Drawing at the top of other apps

Need to add android.permission.SYSTEM_ALERT_WINDOW permission in manifest.xml before using.

WindowManager.LayoutParams.TYPE_SYSTEM_ALERT requires user authorization at Android 6.0 or higher, it is replaced by WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY in Android 8.0 and requires user authorization.

In the simplest case, you just need to execute the dangerous code in the callback method of onGrant():

void showAlertWindow() {
    Dialog dialog = new Dialog(this);
    Window window = dialog.getWindow();

    int overlay = WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY;
    int alertWindow = WindowManager.LayoutParams.TYPE_SYSTEM_ALERT;
    int type = Build.VERSION.SDK_INT >= Build.VERSION_CODES.O ? overlay : alertWindow;
    window.setType(type);

    ...
    dialog.show();
}

AndPermission.with(this)
    .overlay()
    .onGranted(new Action<Void>() {
        @Override
        public void onAction(Void data) {
            showAlertWindow();
        }
    })
    .onDenied(new Action<Void>() {
        @Override
        public void onAction(Void data) {
            // TODO ...
        }
    })
    .start();

At this point, if you don't have permission to draw on other apps, the setup page is automatically opened to request the user to open it.

If the developer wants to improve the experience. For example, if there is no permission, a dialog will be played to let the user choose whether to start the setting screen:

AndPermission.with(this)
    .overlay()
    .rationale(new Rationale<Void>() {
        @Override
        public void showRationale(Context c, Void d, RequestExecutor e) {
            // Startup settings: e.execute();
            // Cancel: e.cancel();
        }
    })
    .onGranted(new Action<Void>() {
        @Override
        public void onAction(Void data) {
            showAlertWindow();
        }
    })
    .onDenied(new Action<Void>() {
        @Override
        public void onAction(Void data) {
            // TODO ...
        }
    })
    .start();

When showRationale() is called back without the SYSTEM_ALERT_WINDOW permission, the developer must call RequestExecutor#execute() to initiate the setting or RequestExecutor#cancel() to cancel the startup setting, ortherwise AndPermission will have no response.