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，该函数为每个图像生成一块 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，听起来合乎逻辑，galleryHtml（已在代码的上半部分定义，此处未包含），returns未定义，因为它在所有先前的承诺之前运行已解决。

我读过 Promise.all 方法，我应该传递一个承诺数组，问题是我不知道会返回多少承诺，在我的代码中 "renameTask" 是充当承诺接收者的变量。

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

提前致谢。

更新：

菲利克斯，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。再次感谢求助！

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，向其添加承诺并将该数组传递给 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

在所有 promise 都 resolve 之后执行 promise

Execute promise after all promises were resolved

javascript

node.js

promise

ecmascript-6

es6-promise