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

Question

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?

Answer 1

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.

Answer 2

Another aspect in addition to michael's excellent answer is overriding custom attributes in themes. Suppose you have a number of custom views that all refer to the custom attribute "custom_background".

<declare-styleable name="MyCustomStylables">     <attr name="custom_background" format="color"/> </declare-styleable> 

In a theme you define what the value is

<style name="MyColorfulTheme" parent="AppTheme">     <item name="custom_background">#ff0000</item> </style> 

or

<style name="MyBoringTheme" parent="AppTheme">     <item name="custom_background">#ffffff</item> </style> 

You can refer to the attribute in a style

<style name="MyDefaultLabelStyle" parent="AppTheme">     <item name="android:background">?background_label</item> </style> 

Notice the question mark, as also used for reference android attribute as in

?android:attr/colorBackground 

As most of you have noticed, you can -and probably should- use @color references instead of hard coded colors.

So why not just do

<item name="android:background">@color/my_background_color</item> 

You can not change the definition of "my_background_color" at runtime, whereas you can easily switch themes.