gistfile1.txt

from PyQt4.QtGui import *
from qgis.gui import *

map = iface.mapCanvas().map()
othermap = QgsMapCanvasMap(iface.mapCanvas())
othermap.setZValue(-9.0)
legend = iface.legendInterface()

def updatebackmap():
    def updatemap():
        othermap.setContent(job.renderedImage(), settings.extent())
    settings = QgsMapSettings(iface.mapCanvas().mapSettings())
    groups = legend.groupLayerRelationship()
    slidergroup = [layers for group, layers in groups if group == 'Slider'][0]
    settings.setLayers(slidergroup)
    job = QgsMapRendererParallelJob(settings)
    job.start()
    job.finished.connect(updatemap)

def value(i):
    i = float(i) / 100
    othermap.setOpacity(i)

iface.mapCanvas().extentsChanged.connect(updatebackmap)

slider = QSlider(1)
slider.setTickInterval(0.1)
slider.show()
slider.valueChanged.connect(value)

sipe.py

from PyQt4.QtGui import *
from PyQt4.QtCore import *
from qgis.gui import *
 
class SwipeMap(QgsMapCanvasMap):
    def __init__(self, canvas):
        super(SwipeMap, self).__init__(canvas)
        self.width = 0
        self.setZValue(-9.0)
        self.image = QImage()
        
    def setContent(self, image, rect):
        self.image = image
        self.setRect(rect)
        
    def setWidth(self, width):
        self.width = width
        self.update()
        
    def paint(self, painter, *args):
        w = self.width
        h = self.boundingRect().height() - 2
        image = self.image.copy(0,0, self.width, h)
        painter.drawImage(QRect(0,0,w,h), image )
        painter.drawLine(w-1,0,w-1,h-1)

map = iface.mapCanvas().map()
othermap = SwipeMap(iface.mapCanvas())
legend = iface.legendInterface()

def updatebackmap():
    def updatemap():
        othermap.setContent(job.renderedImage(), settings.extent())
    settings = QgsMapSettings(iface.mapCanvas().mapSettings())
    groups = legend.groupLayerRelationship()
    slidergroup = [layers for group, layers in groups if group == 'Slider'][0]
    settings.setLayers(slidergroup)
    job = QgsMapRendererParallelJob(settings)
    job.start()
    job.finished.connect(updatemap)

def value(i):
    w = (i * iface.mapCanvas().map().boundingRect().width()) / 100
    othermap.setWidth(w)

iface.mapCanvas().mapCanvasRefreshed.connect(updatebackmap)

slider = QSlider(1)
slider.setTickInterval(1)
slider.setRange(0,100)
slider.show()
slider.valueChanged.connect(value)