网络和并发

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1GET / HTTP/1.1
2Accept:text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8
3Accept-Encoding:gzip, deflate, br
4Accept-Language:zh-CN,zh;q=0.9,en;q=0.8
5Cache-Control:max-age=0
6Connection:keep-alive
7Host:www.imooc.com
8Referer:https://www.baidu.com/

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1const urls = [
2  {
3    info: "link1",
4    time: 3000,
5  },
6  {
7    info: "link2",
8    time: 2000,
9  },
10  {
11    info: "link3",
12    time: 5000,
13  },
14  {
15    info: "link4",
16    time: 1000,
17  },
18  {
19    info: "link5",
20    time: 1200,
21  },
22  {
23    info: "link6",
24    time: 2000,
25  },
26  {
27    info: "link7",
28    time: 800,
29  },
30  {
31    info: "link8",
32    time: 3000,
33  },
34];
35
36// 设置我们要执行的任务
37function loadImg(url) {
38  return new Promise((resolve, reject) => {
39    console.log("----" + url.info + " start!");
40    setTimeout(() => {
41      console.log(url.info + " OK!!!");
42      resolve();
43    }, url.time);
44  });
45}
46
47module.exports = {
48  urls,
49  loadImg,
50};

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1//index.js
2const { urls, loadImg } = require("./mock");
3
4function promiseLimit(arr, maxCount) {
5  let current = 0;
6  let pendingList = [];
7  for (let i = 0; i < arr.length; i++) {
8    doSend(arr[i]);
9  }
10
11  function doSend(item) {
12    if (current < maxCount) {
13      current++;
14      loadImg(item).then(() => {
15        current--;
16        if (pendingList.length > 0) {
17          doSend(pendingList.shift());
18        }
19      });
20    } else {
21      pendingList.push(item);
22    }
23  }
24}
25promiseLimit(urls, 3);

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1const { urls, loadImg } = require("./mock");
2
3class PromiseLimit {
4  constructor(props) {
5    const { concurrency } = props;
6    this.concurrency = concurrency || 1;
7    this.pendingList = [];
8    this.limitCount = 0;
9  }
10
11  add(task) {
12    this.pendingList.push(task);
13    this.run();
14  }
15
16  run() {
17    if (this.pendingList.length === 0 || this.limitCount === this.concurrency) {
18      return;
19    }
20
21    this.limitCount++;
22
23    const fn = this.pendingList.shift();
24    const promise = fn();
25    promise.then(this.complete.bind(this)).catch(this.complete.bind(this));
26  }
27
28  complete() {
29    this.limitCount--;
30    this.run();
31  }
32}
33
34const Limit = new PromiseLimit({ concurrency: 3 });
35
36for (let i = 0; i < urls.length; i++) {
37  Limit.add(() => loadImg(urls[i]));
38}