Question

Pretty sure this has been asked already, but I don’t know what to search for. Anyway,

var livemarks = [];

var livemarkIds = PlacesUtils.annotations.getItemsWithAnnotation("livemark/feedURI", {});

for (var i = 0; i < livemarkIds.length; i++){

    PlacesUtils.livemarks.getLivemark( {id : livemarkIds[i]}, function(result, livemark){

        if (result == Components.results.NS_OK){
            livemarks.push(livemark);
        }

    });

}

alert(livemarks.length);

I am trying to play a bit with a Firefox addon that’s no longer maintained by its creator, just to learn a bit. I recently got an error saying getFeedURI is going to be deprecated and I want to change his old function.

EDIT:

From a function defined in a function (inner function), I am unable to access a var defined in the parent. Why?
E.g. I cannot access var livemarks from inside getLivemark(), or other similar internal functions.

I was checking (scroll down completely): this and his code works fine. So what’s wrong with my code? I just wanted to avoid the recursion, if possible.

Answer 1

I suspect the PlacesUtils.livemarks.getLivemark function does its work asynchronously, so your callback is called after you alert the length. Put your alert inside the callback and you should see the correct length (eventually). Here’s one way:

var expecting = livemarkIds.length;
for (var i = 0; i < livemarkIds.length; i++){

    PlacesUtils.livemarks.getLivemark( {id : livemarkIds[i]}, function(result, livemark){

        if (result == Components.results.NS_OK){
            livemarks.push(livemark);

            // ***New stuff***
            if (livemarks.length === expecting) {
                // Now you have them all, now you can do the next thing
                doSomethingWithTheLiveMarks(livemarks);
            }
        }

    });
}

Note that there I put livemarkIds.length into expecting, just in case you do other things with livemarkIds while the function is running. If you aren’t, you can just use that directly.

---

Re your comment below:

However, the system works like this: I get the livemarks in an array. This code is in a class (and method) actually, so another class initializes this one and will call the function getFeeds(), which will return that array of livemarks.

If PlacesUtils.livemarks.getLivemark is asynchronous, it’s impossible for getFeeds to return the array as a return value. E.g., it cannot be used like this:

a = b;
c = 42;
feeds = getFeeds(feedIds);
if (feeds.length === 0) {
    // Do something
}
else {
    // Do something else
}

The good news is it’s really easy to fix: Have getFeeds accept a callback function that it calls when it has the feeds. The code above changes to look like this:

a = b;
c = 42;
feeds = getFeeds(feedIds, function(feeds) {
    if (feeds.length === 0) {
        // Do something
    }
    else {
        // Do something else
    }
});

As you can see, it’s a pretty straightforward change. Assuming the loop above is all of getFeeds, then getFeeds ends up looking something like this:

function getFeeds(livemarkIds, callback) {

    var livemarks = [];
    for (var i = 0; i < livemarkIds.length; i++){

        PlacesUtils.livemarks.getLivemark( {id : livemarkIds[i]}, function(result, livemark){

            if (result == Components.results.NS_OK){
                livemarks.push(livemark);

                if (livemarks.length === livemarkIds.length) {
                    // Done, trigger the callback
                    callback(livemarks);
                }
            }

        });
    }
}

And the pattern continues: If the code calling getFeeds is being called by something else that’s expecting a return value from the async stuff, instead of returning that value, you have that code accept a callback, and call the callback from the getFeeds callback. And so on.

Once you get used to it, it’s very easy to do. Getting used to it can be tricky. 🙂