$.when.apply($, someArray) 有什么作用?

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问题描述

我正在阅读关于 Deferreds 和 Promises 并不断遇到 $.when.apply($, someArray).我有点不清楚这到底是做什么的，正在寻找一行完全有效的解释(不是整个代码片段).这是一些上下文:

I'm reading about Deferreds and Promises and keep coming across $.when.apply($, someArray). I'm a little unclear on what this does exactly, looking for an explanation that one line works exactly (not the entire code snippet). Here's some context:

var data = [1,2,3,4]; // the ids coming back from serviceA
var processItemsDeferred = [];

for(var i = 0; i < data.length; i++){
  processItemsDeferred.push(processItem(data[i]));
}

$.when.apply($, processItemsDeferred).then(everythingDone); 

function processItem(data) {
  var dfd = $.Deferred();
  console.log('called processItem');

  //in the real world, this would probably make an AJAX call.
  setTimeout(function() { dfd.resolve() }, 2000);    

  return dfd.promise();
}

function everythingDone(){
  console.log('processed all items');
}

推荐答案

.apply 用于调用带有参数数组的函数.它获取数组中的每个元素，并将每个元素用作函数的参数..apply 还可以更改函数内的上下文 (this).

.apply is used to call a function with an array of arguments. It takes each element in the array, and uses each as a parameter to the function. .apply can also change the context (this) inside a function.

那么，让我们以 $.when 为例.它习惯于说当所有这些承诺都得到解决时……做点什么".它需要无限(可变)数量的参数.

So, let's take $.when. It's used to say "when all these promises are resolved... do something". It takes an infinite (variable) number of parameters.

就您而言，您有一系列承诺；你不知道你传递给 $.when 多少参数.将数组本身传递给 $.when 是行不通的，因为它希望它的参数是 promise，而不是数组.

In your case, you have an array of promises; you don't know how many parameters you're passing to $.when. Passing the array itself to $.when wouldn't work, because it expects its parameters to be promises, not an array.

这就是 .apply 的用武之地.它接受数组，并使用每个元素作为参数调用 $.when(并确保 this 设置为 jQuery/$)，然后一切正常:-)

That's where .apply comes in. It takes the array, and calls $.when with each element as a parameter (and makes sure the this is set to jQuery/$), so then it all works :-)

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