Geek asks and answers

Add then method to function for callbacks

I recently saw some code snippets that look like this:

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

I find this callback syntax really elegant. I understand that this is not part of vanilla JS, and I would like to use it on occasion, regardless of specific libraries, so I'm interested in how to implement this myself. So how does this work, and how to implement it to call a function?

+5
source share
1 answer

This template is called "promises". It is implemented by jQuery and dojo, among others, and one approach is to look at their code and see how they implement it.

, , (), , , . MSDN promises

gitHub : 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;
    }

};

( , . , , )

+4

More articles:


All Articles