This is a subclass of QgsProcessingParameterNumber, but specifically
for numeric parameters which represent distances. It is linked
to a parent parameter, from which the distance unit will
be determined, and is shown using a dedicated distance widget
within the processing parameters panel. This widget shows
the distance unit.
This avoids the confusion when running algorithms which
use distances where the unit depends on a layer or CRS parameter -
e.g. the distance parameter in the buffer algorithm gives
the distance in layer units... so now we can show those units
directly within the dialog. Hopefully this leads to less
user confusion and accidental "1000 degree buffers"!
Additionally - if the unit is in degrees, a small warning
icon is shown next to the parameter. The tooltip for this
icon advises users to reproject data into a suitable
projected local coordinate system.
Initially implemented for the native buffer and single
sided buffer algorithm only - but more will be added.
Fixes#16290
This is basically the equivalent of the dissolve algorithm, but
instead of a dissolving overlapping geometries the geometries
are instead just collected together into a multipart geometry.
It's designed to slot between the 'promote to multipart' algorithm
(which performs no collection of geometries - it just converts
singleparts to multiparts with 1 part) and the more complex
all-encompassing 'aggregate' algorithm.
initAlgorithm() method
This allows 2 benefits:
- algorithms can be subclassed and have subclasses add additional
parameters/outputs to the algorithm. With the previous approach
of declaring parameters/outputs in the constructor, it's not
possible to call virtual methods to add additional parameters/
outputs (since you can't call virtual methods from a constructor).
- initAlgorithm takes a variant map argument, allowing the algorithm
to dynamically adjust its declared parameters and outputs according
to this configuration map. This potentially allows model algorithms which
can be configured to have variable numbers of parameters and
outputs at run time. E.g. a "router" algorithm which directs
features to one of any number of output sinks depending on some
user configured criteria.
This somewhat changes the meaning of outputs from processing 2.x.
In 2.x processing outputs were used both as a method of specifying
inputs to algorithms (file paths to destination layers created
by the algorithm) AND pure outputs (such as statistics calculated
by the algorithm).
This is now split. The old input-type-outputs (destination layers)
are now input parameters (since the parameter value IS an input to the
algorithm). To differentiate them from parameters indicating pure
input layers a new "isDestination()" method was added to
QgsProcessingParameterDefinition.
Output definitions are now purely indications of values CREATED
by the algorithms. Suitable candidates are the existing calculated
stats and actual file path/URI of any layers created by the algorithm.
Moving forward we should ensure all algorithms output as much
useful information as possible - e.g. number of features processed,
number of skipped features, count null geometries encountered, etc...
