Programatically check for mouse click in PyQGIS

Question

I want to know how to check for a mouse click in QGIS. I am trying to write a Python plugin and want to provide functionality similar to the "Select Single Feature" tool that already exists in QGIS.

I checked the QGIS API docs and found

QgsMapCanvas::CanvasProperties::mouseButtonDown

This sounds promising. I have a QgsMapCanvas object but I can't see how to access the mouseButtonDown attribute.

Answer 1

The best way to make a new tool like the Select Single Feature tool is to inherit from the QgsMapTool class. When your tool is active, which can be set using QgsMapCanvas::setMapTool, any keyboard or click events the canvas gets will be passed onto your custom tool.

Here is a basic QgsMapTool class

class PointTool(QgsMapTool): 
 def __init__(self, canvas):
 QgsMapTool.__init__(self, canvas)
 self.canvas = canvas 
 def canvasPressEvent(self, event):
 pass
 def canvasMoveEvent(self, event):
 x = event.pos().x()
 y = event.pos().y()
 point = self.canvas.getCoordinateTransform().toMapCoordinates(x, y)
 def canvasReleaseEvent(self, event):
 #Get the click
 x = event.pos().x()
 y = event.pos().y()
 point = self.canvas.getCoordinateTransform().toMapCoordinates(x, y)
 def activate(self):
 pass
 def deactivate(self):
 pass
 def isZoomTool(self):
 return False
 def isTransient(self):
 return False
 def isEditTool(self):
 return True

You can do what you need in canvasReleaseEvent, etc

To set this tool active you just do:

tool = PointTool(qgis.iface.mapCanvas())
qgis.iface.mapCanvas().setMapTool(tool)

• robert: Thank you for your response. Its exactly what I need. However, when I try implementing this solution I get the following error: class PointTool(QgsMapTool): NameError: name 'QgsMapTool' is not defined. Any ideas?
• Nathan W: You will need to use from qgis.gui import QgsMapTool at the top
• robert: Last question... How do you then deactivate this tool?
• Nathan W: Set maptool to something else, or to None. I would save what the user had selected using QgsMapCanvas.mapTool() restoring it after you are done.
• wannik: @NathanW "To set maptool to something else" also means that I click 'Pan Map' on the toolbar, right?

Answer 2

I think you can do this with a combination of using QGIS "canvasClicked" but also SIGNAL/SLOTS to deal with the response:

result = QObject.connect(self.clickTool, SIGNAL("canvasClicked(const QgsPoint &, Qt::MouseButton)"), self.handleMouseDown)

Not tried but should give you some more information to start looking at. There is a tutorial here where someone is using this to build a very basic plugin.

• Nathan W: They are using the built in QgsMapToolEmitPoint class which will give you the basic start for a tool. A good way to connect to signals in PyQt is using this syntax self.clickTool.canvasClicked.connect(self.handleMouseDown)

Answer 3

Try something like this (this is to select a point):

def run(self):
 self.pointEmitter = QgsMapToolEmitPoint(self.iface.mapCanvas())
 QObject.connect( self.pointEmitter, SIGNAL("canvasClicked(const QgsPoint, Qt::MouseButton)"), self.selectNow)
 self.iface.mapCanvas().setMapTool( self.pointEmitter )
def selectNow(self, point, button):
 #QMessageBox.information(None, "Clicked coords", " x: " + str(point.x()) + " Y: " + str(point.y()) )
 layer = self.iface.activeLayer()
 if not layer or layer.type() != QgsMapLayer.VectorLayer:
 QMessageBox.warning(None, "No!", "Select a vector layer")
 return
 width = self.iface.mapCanvas().mapUnitsPerPixel() * 2
 rect = QgsRectangle(point.x() - width,
 point.y() - width,
 point.x() + width,
 point.y() + width)
 rect = self.iface.mapCanvas().mapRenderer().mapToLayerCoordinates(layer, rect)
 layer.select([], rect)
 feat = QgsFeature()
 ids = []
 while layer.nextFeature(feat):
 ids.append( feat.id() )
 layer.setSelectedFeatures( ids )

• Nathan W: I would use the self.clickTool.canvasClicked.connect(self.handleMouseDown) syntax to connect to signals as it's much cleaner.