How to filter the MouseWheel event in a QGIS C++ plugin?

Question

I’m developing a C++ plugin for Qgis and I want to filter/override the mouse-wheel event.

I created a “maptool” to select a polygon feature on a vector layer, the feature has two attributes : an id & an integer H.
I want to override the mouse wheel event so that once I select a polygon, the wheel will allow me to change (increment/decrement) the value of attribute H and not zoom/unzoom the mapcanvas!

I created and “eventFilter” on my plugin’s base class which I install on the mapcanvas (QgsMapCanvas::installEventFilter()) once my maptool is activated, works fine to filter a key press for example but when it comes to the mouse-wheel it seems that it bypasses the eventFilter and does the usual zoom/unzoom action !!

Any ideas on how to solve this problem?

Thank you !

Answer 1

This code should do what you need:

from PyQt4.QtCore import QEvent, QObject
from PyQt4.QtGui import QWheelEvent
from qgis.gui import QgsMapCanvas
from qgis.core import QgsMessageLog
import qgis

class EventHook(QObject):
  def eventFilter(self, object, event):
    if event.type() == QEvent.Wheel:
      QgsMessageLog.logMessage("Wheel event delta was %s" % event.delta(),"plugin")
      return True
    else:
      return False

hook = EventHook()
qgis.utils.iface.mapCanvas().viewport().installEventFilter(hook)

I would be careful how you use this as you are changing how QGIS behaves.

Like @dakcarto said, a better method is to popup a dialog, or widget, that grabs mouse wheel focus to adjust the required value.

Make sure you clean up using qgis.utils.iface.mapCanvas().viewport().removeEventFilter(hook)

Answer 2

@Name - It seems the event filter is the way to go. Be careful, though, they can slow down an app, as they filter many other events. Can't help you without any code to go on.

Alternatively, you could have your plugin pop up a 'zoom pad' to receive the wheeling focus. This small window, or dock widget, could be a down-and-dirty zoom-in on your selected object, or just a widget that accepts mouse wheeling, with feedback info on the wheeling (scaling). This could directly affect the selected object, instead of the map canvas, and offer other controls beyond mouse wheeling.

Creating another window also allows you to override the protected member functions (like the wheelEvent) of a widget (like QgsMapCanvas) to do what you want.

A new window/dock could also offer the user an interface from which to chose which attribute is being incremented by the mouse wheel.