Run QgsProcessingAlgorithm with explicitely defined QgsProcessingContext and QgisInterfaces.

[jakimowb]

Description

This PR addresses #60764 and enables the processing plugin to execute QgsProcessingAlgorithms with none-standard QgsProcessingContext and QgisInterface.

It is now possible use the algorithm dialogs with a set of map layers that differs from layers inside the QgsProject.instance().
This allows plugin developers to focus on layers relevant for their QGIS plugin, or to avoid messing up with layers inside the global project QgsProject.instance(). Developers can now provide a QgsProcessingContext object (with its own context.project()).

Some algorithms callers of processing algorithms may want to add processing results back to a QgsMapCanvas, or read a mapcanvas extent.
By providing an own QgsInterface aka iface different to that from the main QGIS GUI (from qgis.utils import iface), it becomes possible to provide an explicit mapcanvas and layer handling.

Examples for the new functionality are given by two new unit test in
python/plugins/processing/tests/GuiTest.py:

1. def testIndividualProcessingContext(self): shows how AlgorithmDialog and BatchAlgorithmDialog can be called with a self-defined context + iface,

2. def testProcessingPlugin_executeAlgorithm(self): shows how ProcessingPlugin.executeAlgorithm(...) can be called with an explicitly defined context and iface. As this test creates a blocking dialogs it is disable for running in a CI.

[github-actions]

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[nyalldawson]

Some algorithms may want to add processing results back to a QgsMapCanvas, or read a mapcanvas extent.
By providing an own QgsInterface aka iface different to that from the main QGIS GUI (from qgis.utils import iface), it becomes possible to provide an explicit mapcanvas and layer handling.

Is this just theoretical? None of the built-in algorithms do this. And if an algorithm does try this, it still won't have access to the iface object as that's not part of the context passed to algorithms anyway.

[jakimowb]

Sorry, I actually mean the callers of QgsProcessingAlgorithms and have corrected my description above.

The ProcessingPanel uses the iface to create the QgsWidgetContext for parameter widgets, and Postprocessing.py handleAlgorithmResults calls iface.setActiveLayer(<produced layer>). If both were enabled to use a different context.project() and iface, then a much more flexible use of the processing framework would be possible.

For example, the EnMAP-Box and the EO TimeSeriesViewer plugins extend QGIS by additional QMainWindows, e.g. to be used on a 2nd screen, and visualize map layers in multiple QgsMapCanvases side by side.

Both implement a QgisInterface and have their own QgsLayerTreeView and QgsProject to not mess-up the standard QGIS layer tree and QgsProject.instance():

class EnMAPBox(QgisInterface, QObject, QgsExpressionContextGenerator, QgsProcessingContextGenerator):
   
class EOTimeSeriesViewer(QgisInterface, QObject):

If QgsProcessingAlgorithms like Create Layer from extent (native:extenttolayer) are started from them, users expect that parameterization dialogs also use one of the EnMAP-Box/EO Time Series map canvases instead of the main QGIS map canvas, e.g. to get the canvas extent. If there is an "open after running" option, user expect that produced layers are added to the EnMAP-Box/EO Time Series Viewer GUI (as now they are added to the QGIS layer tree and different steps are required to get them into the EnMAP-Box).

To comply with the QgisInterface, some of the QgisInterface functions can be implemented as (pseudo-code):

    def mapCanvas(self) -> QgsMapCanvas:
        return <last map canvas that was used / touched by a user>
    def activeLayer(self) -> QgsMapCanvas:
        return <last layer selected in a layer tree>
    def mainWindow(self) -> QWidget:
        return <main EnMAP-Box / EO Time Series Viewer window or other widget>