为什么我的诺言没有得到解决？

Question

I have two functions - A helper function for downloading files which is as follows

var downloadHelper = function(url, saveDir) {
    var deferred = Q.defer();

    setTimeout(function() {
        deferred.resolve("success");    
    }, 2000);


    return deferred.promise;
}

Now I have a list of files to be downloaded in parallel. I have the logic for that function as follows:

var downloadAll = function() {
    var fileDownloadList = []
    for(var key in config.files) {

        var deferred = Q.defer();
        var saveLocation = __base + config.localDir
        downloadHelper(
            config.files[key], 
            saveLocation
        ).then(function() {
            deferred.resolve("downloaded: " + fileUrl);
        }).catch(function(err) {
            deferred.reject(err);
        });

        fileDownloadList.push(deferred.promise);
    }

    Q.all(fileDownloadList).done(function() {
        console.log("All downloaded");
    },function(err) {
        console.log(err);
    });

    setTimeout(function() {
        console.log(fileDownloadList);
    }, 10000);
}

The done is never getting called!

For debugging purposes, I added a setTimeout that will be called after 10 seconds and what I see is that out of 2 files, the second promise is resolved and the first one is still in pending state.

Any ideas?

Thanks in advance

Answer 1

One way to make your code work

for(var key in config.files) {
    (function() {
        var deferred = Q.defer();
        var saveLocation = __base + config.localDir
        downloadHelper(
            config.files[key], 
            saveLocation
        ).then(function() {
            deferred.resolve("downloaded: " + fileUrl);
        }).catch(function(err) {
           deferred.reject(err);
        });
        fileDownloadList.push(deferred.promise);
    }());
}

But since downloadhelper returns a promise, no need to create yet another one

for (var key in config.files) {
    var saveLocation = __base + config.localDir
    fileDownloadList.push(downloadHelper(
        config.files[key],
        saveLocation
    ).then(function () {
        return("downloaded: " + fileUrl);
    }));
} 

You'll see I removed

.catch(function(err) {
    deferred.reject(err);
})

That's redundant, it's the same as not having the catch at all