Android: How to obtain the resource ID of the current theme?

In Android, you can get the current theme of an activity as a Resource.Theme object from getTheme(). Also, you can set the theme to a different one via that other theme's resource id, as in setTheme(R.style.Theme_MyTheme).

But how do I find out whether it's worth it -- whether the current theme is already the one that I would want to set? I am looking for something like getTheme().getResourceId(), in order to write something like:

protected void onResume() {
    int newThemeId = loadNewTheme();
    if (newThemeId != getTheme().getResourceId()) { // !!!! How to do this?
        setTheme(newThemeId);
        // and rebuild the gui, which is expensive
    }
}

Any ideas?

I found a way to solve the requirement without getting the resource id.

I'm adding an item to each of my themes with the name of the string:

<item name="themeName">dark</item>

And in the code I check the name like so:

TypedValue outValue = new TypedValue();
getTheme().resolveAttribute(R.attr.themeName, outValue, true);
if ("dark".equals(outValue.string)) {
   ...
}

OK here's one puzzle piece: we can get the default theme, as set in the AndroidManifest.xml, as context.getApplicationInfo().theme for the theme set at application level, and from within an Activity, as getPackageManager().getActivityInfo(getComponentName(), 0).theme for that activity.

I guess that gives us a starting point to do our own wrapper for a custom getTheme() and setTheme().

Still that feels like working around rather than with the API. So I'll leave the question open to see if someone comes up with a better idea.

EDIT: There's

getPackageManager().getActivityInfo(getComponentName(), 0).getThemeResource()

which will automatically fallback to application theme if the activity doesn't override it.

There is a way to do this via reflection. Put this in your Activity:

int themeResId = 0;
try {
    Class<?> clazz = ContextThemeWrapper.class;
    Method method = clazz.getMethod("getThemeResId");
    method.setAccessible(true);
    themeResId = (Integer) method.invoke(this);
} catch (NoSuchMethodException e) {
    Log.e(TAG, "Failed to get theme resource ID", e);
} catch (IllegalAccessException e) {
    Log.e(TAG, "Failed to get theme resource ID", e);
} catch (IllegalArgumentException e) {
    Log.e(TAG, "Failed to get theme resource ID", e);
} catch (InvocationTargetException e) {
    Log.e(TAG, "Failed to get theme resource ID", e);
}
// use themeResId ...

[Insert disclaimer here regarding non-public apis]

According to sources Activity.setTheme is called before Activity.onCreate, so you can save the themeId when android set it:

public class MainActivity extends Activity {
    private int themeId;
    
    @Override
    public void setTheme(int themeId) {
        super.setTheme(themeId);
        this.themeId = themeId;
    }
    
    public int getThemeId() {
        return themeId;
    }
}

If you have specified android:theme="@style/SomeTheme" in the <activity/> element in your manifest, then you can get the resource ID of the activity's theme like this:

int themeResId;
try {
    PackageManager packageManager = getPackageManager();
    //ActivityInfo activityInfo = packageManager.getActivityInfo(getCallingActivity(), PackageManager.GET_META_DATA);
    ActivityInfo activityInfo = packageManager.getActivityInfo(getComponentName(), PackageManager.GET_META_DATA);
    themeResId = activityInfo.theme;
}
catch(PackageManager.NameNotFoundException e) {
    Log.e(LOG_TAG, "Could not get themeResId for activity", e);
    themeResId = -1;
}

Don't forget, if you are going to then call setTheme(themeResId) in your activity's onCreate() method, you need to do it before you call setContentView(...).