This commit adds the ability for expressions to be evaluated against
specific contexts. It replaces the previous behaviour where
expressions were evaluated against a specific feature and could
utilise fragile global "special columns".
Now, expressions are instead evaluated using a context designed for
each individual expression. This is done via QgsExpressionContext
and QgsExpressionContextScope objects.
A QgsExpressionContextScope encapsulates the variables and functions
relating to a specific context. For instance, scopes can be created
for "global" variables (such as QGIS version, platform, and user-set
variables specified within the QGIS options dialog. Think things
like user name, work department, etc), or for "project" variables
(eg project path, title, filename, and user-set variables set
through the project properties dialog. Project version, reference
number, that kind of thing). Many more scopes are planned, including
map layer scopes (variables for layer name, id, user-set variables
through the layer properties dialog), composer scopes, etc...
QgsExpressionContextScopes are 'stacked' into a QgsExpressionContext
object. Scopes added later to a QgsExpressionContext will override
any variables or functions provided by earlier scopes, so for
instance a user could override their global 'author' variable set
within QGIS options with a different 'author' set via the project
properties dialog.
The intended use is that a QgsExpressionContext is created before
a batch set of QgsExpression evaluations. Scopes are then added to
the context based on what makes sense for that particular
expression. Eg, almost all contexts will consist of the global
scope and project scope, and then additional scopes as required.
So a composer label would be evaluated against a context
consisting of the global scope, project scope, composition scope
and finally composer item scope. The batch set of expression
evaluations would then be performed using this context, after which
the context is discarded. In other words, a context is designed
for use for one specific set of expression evaluations only.
Adds an argument to QgsExpression::replaceExpressionText to allow
passing an optional QgsDistanceArea to use during calculations.
Ensure that html and label composer items correctly specify this
argument.
- update methods of existing classes
- add comment to methods missing in the sip bindings
- split up collective sip files into single files and use
same directory structure in python/ as in src/
- add a lot of missing classes (some might not make sense because of
missing python methods in those classes)
- remove some non-existing methods from the header files
- add scripts/sipdiff
- replace some usages of std::vector and std::set with QVector/QSet
- year,month,day,hour,minute,second extraction
- new QgsInterval objects - handles time amounts
- age() function returns the difference between two datetimes
- $now returns the current date and time
- support for datetime - interval = new datetime
- tointerval, todate, totime functions
- tests ;)
- quoting of columns (#5536 and #5542)
- quoting of string
- do not quote of numbers (avoids type cast during evaluation)
- use at least 4 floating point digits for doubles
