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javascript - 将 then 方法添加到回调函数

转载 作者:搜寻专家 更新时间:2023-11-01 04:18:41 26 4
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我最近看到一些代码是这样的:

myFunc(args).then(function() { ... });

我发现这个回调语法非常优雅。我的理解是这不是 vanilla JS 的一部分,我希望能够在不依赖特定库的情况下偶尔使用它,所以我对如何自己实现它很感兴趣。那么,这种东西是如何工作的,您将如何为函数调用实现它?

最佳答案

这种模式称为“ promise ”。它由 jQuery 和 dojo 等实现,一种方法是查看它们的代码并了解它们是如何实现的。

一般的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先前的方法,该方法将在成功或失败时运行。 MSDN 在 blog post here 中有更多关于 promise 的信息

在 gitHub 上发布了一个极简主义的实现: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。我仔细查看了它,它作为一个起点看起来不错,但所有功劳归于 original author )

关于javascript - 将 then 方法添加到回调函数,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/14793830/

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