使用 AngularJS 立即返回已解决的承诺 [英] Immediately return a resolved promise using AngularJS

问题描述

我正在尝试了解 JavaScript(特别是 AngularJS)中的 promise.

I'm trying to get my head around promises in JavaScript (in particular AngularJS).

我在服务中有一个函数，我们称之为fooService，它检查我们是否已经加载了一些数据.如果有，我只希望它返回，如果没有，我们需要加载数据并返回一个承诺:

I have a function in a service, let's call it fooService, that checks if we've loaded some data. If it has, I just want it to return, and if we haven't, we need to load the data and return a promise:

this.update = function(data_loaded) {
    if (data_loaded) return;  // We've loaded the data, no need to update

    var promise = Restangular.all('someBase').customGet('foo/bar').then(function(data) {
        // Do something with the data here
    }

    return promise;
}

我有另一个函数，然后调用 fooService 的 update 函数，如下所示:

I have another function that then calls the update function of fooService like so:

fooService.update(data_loaded).then(function() {
    // Do something here when update is finished
})

我的问题是，如果我们不需要在 update 函数中加载数据，则不会返回承诺，因此 .then()在我的其他函数中没有调用.这里应该采用什么方法 - 基本上，如果我们不需要从 Retangular 调用中获取数据，我想立即从 update() 函数返回一个已解决的承诺?

My issue here is that if we don't need to load the data in the update function, a promise isn't returned, so the .then() is not called in my other function. What should the approach be here - basically I want to return a resolved promise immediately from the update() function if we do not need to get data from the Restangular call?

推荐答案

目前接受的答案过于复杂，滥用了延迟反模式.这是一个更简单的方法:

The current accepted answer is overly complicated, and abuses the deferred anti pattern. Here is a simpler approach:

this.update = function(data_loaded) {
    if (data_loaded) return $q.when(data);  // We've loaded the data, no need to update

    return Restangular.all('someBase').customGet('foo/bar')
                             .then(function(data) {
        // Do something with the data here 
    });
};

或者，更进一步:

this._updatep = null;
this.update = function(data_loaded) { // cached
    this._updatep = this._updatep || Restangular.all('someBase') // process in
                                                .customGet('foo/bar'); //.then(..
    return this._updatep;
};

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