Dojo AMD 模块更改了“this”的引用

Question

对于我在尝试学习 AMD 风格的 Dojo 时看到的一些行为，我真的很困惑。当我实例化我的模块/对象时，“this”指的是我的构造函数中的对象。我调用了一个内部函数，该内部函数中的“this”指的是 Window 对象。因此，当我到达 this.attachMapEventHandlers 时，我得到一个“对象 [对象全局] 没有方法 'attachMapEventHandlers'”错误。我究竟做错了什么？更新：我找到了 lang.hitch，这似乎表明异步性质是让我绊倒的原因，但我对如何实施解决方案感到困惑。

我在 index.html 中的脚本：

require(["javascript/layout", "dijit/layout/ContentPane", "dijit/layout/BorderContainer", "dijit/layout/AccordionContainer", 
        "dojo/dom", "dojo/dom-attr", "dijit/Toolbar", "dijit/form/Button", "dijit/Dialog","dijit/ProgressBar", "dojo/domReady!"],
        function (layout, dom, domAttr) {
            mapControl = new layout();

layout.js：

define(["dojo/_base/declare"], function(declare) {
return declare(null, {
    action:"pan",
    activeMeasureTool:"",
    aerialLayer:"",
    legendLayers:"",
    loadedServices:"",
    popup:"",
    resizeId:0,
    constructor: function() {
        this.init();
    },
    init: function() {
        require(["esri/map", "esri/config", "esri/SpatialReference", "esri/geometry/Extent"], 
            function(Map, config, SpatialReference, Extent) {
            //custom map requires a proxy to function properly.
            esri.config.defaults.io.proxyUrl = "../sdc_devdata/proxy.php";

            var spatRef = new SpatialReference(2276);

            var startExtent = new Extent(2481416.32087491, 6963246.42495962, 2501196.36936991, 6980267.92469462, spatRef);
            var appFullExtent = new Extent(2396699.46935379, 6872369.60195443, 2607745.94404633, 7107335.22319087, spatRef);

            map = new Map("map", {extent: startExtent, isZoomSlider:true, logo:false, sliderStyle:"large"});
            this.attachMapEventHandlers();
            this.createLayers();
            this.handleLayerVisibilityChange();
        });
    },

Answer 1

您可以采取一些措施来解决此问题。

首先，您可以将所需的依赖项添加到 define 数组中，这样您就无需在类的构造函数中执行异步请求。

看起来像：

define(["dojo/_base/declare", "esri/map", "esri/config", "esri/SpatialReference", "esri/geometry/Extent"], function (declare, Map, config, SpatialReference, Extent) {
  return declare(null, {
    action: "pan",
    activeMeasureTool: "",
    aerialLayer: "",
    legendLayers: "",
    loadedServices: "",
    popup: "",
    resizeId: 0,
    constructor: function () {
      this.init();
    },
    init: function () {
      //custom map requires a proxy to function properly.
      esri.config.defaults.io.proxyUrl = "../sdc_devdata/proxy.php";

      var spatRef = new SpatialReference(2276);

      var startExtent = new Extent(2481416.32087491, 6963246.42495962, 2501196.36936991, 6980267.92469462, spatRef);
      var appFullExtent = new Extent(2396699.46935379, 6872369.60195443, 2607745.94404633, 7107335.22319087, spatRef);

      map = new Map("map", {
        extent: startExtent,
        isZoomSlider: true,
        logo: false,
        sliderStyle: "large"
      });
      this.attachMapEventHandlers();
      this.createLayers();
      this.handleLayerVisibilityChange();
    }
  });
});

或者您可以在执行 require 时将 this 的当前范围保存到闭包中的某个内容

init: function () {
  var that = this;
  require(["esri/map", "esri/config", "esri/SpatialReference", "esri/geometry/Extent"],
    function (Map, config, SpatialReference, Extent) {
      //custom map requires a proxy to function properly.
      esri.config.defaults.io.proxyUrl = "../sdc_devdata/proxy.php";

      var spatRef = new SpatialReference(2276);

      var startExtent = new Extent(2481416.32087491, 6963246.42495962, 2501196.36936991, 6980267.92469462, spatRef);
      var appFullExtent = new Extent(2396699.46935379, 6872369.60195443, 2607745.94404633, 7107335.22319087, spatRef);

      map = new Map("map", {
        extent: startExtent,
        isZoomSlider: true,
        logo: false,
        sliderStyle: "large"
      });
      that.attachMapEventHandlers();
      that.createLayers();
      that.handleLayerVisibilityChange();
    });
},

编辑：您的第三个选项是使用 lang.hitch，它允许您在回调函数中指定 this 的范围。要使用它，您需要将 dojo/_base/lang 添加到您的 define() 依赖列表中，并将 require 回调包装在 lang.hitch(this, function(){});

define(["dojo/_base/declare", "dojo/_base/lang"], function (declare, lang) {
  return declare(null, {
    //...
    init: function () {
      require(["esri/map", "esri/config", "esri/SpatialReference", "esri/geometry/Extent"],
        lang.hitch(this, function (Map, config, SpatialReference, Extent) {
          //this now refers to the instance of the class
        }));
    }

  });
});

我强烈建议使用第一个选项，因为它与 AMD 的整个使用一致（在执行之前声明模块需要哪些依赖项，而不是动态加载它）。

Answer 2

你可以把'this'放在另一个变量中，比如_this，并在你的内部函数中使用_this，比如

init: function() {
         var _this= this;
        require(["esri/map", "esri/config", "esri/SpatialReference", "esri/geometry/Extent"], 
            function(Map, config, SpatialReference, Extent) {
            ...
            _this.attachMapEventHandlers();
            _this.createLayers();
            _this.handleLayerVisibilityChange();