异步递归函数结束后的回调

Question

下面的函数以递归方式打印文件夹中的 Chrome 书签。在处理最终递归循环后，如何更改以下函数以调用另一个函数？ chrome.bookmarks.getChildren() 是异步的，这使得很难知道函数何时处理完所有内容。

谢谢。

    for (var i = 0; i < foldersArray.length; i++) {
        // The loop makes several calls with different folder IDs.
        printBookmarks(foldersArray[i]);  
    }

    // I'd like any code here to be run only after the above has 
    //finished processing    

    function printBookmarks(id) {
        chrome.bookmarks.getChildren(id, function(children) {
           children.forEach(function(bookmark) { 
               console.debug(bookmark.title);
               printBookmarks(bookmark.id);
           });
        });
    }

编辑：抱歉，我认为我在最初的代码示例中不清楚。我已经更新了代码，通过多次调用该函数来显示异步函数遇到的问题。我希望 printBookmarks 函数调用之后的任何代码都等待所有 printBookmarks 函数完成处理。

Answer 1

您的异步方法实例可能会同时执行，并且您事先不知道会有多少个。因此，您必须进行计数，然后在最后一个异步方法完成时使用回调。

for (var i = 0; i < foldersArray.length; i++) {
    // The loop makes several calls with different folder IDs.
    printBookmarks(foldersArray[i], thingsToDoAfter);
}

function thingsToDoAfter() {
    // I'd like any code here to be run only after the above has 
    // finished processing.
}

var count = 0;

function printBookmarks(id, callback) {
    count++;
    chrome.bookmarks.getChildren(id, function(children) {
        children.forEach(function(bookmark) { 
            console.debug(bookmark.title);
            printBookmarks(bookmark.id, callback);
        });
        count--;
        if (count === 0 && callback)
            callback();
    });
}

Answer 2

我最近不得不解决这个问题。该解决方案与 Eric 的类似，但我发现我需要 count 变量位于函数的本地。这是我解决这个问题的方法：

for(var i=0;i<foldersArray.length; i++){
  // The loop make's several call's with different folder ID's.
  var printed_children = 0;
  printBookmarks(foldersArray[i],function() {
    printed_children++;
    if(printed_children == foldersArray.length){
      // You know you are done!
    }
  });  
}
// I'd like any code here to be run only after the above has 
//finished processing.


function printBookmarks(id,finished_callback) {
  // the printed_children count is local so that it can keep track of 
  // whether or not this level of recursion is complete and should call 
  // back to the previous level
  var printed_children = 0;
  chrome.bookmarks.getChildren(id, function(children) {
    children.forEach(function(bookmark) { 
      console.debug(bookmark.title);
      // added a callback function to the printBookmarks so that it can
      // check to see if all upstream recursions have completed.
      printBookmarks(bookmark.id,function() {
        printed_children++;
        if(printed_children == children.length){
          finished_callback();
        }
      });
    });
    if(children.length == 0){
      finished_callback();
    }
  });
}

它有点难看，但应该可行。

Answer 3

您可以将所有已完成的调用保存到一个变量中，并根据要处理的书签数量进行测试。当您到达末尾时（完成的计数等于要处理的书签的数量），您将执行最终的函数。

此处有类似问题的答案，其中包含可用作指南的代码：

如何在运行 JavaScript 代码之前使用 jQuery.getScript() 加载可变数量的脚本？

Answer 4

你可以这样做 JQFAQ.com。我正在更新以供将来使用。

Answer 5

可能是一个更好的方法来解决这个问题，但您可以添加一个深度参数，例如

printBookmarks('0', 0);

function printBookmarks(id, depth) {
    chrome.bookmarks.getChildren(id, function(children) {
        children.forEach(function(bookmark) { 
            console.debug(bookmark.title);
            printBookmarks(bookmark.id, depth + 1);
        });
        if(depth == 0) yourFunction();
    });     
}

编辑以响应评论

这是另一个答案的变体，方法略有不同。

runCount = 0;
for (var i = 0; i < foldersArray.length; i++) {
    // The loop makes several calls with different folder IDs.
    printBookmarks(foldersArray[i]);  
    runCount++;
}

while(runCount > 0) { // sleep for 10ms or whatnot}
// I'd like any code here to be run only after the above has 
// finished processing.    

function printBookmarks(id) {
    chrome.bookmarks.getChildren(id, function(children) {
        children.forEach(function(bookmark) { 
            console.debug(bookmark.title);
            printBookmarks(bookmark.id);
            runCount--;
        });
    });
}