[FEATURE][processing] Allow expression variables to be set for a model

This adds a new "Model Variables" dock panel to the model editor, allowing
users to create and set custom expression variables for use in the model.
These variables are available anywhere expressions are (correctly) evaluated
within the model, so can be used as input parameter values for child
algorithms, within data-defined dynamic parameters, etc.

The use case here is for models which use a constant value throughout
multiple steps within the model (e.g. @target_resolution: a target
raster resolution, @max_simplification: a simplification value for
input features coming from different sources, etc), allowing users
one single place to define and edit these constant values (instead
of hunting down and setting them in multiple places throughout the model).

These variables are stored within the model itself, and are not exposed
outside of the model designer dialog.

python/plugins/processing/modeler/ModelerDialog.py

@@ -79,14 +79,17 @@ from qgis.core import (Qgis,
                        QgsProcessingUtils,
                        QgsProcessingModelAlgorithm,
                        QgsProcessingModelParameter,
-                       QgsProcessingParameterType
+                       QgsProcessingParameterType,
+                       QgsExpressionContextScope,
+                       QgsExpressionContext
                        )
 from qgis.gui import (QgsMessageBar,
                       QgsDockWidget,
                       QgsScrollArea,
                       QgsFilterLineEdit,
                       QgsProcessingToolboxTreeView,
-                      QgsProcessingToolboxProxyModel)
+                      QgsProcessingToolboxProxyModel,
+                      QgsVariableEditorWidget)
 from processing.gui.HelpEditionDialog import HelpEditionDialog
 from processing.gui.AlgorithmDialog import AlgorithmDialog
 from processing.modeler.ModelerParameterDefinitionDialog import ModelerParameterDefinitionDialog
@@ -140,6 +143,8 @@ class ModelerDialog(BASE, WIDGET):
 
         self.setupUi(self)
 
+        self._variables_scope = None
+
         # LOTS of bug reports when we include the dock creation in the UI file
         # see e.g. #16428, #19068
         # So just roll it all by hand......!
@@ -181,8 +186,7 @@ class ModelerDialog(BASE, WIDGET):
         self.scrollArea_1.setWidget(self.scrollAreaWidgetContents_1)
         self.verticalDockLayout_1.addWidget(self.scrollArea_1)
         self.propertiesDock.setWidget(propertiesDockContents)
-        self.addDockWidget(Qt.DockWidgetArea(1), self.propertiesDock)
-        self.propertiesDock.setWindowTitle(self.tr("Model properties"))
+        self.propertiesDock.setWindowTitle(self.tr("Model Properties"))
 
         self.inputsDock = QgsDockWidget(self)
         self.inputsDock.setFeatures(QDockWidget.DockWidgetFloatable | QDockWidget.DockWidgetMovable)
@@ -243,6 +247,22 @@ class ModelerDialog(BASE, WIDGET):
         self.searchBox.setToolTip(self.tr("Enter algorithm name to filter list"))
         self.searchBox.setShowSearchIcon(True)
 
+        self.variables_dock = QgsDockWidget(self)
+        self.variables_dock.setFeatures(QDockWidget.DockWidgetFloatable | QDockWidget.DockWidgetMovable)
+        self.variables_dock.setObjectName("variablesDock")
+        self.variables_dock_contents = QWidget()
+        vl_v = QVBoxLayout(self.algorithmsDockContents)
+        vl_v.setContentsMargins(0, 0, 0, 0)
+        self.variables_editor = QgsVariableEditorWidget()
+        vl_v.addWidget(self.variables_editor)
+        self.variables_dock_contents.setLayout(vl_v)
+        self.variables_dock.setWidget(self.variables_dock_contents)
+        self.addDockWidget(Qt.DockWidgetArea(1), self.variables_dock)
+        self.variables_dock.setWindowTitle(self.tr("Variables"))
+        self.addDockWidget(Qt.DockWidgetArea(1), self.propertiesDock)
+        self.tabifyDockWidget(self.propertiesDock, self.variables_dock)
+        self.variables_editor.scopeChanged.connect(self.variables_changed)
+
         self.bar = QgsMessageBar()
         self.bar.setSizePolicy(QSizePolicy.Minimum, QSizePolicy.Fixed)
         self.centralWidget().layout().insertWidget(0, self.bar)
@@ -455,6 +475,7 @@ class ModelerDialog(BASE, WIDGET):
         else:
             self.model = QgsProcessingModelAlgorithm()
             self.model.setProvider(QgsApplication.processingRegistry().providerById('model'))
+        self.update_variables_gui()
 
         self.fillInputsTree()
 
@@ -492,6 +513,18 @@ class ModelerDialog(BASE, WIDGET):
             self.model.setHelpContent(dlg.descriptions)
             self.hasChanged = True
 
+    def update_variables_gui(self):
+        variables_scope = QgsExpressionContextScope(self.tr('Model Variables'))
+        for k, v in self.model.variables().items():
+            variables_scope.setVariable(k, v)
+        variables_context = QgsExpressionContext()
+        variables_context.appendScope(variables_scope)
+        self.variables_editor.setContext(variables_context)
+        self.variables_editor.setEditableScopeIndex(0)
+
+    def variables_changed(self):
+        self.model.setVariables(self.variables_editor.variablesInActiveScope())
+
     def runModel(self):
         if len(self.model.childAlgorithms()) == 0:
             self.bar.pushMessage("", self.tr("Model doesn't contain any algorithm and/or parameter and can't be executed"), level=Qgis.Warning, duration=5)
@@ -722,6 +755,8 @@ class ModelerDialog(BASE, WIDGET):
             self.textName.setText(alg.name())
             self.repaintModel()
 
+            self.update_variables_gui()
+
             self.view.centerOn(0, 0)
             self.hasChanged = False
         else:

src/gui/processing/qgsprocessingmodelerparameterwidget.cpp

@@ -198,6 +198,7 @@ QgsExpressionContext QgsProcessingModelerParameterWidget::createExpressionContex
     QStringList highlightedVariables = childScope->variableNames();
     QStringList highlightedFunctions = childScope->functionNames();
     highlightedVariables += algorithmScope->variableNames();
+    highlightedVariables += mModel->variables().keys();
     highlightedFunctions += algorithmScope->functionNames();
     c.setHighlightedVariables( highlightedVariables );
     c.setHighlightedFunctions( highlightedFunctions );

src/gui/processing/qgsprocessingwidgetwrapper.cpp

@@ -326,6 +326,7 @@ QgsExpressionContext QgsProcessingGuiUtils::createExpressionContext( QgsProcessi
     QStringList highlightedVariables = childScope->variableNames();
     QStringList highlightedFunctions = childScope->functionNames();
     highlightedVariables += algorithmScope->variableNames();
+    highlightedVariables += widgetContext.model()->variables().keys();
     highlightedFunctions += algorithmScope->functionNames();
     c.setHighlightedVariables( highlightedVariables );
     c.setHighlightedFunctions( highlightedFunctions );