javascript - 在所有 promise 都 resolve 之后执行 promise

Question

我编写了一些代码来遍历目录，选择所有 jpg 文件，重命名它们，创建 thumbs 文件夹并将它们放入该文件夹中。我还编写了一个函数，可以为每个图像生成一大块 html，并且在 forEach 循环结束时应该将所有输出写入一个文件。

// Read the directory
fs.readdir(dir, function (err, files) {

// Return the error if something went wrong
if (err) {
    console.log("Something went wrong");
}

//Make thumbs dir
fs.mkdirSync(thumbsFolder);

// For every file in the list
files.forEach(function (file) {
    var extension = path.extname(file);

    if (extension === ".jpg") {

        //Rename images
        var renameTask = renameImages(file, newFileName, extension);

        //Generating thumbs
        renameTask.then(function (newFileName) {
            generateThumbs(dir, newFileName, thumbsFolder)
            console.log("Promise resolved! Generating thumb for: " + newFileName);
            galleryHtml += generateThumbsGallery(newFileName, articleTitle);

        }, function (err) {
            console.log(err);
        });

    }

});

//Gallery output
console.log("Gallery Output: ");
console.log(galleryHtml);

});

我的问题是，按照听起来的逻辑，galleryHtml(已在代码的上半部分定义，此处未包含)返回未定义，因为它在所有先前的 promise 都已解决之前运行。

我读过 Promise.all 方法，我应该传递一个 promises 数组，问题是我不知道会返回多少个 promises，在我的代码中“renameTask”是作用的变量作为 promise 接收者。

因此，我需要一些帮助 - 我提前非常感谢 - 了解如何将 forEach 本身作为一个 promise ，并且在所有 forEach promise 得到解决之后，然后执行最终的 promise 。

提前致谢。

更新:

Felix，非常感谢，我现在更近了

// Read the directory
fs.readdir(dir, function (err, files) {

    // Return the error if something went wrong
    if (err) {
        console.log("Something went wrong");
    }

    var promises = [];

    //Make thumbs dir
    fs.mkdirSync(thumbsFolder);

    // For every file in the list
    files.forEach(function (file) {
        var extension = path.extname(file);

        if (extension === ".jpg") {

            //Rename images
            var renameTask = renameImages(file, newFileName, extension);


            promises.push(renameTask.then(function (newFileName) {
                    generateThumbs(dir, newFileName, thumbsFolder)
                    console.log("Promise resolved! Generating thumb for: " + newFileName);
                    galleryHtml += generateThumbsGallery(newFileName, articleTitle);
                }, function (err) {
                    console.log(err);
                }) 
            );

            console.log("<<<< Promises length: " + promises.length + " >>>>");

        }

    });

    Promise.all(promises).then(function () {
        console.log("<<<<< All promises were resolved >>>>");
    });

});

问题是我从来没有收到消息“<<<<< All promises were resolved >>>>”，我知道这应该是我遗漏的一件小事。再次感谢您帮助!

Answer 1

I've read about Promise.all method, and that I should pass a promises array, the question is that I don´t know how many promises would be returned, and in my code "renameTask" is the variable that acts as the promise receiver.

它并不像您想象的那么复杂。创建一个数组，向其添加 promise 并将该数组传递给 Promise.all:

var promises = []; // array of promises

files.forEach(function (file) {
    var extension = path.extname(file);
    if (extension === ".jpg") {
        var renameTask = renameImages(file, newFileName, extension);
        promises.push(renameTask.then(...)); // add promise
    }

});

Promise.all(promises).then(...); // use array