如何从异步回调函数返回值？ [英] How to return value from an asynchronous callback function?

问题描述

这个问题在SO中被问过很多次。但我还是不能得到东西。

This question is asked many times in SO. But still I can't get stuff.

我想从回调中得到一些值。看看下面的脚本澄清。

I want to get some value from callback. Look at the script below for clarification.

function foo(address){

      // google map stuff
      geocoder.geocode( { 'address': address}, function(results, status) {
          results[0].geometry.location; // I want to return this value
      })

    }
    foo(); //result should be results[0].geometry.location; value

如果我尝试返回该值，只是得到undefined。我跟着一些想法从SO，但
仍然失败。

If I try to return that value just getting "undefined". I followed some ideas from SO, but still fails.

这些是：

function foo(address){
    var returnvalue;    
    geocoder.geocode( { 'address': address}, function(results, status) {
        returnvalue = results[0].geometry.location; 
    })
    return returnvalue; 
}
foo(); //still undefined

推荐答案

在这种情况下，您需要向foo传递一个回调，该回调将接收返回值

In this case you need to pass a callback to foo that will receive the return value

function foo(address, fn){
  geocoder.geocode( { 'address': address}, function(results, status) {
     fn(results[0].geometry.location); 
  });
}

foo("address", function(location){
  alert(location); // this is where you get the return value
});

事情是，如果内部函数调用是异步的，那么所有的函数'必须也是异步的，以便返回响应。

The thing is, if an inner function call is asynchronous, then all the functions 'wrapping' this call must also be asynchronous in order to 'return' a response.

如果你有很多回调，你可以考虑采取暴跌，并使用 promise库like Q 。

If you have a lot of callbacks you might consider taking the plunge and use a promise library like Q.

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