我最近看到过几段代码看起来像这样：
myFunc(args).then(function() { ... });
我觉得这个回调语法非常优雅.我的理解是这不是vanilla JS的一部分,我希望能够偶尔使用它而不依赖于特定的库,所以我对如何自己实现它感兴趣.那么,这种事情是如何工作的,你将如何为函数调用实现它？
最佳答案
这种模式称为“承诺”.它由jQuery和dojo等实现,一种方法是查看它们的代码并看看它们是如何实现它的.
原文链接：https://www.f2er.com/js/429699.html一般实现模式是创建一个函数,该函数返回一个对象,该对象包含一个函数(然后),将一对函数作为回调传递给前一个方法,然后在成功或失败时运行. MSDN在blog post here中有更多关于承诺的内容
在gitHub上发布了一个简约的实现：Promises GIST
function Promise () {
this._thens = [];
}
Promise.prototype = {
/* This is the "front end" API. */
// then(onResolve,onReject): Code waiting for this promise uses the
// then() method to be notified when the promise is complete. There
// are two completion callbacks: onReject and onResolve. A more
// robust promise implementation will also have an onProgress handler.
then: function (onResolve,onReject) {
// capture calls to then()
this._thens.push({ resolve: onResolve,reject: onReject });
},// Some promise implementations also have a cancel() front end API that
// calls all of the onReject() callbacks (aka a "cancelable promise").
// cancel: function (reason) {},/* This is the "back end" API. */
// resolve(resolvedValue): The resolve() method is called when a promise
// is resolved (duh). The resolved value (if any) is passed by the resolver
// to this method. All waiting onResolve callbacks are called
// and any future ones are,too,each being passed the resolved value.
resolve: function (val) { this._complete('resolve',val); },// reject(exception): The reject() method is called when a promise cannot
// be resolved. Typically,you'd pass an exception as the single parameter,// but any other argument,including none at all,is acceptable.
// All waiting and all future onReject callbacks are called when reject()
// is called and are passed the exception parameter.
reject: function (ex) { this._complete('reject',ex); },// Some promises may have a progress handler. The back end API to signal a
// progress "event" has a single parameter. The contents of this parameter
// could be just about anything and is specific to your implementation.
// progress: function (data) {},/* "Private" methods. */
_complete: function (which,arg) {
// switch over to sync then()
this.then = which === 'resolve' ?
function (resolve,reject) { resolve(arg); } :
function (resolve,reject) { reject(arg); };
// disallow multiple calls to resolve or reject
this.resolve = this.reject =
function () { throw new Error('Promise already completed.'); };
// complete all waiting (async) then()s
var aThen,i = 0;
while (aThen = this._thens[i++]) { aThen[which] && aThen[which](arg); }
delete this._thens;
}
};
(请注意,这不是我的代码.我仔细研究了它,它看起来很好作为一个起点,但所有功劳都归功于original author)
