, upon shipping this feature to production, we
start getting complaints from users that the application "`feels slow.`" Like all good developers faced with
a performance issue, rather than guessing what the issue might be, we try to get a performance profile of the application
to see what exactly is causing the problem.

It turns out, surprisingly, that the problem is that innocent looking `Contacts.count()` call, which is taking up to
a second and a half to complete.  Unfortunately, for reasons beyond the scope of this book, it is not possible to improve
that load time, nor is possible to cache the result.

This leaves us with two options:

* Remove the feature.
* Come up with some other way to mitigate the performance issue.

Let's assume that we can't remove the feature, and therefore look at how we can mitigate this performance issue by
using htmx instead.

=== Pulling Out The Expensive Code

The first step in implementing the Lazy Load pattern is to pull the expensive code -- that is, the call to `Contacts.count()` -- out of the request handler for the `/contacts` endpoint.

Let's put this function call into its own HTTP request handler as a new HTTP endpoint that we will put at `/contacts/count`.
For this new endpoint, we won't need to render a template at all: its sole job is going to be to render that small bit of text
that is in the span, "`(22 total Contacts).`"

Here is what the new code will look like:

.Pulling the expensive code out
[source,python]
----
@app.route("/contacts")
def contacts():
    search = request.args.get("q")
    page = int(request.args.get("page", 1)) <1>
    if search is not None:
        contacts_set = Contact.search(search)
        if request.headers.get('HX-Trigger') == 'search':
            return render_template("rows.html", contacts=contacts_set, page=page)
    else:
        contacts_set = Contact.all(page)
    return render_template("index.html", contacts=contacts_set, page=page) <2>

@app.route("/contacts/count")
def contacts_count():
    count = Contact.count() <3>
    return "(" + str(count) + " total Contacts)" <4>

----
<1> We no longer call `Contacts.count()` in this handler.
<2> `Count` is no longer passed out to the template to render in the `/contacts` handler.
<3> We create a new handler at the `/contacts/count` path that does the expensive calculation.
<4> Return the string with the total number of contacts.

So now we have moved the performance issue out of the `/contacts` handler code, which renders the main contacts table,
and created a new HTTP end point that will produce this expensive-to-create count string for us.

Now we need to get the content from this new handler _into_ the span, somehow.  As we said earlier, the default behavior
of htmx is to place any content it receives for a given request into the `innerHTML` of an element, and that turns out
to be exactly what we want here: we want to retrieve this text and put it into the `span`.  So we can simply place an
`hx-get` attribute on the span, pointing to this new path, and do exactly that.

However, recall that the default _event_ that will trigger a request for a `span` element in htmx is the `click` event.
Well, that's not what we want!  Instead, we want this request to trigger immediately, when the page loads.

To do this, we can add the `hx-trigger` attribute to update the trigger of the requests for the element, and use the
`load` event.

The `load` event is a special event that htmx triggers on all content when it is loaded into the DOM.  By setting `hx-trigger`
to `load`, we will cause htmx to issue the `GET` request when the `span` element is loaded into the page.

Here is our updated template code:

.Adding a contact count element to the application
[source, html]
----
<p>
    <a href="/contacts/new">Add Contact</a> <span hx-get="/contacts/count" hx-trigger="load"></span><1>
</p>
----
<1> Issue a `GET` to `/contacts/count` when the `load` event occurs.

Note that the `span` starts empty: we have removed the content from it, and we are allowing the request to `/contacts/count`
to populate it instead.

And, check it out, our `/contacts` page is fast again!  When you navigate to the page it feels very snappy and
profiling shows that yes, indeed, the page is loading much more quickly.  Why is that?  Well, we've deferred the
expensive calculation to a secondary request, allowing the initial request to finish loading faster.

You might say "`OK, great, but it's still taking a second or two to get the total count on the page.`"  True, but
often the user may not be particularly interested in the total count.  They may just want to come to the page and
search for an existing user, or perhaps they may want to edit or add a user.  The total count of contacts
is just a "`nice to have`" bit of information in these cases.

By deferring the calculation of the count in this manner we let users get on with their use of the application while we
perform the expensive calculation.

Yes, the total time to get all the information on the screen takes just as long.  It actually will be a bit longer, since
we now need two HTTP requests to get all the information for the page.  But the _perceived performance_ for the end user will
be much better: they can do what they want nearly immediately, even if some information isn't available instantaneously.

Lazy Loading is a great tool to have in your belt when optimizing web application performance.

=== Adding An Indicator

A shortcoming of the current implementation is that currently there is no indication that the count request is in flight,
it just appears at some point when the request finishes.

This isn't ideal.  What we want here is an indicator, just like we added in our Active Search example.  And, in fact, we can
simply reuse that same exact spinner image, copy-and-pasted into the new HTML we have created.

Now, in this case, we have a one-time request and, once the request is over, we are not going to need the spinner anymore.
So it doesn't make sense to use the exact same approach we did with the active search example.  Recall that in that
case we placed a spinner _after_ the span and using the `hx-indicator` attribute to point to it.

In this case, since the spinner is only used once, we can put it _inside_ the content of the span.  When the request
completes the content in the response will be placed inside the span, replacing the spinner with the computed contact
count.  It turns out that htmx allows you to place indicators with the `htmx-indicator` class on them inside of elements
that issue htmx-powered requests.  In the absence of an `hx-indicator` attribute, these internal indicators will be shown
when a request is in flight.

So let's add that spinner from the active search example as the initial content in our span:

.Adding an indicator to our lazily loaded content
[source, html]
----
<span hx-get="/contacts/count" hx-trigger="load">
  <img id="spinner" class="htmx-indicator" src="/static/img/spinning-circles.svg"/><1>
</span>
----
<1> Yep, that's it.

Now when the user loads the page, rather than having the total contact count magically appear,
there is a nice spinner indicating that something is coming.  Much better.

Note that all we had to do was copy and paste our indicator from the active search example into the `span`.  Once again
we see how htmx provides flexible, composable features and building blocks. Implementing a new feature is often just copy-and-paste, maybe a tweak or two, and you are done.

=== But That's Not Lazy!

You might say "`OK, but that's not really lazy.  We are still loading the count immediately when the page is loaded,
we are just doing it in a second request.  You aren't really waiting until the value is actually needed.`"

Fine.  Let's make it _lazy_ lazy: we'll only issue the request when the `span` scrolls into view.

To do that, lets recall how we set up the infinite scroll example: we used the `revealed` event for our trigger.  That's
all we want here, right?  When the element is revealed we issue the request?

Yep, that's it.  Once again, we can mix and match concepts across various UX patterns to come up with solutions to
new problems in htmx.

.Making it truly lazy
[source, html]
----
<span hx-get="/contacts/count" hx-trigger="revealed"> <1>
  <img id="spinner" class="htmx-indicator" src="/static/img/spinning-circles.svg"/>
</span>
----
<1> Change the `hx-trigger` to `revealed`.

Now we have a truly lazy implementation, deferring the expensive computation until we are absolutely sure we need it. A
pretty cool trick, and, again, a simple one-attribute change demonstrates the flexibility of both htmx and the hypermedia
approach.

== Inline Delete

For our next hypermedia trick, we are going to implement the "`Inline Delete`" pattern.  With this feature, a contact can
be deleted directly from the table of all contacts, rather than requiring the user to navigate all the way to the edit view
of particular contact, in order to access the "`Delete Contact`" button we added in the last chapter.

Recall that we already have "`Edit`" and "`View`" links for each row, in the `rows.html` template:

.The existing row actions
[source, html]
----
<td>
    <a href="/contacts/{{ contact.id }}/edit">Edit</a>
    <a href="/contacts/{{ contact.id }}">View</a>
</td>
----

Now we want to add a "`Delete`" link as well.  And, thinking on it, we want that link to act an awful lot like the
"`Delete Contact`" button from `edit.html`, don't we?  We'd like to issue an HTTP `DELETE` to the URL for the given
contact and we want a confirmation dialog to ensure the user doesn't accidentally delete a contact.

Here is the "`Delete Contact`" button html:

.The existing row actions
[source, html]
----
<button hx-delete="/contacts/{{ contact.id }}"
        hx-push-url="true"
        hx-confirm="Are you sure you want to delete this contact?"
        hx-target="body">
    Delete Contact
</button>
----

As you may suspect by now, this is going to be another copy-and-paste job.

One thing to note is that, in the case of the "`Delete Contact`" button, we wanted to re-render the whole screen and update
the URL, since we are going to be returning from the edit view for the contact to the list view of all contacts.  In
the case of this link, however, we are already on the list of contacts, so there is no need to update the URL, and
we can omit the `hx-push-url` attribute.

Here is the code for our inline "`Delete`" link:

.The existing row actions
[source, html]
----
<td>
    <a href="/contacts/{{ contact.id }}/edit">Edit</a>
    <a href="/contacts/{{ contact.id }}">View</a>
    <a href="#" hx-delete="/contacts/{{ contact.id }}"
        hx-confirm="Are you sure you want to delete this contact?"
        hx-target="body">Delete</a> <1>
</td>
----
<1> Almost a straight copy of the "`Delete Contact`" button.

As you can see, we have added a new anchor tag and given it a blank target (the `#` value in its `href` attribute) to
retain the correct mouse-over styling behavior of the link.  We've also copied the `hx-delete`, `hx-confirm` and
`hx-target` attributes from the "`Delete Contact`" button, but omitted the `hx-push-url` attributes since we don't want
to update the URL of the browser.

We now have inline delete working, even with a confirmation dialog.  A user can click on the "`Delete`" link and the
row will disappear from the UI as the entire page is re-rendered.

.A Style Sidebar
****
One side effect of adding this delete link is that we are starting to pile up the actions in a contact row:

.That's a lot of actions
image::screenshot_stacked_actions.png["All 3 table rows have Edit, View, Delete links at the end"]

It would be nice if we didn't show the actions all in a row, and, additionally, it would be nice if we only showed the
actions when the user indicated interest in a given row.  We will return to this problem after we look at the relationship
between scripting and a Hypermedia-Driven Application in a later chapter.

For now, let's just tolerate this less-than-ideal user interface, knowing that we will fix it later.
****

=== Narrowing Our Target

We can get even fancier here, however.  What if, rather than re-rendering the whole page, we just removed the row
for the contact?  The user is looking at the row anyway, so is there really a need to re-render the whole page?

To do this, we'll need to do a couple of things:

* We'll need to update this link to target the row that it is in.
* We'll need to change the swap to `outerHTML`, since we want to replace (really, remove) the entire row.
* We'll need to update the server side to render empty content when the `DELETE` is issued from a "`Delete`" link rather
than from the "`Delete Contact`" button on the contact edit page.

First things first, update the target of our "`Delete`" link to be the row that the link is in, rather than the entire
body.  We can once again take advantage of the relative positional `closest` feature to target the closest `tr`, like
we did in our "`Click To Load`" and "`Infinite Scroll`" features:

.The existing row actions
[source, html]
----
<td>
    <a href="/contacts/{{ contact.id }}/edit">Edit</a>
    <a href="/contacts/{{ contact.id }}">View</a>
    <a href="#" hx-delete="/contacts/{{ contact.id }}"
        hx-swap="outerHTML"
        hx-confirm="Are you sure you want to delete this contact?"
        hx-target="closest tr">Delete</a> <1>
</td>
----
<1> Updated to target the closest enclosing `tr` (table row) of the link.

=== Updating The Server Side

Now we need to update the server side.  We want to keep the "`Delete Contact`" button working as well, and in
that case the current logic is correct.  So we'll need some way to differentiate between `DELETE` requests that are
triggered by the button and `DELETE` requests that come from this anchor.

The cleanest way to do this is to add an `id` attribute to the "`Delete Contact`" button, so that we can inspect the
`HX-Trigger` HTTP Request header to determine if the delete button was the cause of the request.  This is a simple
change to the existing HTML:

.Adding an `id` to the "`delete contact`" button
[source, html]
----
    <button id="delete-btn" <1>
            hx-delete="/contacts/{{ contact.id }}"
            hx-push-url="true"
            hx-confirm="Are you sure you want to delete this contact?"
            hx-target="body">
        Delete Contact
    </button>
----
<1> An `id` attribute has been added to the button.

By giving this button an id attribute, we now have a mechanism for differentiating between the delete button in the
`edit.html` template and the delete links in the `rows.html` template.  When this button issues a request, it will
look something like this:

[source, http]
----
DELETE http://example.org/contacts/42 HTTP/1.1
Accept: text/html,*/*
Host: example.org
...
HX-Trigger: delete-btn
...
----

You can see that the request now includes the `id` of the button. This allows us to write code very similar to what we did
for the active search pattern, using a conditional on the `HX-Trigger` header to determine what we want to do.  If that
header has the value `delete-btn`, then we know the request came from the button on the edit page, and we can do what we
are currently doing: delete the contact and redirect to `/contacts` page.

If it _does not_ have that value, then we can simply delete the contact and return an empty string.  This empty string
will replace the target, in this case the row for the given contact, thereby removing the row from the UI.

Let's refactor our server-side code to do this:

.Updating our server code to handle two different delete patterns
[source, python]
----
@app.route("/contacts/<contact_id>", methods=["DELETE"])
def contacts_delete(contact_id=0):
    contact = Contact.find(contact_id)
    contact.delete()
    if request.headers.get('HX-Trigger') == 'delete-btn': <1>
        flash("Deleted Contact!")
        return redirect("/contacts", 303)
    else:
        return "" <2>
----
<1> If the delete button on the edit page submitted this request, then continue to do the previous logic.
<2> If not, simply return an empty string, which will delete the row.

And that's our server-side implementation:  when a user clicks "`Delete`" on a contact row and confirms the delete, the row will
disappear from the UI.  Once again, we have a situation where just changing a few lines of simple code gives us a
dramatically different behavior. Hypermedia is powerful in this manner.

=== The Htmx Swapping Model

This is pretty cool, but there is another improvement we can make if we take some time to understand the htmx content
swapping model: it would nice if, rather than just instantly deleting the row, we faded it out before we removed
it.  The fade would make it clear that the row is being removed, giving the user some nice visual feedback on the
deletion.

It turns out we can do this pretty easily with htmx, but to do so we'll need to dig in to exactly how htmx swaps content.

You might think that htmx simply puts the new content into the DOM, but that's not in fact how it works.  Instead, content
goes through a series of steps as it is added to the DOM:

* When content is received and about to be swapped into the DOM, the `htmx-swapping` CSS class is added to the target
  element.
* A small delay then occurs (we will discuss why this delay exists in a moment).
* Next, the `htmx-swapping` class is removed from the target and the `htmx-settling` class is added.
* The new content is swapped into the DOM.
* Another small delay occurs.
* Finally, the `htmx-settling` class is removed from the target.

There is more to the swap mechanic (settling, for example, is a more advanced topic that we will discuss in a later chapter)
but this is enough for now.

Now, there are small delays in the process here, typically on the order of a few milliseconds.  Why so?  It turns out
that these small delays allow _CSS transitions_ to occur.

.CSS Transitions
****
CSS transitions are a technology that allow you to animate a transition from one style to another.  So, for example, if
you changed the height of something from 10 pixels to 20 pixels, by using a CSS transition you can make the element
smoothly animate to the new height.  These sorts of animations are fun, often increase application usability, and are
a great mechanism to add polish to your web application.
****

Unfortunately, CSS transitions are difficult to access in plain HTML: you usually have to use JavaScript and add or remove classes
to get them to trigger.  This is why the htmx swap model is more complicated than you might initially think. By swapping
in classes and adding small delays, you can access CSS transitions purely within HTML, without needing to write any
JavaScript!

=== Taking Advantage of "`htmx-swapping`"

OK, so, let's go back and look at our inline delete mechanic:  we click an htmx-enhanced link which deletes the contact
and then swaps some empty content in for the row.  We know that before the `tr` element is removed, it will have the
`htmx-swapping` class added to it.  We can take advantage of that to write a CSS transition that fades the opacity of
the row to 0.  Here is what that CSS looks like:

.Adding a fade out transition
[source, css]
----
tr.htmx-swapping { <1>
  opacity: 0; <2>
  transition: opacity 1s ease-out; <3>
}
----
<1> We want this style to apply to `tr` elements with the `htmx-swapping` class on them.
<2> The `opacity` will be 0, making it invisible.
<3> The `opacity` will transition to 0 over a 1 second time period, using the `ease-out` function.

Again, this is not a CSS book and we are not going to go deeply into the details of CSS transitions, but hopefully the
above makes sense to you, even if this is the first time you've seen CSS transitions.

So, think about what this means from the htmx swapping model:  when htmx gets content back to swap into the row it will
put the `htmx-swapping` class on the row and wait a bit.  This will allow the transition to a zero opacity to occur,
fading the row out.  Then the new (empty) content will be swapped in, which will effectively remove the row.

Sounds good, and we are nearly there.  There is one more thing we need to do: the default "`swap delay`" for htmx is very
short, a few milliseconds.  That makes sense in most cases: you don't want to have much of a delay before you put the
new content into the DOM.  But, in this case, we want to give the CSS animation time to complete before we do the swap,
we want to give it a second, in fact.

Fortunately htmx has an option for the `hx-swap` annotation that allows you to set the swap delay: following the swap
type you can add `swap:` followed by a timing value to tell htmx to wait a specific amount of time before it swaps.  Let's
update our HTML to allow a one second delay before the swap is done for the delete action:

.The existing row actions
[source, html]
----
<td>
    <a href="/contacts/{{ contact.id }}/edit">Edit</a>
    <a href="/contacts/{{ contact.id }}">View</a>
    <a href="#" hx-delete="/contacts/{{ contact.id }}"
        hx-swap="outerHTML swap:1s" <1>
        hx-confirm="Are you sure you want to delete this contact?"
        hx-target="closest tr">Delete</a>
</td>
----
<1> A swap delay changes how long htmx waits before it swaps in new content.

With this modification, the existing row will stay in the DOM for an additional second, with the `htmx-swapping` class
on it.  This will give the row time to transition to an opacity of zero, giving the fade out effect we want.

Now, when a user clicks on a "`Delete`" link and confirms the delete, the row will slowly fade out and then, once it has
faded to a 0 opacity, it will be removed.  Pretty fancy, and all done in a declarative, hypermedia-oriented manner, no
JavaScript required.  (Well, obviously htmx is written in JavaScript, but you know what we mean: we didn't have to write
any JavaScript to implement the feature.)

== Bulk Delete

The final feature we are going to implement in this chapter is a "`Bulk Delete.`"  The current mechanism for deleting users
is nice, but it would be annoying if a user wanted to delete five or ten contacts at a time, wouldn't it?  For the bulk
delete feature, we want to add the ability to select rows via a checkbox input and delete them all in a single go by clicking
a "`Delete Selected Contacts`" button.

To get started with this feature, we'll need to add a checkbox input to each row in the `rows.html` template.  This input
will have the name `selected_contact_ids` and its value will be the `id` of the contact for the current row.

Here is what the updated code for `rows.html` looks like:

.Adding a checkbox to each row
[source, html]
----
{% for contact in contacts %}
<tr>
  <td><input type="checkbox" name="selected_contact_ids" value="{{ contact.id }}"></td> <1>
  <td>{{ contact.first }}</td>
  ... omitted
</tr>
{% endfor %}
----
<1> A new cell with the checkbox input whose value is set to the current contact's id.

We'll also need to add an empty column in the header for the table to accommodate the checkbox column.  With that
done we now get a series of check boxes, one for each row, a pattern no doubt familiar to you from the web:

.Checkboxes for our contact rows
image::screenshot_checkboxes.png[Table rows each have a checkbox in the first column]

If you are not familiar with or have forgotten the way checkboxes work in HTML: a checkbox will submit its value associated
with the name of the input if and only if it is checked.  So if, for example, you checked the contacts with the ids 3,
7 and 9, then those three values would all be submitted to the server.  Since all the checkboxes in this case have
the same name, `selected_contact_ids`, all three values would be submitted with the name `selected_contact_ids`.

=== The "`Delete Selected Contacts`" Button

The next step is to add a button below the table that will delete all the selected contacts.  We want this button, like
our delete links in each row, to issue an HTTP `DELETE`, but rather than issuing it to the URL for a given contact, like
we do with the inline delete links and with the delete button on the edit page, here we want to issue the `DELETE` to
the `/contacts` URL.

As with the other delete elements, we want to confirm that the user wishes to delete the contacts,
and, for this case, we are going to target the body of page, since we are going to re-render the whole table.

Here is what the button code looks like:

.The "`delete selected contacts`" button
[source, html]
----
<button hx-delete="/contacts" <1>
        hx-confirm="Are you sure you want to delete these contacts?" <2>
        hx-target="body"> <3>
    Delete Selected Contacts
</button>
----
<1> Issue a `DELETE` to `/contacts`.
<2> Confirm that the user wants to delete the selected contacts.
<3> Target the body.

Pretty easy.  One question though: how are we going to include the values of all the selected checkboxes in the
request?  As it stands right now, this is just a stand-alone button, and it doesn't have any information indicating that
it should include any other information in the `DELETE` request it makes.

Fortunately, htmx has a few different ways to include values of inputs with a request.

One way would be to use the `hx-include` attribute, which allows you to use a CSS selector to specify the elements
you want to include in the request.  That would work fine here, but we are going to use another approach that is a bit
simpler in this case.

By default, if an element is a child of a `form` element and makes a non-`GET` request, htmx will include all the values of
inputs within that form.  In situations like this, where there is a bulk operation for a table, it is common to enclose
the whole table in a form tag, so that it is easy to add buttons that operate on the selected items.

Let's add that form tag around the form, and be sure to enclose the button in it as well:

.The "`delete selected contacts`" button
[source, html]
----
    <form> <1>
        <table>
          ... omitted
        </table>
        <button hx-delete="/contacts"
                hx-confirm="Are you sure you want to delete these contacts?"
                hx-target="body">
            Delete Selected Contacts
        </button>
    </form> <2>

----
<1> The form tag encloses the entire table.
<2> The form tag also encloses the button.

Now, when the button issues a `DELETE`, it will include all the contact ids that have been selected as the
`selected_contact_ids` request variable.

=== The Server Side for Delete Selected Contacts

The server-side implementation is going to look like our original server-side code for deleting a contact.
In fact, once again, we can just copy and paste, and make a few fixes:

* We want to change the URL to `/contacts`.
* We want the handler to get _all_ the ids submitted as `selected_contact_ids` and iterate over each one, deleting the
  given contact.

Those are the only changes we need to make!  Here is what the server-side code looks like:

.The "`delete selected contacts`" button
[source, python]
----
@app.route("/contacts/", methods=["DELETE"]) <1>
def contacts_delete_all():
    contact_ids = list(map(int, request.form.getlist("selected_contact_ids"))) <2>
    for contact_id in contact_ids: <3>
        contact = Contact.find(contact_id)
        contact.delete() <4>
    flash("Deleted Contacts!") <5>
    contacts_set = Contact.all()
    return render_template("index.html", contacts=contacts_set)
----
<1> We handle a `DELETE` request to the `/contacts/` path.
<2> Convert the `selected_contact_ids` values submitted to the server from a list of strings to a list integers.
<3> Iterate over all of the ids.
<4> Delete the given contact with each id.
<5> Beyond that, it's the same code as our original delete handler: flash a message and render the `index.html` template.

So, we took the original delete logic and slightly modified it to deal with an array of ids, rather than a single id.

You might notice one other small change: we did away with the redirect that was in the original
delete code.  We did so because we are already on the page we want to re-render, so there is no reason
to redirect and have the URL update to something new.  We can just re-render the page, and the new list of contacts (sans the
contacts that were deleted) will be re-rendered.

And there we go, we now have a bulk delete feature for our application.  Once again, not a huge amount of code, and we
are implementing these features entirely by exchanging hypermedia with a server in the traditional, RESTful manner of
the web.


[.design-note]
.HTML Note: HTML5 Soup
****
[quote,Confucius]
The beginning of wisdom is to call things by their right names.

Elements like `<section>`, `<article>`, `<nav>`, `<header>`, `<footer>`, `<figure>` have become a sort of shorthand for HTML.

By using these elements, a page can make false promises, like `<article>` elements being self-contained, reusable entities, to clients like browsers, search engines and scrapers that can't know better. To avoid this:

* Make sure that the element you're using fits your use case. Check the HTML spec.
* Don't try to be specific when you can't or don't need to.
  Sometimes, `<div>` is fine.

The most authoritative resource for learning about HTML is the HTML specification.
The current specification lives on link:https://html.spec.whatwg.org/multipage[].footnote:[
The single-page version is too slow to load and render on most computers. 
There's also a developers' edition at /dev, but the standard version has nicer styling.]
There's no need to rely on hearsay to keep up with developments in HTML.

Section 4 features a list of all available elements,
including what they represent, where they can occur, and what they are allowed to contain.
It even tells you when you're allowed to leave out closing tags!
****