TransitionDrawable

TransitionDrawable is a subclass of Drawable that provides animated transitions between a sequence of drawables. It is part of the Android framework (android.graphics.drawable) and can be used to create visual effects such as fades or cross‑fades in user interfaces.

A TransitionDrawable is constructed from an array of Drawable objects. The array defines the order of the layers, with the first drawable visible by default. When the transition is started, the drawable gradually changes the opacity of the layers until the final one is fully opaque. The transition is controlled by startTransition(int durationMillis) and stopTransition(). The durationMillis parameter specifies the time in milliseconds for the cross‑fade animation. After the transition finishes, the drawable retains the final state unless reset.

The drawable also supports other operations inherited from Drawable, such as setBounds(Rect), setAlpha(int), and setColorFilter(ColorFilter). It can be referenced in XML using a <transition> tag inside a <layer-list>, which allows designers to define the drawable sequence without code.

Typical usage examples include:

* Changing background colors or images when a button is pressed.

* Animating theme changes in a settings screen.

* Providing smooth visual feedback during configuration changes.

To use a TransitionDrawable programmatically:

1. Create an array of Drawable objects.

2. Instantiate the TransitionDrawable with the array.

3. Attach the drawable to a view using setBackground() or setImageDrawable().

4. Call startTransition() with the desired duration to begin the animation.

TransitionDrawable can be combined with animation listeners if finer control is required, for example by wrapping the state change in an AnimationListener or using the LayerDrawable's callback mechanism. The effect is lightweight and fully integrated with Android's drawable caching and state system.