For loop not waiting for winjs promise completion

问题

For loop not waiting for winjs promise completion

for (var j = 0; j < magazineResult[0].data.length; j++) {
        downRequest[0].data[j].COVER_PAGE_THUMB = parentUrl + eval(JSON.stringify(downRequest[0].data[j].COVER_PAGE_THUMB));

        // Create a new download operation.
        downloadFile(eval(magazineResult[0].data[j].COVER_PAGE_THUMB),eval(JSON.stringify(magazineResult[0].data[j].COVER_PAGE_THUMB)));
        var url = downRequest[0].data[j].COVER_PAGE_THUMB;
        var imgPath = downRequest[0].data[j].ISSUE_ID;
        var imgExtension = url.substring(url.lastIndexOf('.') + 1);
        var fileName = imgPath + "." + imgExtension;
        var promise = Windows.Storage.ApplicationData.current.localFolder.createFileAsync(fileName, Windows.Storage.CreationCollisionOption.replaceExisting);
        // Assign the completion handler function.
        promise.done(function (newFile) {
            MagazineDownLoad.downloadFile(url, fileName, j, newFile);
        });


    }

回答1:

If you're trying to get MagazineDownLoad.downloadFile to operate asynchronously then you'll have to modify it's definition:

// in MagazineDownload
function downloadFile(url, filename, j, newfile){
    return new WinJS.Promise(function (complete, error, progress) {
        var returnValue;
        //do the stuff that you do and assign something to returnValue
        complete(returnValue);
    });
}

Then you can use it asynchronously:

for (var j = 0; j < magazineResult[0].data.length; j++) {
    downRequest[0].data[j].COVER_PAGE_THUMB = parentUrl + eval(JSON.stringify(downRequest[0].data[j].COVER_PAGE_THUMB));

    // Create a new download operation.
    downloadFile(eval(magazineResult[0].data[j].COVER_PAGE_THUMB),eval(JSON.stringify(magazineResult[0].data[j].COVER_PAGE_THUMB)));
    var url = downRequest[0].data[j].COVER_PAGE_THUMB;
    var imgPath = downRequest[0].data[j].ISSUE_ID;
    var imgExtension = url.substring(url.lastIndexOf('.') + 1);
    var fileName = imgPath + "." + imgExtension;
    var promise = Windows.Storage.ApplicationData.current.localFolder.createFileAsync(fileName, Windows.Storage.CreationCollisionOption.replaceExisting);
    // Assign the completion handler function.
    promise.done(function (newFile) {
        MagazineDownLoad.downloadFile(url, fileName, j, newFile).done(function(result){
            //do some more stuff with the result
        });
    });
}

回答2:

The WinJS.Promise() runs asynchronously and your for-loop runs in sync. What you are experiencing is expected. If you want to queue your actions you should not perform a loop, but rather queue a new action when your done() is called. Something like this:

var index = 0, data = magazineResult[0].data;
function queueDownload() {
    // Duplicate all needed logic here from your question
    var promise = Windows.Storage.ApplicationData.current.localFolder.createFileAsync(fileName, Windows.Storage.CreationCollisionOption.replaceExisting);
    // Assign the completion handler function.
    promise.done(function (newFile) {
        MagazineDownLoad.downloadFile(url, fileName, j, newFile);
        if (index < data.length) {
            queueDownload(++index);
        }
    });
}
queueDownload(index);

来源：https://stackoverflow.com/questions/15583972/for-loop-not-waiting-for-winjs-promise-completion

标签

microsoft-metro

winjs