How to: Define theme (style) item for custom widget

android, android-theme

Solution

Yes, there's one way:

Suppose you have a declaration of attributes for your widget (in `attrs.xml`):

<declare-styleable name="CustomImageButton">
    <attr name="customAttr" format="string"/>
</declare-styleable>

Declare an attribute you will use for a style reference (in `attrs.xml`):

<declare-styleable name="CustomTheme">
    <attr name="customImageButtonStyle" format="reference"/>
</declare-styleable>

Declare a set of default attribute values for the widget (in `styles.xml`):

<style name="Widget.ImageButton.Custom" parent="android:style/Widget.ImageButton">
    <item name="customAttr">some value</item>
</style>

Declare a custom theme (in `themes.xml`):

<style name="Theme.Custom" parent="@android:style/Theme">
    <item name="customImageButtonStyle">@style/Widget.ImageButton.Custom</item>
</style>

Use this attribute as the third argument in your widget's constructor (in `CustomImageButton.java`):

public class CustomImageButton extends ImageButton {
    private String customAttr;

    public CustomImageButton( Context context ) {
        this( context, null );
    }

    public CustomImageButton( Context context, AttributeSet attrs ) {
        this( context, attrs, R.attr.customImageButtonStyle );
    }

    public CustomImageButton( Context context, AttributeSet attrs,
            int defStyle ) {
        super( context, attrs, defStyle );

        final TypedArray array = context.obtainStyledAttributes( attrs,
            R.styleable.CustomImageButton, defStyle,
            R.style.Widget_ImageButton_Custom ); // see below
        this.customAttr =
            array.getString( R.styleable.CustomImageButton_customAttr, "" );
        array.recycle();
    }
}

Now you have to apply `Theme.Custom` to all activities that use `CustomImageButton` (in AndroidManifest.xml):

<activity android:name=".MyActivity" android:theme="@style/Theme.Custom"/>

That's all. Now `CustomImageButton` tries to load default attribute values from `customImageButtonStyle` attribute of current theme. If no such attribute is found in the theme or attribute's value is `@null` then the final argument to `obtainStyledAttributes` will be used: `Widget.ImageButton.Custom` in this case.

You can change names of all instances and all files (except `AndroidManifest.xml`) but it would be better to use Android naming convention.

Problem

I've written a custom widget for a control that we use widely throughout our application. The widget class derives from `ImageButton` and extends it in a couple of simple ways. I've defined a style which I can apply to the widget as it's used, but I'd prefer to set this up through a theme. In `R.styleable` I see widget style attributes like `imageButtonStyle` and `textViewStyle`. Is there any way to create something like that for the custom widget I wrote?

Original source