Add alpha change listener

[jenzz]

I recently needed a listener for the ActionBar's alpha value.
Reason being that I can fade in/out the ActionBar title and subtitle along with the background drawable.

Simple addition. Feel free to merge if you find it useful.

[deRonbrown]

+1 I've used this in my implementation as well. Perhaps you could implement it as a factory method so you can chain it all together.

[MariusVolkhart]

👍 I'm using this in our implementation also. I too would like to see this as a chainable method.

[SimonStefan]

+1

[lbloder]

+1

[NikolaDespotoski]

I needed something similar to this. Here is a workaround:
Uses reflection to get internal scroll listener then it dispatches the event's down to the super class. It also gives possibility to chain more than one scroll listener. Order scroll event receivers is: superclass -> class -> list of partner scroll listeners

Usage:

NotifyingFadingActionBar fadingActionBarHelper = new NotifyingFadingActionBar(this)
                .withCallback(this)
                .headerLayoutResId(R.layout.place_header)
                .addOnScrollListener(myScrollListener)
                .actionBarBackground(mActionbarDrawable)
                .contentLayout(R.layout.activity_waiting_list);
        setContentView(fadingActionBarHelper.createView(this));

https://gist.github.com/NikolaDespotoski/009595717408e7854efd