Run function from QGIS plugin with canvasReleaseEvent

Question

In this topic https://gis.stackexchange.com/a/430301/192997 it is explained how to run a basic function from a canvas event :

Code edited by changing QgsMapTool by QgsMapToolPan:

def test(event):
    print('canvas release')
tool = QgsMapToolPan(iface.mapCanvas())
tool.canvasReleaseEvent = test
iface.mapCanvas().setMapTool(tool)

I try to do the same to run a function from a plugin (made with PluginBuilder) with canvasReleaseEvent:

class MyPlugin:
    """QGIS Plugin Implementation."""
def __init__(self, iface):
    """Constructor."""

    self.iface = iface
    self.dlg = MyPluginDialog()

    #code......


def tr(self, message):

    #code......

def add_action(
    self,
    icon_path,
    text,
    callback,
    enabled_flag=True,
    add_to_menu=True,
    add_to_toolbar=True,
    status_tip=None,
    whats_this=None,
    parent=None):

    #code......

def initGui(self):

    #code......


def unload(self):

    #code......


def run(self):

if self.first_start == True:
    self.first_start = False

# show the dialog
self.dlg.show()
# Run the dialog event loop
result = self.dlg.exec_()
# See if OK was pressed
if result:
    # Do something useful here - delete the line containing pass and
    # substitute with your code.
    pass

tool = QgsMapToolPan(self.iface.mapCanvas())
tool.canvasReleaseEvent = lambda event self.function_from_plugin(event)
self.iface.mapCanvas().setMapTool(tool)


def function_from_plugin(self, event):
    if self.dlg.horizontalSlider.value() == 1:
    ### rest of the code

but I get this error : AttributeError: 'QgsMapMouseEvent' object has no attribute 'dlg'

I looked other topics but I can't manage to understand everything:

How to link my QGIS plugin main class to mouse events from a custom Tool?

https://gis-ops.com/qgis-3-plugin-tutorial-pyqt-signal-slot-explained/

Answer 1

In your plugin script, you define tool in the main scope of the class. It means that tool runs every time QGIS loads the plugin. That means the plugin button will have no function.

You must define tool in run method to bind the tool to the button in the plugin toolbar. Because the plugin button runs run method.

You can use the following structure:

...
def function_from_plugin(self, event):
    if self.dlg.horizontalSlider.value() == 1:
        ### rest of the code
def run(self):
    tool = QgsMapTool(self.iface.mapCanvas())
    tool.canvasReleaseEvent = lambda event: self.function_from_plugin(event)
    self.iface.mapCanvas().setMapTool(tool)

If your plugin is not very complex, I recommend using Minimal QGIS Plugin, instead of Plugin Builder, as follows. In this case, you just need __init__.py and metadata.txt.

from qgis.gui import QgsMapTool
from PyQt5.QtWidgets import QAction
def classFactory(iface):
    return MinimalPlugin(iface)
class MinimalPlugin:
    def init(self, iface):
        self.iface = iface
def initGui(self):
    self.action = QAction('Go!', self.iface.mainWindow())
    self.action.triggered.connect(self.run)
    self.iface.addToolBarIcon(self.action)

def unload(self):
    self.iface.removeToolBarIcon(self.action)
    del self.action

def function_from_plugin(self, event):
    print("canvas released")

def run(self):
    tool = QgsMapTool(self.iface.mapCanvas())
    tool.canvasReleaseEvent = lambda event: self.function_from_plugin(event)
    self.iface.mapCanvas().setMapTool(tool)