How to set the theme for the application, to avoid wrong color transitions?

Question

Background

I\'m developing a themes chooser feature in my \"app manager\" app, and I\'ve succeeded setting the theme dynamically for each of the activities.

<

Answer 1

The transition color is retrieved from the activity theme on the manifest (or the application if not set).

Currently the only way around this limitation is to create a dummy subclass for each real Activity, eg. MyActivityLight, to declare a different theme. Activity alias won't work, the attribute will be ignored.

For activities with IntentFilter's, you should only maintain one of each "type" enabled, using PackageManager#setComponentEnabledSetting(). Note that the change may take some seconds.

For activities that are started by class name, you can infer the correct prefix according to the user's theme.

---

So lets suppose you have 2 themes: AppTheme.Dark and AppTheme.Light and some activities. The dark theme is the default one.

Original manifest:

Change all activities above as abstract classes and create dummy subclasses suffixed by Light and Dark.

Then the manifest should be changed like this:

Then you could have something like this to get the themed Activity class, given an abstract Activity:

public static ComponentName getThemedActivityName(
        Context ctx, 
        Class clazz) {

    // Probably gets some value off SharedPreferences
    boolean darkTheme = isUsingDarkTheme(ctx);

    String baseName = clazz.getName();
    String name += (darkTheme) ? "Dark" : "Light";
    return new ComponentName(ctx, name);
}

public static void startThemedActivity(
        Activity ctx, 
        Class clazz) {
    Intent intent = new Intent();
    intent.setComponent(getThemedActivityName(ctx, clazz));
    ctx.startActivity(intent);
}

And also change the enabled status where needed, when the theme is changed.

public void onThemeChanged(Context ctx, boolean dark) {
    // save theme to SharedPreferences or similar and...

    final PackageManager pm = ctx.getPackageManager();
    final String pckgName = ctx.getPackageName();

    final PackageInfo pckgInfo;
    try {
        final int flags = PackageManager.GET_ACTIVITIES 
                             | PackageManager.GET_DISABLED_COMPONENTS;
        pckgInfo = pm.getPackageInfo(pckgName, flags);
    } catch (PackageManager.NameNotFoundException e) {
        throw new RuntimeException(e);
    }

    final ActivityInfo[] activities = pckgInfo.activities;

    for (ActivityInfo info: activities) {
        final boolean enable;
        if (info.theme == R.style.AppTheme_Light) {
            enable = !dark;
        } else if (info.theme == R.style.AppTheme_Dark) {
           enable = dark;
        } else {
           continue;
        }

        final int state = (enable) ? 
                PackageManager.COMPONENT_ENABLED_STATE_ENABLED :
                PackageManager.COMPONENT_ENABLED_STATE_DISABLED;

        final String name = info.targetActivity;
        final ComponentName cmp = new ComponentName(pckgName, name);
        pm.setComponentEnabledSetting(cmp, state, PackageManager.DONT_KILL_APP);
    }
}

If doing IPC on a loop scares you, you can do this asynchronously on a helper thread, as long as multiple calls to onThemeChanged() run sequentially.

Note that in this example I change the enabled status of all activities (that have a known theme) but only had to do that for the ones with intent filters. If the activities aren't hardcoded its easier this way.

Important Note: As Richard Le Mesurier and other have pointed out, using this technique on Launcher Activities removes or disables the shortcut on the home screen, if it exists. This is just a solution for non launcher Activities.