实现一个proimse

[jackieli123723]

(function(window,undefined){

// resolve 和 reject 最终都会调用该函数
var final = function(status,value){
    var promise = this, fn, st;

    if(promise._status !== 'PENDING') return;

    // 所以的执行都是异步调用，保证then是先执行的
    setTimeout(function(){
        promise._status = status;
        st = promise._status === 'FULFILLED'
        queue = promise[st ? '_resolves' : '_rejects'];

        while(fn = queue.shift()) {
            value = fn.call(promise, value) || value;
        }

        promise[st ? '_value' : '_reason'] = value;
        promise['_resolves'] = promise['_rejects'] = undefined;
    });
}

//参数是一个函数，内部提供两个函数作为该函数的参数,分别是resolve 和 reject
var Promise = function(resolver){
    if (!(typeof resolver === 'function' ))
        throw new TypeError('You must pass a resolver function as the first argument to the promise constructor');
    //如果不是promise实例，就new一个
    if(!(this instanceof Promise)) return new Promise(resolver);

    var promise = this;
    promise._value;
    promise._reason;
    promise._status = 'PENDING';
    //存储状态
    promise._resolves = [];
    promise._rejects = [];

    //
    var resolve = function(value) {
        //由於apply參數是數組
        final.apply(promise,['FULFILLED'].concat([value]));
    }

    var reject = function(reason){
        final.apply(promise,['REJECTED'].concat([reason]));
    }

    resolver(resolve,reject);
}

Promise.prototype.then = function(onFulfilled,onRejected){
    var promise = this;
    // 每次返回一个promise，保证是可thenable的
    return new Promise(function(resolve,reject){

        function handle(value) {
            // 這一步很關鍵，只有這樣才可以將值傳遞給下一個resolve
            var ret = typeof onFulfilled === 'function' && onFulfilled(value) || value;

            //判断是不是promise 对象
            if (ret && typeof ret ['then'] == 'function') {
                ret.then(function(value) {
                    resolve(value);
                }, function(reason) {
                    reject(reason);
                });
            } else {
                resolve(ret);
            }
        }

        function errback(reason){
            reason = typeof onRejected === 'function' && onRejected(reason) || reason;
            reject(reason);
        }

        if(promise._status === 'PENDING'){
            promise._resolves.push(handle);
            promise._rejects.push(errback);
        }else if(promise._status === FULFILLED){ // 状态改变后的then操作，立刻执行
            callback(promise._value);
        }else if(promise._status === REJECTED){
            errback(promise._reason);
        }
    });
}

Promise.prototype.catch = function(onRejected){
    return this.then(undefined, onRejected)
}

Promise.resolve = function(arg){
    return new Promise(function(resolve,reject){
        resolve(arg)
    })
}

Promise.reject = function(arg){
    return Promise(function(resolve,reject){
        reject(arg)
    })
}

window.Promise = Promise;

})(window);

/**********************************************************************/
//实例
var getData100 = function(){
    return new Promise(function(resolve,reject){
        setTimeout(function(){
            resolve('100ms');
        },1000);
    });
}

var getData200 = function(){
    return new Promise(function(resolve,reject){
        setTimeout(function(){
            resolve('200ms');
        },2000);
    });
}
var getData300 = function(){
    return new Promise(function(resolve,reject){
        setTimeout(function(){
            reject('reject');
        },3000);
    });
}

getData100().then(function(data){
    console.log(data); // 100ms
    return getData200();
}).then(function(data){
    console.log(data); // 200ms
    return getData300();
}).then(function(data){
    console.log(data); // 100ms
}, function(data){
    console.log(data);
});