Showing transparency slider by default in Layers Panel of QGIS?

Is it possible to have a transparency slider shown by default in the Layer window, i.e. directly after adding a new layer, instead of having it to enable for each layer manually via layer-properties?

Answer

This was briefly mentioned in QGIS - Developer forum where you need to use QgsMapLayer::setCustomPropertylayer to enable the embedded widget for your layer. We can then add an itemAdded event so that whenever a layer is added, it will automatically be shown with the transparency widget.

So you could use something like the following in the Python Console:

def transparency_slider():
    for layer in QgsMapLayerRegistry.instance().mapLayers().values():
        if layer.customProperty("embeddedWidgets/count") != 1 or layer.customProperty("embeddedWidgets/0/id") != u'transparency':
            layer.setCustomProperty("embeddedWidgets/count", 1)
            layer.setCustomProperty("embeddedWidgets/0/id", "transparency")         

        else:
            pass
        qgis.utils.iface.legendInterface().refreshLayerSymbology(layer)

# Connect "itemAdded" event to "transparency_slider" function
legend = qgis.utils.iface.legendInterface()
legend.itemAdded.connect(transparency_slider)

Tested on QGIS 2.18.3 for Win7 64-bit.

---

Example:

Inserting code into python console and before adding shapefiles:

Result: