Promise源码深入理解

)

) });

)

}, )

});

)

)

}, )

});

)

}, )

});

p.then(function (x) {

console.log(x)

}) //输出 1 链式调用1 链式调用2

// 4.v4.0 then函数中的onFullfilled和onRejected方法的返回值问题

function resolvePromise(promise, x, resolve, reject) {

if (promise === x) { throw new TypeError("type error") }

let isUsed;

if (x !== null && (typeof x === "object" || typeof x === "function")) {

try {

let then = x.then;

if (typeof then === "function") {

//是一个promise的情况

then.call(x, function (y) {

if (isUsed) return;

isUsed = true;

resolvePromise(promise, y, resolve, reject);

}, function (e) {

if (isUsed) return;

isUsed = true;

reject(e);

})

} else {

//仅仅是一个函数或者是对象

resolve(x)

}

} catch (e) {

if (isUsed) return;

isUsed = true;

reject(e);

}

} else {

//返回的基本类型，直接resolve

resolve(x)

}

}

myPromise.prototype.then = function (onFullfilled, onRejected) {

let self = this;

let promise2;

switch (self.status) {

case "pending":

promise2 = new myPromise(function (resolve, reject) {

self.onFullfilledArray.push(function () {

setTimeout(function () {

try {

let temple = onFullfilled(self.value);

resolvePromise(temple)

} catch (e) {

reject(e) //error catch     

}

})

});

self.onRejectedArray.push(function () {

setTimeout(function () {

try {

let temple = onRejected(self.reason); resolvePromise(temple)

} catch (e) {

reject(e)// error catch     

}

})

});

})

case "resolved":

promise2 = new myPromise(function (resolve, reject) {

setTimeout(function () {

try {

let temple = onFullfilled(self.value);

//将上次一then里面的方法传递进下一个Promise状态  

resolvePromise(temple);

} catch (e) {

reject(e);//error catch

}

})

})

break;

case "rejected":

promise2 = new myPromise(function (resolve, reject) {

setTimeout(function () {

try {

let temple = onRejected(self.reason);

//将then里面的方法传递到下一个Promise的状态里

resolvePromise(temple);

} catch (e) {

reject(e);

}

})

})

break;

default:

}

return promise2;

}