在WindowManager中使用TYPE_APPLICATION时窗口未显示

Question

将windowview添加到windowmanager时窗口未显示。但它显示我何时使用TYPE_PHONE而不是TYPE_APPLICATION。

windowManager = (WindowManager) ctx.getApplicationContext().getSystemService(WINDOW_SERVICE);      
WindowManager.LayoutParams params = new WindowManager.LayoutParams( 300,300,WindowManager.LayoutParams.TYPE_PHONE,WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE,PixelFormat.TRANSPARENT);  
params.gravity = Gravity.TOP | Gravity.LEFT;  params.x = 0;  params.y = 100;  
windowManager.addView(surfaceView, params);