How to add a custom SymbolLayerV2 to a Vectorlayer in QGIS

Question

I'm trying to add a symbolLayer with a line-decoration (line-arrow) to a vectorlayer of linestrings. what i am trying at the moment is to create a new layer and set it's QgsLineSymbol with a list of symbollayers.

# Symbology
        simple_props = { 'width' : '1', 'color' : '255,255,255' }
        deco_props = { 'width' : '0.26', 'color' : '0,0,255' }

        simple = QgsSymbolLayerV2Registry.instance().symbolLayerMetadata("SimpleLine").createSymbolLayer(simple_props)
        deco = QgsSymbolLayerV2Registry.instance().symbolLayerMetadata("LineDecoration").createSymbolLayer(deco_props)
        s = QgsLineSymbolV2([simple, deco])

        self.ll.setRendererV2( QgsSingleSymbolRendererV2( s ) ) 

Every time QGIS trys to set the QgsLineSymbolV2 it crashes with a win32-exception. what am i doing wrong, or is there a possibilty to just add a new symbollayer to a existing vectorlayer?

Answer 1

So i don't know what was wrong, but i found a workaround. now i just add a second symbollayer to the existing symbol in the layer-renderer.

    sl = QgsSymbolLayerV2Registry.instance().symbolLayerMetadata("LineDecoration").createSymbolLayer({ 'width' : '0.26', 'color' : '0,0,0' })
    symbollist = layer.rendererV2().symbols()
    symbol = symbollist[0]
    symbol.appendSymbolLayer(sl)

Answer 2

Over 5 years later I realise, but when I tried the accepted answer (in QGIS 2.18), I got a message:

AttributeError: 'NoneType' object has no attribute 'createSymbolLayer'

To fix this, I needed to use ArrowLine instead of LineDecoration as follows:

sl = QgsSymbolLayerV2Registry.instance().symbolLayerMetadata("ArrowLine").createSymbolLayer({ 'width' : '0.26', 'color' : '0,0,0' })  
symbollist = layer.rendererV2().symbols()  
symbol = symbollist[0]  
symbol.appendSymbolLayer(sl)