1. 在Manifest中添加权限

<!--悬浮窗权限-->
    <uses-permission android:name="android.permission.SYSTEM_ALERT_WINDOW"/>

2. 在代码中判断权限，并添加

private void startFloatingService() {
        if (Settings.canDrawOverlays(this)) {
            Toast.makeText(this, "授权成功", Toast.LENGTH_SHORT).show();
        } else {
            Toast.makeText(this, "请授权", Toast.LENGTH_SHORT).show();
            startActivityForResult(new Intent(Settings.ACTION_MANAGE_OVERLAY_PERMISSION, Uri.parse("package:" + getPackageName())), 0);
        }
    }


    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        if(requestCode == 0){
            if (Settings.canDrawOverlays(this)) {
                Toast.makeText(this, "授权成功", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(this, "请授权", Toast.LENGTH_SHORT).show();
                startActivityForResult(new Intent(Settings.ACTION_MANAGE_OVERLAY_PERMISSION, Uri.parse("package:" + getPackageName())), 0);
            }
        }
    }

3. 调用

   @Override
    protected void initView() {
        startFloatingService();

        final WindowManager mWm = (WindowManager) getSystemService(Context.WINDOW_SERVICE);
        final Button view = new Button(this);
        view.setAllCaps(false);
        view.setText("我是悬浮窗啊!");
        final WindowManager.LayoutParams mParams = new WindowManager.LayoutParams();
        mParams.width = 800;
        mParams.height = 800;
        mParams.type = WindowManager.LayoutParams.TYPE_APPLICATION_OVERLAY;
        mParams.flags = WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL
                | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                | WindowManager.LayoutParams.FLAG_SHOW_WHEN_LOCKED;
        mParams.format = PixelFormat.RGBA_8888;
        mWm.addView(view, mParams);

        view.setOnTouchListener(new View.OnTouchListener() {
            private int x;
            private int y;

            @Override
            public boolean onTouch(View v, MotionEvent event) {
                switch (event.getAction()) {
                    case MotionEvent.ACTION_DOWN:
                        x = (int) event.getRawX();
                        y = (int) event.getRawY();
                        break;
                    case MotionEvent.ACTION_MOVE:
                        int moveX = (int) event.getRawX();
                        int moveY = (int) event.getRawY();
                        int diffX = moveX - x;
                        int diffY = moveY - y;
                        mParams.x = mParams.x + diffX;
                        mParams.y = mParams.y + diffY;

                        x = moveX;
                        y = moveY;

                        mWm.updateViewLayout(view, mParams);
                        break;
                }
                return false;
            }
        });
    }

参考1
参考2
参考3