How to create always-top fullscreen overlay activity in Android

I'd like to be able to create an Activity that is always at the front of display of Android. It should receive no input, just pass it down to whatever application is next below it. Something like a HUD.

I was able to research that I need to set underlying window type to TYPE_SYSTEM_ALERT but it looks like Android is ignoring my code - no exception thrown even if I delete android.permission.SYSTEM_ALERT_WINDOW permission from manifest. (it is required to use this window type). When I tried to use ALERT type on dialog, it worked OK, but I cannot make dialog into full screen transparent entity. Here is my code, maybe there is something simple missing.

public void onCreate(Bundle savedInstanceState) {       
     super.onCreate(savedInstanceState);        

     requestWindowFeature(Window.FEATURE_NO_TITLE);
     getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
     getWindow().setType(WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);             
     getWindow().setBackgroundDrawableResource(android.R.color.transparent);
     getWindow().setFormat(PixelFormat.TRANSLUCENT);
     setContentView(R.layout.main);
}

Translucent setting has to be enabled externally in xml manifest, otherwise it also didn't work.

 <item name="android:windowIsTranslucent">true</item>

Solution 1:

final WindowManager.LayoutParams params = new WindowManager.LayoutParams(
WindowManager.LayoutParams.FLAG_FULLSCREEN,
WindowManager.LayoutParams.FLAG_FULLSCREEN,
WindowManager.LayoutParams.TYPE_SYSTEM_ALERT,
WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN,
PixelFormat.TRANSLUCENT);

WindowManager wm = (WindowManager) getApplicationContext()
    .getSystemService(Context.WINDOW_SERVICE);

ViewGroup mTopView = (ViewGroup) App.inflater.inflate(R.layout.main, null);
getWindow().setAttributes(params);
wm.addView(mTopView, params);

Solution 2:

I found that the accepted answer (by tmouse) did not quite work for me. Maybe it worked for an older API level? I'm using API level 24.

That particular answer had recommended:

final WindowManager.LayoutParams params = new WindowManager.LayoutParams(
    WindowManager.LayoutParams.FLAG_FULLSCREEN,
    WindowManager.LayoutParams.FLAG_FULLSCREEN,
    WindowManager.LayoutParams.TYPE_SYSTEM_ALERT,
    WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN,
    PixelFormat.TRANSLUCENT
);

There is a problem with this. The constructors that exist for WindowManager.LayoutParams are as follows:

So the WindowManager.LayoutParams.FLAG_FULLSCREEN flag gets used as an explicit value for int w and int h. This is no good! Your view's dimensions get set to 1024*1024 — so it does not fill the screen. And even if you explicitly set the layout's width and height to match the device's dimensions: they will not update when the screen orientation changes. We need a different approach…

I found that the correct construction for a full-screen overlay is like so:

final WindowManager.LayoutParams params = new WindowManager.LayoutParams(
    WindowManager.LayoutParams.TYPE_SYSTEM_OVERLAY, // TYPE_SYSTEM_ALERT is denied in apiLevel >=19
    WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN | WindowManager.LayoutParams.FLAG_FULLSCREEN,
    PixelFormat.TRANSLUCENT
);

This means we no longer explicitly specify a width and height. The layout relies entirely on our flags instead.

Yes, WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN is a required flag still; it is necessary if you want to draw over decorations such as the status bar.

WindowManager.LayoutParams.TYPE_SYSTEM_OVERLAY should be used instead of TYPE_SYSTEM_ALERT in API level >=19. This is a note I wrote a while ago, sadly I've been unable to find citation to corroborate why I thought that, so take that with a pinch of salt.

Bonus notes (if you're reading this, you're probably trying to make a full-screen overlay):

Your manifest will need these permissions (explanation here):

<uses-permission android:name="android.permission.ACTION_MANAGE_OVERLAY_PERMISSION"/>
<uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>

My understanding is that SYSTEM_ALERT_WINDOW is the actual permission required, but that ACTION_MANAGE_OVERLAY_PERMISSION is needed also: it lets you request at runtime that the user grant the SYSTEM_ALERT_WINDOW privilege.

I provide here the source code to my working API level 24 app that creates a full-screen overlay.