为什么plus.downloader.createDownload()里面的代码不会执行啊？哪怕是alert也不会执行

function downLoad() {  
  var url = 'http://39.106.2.140:8091/appdownload/app.apk'  
  var dtask = plus.downloader.createDownload(url,{}, function (d, status) {  
    alert(111)  
    if (status == 200) {  
      var path = d.filename  
      plus.runtime.install(path);        
    } else {  
      plus.downloader.clear();          
    }  
  })  
  dtask.start();  

}  

document.addEventListener('plusready', function () {  
  downLoad()  

  var webview = plus.webview.currentWebview();  
  plus.key.addEventListener('backbutton', function () {  
    webview.canBack(function (e) {  
      if (e.canBack) {  
        webview.back();  
      } else {  
        webview.close();  
      }  
    })  
  });  
}, false);