使用 promises 同步执行 ajax

Question

这是我处理文件上传的代码：

但由于我没有使用 promises，因此运行顺序不正确！当我上传多个文件时，代码运行的 ajax 部分位于最后（我正在使用 krajee bootstrap 并且此代码执行实际的文件上传！）。这是代码：

$('#uploadNoteImage').on('fileloaded', function (event, file, previewId, index, reader) {
    var url = noTrailingSlash(window.location.href) + '/user/notes';
    console.log("1) Notes upload number: "+index);

    var imgpgno = parseInt(getNotesLength(noteTitle, notesData));
    console.log("2) Got page number of image "+imgpgno);

    var data = {
        "subject": noteTitle,
        "pgno": imgpgno + 1,
        "note": reader.result
    };

    console.log("3) Formed data object:");
    console.log(data);
    $.ajax({
        url: url,
        method: "PUT",
        data: data,
        success: function (data) {
            console.log("4) Successfully uploaded data");
            console.log(data);
            toastr.success('', 'Added!');
            var order = getIndexToDelete(noteTitle, notesData);
            console.log("5) Fetched order number: "+order);
            var id = Math.floor(Math.random() * 1000);
            if (imgpgno == 0) {
                console.log("6)(No notes uploaded yet state) Images before uploading"+images);
                modalImg.src = reader.result;
                notesData[order].data.push({
                    "id": id, "pgno": imgpgno + 1,
                    "note": reader.result
                });
                images = imgpgno + 1;
                console.log("7)(No notes uploaded yet state) Images after uploading: "+images);
                // imgpgno++;

            }
            else if(imgpgno!=0) {
                var newPageNo=imgpgno + 1;
                console.log("6)(1 note uploaded state) Pushing data with pgno: "+newPageNo);
                notesData[order].data.push({
                    "id": id, "pgno": newPageNo,
                    "note": reader.result
                });
                images = imgpgno + 1;
                console.log("7)(1 note uploaded state) Images after uploading: "+images);
            }
        },
        error: function (err) {
            toastr.error('Try again!', 'Something went wrong in uploading note!');
            console.log(err);
        }
    });
});

如何在此处包含承诺，以便代码运行以正确的顺序排列？我在这里详细描述了这个问题：Late execution of AJAX code

Answer 1

最简单的方法需要一点 "setup" -

var uploadInProgress = $.when();

这个 "primes" uploadInProgress 变量以这样的方式第一次上传将在我们想要的时候开始

整个代码与您已有的代码差别不大：

var uploadInProgress = $.when();
$('#uploadNoteImage').on('fileloaded', function (event, file, previewId, index, reader) {
    // here, we chain the "pending" upload to this one
    uploadInProgress = uploadInProgress.then(function() {
        var url = noTrailingSlash(window.location.href) + '/user/notes';
        console.log("1) Notes upload number: "+index);

        var imgpgno = parseInt(getNotesLength(noteTitle, notesData));
        console.log("2) Got page number of image "+imgpgno);

        var data = {
            "subject": noteTitle,
            "pgno": imgpgno + 1,
            "note": reader.result
        };

        console.log("3) Formed data object:");
        console.log(data);
        return $.ajax({
            url: url,
            method: "PUT",
            data: data
        // since we are using Promise pattern, use .then for success, and .catch for error conditions
        }).then(function (data) {
            console.log("4) Successfully uploaded data");
            console.log(data);
            toastr.success('', 'Added!');
            var order = getIndexToDelete(noteTitle, notesData);
            console.log("5) Fetched order number: "+order);
            var id = Math.floor(Math.random() * 1000);
            if (imgpgno == 0) {
                console.log("6)(No notes uploaded yet state) Images before uploading"+images);
                modalImg.src = reader.result;
                notesData[order].data.push({
                    "id": id, "pgno": imgpgno + 1,
                    "note": reader.result
                });
                images = imgpgno + 1;
                console.log("7)(No notes uploaded yet state) Images after uploading: "+images);
                // imgpgno++;

            }
            else { // if(imgpgno!=0) { // the check is redundant since when the above if condition is false, this has to be true
                var newPageNo=imgpgno + 1;
                console.log("6)(1 note uploaded state) Pushing data with pgno: "+newPageNo);
                notesData[order].data.push({
                    "id": id, "pgno": newPageNo,
                    "note": reader.result
                });
                images = imgpgno + 1;
                console.log("7)(1 note uploaded state) Images after uploading: "+images);
            }
        }).catch(function (err) {
            // this catch ensures that the next upload will be able to run regardless of this upload's failure
            toastr.error('Try again!', 'Something went wrong in uploading note!');
            console.log(err);
        });
    });
});

而不是 success:/error: 回调，使用 $.ajax returns a Promise 的事实，并使用 .then/ .catch - 这使得将上传请求一个接一个地链接起来变得容易

注意：确保 .catch 确实处理了错误（即不抛出一个错误），否则承诺链将中断 - 您在 error: 中的代码没有问题

使用 promises 同步执行 ajax

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