JavaScript 中斷請求幾種方案詳解

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1 Promise

Promise有一個缺點是一旦創建無法取消,所以本質上Promise是無法被終止的.

但是我們可以通過中斷調用鏈或中斷Promise來模擬請求的中斷.

中斷調用鏈

中斷調用鏈就是在某一個then/catch執行之後,後續的鏈式調用(包括then,catch,finally)不再繼續執行.

方法是在then/catch返回一個新的Promise實例,並保持pending狀態:

new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve('result');
    });
}).then(res => {
    // 達到某種條件,return一個pending狀態的Promise實例,以中斷調用鏈
    if (res === 'result') {
        return new Promise(() => {});
    }
    console.log(res); // 不打印
}).then(() => {
    console.log('then不執行'); // 不打印
}).catch(() => {
    console.log('catch不執行'); // 不打印
}).finally(() => {
    console.log('finally不執行'); // 不打印
});

中斷Promise

中斷Promise不等同於中止Promise,因為Promise是無法被終止的.

這裡的中斷指的是,在合適的時機,把pending狀態的promise給reject掉.例如一個常見的應用場景就是給網絡請求設置超時時間,一旦超時就中斷.

老規矩,用setTimeout來模擬網絡請求.閥值設置為Math.random() * 3000表示隨機3秒之內返回結果.

const request = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('收到服務端數據')
  }, Math.random() * 3000)
})

假設超過2秒就是網絡超時,我們可以封裝一個超時處理函數.

由於網絡請求所需的事件是隨機的,因此可以利用Promise.race方法,達到超時reject的目的.

const timeoutReject = (p1, timeout = 2000) => {
    const p2 = new Promise((resolve, reject) => {
        setTimeout(() => {
            reject('網絡超時');
        }, timeout);
    });
    return Promise.race([p1, p2]);
};

timeoutReject(request).then(res => {
    console.log(res);
}).catch(err => {
    console.log(err);
});

包裝abort方法——仿照Axios的CancelToken

上面實現的方式並不靈活,因為中斷Promise的方式有很多,不單單是網絡超時.

我們可以仿照Axios中CancelToken的核心源碼,簡單包裝一個abort方法,供使用者隨時調用.

function abortWrapper(p1) {
    let abort;
    const p2 = new Promise((resolve, reject) => {
        abort = reject;
    });
    // 如果沒有resolve或reject,p2的狀態永遠是pending
    const p = Promise.race([p1, p2]);
    p.abort = abort;
    return p;
}

const req = abortWrapper(request);
req.then(res => {
    console.log(res);
}).catch(err => {
    console.log(err);
});

setTimeout(() => {
    // 手動調用req.abort,將p2的狀態改變為rejected
    req.abort('手動中斷請求');
}, 2000);

如此封裝的主要目的就是為瞭能夠在Promise外部控制其resolve或reject,讓使用者可以隨時手動調用resolve(觸發.then)或reject(觸發.catch).

需要註意的是,雖然Promise請求被中斷瞭,但是promise並沒有終止,網絡請求依然可能返回,隻不過那時我們已經不關心請求結果瞭.

2 RXJS的unsubscribe方法

rxjs本身提供瞭取消訂閱的方法,即unsubscribe.

let stream1$ = new Observable(observer => {
    let timeout = setTimeout(() => {
        observer.next('observable timeout');
    }, 2000);

    return () => {
        clearTimeout(timeout);
    }
});
let disposable = stream1$.subscribe(value => console.log(value));
setTimeout(() => {
    disposable.unsubscribe();
}, 1000);

3 Axios的CancelToken

Axios的CancelToken有兩種使用方法:

  • 方法一
import axios from 'axios';
const CancelToken = axios.CancelToken;
const source = CancelToken.source();

axios.get('/user/12345', {
  cancelToken: source.token
}).catch(function (thrown) {
  if (axios.isCancel(thrown)) {
    console.log('Request canceled', thrown.message);
  } else {
    // handle error
  } 
});

source.cancel('Operation canceled by the user.');
  • 方法二
import axios from 'axios';
const CancelToken = axios.CancelToken;

// 創建一個變量如 cancel 用於存儲這個中斷某個請求的方法
let cancel;

axios.get('/user/12345', {
  cancelToken: new CancelToken(function executor(c) {
    cancel = c; // 將參數 c 賦值給 cancel
  })
});

// 判斷 cancel 是否為函數,確保 axios 已實例化一個CancelToken
if (typeof cancel === 'function') {
    cancel();
    cancel = null;
}

CancelToken的核心源碼:(axios/lib/cancel/CancelToken.js)

'use strict';

var Cancel = require('./Cancel');

/**
 * A `CancelToken` is an object that can be used to request cancellation of an operation.
 *
 * @class
 * @param {Function} executor The executor function.
 */
function CancelToken(executor) {
  if (typeof executor !== 'function') {
    throw new TypeError('executor must be a function.');
  }

  var resolvePromise;
  this.promise = new Promise(function promiseExecutor(resolve) {
    resolvePromise = resolve;
  });

  var token = this;
  executor(function cancel(message) {
    if (token.reason) {
      // Cancellation has already been requested
      return;
    }

    token.reason = new Cancel(message);
    resolvePromise(token.reason);
  });
}

/**
 * Throws a `Cancel` if cancellation has been requested.
 */
CancelToken.prototype.throwIfRequested = function throwIfRequested() {
  if (this.reason) {
    throw this.reason;
  }
};

/**
 * Returns an object that contains a new `CancelToken` and a function that, when called,
 * cancels the `CancelToken`.
 */
CancelToken.source = function source() {
  var cancel;
  var token = new CancelToken(function executor(c) {
    cancel = c;
  });
  return {
    token: token,
    cancel: cancel
  };
};

module.exports = CancelToken;

可以看到,在Axios底層,CancelToken的核心源碼所體現的思想,與上面中斷Promise包裝abort方法的思想一致.

隻不過Axios在外部手動調用resolve(用戶觸發cancel方法),而resolve一旦調用,就會觸發promise的then方法,來看這個promise.then的源碼:(axios/lib/adapters/xhr.js)

if (config.cancelToken) {
  // Handle cancellation
  config.cancelToken.promise.then(function onCanceled(cancel) {
    if (!request) {
      return;
    }

    request.abort();
    reject(cancel);
    // Clean up request
    request = null;
  });
}

可以看到then方法中會執行abort方法取消請求,同時調用reject讓外層的promise失敗.

到此這篇關於JavaScript 中斷請求幾種方案詳解的文章就介紹到這瞭,更多相關js中斷請求內容請搜索WalkonNet以前的文章或繼續瀏覽下面的相關文章希望大傢以後多多支持WalkonNet！

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