For循环不等待winjs承诺完成答案

【问题标题】：For loop not waiting for winjs promise completionFor循环不等待winjs承诺完成
【发布时间】：2013-03-23 06:14:23
【问题描述】：

for 循环不等待 winjs 承诺完成

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);
        });


    }

【问题讨论】：

WinJS 承诺在哪里？
我需要为 MagazineDownLoad.downloadFile 操作实现承诺。只有在此操作完成后for循环才会继续。如何在这里实现承诺..？

标签： microsoft-metro winjs

【解决方案1】：

如果你想让MagazineDownLoad.downloadFile 异步操作，那么你必须修改它的定义：

// 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);
    });
}

然后就可以异步使用了：

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
        });
    });
}

【讨论】：

【解决方案2】：

WinJS.Promise() 异步运行，您的 for 循环同步运行。您正在经历的事情是意料之中的。如果您想将您的操作排队，您不应该执行循环，而是在您的done() 被调用时排队一个新操作。像这样的：

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);

【讨论】：