Android - reference the value of an attribute in the currently-applied theme from code

Question

the Android devGuide explains how it is possible to reference the value of an attribute in the currently-applied theme, using the question-mark (?) instead of at (@).

Does anyone know how to do this from code, e.g. in a customized component?

Answer 1

In XML, it would look something like this:

style="?header_background"

programmatically, it's a little trickier. In your activity:

private static Theme theme = null;

protected void onCreate(Bundle savedInstanceState) {
   ...
   theme = getTheme();
   ...
}

public static int getThemeColors(int attr){
   TypedValue typedvalueattr = new TypedValue();
   theme.resolveAttribute(attr, typedvalueattr, true);
   return typedvalueattr.resourceId;
}

And when you want to access an attribute of the theme, you would do something like this:

int outside_background = MyActivity.getThemeColors(R.attr.outside_background);
setBackgroundColor(getResources().getColor(outside_background));

It's a little more convoluted, but there you go ;-)

Answer 2

The above is not a good way of doing this for many reasons. NullPointerExceptions is one.

Below is the correct way of doing this.

public final class ThemeUtils {
    // Prevent instantiation since this is a utility class
    private ThemeUtils() {}

   /**
    * Returns the color value of the style attribute queried.
    *
    * <p>The attribute will be queried from the theme returned from {@link Context#getTheme()}.</p>
    *
    * @param context the caller's context
    * @param attribResId the attribute id (i.e. R.attr.some_attribute)
    * @param defaultValue the value to return if the attribute does not exist
    * @return the color value for the attribute or defaultValue
    */
    public static int getStyleAttribColorValue(final Context context, final int attribResId, final int defaultValue) {
        final TypedValue tv = new TypedValue();
        final boolean found = context.getTheme().resolveAttribute(attribResId, tv, true);
        return found ? tv.data : defaultValue;
    }
}

Then to use just do:

final int attribColor = ThemeUtils.getStyleAttribColorValue(context, R.attr.some_attr, 0x000000 /* default color */);

Just make sure the context that you pass in came from the Activity. Don't do getApplicationContext() or the theme returned will be from the Application object and not the Activity.