QGIS - Change project variable using a dropdown menu

Question

This is my first time in trying to develop any automation within the QGIS environment.

I am trying to change a user defined project variable – DMA select – programmatically.

The changed project variable is then used to change the parameter in rule based symbologies for 2 layers using the single variable.

I have written a script that can change the project variable on the Python console when I change the variable manually in the script.

project = QgsProject.instance()
QgsExpressionContextUtils.setProjectVariable(project,'DMA_Select','NORTHMEAD')
iface.mapCanvas().refreshAllLayers()

I have also created a form in QT Designer with a dropdown of the possible values for the project variable.

What do I do next to combine this together so that the selected vale from the combo box can change the project variable?

I've searched around the internet and guides/books for this particular solution and have not came up with anything. I can see that we can bind the form/UI to layers but cannot find an example where it is used to change a project or global variable.

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I have inserted in the file change_variable.py - between def initGui(self): and def unload(self):

    def VariableValue(self):
        self.setVariableValue(value)):
        project = QgsProject.instance()
        QgsExpressionContextUtils.setProjectVariable(project,'DMA_Select', value)

        #iface is accessed by self.iface inside the plugin
        self.iface.mapCanvas().refreshAllLayers()

and then in the def run(self): section after

#comboBox is the UI objectName
self.dlg.comboBox.currentTextChanged.connect(lambda currentText: setVariableValue(currentText))

When I open QGIS i get the error message on the plugin load

---

change_variable.py changed

The plugin loads okay but when run and dropdown is selected I get this error

Answer 1

I would solve this issue by creating my own QGIS plugin using PluginBuilder. It is not complicated and there are many guides on how to do that. You need it to be a plugin so that the main QGIS app components can be accessible.

After creating the plugin find the .ui file in the main plugin directory and add the UI components the way you created them before in Qt Designer.

Now, in the main plugin code (PluginName.py):

Define a setVariableValue function somewhere in the Plugin class - it is necessary for the function to have access to the main QGIS application components.

def setVariableValue(self, value):
    project = QgsProject.instance()
    QgsExpressionContextUtils.setProjectVariable(project,'DMA_Select', value)
    #iface is accessed by self.iface inside the plugin
    self.iface.mapCanvas().refreshAllLayers()

Finally, connect the function to the comboBox currentTextChanged slot in the run(self) method in the main plugin class. The variable value will be changed every time you select a new value from the list.

#comboBox is the UI objectName
self.dlg.comboBox.currentTextChanged.connect(lambda currentText: self.setVariableValue(currentText))

Then you need to reload the plugin.