javascript - 动态脚本加载同步

Question

我有一个脚本，它知道动态加载包含 javascript 类的脚本。 我正在使用以下代码加载类脚本:

var head = document.getElementsByTagName("head")[0];
var script = document.createElement("script");
script.type = "text/javascript";
script.src = "myscript.js";
head.appendChild(script);

然后我尝试使用 eval 创建新类:

var classObj = eval(" new MyClass()" );

问题是，在脚本加载到内存之前，正在执行 eval 的代码，并且我收到一条错误，指出 MyClass 未定义。

有办法同步这些事件吗？我需要确保脚本已完全加载到内存中，然后才能开始从中分配类。

Answer 1

您需要将事件处理程序附加到 onload 方法(在符合 Web 标准的浏览器中)或 onreadystatechange，在 Internet Explorer 中检查 script.readyState 属性是否等于“已加载”或“完成”。

在收到脚本已加载的通知之前，您可能正在尝试访问尚未声明或创建的对象、函数和属性。

这是一个示例函数，从模块 bezen.dom.js 中提取在我的Javascript library, bezen.org :

var appendScript = function(parent, scriptElt, listener) {
    // append a script element as last child in parent and configure 
    // provided listener function for the script load event
    //
    // params:
    //   parent - (DOM element) (!nil) the parent node to append the script to
    //   scriptElt - (DOM element) (!nil) a new script element 
    //   listener - (function) (!nil) listener function for script load event
    //
    // Notes:
    //   - in IE, the load event is simulated by setting an intermediate 
    //     listener to onreadystate which filters events and fires the
    //     callback just once when the state is "loaded" or "complete"
    //
    //   - Opera supports both readyState and onload, but does not behave in
    //     the exact same way as IE for readyState, e.g. "loaded" may be
    //     reached before the script runs.

    var safelistener = catchError(listener,'script.onload');

    // Opera has readyState too, but does not behave in a consistent way
    if (scriptElt.readyState && scriptElt.onload!==null) {
      // IE only (onload===undefined) not Opera (onload===null)
      scriptElt.onreadystatechange = function() {
        if ( scriptElt.readyState === "loaded" || 
             scriptElt.readyState === "complete" ) {
          // Avoid memory leaks (and duplicate call to callback) in IE
          scriptElt.onreadystatechange = null;
          safelistener();
        }
      };
    } else {
      // other browsers (DOM Level 0)
      scriptElt.onload = safelistener;
    }
    parent.appendChild( scriptElt );
};

为了适应您的需要，您可以替换对 catchError 的调用，它包装监听器以捕获和记录错误，并使用修改后的函数:

var appendScript = function(parent, scriptElt, listener) {
    // append a script element as last child in parent and configure 
    // provided listener function for the script load event
    //
    // params:
    //   parent - (DOM element) (!nil) the parent node to append the script to
    //   scriptElt - (DOM element) (!nil) a new script element 
    //   listener - (function) (!nil) listener function for script load event
    //
    // Notes:
    //   - in IE, the load event is simulated by setting an intermediate 
    //     listener to onreadystate which filters events and fires the
    //     callback just once when the state is "loaded" or "complete"
    //
    //   - Opera supports both readyState and onload, but does not behave in
    //     the exact same way as IE for readyState, e.g. "loaded" may be
    //     reached before the script runs.

    var safelistener = function(){
      try {
        listener();
      } catch(e) {
        // do something with the error
      }
    };

    // Opera has readyState too, but does not behave in a consistent way
    if (scriptElt.readyState && scriptElt.onload!==null) {
      // IE only (onload===undefined) not Opera (onload===null)
      scriptElt.onreadystatechange = function() {
        if ( scriptElt.readyState === "loaded" || 
             scriptElt.readyState === "complete" ) {
          // Avoid memory leaks (and duplicate call to callback) in IE
          scriptElt.onreadystatechange = null;
          safelistener();
        }
      };
    } else {
      // other browsers (DOM Level 0)
      scriptElt.onload = safelistener;
    }
    parent.appendChild( scriptElt );
};