Javascript for循环完成后如何执行代码

Question

I'm trying to work through this js/async scenario and i'm trying to know how the rest of the js world handles this.

function doStuff(callback) {

  cursor.each(function(err, blahblah) {
    ...doing stuff here takes some time
  });

  ... Execute this code ONLY after the `cursor.each` loop is finished
  callback();

EDIT

Here's a more concrete example updated using most of the suggestions below which still doesn't work.

function doStuff(callback) {

  MongoClient.connect(constants.mongoUrl, function(err, db) {

    var collection = db.collection('cases2');
    var cursor = collection.find();

    var promises = [];  // array for storing promises

    cursor.each(function(err, item) {

      console.log('inside each'); // NEVER GETS LOGGED UNLESS I COMMENT OUT THIS LINE: return Q.all(promises).then(callback(null, items));

      var def = Q.defer();        // Create deferred object and store
      promises.push(def.promise); // Its promise in the array

      if(item == null) {
        return def.resolve();
      }

      def.resolve();  // resolve the promise
    });

    console.log('items'); // ALWAYS GETS CALLED
    console.log(items);

    // IF I COMMENT THIS LINE OUT COMPLETELY, 
    // THE LOG STATEMENT INSIDE CURSOR.EACH ACTUALLY GETS LOGGED
    return Q.all(promises).then(callback(null, items));
  });
}

Answer 1

without using promises or any other dependencies/libraries you can simply

function doStuff(callback) {

add a counter

    var cursor = new Array(); // init with some array data
    var cursorTasks = cursor.length;

    function cursorTaskComplete()
    {
        cursorTasks--;

        if ( cursorTasks <= 0 ) {
            // this gets get called after each task reported to be complete
            callback();
        }
    }

    for ( var i = 0; i < cursor.length; i++ ) {
        ...doing stuff here takes some time and does some async stuff

check after each async request

        ...when async operation is complete call
        cursorTaskComplete()
  }
}

Answer 2

Without knowing the details of the async calls you're making within the cursor.each loop, I shall assume that you have the ability to invoke a callback each time the functions invoked therein have completed their async task:

function doStuff() {
    var promises = [];  // array for storing promises

    cursor.each(function(err, blahblah) {
        var def = Q.defer();        // create deferred object and store
        promises.push(def.promise); // its promise in the array

        call_async_function(..., def.resolve);  // resolve the promise in the async function's callback
    });

    // pass the array to Q.all, only when all are resolved will "callback" be called
    return Q.all(promises);
} 

and the usage then becomes:

doStuff().then(callback)

Note how the invocation of the callback now never touches the doStuff function - that function now also returns a promise. You can now register multiple callbacks, failure callbacks, etc, all without modifying doStuff . This is called "separation of concerns".

[NB: all the above based on the Q promises library - https://github.com/kriskowal/q]

EDIT further discussion and experimentation has determined that the .each call is itself async, and gives no indication to the outside when the last row has been seen. I've created a Gist that demonstrates a resolution to this problem.

Answer 3

if you want to do it with the async module, you can make use of the async forEachSeries function

Code snippet:

function doStuff(callback) {

  async.forEachSeries(cursor, function(cursorSingleObj,callbackFromForEach){
      //...do stuff which takes time
      //this callback is to tell when everything gets over execute the next function
      callbackFromForEach();
  },function(){
     //over here the execution of forEach gets over and then the main callback is called
    callback();
  });
}

Answer 4

In my mind an elegant/ideal solution would be to have something like

 cursor.each(........).then( function() { ....your stuff});

But without that you can do this....UPDATED

http://plnkr.co/edit/27l7t5VLszBIW9eFW4Ip?p=preview

The gist of this is as shown below...notice....when

var doStuff = function(callback) {
      cursor.forEach(function(cursorStep) {
        var deferred = $q.defer();
        var promise = deferred.promise;
        allMyAsyncPromises.push(promise);
        cursorStep.execFn(cursorStep.stepMeta);
        promise.resolve;
      });

      $q.when(allMyAsyncPromises).then(callback);
}

After hitting the start button wait for few seconds...the async tasks have been simulated to finish in 5 seconds so the status will update accordingly.

Not having access to a real cursor object..I had to resort of fake cursor like and array.