使用jQuery Deferred查找第一个可用的数据源

So I was asked this at an interview, but it brought up a good use case. Assume that you have a bunch of data sources. You want to find the first available one and process it and ignore the rest.

所以我在接受采访时被问到这个问题，但它提出了一个很好的用例。假设您有一堆数据源。你想找到第一个可用的并处理它而忽略其余的。

So something like:

所以类似于：

var datasources = new Array("somedatabase1/pizza","somedatabase2/beer","somedatabase3/llama");
var dfds = new Array();
$.each(datasources,function(source){
    dfds.push($.getJSON(source));
});

$.when(dfds).done(function(){alert("they are all done");});

Ignore that I really don't think when accepts an array (maybe it does). This of course would make it wait till they are all completed. I am looking for some code that would make it wait until one, any of them is done, and then not worry about the others.

忽略我真的不认为什么时候接受一个数组（也许它）。这当然会让它等到它们全部完成。我正在寻找一些代码，让它等到一个，其中任何一个完成，然后不用担心其他的。

I was informed that it would only work recursively.

我被告知它只能递归地工作。

2 个解决方案

#1

This doesn't use recursion but fits the requirement to fetch from multiple datasources and only care about the first that returns a successful response.

这不使用递归，但符合从多个数据源获取的要求，只关心返回成功响应的第一个。

http://jsfiddle.net/mNJ6D/

function raceToIt(urls) {
    var deferred = $.Deferred(),
        promises;

    function anyComplete(data) {
        if (!deferred.isResolved()) {
            deferred.resolveWith(this, [data]);
            promises.forEach(function(promise) {
                promise.abort();
            });
        }
    }
    promises = urls.map(function(url) {
        return $.getJSON(url).then(anyComplete);
    });
    return deferred.promise();
}
raceToIt(["/echo/json/", "/echo/json/", "/echo/json/"]).then(function(data) {
    console.log(data);
});

#2

I've made a plugin which provides another version of $.when() with reversed semantics. It's modified from the actual jQuery implementation of $.when() so it's exactly the same as the original except that it waits for either the first resolved promise, or all promised to be rejected.

我已经制作了一个插件，它提供了另一个带有反向语义的$ .when（）版本。它是从$ .when（）的实际jQuery实现中修改的，所以它与原始的完全相同，只是它等待第一个已解决的promise，或者所有承诺被拒绝。

Just drop this code in right after you load jQuery:

只需在加载jQuery后立即删除此代码：

(function($) {
  $.reverseWhen = function( subordinate /* , ..., subordinateN */ ) {
    var i = 0,
      rejectValues = Array.prototype.slice.call( arguments ),
      length = rejectValues.length,

      // the count of uncompleted subordinates
      remaining = length !== 1 || ( subordinate && jQuery.isFunction( subordinate.promise ) ) ? length : 0,

      // the master Deferred. If rejectValues consist of only a single Deferred, just use that.
      deferred = remaining === 1 ? subordinate : jQuery.Deferred(),

      // Update function for both reject and progress values
      updateFunc = function( i, contexts, values ) {
        return function( value ) {
          contexts[ i ] = this;
          values[ i ] = arguments.length > 1 ? Array.prototype.slice.call( arguments ) : value;
          if( values === progressValues ) {
            deferred.notifyWith( contexts, values );
          } else if ( !( --remaining ) ) {
            deferred.rejectWith( contexts, values );
          }
        };
      },

      progressValues, progressContexts, rejectContexts;

    // add listeners to Deferred subordinates; treat others as rejected
    if ( length > 1 ) {
      progressValues = new Array( length );
      progressContexts = new Array( length );
      rejectContexts = new Array( length );
      for ( ; i < length; i++ ) {
        if ( rejectValues[ i ] && jQuery.isFunction( rejectValues[ i ].promise ) ) {
          rejectValues[ i ].promise()
            .done( deferred.resolve )
            .fail( updateFunc( i, rejectContexts, rejectValues ) )
            .progress( updateFunc( i, progressContexts, progressValues ) );
        } else {
          --remaining;
        }
      }
    }

    // if we're not waiting on anything, reject the master
    if ( !remaining ) {
      deferred.rejectWith( rejectContexts, rejectValues );
    }

    return deferred.promise();
  };
})(jQuery);

#1