LayerDrawable

LayerDrawable is a subclass of the Android Drawable class that allows developers to stack multiple drawable resources on top of each other in a single object. Each individual drawable is referred to as a layer, and the layers are rendered in the order they are added—layer zero is drawn first, then layer one on top, and so on. Because LayerDrawable inherits from Drawable, it can be used wherever a Drawable is expected, such as in ImageView, background attributes, and layout XML.

A LayerDrawable is typically instantiated either in XML or programmatically. In XML, a <layer-list> element defines the child drawables and their optional attributes such as insets, gravity, and alpha. The <item> tags inside the list correspond to the layers. Example XML:

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

<item android:id="@+id/outline" android:drawable="@drawable/outline" />

<item android:id="@+id/center" android:drawable="@drawable/center" />

<item android:id="@+id/foreground" android:drawable="@drawable/foreground" />

</layer-list>

Programmatic creation involves passing an array of Drawable objects to the LayerDrawable constructor. Layers can be accessed or modified later via getDrawable(index) or setDrawableByLayerId(id, drawable). Alpha changes on individual layers can be made with setAlpha(index, value).

LayerDrawables are useful for creating complex UI components without inflating multiple views. They simplify shading, borders, stateful backgrounds, and composite animations. Because all layers share the same density and padding semantics, they maintain consistent scaling across screen densities. Performance-wise, LayerDrawable avoids additional view hierarchies but involves a small drawing cost for each layer. Overall, LayerDrawable provides a lightweight, efficient way to combine graphical elements in Android apps.