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AnimatedStateListDrawable

When developing an application, there’s an opportunity to polish the user experience by using icon animations to define different states of a particular interaction.

On Android, icon animations can be performed using AnimatedVectorDrawables, which provide us with a way to play them manually. However, when dealing with several icon animations, and when tying them to states, it results in a lot of boilerplate code. With this in mind, another option Android provides is AnimatedStateListDrawables.

AnimatedStateListDrawable is similar to StateListDrawable, as it allows us to change a drawable based on its state, but instead of simply swapping them, AnimatedStateListDrawable can define AnimatedVectorDrawables to animate changes between states.

Let’s take a look at an example. We start by implementing a StateListDrawable and then convert it to an AnimatedStateListDrawable.

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:drawable="@drawable/ic_home_selected" android:state_checked="true" />
    <item android:drawable="@drawable/ic_home_unselected" android:state_checked="false" />
</selector>

We create our StateListDrawable using the selector tag and define the items to change the drawable based on the states defined. This would create a drawable that would switch the home icon based on the checked state of the view.

Now let’s animate the drawable transition, we start by changing the root tag to animated-selector and define a transition that uses AnimatedVectorDrawable.

<?xml version="1.0" encoding="utf-8"?>
<animated-selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/selected"
        android:drawable="@drawable/ic_home_selected"
        android:state_checked="true" />
    <item
        android:id="@+id/unselected"
        android:drawable="@drawable/ic_home_unselected"
        android:state_checked="false" />

    <transition
        android:drawable="@drawable/avd_home_unselected_to_selected"
        android:fromId="@id/unselected"
        android:toId="@id/selected" />
    <transition
        android:drawable="@drawable/avd_home_selected_to_unselected"
        android:fromId="@id/selected"
        android:toId="@id/unselected" />
</animated-selector>

That’s it, now whenever our view state changes the AnimatedStateListDrawable will automatically run the drawable transition to animate the icon.

You can find the sample source code here. In the sample app, I have used the AnimatedStateListDrawable with BottomNavigationView to animate icons when the selected page changes.

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