Creating a System Overlay (Always on Top over all Apps) in Android

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="de.mobilej.overlay"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="14" />

    <application android:label="SystemOverlay" >
        <activity
            android:name="de.mobilej.overlay.Main"
            android:exported="true" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <service
            android:name="de.mobilej.overlay.OverlayShowingService"
            android:exported="false" />
    </application>

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

</manifest>

Main.java

package de.mobilej.overlay;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;

public class Main extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Intent svc = new Intent(this, OverlayShowingService.class);
        startService(svc);
        finish();
    }
}

OverlayShowingService.java

package de.mobilej.overlay;

import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.graphics.PixelFormat;
import android.os.IBinder;
import android.view.Gravity;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnTouchListener;
import android.view.WindowManager;
import android.view.WindowManager.LayoutParams;
import android.widget.Button;
import android.widget.Toast;

public class OverlayShowingService extends Service implements OnTouchListener, OnClickListener {
    
    private View topLeftView;

    private Button overlayedButton;
    private float offsetX;
    private float offsetY;
    private int originalXPos;
    private int originalYPos;
    private boolean moving;
    private WindowManager wm;

    @Override
    public IBinder onBind(Intent intent) {
	return null;
    }

    @Override
    public void onCreate() {
	super.onCreate();

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

	overlayedButton = new Button(this);
	overlayedButton.setText("Overlay button");
	overlayedButton.setOnTouchListener(this);
	overlayedButton.setAlpha(0.0f);
	overlayedButton.setBackgroundColor(0x55fe4444);
	overlayedButton.setOnClickListener(this);

	WindowManager.LayoutParams params = new WindowManager.LayoutParams(WindowManager.LayoutParams.WRAP_CONTENT, WindowManager.LayoutParams.WRAP_CONTENT, WindowManager.LayoutParams.TYPE_SYSTEM_ALERT, WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE | WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL, PixelFormat.TRANSLUCENT);
	params.gravity = Gravity.LEFT | Gravity.TOP;
	params.x = 0;
	params.y = 0;
	wm.addView(overlayedButton, params);

	topLeftView = new View(this);
	WindowManager.LayoutParams topLeftParams = new WindowManager.LayoutParams(WindowManager.LayoutParams.WRAP_CONTENT, WindowManager.LayoutParams.WRAP_CONTENT, WindowManager.LayoutParams.TYPE_SYSTEM_ALERT, WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE | WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL, PixelFormat.TRANSLUCENT);
	topLeftParams.gravity = Gravity.LEFT | Gravity.TOP;
	topLeftParams.x = 0;
	topLeftParams.y = 0;
	topLeftParams.width = 0;
	topLeftParams.height = 0;
	wm.addView(topLeftView, topLeftParams);

    }

    @Override
    public void onDestroy() {
	super.onDestroy();
	if (overlayedButton != null) {
	    wm.removeView(overlayedButton);
	    wm.removeView(topLeftView);
	    overlayedButton = null;
	    topLeftView = null;
	}
    }

    @Override
    public boolean onTouch(View v, MotionEvent event) {

	if (event.getAction() == MotionEvent.ACTION_DOWN) {
	    float x = event.getRawX();
	    float y = event.getRawY();

	    moving = false;

	    int[] location = new int[2];
	    overlayedButton.getLocationOnScreen(location);

	    originalXPos = location[0];
	    originalYPos = location[1];

	    offsetX = originalXPos - x;
	    offsetY = originalYPos - y;

	} else if (event.getAction() == MotionEvent.ACTION_MOVE) {
	    int[] topLeftLocationOnScreen = new int[2];
	    topLeftView.getLocationOnScreen(topLeftLocationOnScreen);

	    System.out.println("topLeftY="+topLeftLocationOnScreen[1]);
	    System.out.println("originalY="+originalYPos);
	    
	    float x = event.getRawX();
	    float y = event.getRawY();

	    WindowManager.LayoutParams params = (LayoutParams) overlayedButton.getLayoutParams();

	    int newX = (int) (offsetX + x);
	    int newY = (int) (offsetY + y);

	    if (Math.abs(newX - originalXPos) < 1 && Math.abs(newY - originalYPos) < 1 && !moving) {
		return false;
	    }

	    params.x = newX - (topLeftLocationOnScreen[0]);
	    params.y = newY - (topLeftLocationOnScreen[1]);

	    wm.updateViewLayout(overlayedButton, params);
	    moving = true;
	} else if (event.getAction() == MotionEvent.ACTION_UP) {
	    if (moving) {
		return true;
	    }
	}

	return false;
    }

    @Override
    public void onClick(View v) {
	Toast.makeText(this, "Overlay button click event", Toast.LENGTH_SHORT).show();
    }

}

Ok for 'permission denied for window type 2038' you just need to allow drawing over other apps in settings on phone.

I am trying to use this to create an app to hide rounded screen corners by adding a black bar across top and bottom of screen.
At the moment these black bars appear below the status bar and above the navigation bar, as show in the image
(https://user-images.githubusercontent.com/77055803/104008998-3aeb8f00-51a2-11eb-90e6-43fa2704ce0d.jpg)

Anyone know how to make the overlays position to the absolute top and bottom of screen?

Also the overlay gets rotated with any app that goes to landscape mode, as shown here
(https://user-images.githubusercontent.com/77055803/104009159-7a19e000-51a2-11eb-89d7-7b4ad5d22660.jpg)

How can I make it always appear at the top and bottom edges of the screen, regardless of app rotation?

use this code it will solve your problem
if (Build.VERSION.SDK_INT <= Build.VERSION_CODES.N) {
wmParams = new WindowManager.LayoutParams(WindowManager.LayoutParams.TYPE_TOAST);
} else {
wmParams = new WindowManager.LayoutParams(WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY);
}