CollapsingToolbarLayout: custom contentScrim similar to Facebook

Question

I want to create a custom contentScrim for my collapsingToolbarLayout. My collapsingToolbarLayout has an imageview inside. When it scrolls, in theory t

Answer 1

This is how I managed to create a custom scrim, which seems to be used in quite a few apps nowadays - if you look at Facebook, you will see they do not use the standard contentScrim for their collapsingToolbarLayout. The code below duplicates what Facebook does in their app.

You must set a AppBarLayout.OnOffsetChangedListener to your activity and then use the following code below to measure the size of the toolbar and the size of the appBarLayout.

When your listener is called, the verticalOffset actually measures the offset of the collapsingToolbarLayout from when it is fully expanded to the point when it touches the pinned toolbar. Therefore, the verticalOffset EXCLUDES the height of the toolbar. To compare apples with apples, we need to also EXCLUDE the toolbar height from the height of the appBarLayout so that when we divide the verticalOffset by the totalHeight, neither the verticalOffset nor the totalHeight contains the toolbar height. This is necessary in order to obtain a percentage to apply your 255 alpha value to.

  @Override
    public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {

        //measuring for alpha
        int toolBarHeight = toolbar.getMeasuredHeight();
        int appBarHeight = appBarLayout.getMeasuredHeight();
        Float f = ((((float) appBarHeight - toolBarHeight) + verticalOffset) / ( (float) appBarHeight - toolBarHeight)) * 255;
        fading_backdrop.getBackground().setAlpha(255 - Math.round(f));
   }

When you scroll now, the fading_backdrop will gradually gain alpha when you scroll upwards so that it overlays nicely with the image in the collapsingToolbarLayout, similar to the facebook custom contentScrim.