关于侧滑遮罩问题

为什么我这样写，侧滑到菜单页时主页面没有起遮罩效果？

代码如下：
plus.webview.startAnimation({
view: menu,
styles: {
fromLeft: "-80%",
toLeft: "0",

                }  
            }, {  
                view: _self,  
                styles: {  
                    fromLeft: "0",  
                    toLeft: "80%",  
                    zindex: 1,  
                    mask: 'rgba(0,0,0,0.5)',    
                 transition: {    
                 duration: 300    
                 },  
                }  
            });  
        }