Fading Toolbar

FadingToolbar.java

public class FadingToolbar extends Toolbar {

    private Drawable mToolbarBackgroundDrawable;

    public FadingToolbar(Context context) {
        super(context);
        initView(context);
    }

    public FadingToolbar(Context context, AttributeSet attrs) {
        super(context, attrs);
        initView(context);
    }

    public FadingToolbar(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        initView(context);
    }

    private void initView(Context context) {
        mToolbarBackgroundDrawable = getContainerView().getBackground().mutate();
    }
    
    /**
     * @param scrollTillOpaque - amount to scroll till toolbar is opaque
     */
    public void attachToScrollView(@NonNull final ScrollView scrollView, final float scrollTillOpaque) {
        scrollView.getViewTreeObserver().addOnScrollChangedListener(new ViewTreeObserver.OnScrollChangedListener() {

            @Override
            public void onScrollChanged() {
                float scrollY = scrollView.getScrollY();
                float maxScroll = scrollTillOpaque - getY();
                float ratio = Math.min(Math.max(scrollY, 0), maxScroll) / maxScroll;
                int alpha = (int) (ratio * 255);
                setBackgroundAlpha(alpha);
            }
        });
    }

    public void setBackgroundAlpha(int alpha) {
        mToolbarBackgroundDrawable.setAlpha(alpha);
    }
}