Dynamically move map canvas and main window using PyQGIS

Question

I am trying to create plugin that will open new window with some layer, Dynamically "pan" my window, when main window is pan. So boundingbox coordinates should be the same in both windows.
I need new window to move with main QGIS window. I've found out in documentation useful code for map canvas:

import sys, os
class MyWnd(QMainWindow):

    def __init__(self, layer):
        QMainWindow.__init__(self)

        self.canvas = QgsMapCanvas()
        self.canvas.setCanvasColor(QtCore.Qt.white)
        self.canvas.setExtent(layer.extent())
        self.canvas.setLayerSet([QgsMapCanvasLayer(layer)])
        self.setCentralWidget(self.canvas)

        actionZoomIn = QAction("Zoom in", self)
        actionZoomOut = QAction("Zoom out", self)
        actionPan = QAction("Pan", self)
        actionZoomIn.setCheckable(True)
        actionZoomOut.setCheckable(True)
        actionPan.setCheckable(True)

        self.connect(actionZoomIn, QtCore.SIGNAL("triggered()"), self.zoomIn)
        self.connect(actionZoomOut, QtCore.SIGNAL("triggered()"), self.zoomOut)
        self.connect(actionPan, QtCore.SIGNAL("triggered()"), self.pan)

        self.toolbar = self.addToolBar("Canvas actions")        
        self.toolbar.addAction(actionZoomIn)
        self.toolbar.addAction(actionZoomOut)
        self.toolbar.addAction(actionPan)

        # create the map tools
        self.toolPan = QgsMapToolPan(self.canvas)
        self.toolPan.setAction(actionPan)
        self.toolZoomIn = QgsMapToolZoom(self.canvas, False) # false = in
        self.toolZoomIn.setAction(actionZoomIn)
        self.toolZoomOut = QgsMapToolZoom(self.canvas, True) # true = out
        self.toolZoomOut.setAction(actionZoomOut)
        self.pan()

    def zoomIn(self):
        self.canvas.setMapTool(self.toolZoomIn)

    def zoomOut(self):
        self.canvas.setMapTool(self.toolZoomOut)

    def pan(self):
        self.canvas.setMapTool(self.toolPan)

And using class below I get coordinates from main canvas and set coordinates to my canvas (part of QGIS plugin):

def run(self):
    """Run method"""
    import re

    wnd = MyWnd(self.iface.activeLayer())
    wnd.show()
    canv = qgis.utils.iface.mapCanvas()
    coords = canv.mapRenderer().extent().toString()
    coords = re.findall("([0-9]*\.[0-9]*)", coords)
    wnd.canvas.setExtent(QgsRectangle(float(coords[0]), float(coords[1]), float(coords[2]),float(coords[3])))
    wnd.canvas.refresh()

I think this part of code can be used in loop (if coordinates change - I will set new coordinates in own canvas). But I don't sure if it is a good way.
Is there any other way to solve this issue? Provide me with right way, please.

Answer 1

When a canvas changes its extent a extentsChanged signal is emitted. When you connect this signal to a method (called slot), then you are able to run this method whenever the extent changes.

To deploy this mechanism I altered your code in the following way.

# Declare a global variable to hold the reference to QGIS canvas
canv = None

# here comes your class definition
class MyWnd(QMainWindow):
    def __init__(self, layer):
        # ...
        self.show()

    # ...

    # new method to sync both canvases
    def sync(self):
        global canv
        self.canvas.setExtent(canv.extent())
        self.canvas.refresh()


# next lines would go into your run(), to run them at python console
# put them in the main code block
canv = iface.mapCanvas()
layer = iface.activeLayer()
wnd = MyWnd(layer)
canv.extentsChanged.connect(wnd.sync)

Now, when you change the extent of mapcanvas in Qgis, extent of your new window changes accordingly. Don't forget to disconnect the signal when your plugin stops working.