android 悬浮activity实现

当需求我们的activity始终在别的应用的最前端的时候，可以向WindowManager 添加需要的view，oncreate中addview，在ondestory removeview，就能实现悬浮activity

wm = (WindowManager) getApplicationContext().getSystemService(
                Context.WINDOW_SERVICE);
        params = new WindowManager.LayoutParams();
        params.systemUiVisibility =  View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN;
        params.type = WindowManager.LayoutParams.TYPE_SYSTEM_ALERT;
        params.flags = WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL
                | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE;
        params.width = -1;
        params.height = -1;
        wm.addView(view, params);`

另statusbar不能下拉：

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

WindowManager.LayoutParams localLayoutParams = new WindowManager.LayoutParams();
localLayoutParams.type = WindowManager.LayoutParams.TYPE_SYSTEM_ERROR;
localLayoutParams.gravity = Gravity.TOP;
localLayoutParams.flags = WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE|

            // this is to enable the notification to recieve touch events
            WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL |

            // Draws over status bar
            WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN;

    localLayoutParams.width = WindowManager.LayoutParams.MATCH_PARENT;
    localLayoutParams.height = (int) (50 * getResources()
            .getDisplayMetrics().scaledDensity);
    localLayoutParams.format = PixelFormat.TRANSPARENT;

    ViewGroup view = new ViewGroup(this);
    manager.addView(view, localLayoutParams);