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

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.