C:/Program Files/QGIS 2.18/apps/qgis/python/plugins/processing/algs/qgis/SetRasterStyle.py

layer.triggerRepaint()

def processAlgorithm(self, progress):
    filename = self.getParameterValue(self.INPUT)
    layer = dataobjects.getObjectFromUri(filename)

    style = self.getParameterValue(self.STYLE)
    if layer is None:
        dataobjects.load(filename, os.path.basename(filename), style=style)
        self.getOutputFromName(self.OUTPUT).open = False
    else:
        with open(style) as f:
            xml = "".join(f.readlines())
        d = QDomDocument()
        d.setContent(xml)
        n = d.firstChild()
        layer.readSymbology(n, '')
        self.setOutputValue(self.OUTPUT, filename)
        iface.mapCanvas().refresh()
        iface.legendInterface().refreshLayerSymbology(layer)
        layer.triggerRepaint()