node.js 实现断点下载器

之前写 eva 下载器的时候，就一直想做断点下载，直到最近才实现。

原理

断点下载其实是依赖 http 的 range 参数：

1	res.setHeader("Range", `bytes=${read}-`);

断点下载解决后，还需要添加文件追加

1	`res.pipe(fs.createWriteStream(out, { flags: "a" }));`

但是并不是所以的资源都支持断点续传，所以要做一下兼容。

实现

const EventEmitter = require("events");
const Request = require("request");
const { getRead, getLength, getTotal, startNum } = require("../utils/index");
const fs = require("fs");

class Downloader extends EventEmitter {
    constructor() {
        super();
        this.instance = null;
        this.request = this.request.bind(this);
    }

    /**
     * 单例初始化
     * @author bubao
     * @date 2019-12-30
     * @static
     * @memberof PromiseRequest
     */
    static init() {
        if (!this.instance) {
            this.instance = new this();
        }
        return this.instance;
    }

    /**
     * request
     * @author bubao
     * @date 2019-12-30
     * @param {{
     * pipe:string, // download path
     * hide:boolean, // hiden ora
     * time:number, // start time
     * size:number, // download size content-length
     * }} options { pipe, hiden, time, size, readable, ...opts }
     * @returns {Promise}
     * @memberof PromiseRequest
     */
    request(options) {
        const that = this;
        const {
            pipe, // download path
            hiden, // hiden ora
            time, // start time
            size, // download size content-length
            ...opts // request options
        } = options;
        const start = startNum(time);
        let read = getRead(options);
        let response = 0;
        let total = 0;
        let speed = 0;

        const res = Request(opts);
        const Interval = setInterval(() => {
            that.emit("progress", {
                completed: read,
                total,
                hiden,
                speed,
                time: { start },
                status: { down: "正在下载。..", end: "完成、n" }
            });
            speed = 0;
        }, 1000);
        read && res.setHeader("Range", `bytes=${read}-`);

        return new Promise(function(resolve) {
            res.on("response", resp => {
                const length = resp.headers["content-length"];
                response = getLength(
                    read && length !== undefined
                        ? read + (length - 0)
                        : length,
                    size
                );
            })
                .on("data", function(data) {
                    speed += data.length;
                    read += data.length;
                    total = getTotal(
                        size,
                        response,
                        read
                    );
                })
                .on("end", () => {
                    that.emit("progress", {
                        completed: read,
                        total: total,
                        hiden,
                        speed,
                        time: { start },
                        status: { down: "正在下载。..", end: "完成、n" }
                    });
                    clearInterval(Interval);
                    resolve();
                });
            // 如果 pipe 参数存在，则下载到指定路径
            download(res, pipe, read);
        });
    }
}

/**
 * 如果存在 piep 则下载
 * @author bubao
 * @date 2019-12-30
 * @param {buffer} data stream
 * @param {string} dir pipe
 * @param {boolean} append
 */
function download(data, dir, append) {
    if (dir && dir.length) {
        const opts = append ? { flags: "a" } : undefined;
        data.pipe(fs.createWriteStream(dir || "./", opts));
    }
}
module.exports = Downloader;