Drawable that shows a ripple effect in response to state changes. The anchoring position of the ripple for a given state may be specified by calling Drawable.SetHotspot(float, float) with the corresponding state attribute identifier.

A touch feedback drawable may contain multiple child layers, including a special mask layer that is not drawn to the screen. A single layer may be set as the mask by specifying its android:id value as mask. If a mask layer is set, the ripple effect will be masked against that layer before it is drawn over the composite of the remaining child layers. If no mask layer is set, the ripple effect is masked against the composite of the child layers. If no child layers or mask is specified and the ripple is set as a View background, the ripple will be drawn atop the first available parent background within the View's hierarchy. In this case, the drawing region may extend outside of the Drawable bounds.

[Android Documentation]