Meteor (Fibers) loop and callback

Question

Meteor fibers "sync" mode is driving me crazy. Here is a simple code example :

var feedsData = feeds.fetch(); // [{_id: "1234"}, {_id: "6789", url: "http://...."}]
for(var i = 0, len = feedsData.length; i < len; i++) {
    var feed = feedsData[i];
    parser.parseURL(feed.url, function(err, out){
        console.log(feed._id, i); // outputs "6789" and "2" each times
    });
}

I don't understand how to make this work. The callback is called after the loop is over, but the internal internal variables such as feed should be preserved... and they are not.

The url parsed are good (the first one, then the second one), but then i can't update my data since I don't have the good _id in the callback.

The wanted output would be: "1234" "0" and "6789" "1", not "6789" "2" both times... How would you make this in Meteor / Fiber code ?

score 1 · Accepted Answer · answered Mar 26 '13 at 21:26

Another way to do it in the "fiber"(and it is probably better than the answer with "future" I posted above) :

var feedsData = feeds.fetch(); // [{_id: "1234"}, {_id: "6789", url: "http://...."}]
Fiber(function() {
    var fiber = Fiber.current;
    for(var i = 0, len = feedsData.length; i < len; i++) {
        var feed = feedsData[i];
        parser.parseURL(feed.url, function(err, out) {
            console.log(feed._id, i);
            if(err) return fiber.throwInto(err);
            fiber.run();
        });
        Fiber.yield();
        console.log('here', i);
    }
    console.log('there');
}).run();
console.log('and there');

The output will be :

"and there"
"1234" "0"
"here" "0"
"6789" "1"
"here" "1"
"there"

Note that everything in the Fiber function is executed in its own fiber as if it was asynchrone, which is why "and there" is outputed first

Rahul · Answer 2 · 2013-03-26T20:54:22.103

0

Not sure this has anything to do with Meteor, Fibers or "sync mode". I think it's just a bug in your javascript. You're looping through an array and then invoking a property of an object within a callback. Of course when the callback eventually is called, it will look at the current value of feed, which will be the most recently assigned one after the loop exited.

So you should rewrite your code to take that into account:

var feedsData = [{_id: "1234"}, {_id: "6789", url: "http://...."}]
for(var i = 0, len = feedsData.length; i < len; i++) {
    var feed = feedsData[i];
    parser.parseURL(feed.url, function(err, out){
        console.log(this._id, arguments[0]); // will output "1234 0" and "6789 1"
    }.bind(feed, i));
}

edited Mar 26 '13 at 20:54

answered Mar 26 '13 at 20:28

Rahul

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Sorry but this doesn't work: TypeError: Cannot call method 'bind' of undefined. The feed variable is internal to each loop, so it should not be reassigned, after 2 loops there is two independant feed variables that each exist for each callbacks. I found the solution for my problem, I'll post it in a minute – Olivier Mar 26 '13 at 20:53
No more error, but it still outputs "6789" and "2" both times... :-/ – Olivier Mar 26 '13 at 20:56
Just posted 2 different answers, both are valid and work as "expected" :) – Olivier Mar 26 '13 at 21:27
Javascript doesn't have block scoping, only function scoping. Your feed variable is being reassigned on each iteration. Rahul's answer is correct - this is not to do with fibers, or node, or meteor, just Javascript :) http://bonsaiden.github.io/JavaScript-Garden#function.scopes – timruffs Jul 16 '13 at 09:37

score 0 · Answer 3 · edited Nov 02 '13 at 16:43

Ok, here is the "fiber" way of doing that :

var Future = require('fibers/future'),
wait = Future.wait,
feedsData = feeds.fetch(); // [{_id: "1234"}, {_id: "6789", url: "http://...."}],
parseUrl = Future.wrap(parser.parseURL);
Fiber(function() {
    for(var i = 0, len = feedsData.length; i < len; i++) {
        var feed = feedsData[i];
        var out = parseUrl(feed.url).wait();
        console.log('here', i, out);
    }
    console.log('there');
}).run();
console.log('and there');

The result output will be :

"and there"
"here" "0" "the out data from the 1st callback"
"here" "1" "the out data from the 2nd callback"
"there"

Just what you would expect. The "future" in Fibers expects that the last parameter given to the function is a callback, and will return err as the first parameter

score 0 · Answer 4 · answered Jul 16 '13 at 09:40

The simplest solution is:

feeds.fetch().forEach(function(feed,i) {
    parser.parseURL(feed.url, function(err, out){
        console.log(feed._id, i);
    });
});

Javascript does not have block scoping (yet, let is coming in ES6), only function scoping.

Meteor (Fibers) loop and callback

4 Answers4

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