Promise
实现一个符合 Promise/A+ 规范的 MyPromise，并实现 resolve、reject、all、race、defer、deferred等静态方法。
MyPromise
- 作用：创建
MyPromise实例。Promise
- MyPromise接收一个回掉函数
executor
- MyPromise状态
- pending
- 可以转换成 fulfilled 或 rejected
- fulfilled
- rejected
onFulfilledCallbacks 和 onRejectedCallbacks
- 两个数组，数组每一项是一个函数。分别接收
then里面的第一个参数和第二个参数。
- 状态是
pending 的回掉函数。
- resolve
promise的状态是fulfilled异常是的处理函数- 接收
value 参数
- 如果是
promise，执行then。
- 如果不是
promise，把value做为参数传给onFulfilledCallbacks里的每个函数。
- reject
promise的状态是rejected异常是的处理函数- 接收
reason 参数，把reason做为参数传给onRejectedCallbacks里的每个函数。
- 执行
executor，如果有异常，抛给reject
- 因为
Promise是在同步代码执行完成后再执行，所以要把Mypromise的执行方法resolve和reject放在异步队列里
function MyPromise(executor) {
if (typeof executor !== 'function') {
throw new TypeError('Promise resolver ' + executor + ' is not a function');
}
let self = this;
this.status = 'pending';
this.value = undefined;
this.reason = undefined;
this.onFulfilledCallbacks = [];
this.onRejectedCallbacks = [];
function resolve(value) {
if (value instanceof MyPromise) {
return value.then(resolve, reject);
}
if (self.status === 'pending') {
self.value = value;
self.status = 'fulfilled';
self.onFulfilledCallbacks.forEach(item => item(value));
}
}
function reject(reason) {
if (self.status === 'pending') {
self.reason = reason;
self.status = 'rejected';
self.onRejectedCallbacks.forEach(item => item(reason));
}
}
try {
executor(resolve, reject);
} catch (e) {
reject(e);
}
}
MyPromise.prototype.then
- 作用：接收两个函数参数，第一个函数的参数是
resolve传入的参数，第二个参数是 reject传入的参数。Promise#then
onFulfilled
MyPromise 成功时执行的方法resolve 的参数会作为value传给 onFulfilled
onRejected
MyPromise 失败时执行的方法reject 的参数会作为value传给 onRejected
- 返回一个
MyPromise 实例 newPromise，方便链式调用
- 对三种状态分别处理
- 每个状态中创建
newPromise
fulfilled
- 直接执行
onFulfilled，返回值x
- 把
newPromise、x以及newPromise里的resolve、reject做为参数传给 resolutionPromise
- 把 MyPromise 的参数放在异步队列里
rejected
- 直接执行
onRejected，返回值x
- 把
newPromise、x以及newPromise里的resolve、reject做为参数传给 resolutionPromise
- 把 MyPromise 的参数放在异步队列里
pending
- 状态待定，把
fulfilled和rejected里的异步函数分别加到 onFulfilledCallbacks 和 onRejectedCallbacks的最后一位
resolutionPromise 后面细说- 用
catch捕获异常，执行 reject
MyPromise.prototype.then = function (onFulfilled, onRejected) {
let self = this;
typeof onFulfilled !== 'function' && (onFulfilled = function (value) {
return value;
});
typeof onRejected !== 'function' && (onRejected = function (reason) {
throw reason;
});
let newPromise;
if (self.status === 'fulfilled') {
newPromise = new MyPromise(function (resolve, reject) {
setTimeout(function () {
try {
let x = onFulfilled(self.value);
resolutionPromise(newPromise, x, resolve, reject);
} catch (e) {
reject(e);
}
});
});
}
if (self.status === 'rejected') {
newPromise = new MyPromise(function (resolve, reject) {
setTimeout(function () {
try {
let x = onRejected(self.reason);
resolutionPromise(newPromise, x, resolve, reject);
} catch (e) {
reject(e);
}
});
});
}
if (self.status === 'pending') {
newPromise = new MyPromise(function (resolve, reject) {
self.onFulfilledCallbacks.push(function (value) {
setTimeout(function () {
try {
let x = onFulfilled(value);
resolutionPromise(newPromise, x, resolve, reject);
} catch (e) {
reject(e);
}
});
});
self.onRejectedCallbacks.push(function (reason) {
setTimeout(function () {
try {
let x = onRejected(reason);
resolutionPromise(newPromise, x, resolve, reject);
} catch (e) {
reject(e);
}
});
});
});
}
return newPromise;
};
MyPromise.prototype.catch
- 作用：捕获异常
- 返回
MyPromise
MyPromise.prototype.catch = function (onRejected) {
return this.then(undefined, onRejected);
};
The Promise Resolution Procedure
- Promise解析过程，是以一个
promise、一个值 x及resolve, reject做为参数的抽象过程
promise 等于 x，reject 抛出异常 new TypeError('循环引用')x如果不是对象（不包括 null）或者函数，执行 resolve(x)- 获取
x.then 赋值给 then
then 如果是 function
- 把
x做为 this 调用then，第一个参数是 resolvePromise，第二个参数是 rejectPromise
resolvePromise和 rejectPromise只有第一次调用有效resolvePromise参数为 y，执行 resolutionPromise(promise, y, resolve, reject)rejectPromise参数为 r，执行 reject(r)
then 如果不是 function
- 用捕获上一步的异常
- 执行
reject(e)
- 如果执行过
resolvePromise或 rejectPromise，忽略
function resolutionPromise(promise, x, resolve, reject) {
if (promise === x) {
reject(new TypeError('循环引用'));
}
let then, called;
if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
try {
then = x.then;
if (typeof then === 'function') {
then.call(x, function (y) {
if (called)
return;
called = true;
resolutionPromise(promise, y, resolve, reject);
}, function (r) {
if (called)
return;
called = true;
reject(r);
})
} else {
resolve(x);
}
} catch (e) {
if (called)
return;
reject(e);
}
} else {
resolve(x);
}
}
MyPromise 静态方法
MyPromise.all
MyPromise.all = function (promises) {
let called = false;
return new MyPromise(function (resolve, reject) {
let newArr = [], count = 0;
for (let i = 0; i < promises.length; i++) {
let item = promises[i];
if (!(item instanceof MyPromise)) {
item = MyPromise.resolve(item);
}
item.then(function (data) {
if (!called) {
newArr[i] = data;
if (i == count) {
resolve(newArr);
count++;
}
}
}, function (e) {
if (!called) {
reject(e);
called = true;
}
});
}
});
};
MyPromise.race
MyPromise.race = function (promises) {
return new MyPromise(function (resolve, reject) {
let called = false;
for (let i = 0; i < promises.length; i++) {
let item = promises[i];
if (!(item instanceof MyPromise)) {
item = MyPromise.resolve(item);
}
item.then(function (data) {
if (!called) {
resolve(data);
called = true;
}
}, function (e) {
if (!called) {
reject(e);
called = true;
}
});
}
})
};
MyPromise.resolve
MyPromise.resolve = function (value) {
if (value instanceof MyPromise) {
return value;
}
return new MyPromise(function (resolve, reject) {
if (typeof value !== null && typeof value === 'object' && typeof value.then === 'function') {
value.then();
} else {
resolve(value);
}
})
};
MyPromise.reject
MyPromise.reject = function (e) {
return new MyPromise(function (resolve, reject) {
reject(e);
})
};
test
- npm i -g promises-aplus-tests
- promises-aplus-tests Promise.js
源码
参考资料
