53 lines
1.6 KiB
JavaScript
53 lines
1.6 KiB
JavaScript
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// 手写一个Promise
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function MyPromise(fun){ // 构造函数接收一个函数
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this.status = MyPromise.PENDING // 设置默认状态为待定
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try {
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fun.call(this, MyPromise.resolve.bind(this), MyPromise.reject.bind(this))
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} catch(err){
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// 如果MyPromise.prototype.then里面有err=>{}处理函数就传递给它
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this.status = MyPromise.REJECT
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this.result = err
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}
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}
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// Promise 状态常量
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MyPromise.PENDING = 0
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MyPromise.FULFILLED = 1
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MyPromise.REJECT = 2
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MyPromise.resolve = function(res) {
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if(this.status === MyPromise.REJECT) return // Promise 的状态一旦确定就不能修改
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this.status = MyPromise.FULFILLED
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this.result = res
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}
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MyPromise.reject = function(err) {
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if(this.status === MyPromise.FULFILLED) return // Promise 的状态一旦确定就不能修改
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this.status = MyPromise.REJECT
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this.result = err
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}
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MyPromise.prototype = {
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then: function(res, err){
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// Promise的.then要加入到任务队列，不能同步执行用setTimeout模拟
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var context = this // 保存的上下文
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setTimeout(function(){
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// 在.then中无论是onFulfilled的返回值还是onReject的返回值，都会交给下一个.then的onFulfilled处理，只有出现异常的时候才后交给下一个.then的onReject处理
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try{
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if(context.status === MyPromise.FULFILLED && res) context.result = res(context.result)
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if(context.status === MyPromise.REJECT && err) {
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context.result =err(context.result)
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context.status = MyPromise.FULFILLED
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}
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}catch(err){
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context.result = err
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context.status = MyPromise.REJECT
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}
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}, 0);
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return this
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}
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}
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