Promise 程式碼實現

/**
 * Promise 實現 遵循promise/A+規範
 * Promise/A+規範譯文:
 * https://malcolmyu.github.io/2015/06/12/Promises-A-Plus/#note-4
 */

// promise 三個狀態
const PENDING = "pending";
const FULFILLED = "fulfilled";
const REJECTED = "rejected";

function Promise(excutor) {
    let that = this; // 快取當前promise例項物件
    that.status = PENDING; // 初始狀態
    that.value = undefined; // fulfilled狀態時 返回的資訊
    that.reason = undefined; // rejected狀態時 拒絕的原因
    that.onFulfilledCallbacks = []; // 儲存fulfilled狀態對應的onFulfilled函式
    that.onRejectedCallbacks = []; // 儲存rejected狀態對應的onRejected函式

    function resolve(value) { // value成功態時接收的終值
        if(value instanceof Promise) {
            return value.then(resolve, reject);
        }

        // 為什麼resolve 加setTimeout?
        // 2.2.4規範 onFulfilled 和 onRejected 只允許在 execution context 棧僅包含平臺程式碼時執行.
        // 注1 這裡的平臺程式碼指的是引擎、環境以及 promise 的實施程式碼。實踐中要確保 onFulfilled 和 onRejected 方法非同步執行，且應該在 then 方法被呼叫的那一輪事件迴圈之後的新執行棧中執行。

        setTimeout(() => {
            // 呼叫resolve 回撥對應onFulfilled函式
            if (that.status === PENDING) {
                // 只能由pedning狀態 => fulfilled狀態 (避免呼叫多次resolve reject)
                that.status = FULFILLED;
                that.value = value;
                that.onFulfilledCallbacks.forEach(cb => cb(that.value));
            }
        });
    }

    function reject(reason) { // reason失敗態時接收的拒因
        setTimeout(() => {
            // 呼叫reject 回撥對應onRejected函式
            if (that.status === PENDING) {
                // 只能由pedning狀態 => rejected狀態 (避免呼叫多次resolve reject)
                that.status = REJECTED;
                that.reason = reason;
                that.onRejectedCallbacks.forEach(cb => cb(that.reason));
            }
        });
    }

    // 捕獲在excutor執行器中丟擲的異常
    // new Promise((resolve, reject) => {
    //     throw new Error('error in excutor')
    // })
    try {
        excutor(resolve, reject);
    } catch (e) {
        reject(e);
    }
}

/**
 * resolve中的值幾種情況：
 * 1.普通值
 * 2.promise物件
 * 3.thenable物件/函式
 */

/**
 * 對resolve 進行改造增強 針對resolve中不同值情況 進行處理
 * @param  {promise} promise2 promise1.then方法返回的新的promise物件
 * @param  {[type]} x         promise1中onFulfilled的返回值
 * @param  {[type]} resolve   promise2的resolve方法
 * @param  {[type]} reject    promise2的reject方法
 */
function resolvePromise(promise2, x, resolve, reject) {
    if (promise2 === x) {  // 如果從onFulfilled中返回的x 就是promise2 就會導致迴圈引用報錯
        return reject(new TypeError('迴圈引用'));
    }

    let called = false; // 避免多次呼叫
    // 如果x是一個promise物件 （該判斷和下面 判斷是不是thenable物件重複 所以可有可無）
    if (x instanceof Promise) { // 獲得它的終值 繼續resolve
        if (x.status === PENDING) { // 如果為等待態需等待直至 x 被執行或拒絕 並解析y值
            x.then(y => {
                resolvePromise(promise2, y, resolve, reject);
            }, reason => {
                reject(reason);
            });
        } else { // 如果 x 已經處於執行態/拒絕態(值已經被解析為普通值)，用相同的值執行傳遞下去 promise
            x.then(resolve, reject);
        }
        // 如果 x 為物件或者函式
    } else if (x != null && ((typeof x === 'object') || (typeof x === 'function'))) {
        try { // 是否是thenable物件（具有then方法的物件/函式）
            let then = x.then;
            if (typeof then === 'function') {
                then.call(x, y => {
                    if(called) return;
                    called = true;
                    resolvePromise(promise2, y, resolve, reject);
                }, reason => {
                    if(called) return;
                    called = true;
                    reject(reason);
                })
            } else { // 說明是一個普通物件/函式
                resolve(x);
            }
        } catch(e) {
            if(called) return;
            called = true;
            reject(e);
        }
    } else {
        resolve(x);
    }
}

/**
 * [註冊fulfilled狀態/rejected狀態對應的回撥函式]
 * @param  {function} onFulfilled fulfilled狀態時 執行的函式
 * @param  {function} onRejected  rejected狀態時 執行的函式
 * @return {function} newPromsie  返回一個新的promise物件
 */
Promise.prototype.then = function(onFulfilled, onRejected) {
    const that = this;
    let newPromise;
    // 處理引數預設值 保證引數後續能夠繼續執行
    onFulfilled =
        typeof onFulfilled === "function" ? onFulfilled : value => value;
    onRejected =
        typeof onRejected === "function" ? onRejected : reason => {
            throw reason;
        };

    // then裡面的FULFILLED/REJECTED狀態時 為什麼要加setTimeout ?
    // 原因:
    // 其一 2.2.4規範 要確保 onFulfilled 和 onRejected 方法非同步執行(且應該在 then 方法被呼叫的那一輪事件迴圈之後的新執行棧中執行) 所以要在resolve里加上setTimeout
    // 其二 2.2.6規範 對於一個promise，它的then方法可以呼叫多次.（當在其他程式中多次呼叫同一個promise的then時 由於之前狀態已經為FULFILLED/REJECTED狀態，則會走的下面邏輯),所以要確保為FULFILLED/REJECTED狀態後 也要非同步執行onFulfilled/onRejected

    // 其二 2.2.6規範 也是resolve函式里加setTimeout的原因
    // 總之都是 讓then方法非同步執行 也就是確保onFulfilled/onRejected非同步執行

    // 如下面這種情景 多次呼叫p1.then
    // p1.then((value) => { // 此時p1.status 由pedding狀態 => fulfilled狀態
    //     console.log(value); // resolve
    //     // console.log(p1.status); // fulfilled
    //     p1.then(value => { // 再次p1.then 這時已經為fulfilled狀態 走的是fulfilled狀態判斷裡的邏輯 所以我們也要確保判斷裡面onFuilled非同步執行
    //         console.log(value); // 'resolve'
    //     });
    //     console.log('當前執行棧中同步程式碼');
    // })
    // console.log('全域性執行棧中同步程式碼');
    //

    if (that.status === FULFILLED) { // 成功態
        return newPromise = new Promise((resolve, reject) => {
            setTimeout(() => {
                try{
                    let x = onFulfilled(that.value);
                    resolvePromise(newPromise, x, resolve, reject); // 新的promise resolve 上一個onFulfilled的返回值
                } catch(e) {
                    reject(e); // 捕獲前面onFulfilled中丟擲的異常 then(onFulfilled, onRejected);
                }
            });
        })
    }

    if (that.status === REJECTED) { // 失敗態
        return newPromise = new Promise((resolve, reject) => {
            setTimeout(() => {
                try {
                    let x = onRejected(that.reason);
                    resolvePromise(newPromise, x, resolve, reject);
                } catch(e) {
                    reject(e);
                }
            });
        });
    }

    if (that.status === PENDING) { // 等待態
        // 當非同步呼叫resolve/rejected時 將onFulfilled/onRejected收集暫存到集合中
        return newPromise = new Promise((resolve, reject) => {
            that.onFulfilledCallbacks.push((value) => {
                try {
                    let x = onFulfilled(value);
                    resolvePromise(newPromise, x, resolve, reject);
                } catch(e) {
                    reject(e);
                }
            });
            that.onRejectedCallbacks.push((reason) => {
                try {
                    let x = onRejected(reason);
                    resolvePromise(newPromise, x, resolve, reject);
                } catch(e) {
                    reject(e);
                }
            });
        });
    }
};

/**
 * Promise.all Promise進行並行處理
 * 引數: promise物件組成的陣列作為引數
 * 返回值: 返回一個Promise例項
 * 當這個陣列裡的所有promise物件全部變為resolve狀態的時候，才會resolve。
 */
Promise.all = function(promises) {
    return new Promise((resolve, reject) => {
        let done = gen(promises.length, resolve);
        promises.forEach((promise, index) => {
            promise.then((value) => {
                done(index, value)
            }, reject)
        })
    })
}

function gen(length, resolve) {
    let count = 0;
    let values = [];
    return function(i, value) {
        values[i] = value;
        if (++count === length) {
            console.log(values);
            resolve(values);
        }
    }
}

/**
 * Promise.race
 * 引數: 接收 promise物件組成的陣列作為引數
 * 返回值: 返回一個Promise例項
 * 只要有一個promise物件進入 FulFilled 或者 Rejected 狀態的話，就會繼續進行後面的處理(取決於哪一個更快)
 */
Promise.race = function(promises) {
    return new Promise((resolve, reject) => {
        promises.forEach((promise, index) => {
           promise.then(resolve, reject);
        });
    });
}

// 用於promise方法鏈時 捕獲前面onFulfilled/onRejected丟擲的異常
Promise.prototype.catch = function(onRejected) {
    return this.then(null, onRejected);
}

Promise.resolve = function (value) {
    return new Promise(resolve => {
        resolve(value);
    });
}

Promise.reject = function (reason) {
    return new Promise((resolve, reject) => {
        reject(reason);
    });
}

/**
 * 基於Promise實現Deferred的
 * Deferred和Promise的關係
 * - Deferred 擁有 Promise
 * - Deferred 具備對 Promise的狀態進行操作的特權方法（resolve reject）
 *
 *參考jQuery.Deferred
 *url: http://api.jquery.com/category/deferred-object/
 */
Promise.deferred = function() { // 延遲物件
    let defer = {};
    defer.promise = new Promise((resolve, reject) => {
        defer.resolve = resolve;
        defer.reject = reject;
    });
    return defer;
}

/**
 * Promise/A+規範測試
 * npm i -g promises-aplus-tests
 * promises-aplus-tests Promise.js
 */

try {
  module.exports = Promise
} catch (e) {
}


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