异步递归函数结束后的回调

Question

The function below prints Chrome bookmarks in a folder recursively. How could I alter the below function to call another function after the final recursive loop is processed? chrome.bookmarks.getChildren() is asynchronous which makes it difficult to know when the function is done processing everything.

Thanks.

    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);
           });
        });
    }

EDIT: Sorry, I don't think I was clear in the initial code example. I've updated the code to show the problem I'm having with the asynchronous function by calling the function multiple times. I'd like any code after the printBookmarks function calls to wait for all the printBookmarks functions to finish processing.

Answer 1

Your asynchronous method instances may all be executing at once, and you don't know how many there will be beforehand. So, you'll have to keep count and then use a callback when the last asynchronous method is done.

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

I've recently had to solve this problem. The solution was similar to Eric's, but I found that I needed the count variable to be local to the function. Here's how I would solve this:

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();
    }
  });
}

It's a bit ugly, but it should work.

Answer 3

您可以执行类似JQFAQ.com的操作 。我正在为将来的使用进行更新。

Answer 4

You could save all your completed calls into a variable and test against the number of bookmarks you want to process. When you reach the end (the count of completions equals the amount of bookmarks to process), then you execute your final function.

An answer to a similar problem is here, with code that you can use as a guide:

How do I load a variable number of scripts with jQuery.getScript() before running javascript code?

Answer 5

Might be a better way to go about this, but you could add a depth parameter, something like

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();
    });     
}

EDIT in response to comment

This is a variation on another answer for a slightly different approach.

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--;
        });
    });
}