潜在的异步函数会返回一个立即解析的承诺吗？ [英] Potentially async function return a promise that immediately resolves?

问题描述

其中 asyncBananaRequest 返回一个承诺 -

Where asyncBananaRequest returns a promise -

function potentiallyAsync () {
  if (cachedBanana) {
    return asyncBananaRequest();
  }
  return ??cachedBanana??;
}

potentiallyAsync().then(function(banana){
  //use banana
})

我想要一个香蕉，我可能已经将它缓存了。有没有办法让我在potentialAsync函数中返回缓存的香蕉作为一个立即用缓存的香蕉解决的承诺？

I want a banana, I might already have it cached. Is there a way for me to return the cached banana in the potentiallyAsync functionas a promise that immediately resolves with the cached bananas?

我目前正在使用Q lib打包的Angular，但我希望有一个通用的实现

I'm currently using the Q lib packaged in Angular, but I'm hoping there's a generic implementation

推荐答案

虽然SomeKittens很棒，但他的答案使用延迟反模式。

While SomeKittens is awesome, his answer uses the deferred anti pattern.

我建议如下：

function potentiallyAsync () {
  return (cachedBanana) ? Promise.resolve(cachedBanana) : asyncBananaRequest();
}

potentiallyAsync().then(function(banana){
  //use banana
});

在Angular的$ q中，你只能使用 $完全相同的东西q.when（cachedBanana）而不是ES6标准 Promise.resolve 。

In Angular's $q you'd use the exact same thing only with $q.when(cachedBanana) instead of the ES6 standards Promise.resolve.

这种形式的链接和使用.resolve（.when in $ q）来创造新的承诺是承诺的面包和黄油。在宣传基于回调的API时，只应在绝对端点使用延迟对象。

This form of chaining and using .resolve (.when in $q) to create new promises are bread and butter of promises. Deferred objects should only be used at absolute endpoints when promisifying callback based APIs.

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