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.
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.