想做悬浮窗的功能，用native.js调用android.view.WindowManager的addView方法，新添加的view为什么不显示？mui新手咨询问题

下面是代码：
mui.init();
mui.plusReady();
document.addEventListener("plusready", function() {
// 扩展API加载完毕，现在可以正常调用扩展API
// 导入android.content.Intent类对象\
try {
var WindowManager = plus.android.importClass("android.view.WindowManager");
var PixelFormat = plus.android.importClass("android.graphics.PixelFormat");
var Gravity = plus.android.importClass("android.view.Gravity");

            var main = plus.android.runtimeMainActivity();  
            var appContext = plus.android.invoke(main, "getApplicationContext");  
            var mWindowManager = plus.android.invoke(appContext, "getSystemService", "window");  
            var w = plus.webview.create("https://www.baidu.com","https://www.baidu.com",{},{preload:'preload webview'});  
            w.show();  
            var webView = w.nativeInstanceObject();  
            var mLayout = new WindowManager.LayoutParams();  
            plus.android.setAttribute(mLayout, "type", WindowManager.LayoutParams.TYPE_SYSTEM_ALERT);  
            plus.android.setAttribute(mLayout, "flags", WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE);  
            plus.android.setAttribute(mLayout, "format", PixelFormat.TRANSLUCENT);  
            plus.android.setAttribute(mLayout, "gravity", Gravity.TOP | Gravity.LEFT);  
            plus.android.setAttribute(mLayout, "width", 140);  
            plus.android.setAttribute(mLayout, "height", 140);  
            plus.android.setAttribute(mLayout, "x", 0);  
            plus.android.setAttribute(mLayout, "y", 0);  
            alert(webView);  
            alert(mLayout);  
            plus.android.invoke(mWindowManager, "addView", webView, mLayout);  
        } catch(e) {  
            console.log(e);  
            alert(e);  
        }  

    }, false);