类和属性问题

Question

我想在Android上使用地理定位API。我知道有一个被定义的“导航器”对象，应该用来获取用户的位置。所以，我创建了这个示例代码：

function GeolocationTester() 
{ 
    // here I want to store all acquired locations 
    this.locations = new Array(); 
    alert("this.locations defined: " + this.locations); 
    this.onSuccess = function(position) 
    { 
     alert("Entered onSuccess"); 
     alert("this.locations defined: " + this.locations); 
    } 

    this.onError = function(error) 
    { 
     alert("error acquiring location"); 
    } 
    navigator.geolocation.watchPosition(this.onSuccess, this.onError, { enableHighAccuracy: true }); 
} 

而且它不适用于我。每当watchPosition调用onSuccess时，this.locations字段没有被定义（并且它在新数组之后被定义）。我知道我做错了什么，但是因为它是我的一个JavaScript尝试，所以不知道是什么。那么，任何人都可以在这里找到问题？

Answer 1

问题出在this的范围。当调用onSuccess或onError时，this未绑定到包含locations数组的对象。您需要创建到该阵列均应分配的职能明确的变量外，然后在回调使用这个变量，像这样：

var allLocations = this.locations = [a, b, c]; 
this.onSuccess = function(position) { 
    alert("allLocations: " + allLocations); 
    alert("this.locations: " + this.locations); 
} 

Answer 2

它使用你的事业this。这将改变，因为它取决于你的函数调用的上下文。只需使用功能的范围，申报地点：

function GeolocationTester() 
{ 
    // here I want to store all acquired locations 
    var locations = []; 
    alert("locations defined: " + locations); 

    function onSuccess(position) { 
     alert("Entered onSuccess"); 
     alert("locations defined: " + locations); 
    } 

    function onError(error){ 
     alert("error acquiring location"); 
    } 


navigator.geolocation.watchPosition(onSuccess, onError, { enableHighAccuracy: true }); 
} 

要真正了解什么this阅读这篇博客http://dmitrysoshnikov.com/ecmascript/chapter-3-this/

Answer 3

尝试定义onSuccess这样的：

this.onSuccess = (function(locations) { 
    return function(position) 
      { 
       alert("Entered onSuccess"); 
       alert("this.locations defined: " + locations); 
      } 
})(this.locations);