与fs和蓝鸟的承诺

Question

我目前正在学习如何在nodejs中使用promises

所以我的第一个挑战是列出目录中的文件,然后使用异步函数获取每个步骤的内容.我提出了以下解决方案,但有一种强烈的感觉,这不是最优雅的方式来做到这一点,尤其是我将异步方法"转变"为承诺的第一部分

// purpose is to get the contents of all files in a directory
// using the asynchronous methods fs.readdir() and fs.readFile()
// and chaining them via Promises using the bluebird promise library [1]
// [1] https://github.com/petkaantonov/bluebird 

var Promise = require("bluebird");
var fs = require("fs");
var directory = "templates"

// turn fs.readdir() into a Promise
var getFiles = function(name) {
    var promise = Promise.pending();

    fs.readdir(directory, function(err, list) {
        promise.fulfill(list)
    })

    return promise.promise;
}

// turn fs.readFile() into a Promise
var getContents = function(filename) {
    var promise = Promise.pending();

    fs.readFile(directory + "/" + filename, "utf8", function(err, content) {
        promise.fulfill(content)
    })

    return promise.promise
}

现在链接两个承诺:

getFiles()    // returns Promise for directory listing 
.then(function(list) {
    console.log("We got " + list)
    console.log("Now reading those files\n")

    // took me a while until i figured this out:
    var listOfPromises = list.map(getContents)
    return Promise.all(listOfPromises)

})
.then(function(content) {
    console.log("so this is what we got: ", content)
})

正如我上面所写,它返回了所需的结果,但我很确定有更优雅的方法.

Answer 1

通过使用泛型promisification和.map方法可以缩短代码:

var Promise = require("bluebird");
var fs = Promise.promisifyAll(require("fs")); //This is most convenient way if it works for you
var directory = "templates";

var getFiles = function () {
    return fs.readdirAsync(directory);
};
var getContent = function (filename) {
    return fs.readFileAsync(directory + "/" + filename, "utf8");
};

getFiles().map(function (filename) {
    return getContent(filename);
}).then(function (content) {
    console.log("so this is what we got: ", content)
});

事实上你可以进一步削减它,因为功能不再拉动它们的重量:

var Promise = require("bluebird");
var fs = Promise.promisifyAll(require("fs")); //This is most convenient way if it works for you
var directory = "templates";

fs.readdirAsync(directory).map(function (filename) {
    return fs.readFileAsync(directory + "/" + filename, "utf8");
}).then(function (content) {
    console.log("so this is what we got: ", content)
});

.map 在处理集合时应该是你的面包和黄油方法 - 它非常强大,因为它适用于任何承诺,一系列承诺映射到中间任何直接值混合的进一步承诺.