标准的方式来做到asyntask为了与承诺

问题描述

所有

我是pretty新的承诺，我想要做的是：

I am pretty new to promise, what I want to do are:

[1]下载文件中的，当A下载，然后下载文件B，B之后已经准备好，然后下载载荷C。

[1] Download file A, when A downloaded, then download file B, after B is ready, then download load C.

[2]制作一个函数来下载文件A，不管它被调用多少次，只降档一次。

[2] Make a function to download file A, no matter how many times it been called, it only down file once.

[1]和[2]是不相关的任务。你能帮助我与其中任何一个或两个。

[1] and [2] are not related tasks. You can help me with either one of them or Both.

谁能给我一份有承诺一个简单的例子？谢谢你。

Could anyone give me a simple example with promise? Thanks.

推荐答案

在使用Node.js的青鸟承诺库，这里有三种方法：

Using the Bluebird promise library in node.js, here are three approaches:

// load and promisify modules
var Promise = require("bluebird");
var fs = Promise.promisifyAll(require('fs'));

完全连续，每一步codeD单独

Purely Sequential, each step coded separately

// purely sequential
fs.readFileAsync("file1").then(function(file1) {
    // file1 contents here
    return fs.readFileAsync("file2");
}).then(function(file2) {
    // file2 contents here
    return fs.readFileAsync("file3");    
}).then(function(file3) {
    // file3 contents here
}).catch(function(err) {
    // error here
});

阅读全部并行收集结果时，他们都做

Read all in parallel, collect results when they are all done

// read all at once, collect results at end
Promise.map(["file1", "file2", "file3"], function(filename) {
    return fs.readFileAsync(filename);
}).then(function(files) {
    // access the files array here with all the file contents in it
    // files[0], files[1], files[2]
}).catch(function(err) {
    // error here
});

从阵列读取顺序

// sequential from an array
Promise.map(["file1", "file2", "file3"], function(filename) {
    return fs.readFileAsync(filename);
}, {concurrency: 1}).then(function(files) {
    // access the files array here with all the file contents in it
    // files[0], files[1], files[2]
}).catch(function(err) {
    // error here
});

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