calls array of async functions and invokes callback when all done - shindig

callAsyncAndJoin

/**                                                                                                                      
 * Calls an array of asynchronous functions and calls the continuation                                                   
 * function when all are done.                                                                                           
 * @param {Array} functions Array of asynchronous functions, each taking                                                 
 *     one argument that is the continuation function that handles the result                                            
 *     That is, each function is something like the following:                                                           
 *     function(continuation) {                                                                                          
 *       // compute result asynchronously                                                                                
 *       continuation(result);                                                                                           
 *     }.                                                                                                                
 * @param {Function} continuation Function to call when all results are in.  It                                          
 *     is pass an array of all results of all functions.                                                                 
 * @param {Object} opt_this Optional object used as "this" when calling each                                             
 *     function.                                                                                                         
 */                                                                                                                      
shindig.callAsyncAndJoin = function(functions, continuation, opt_this) {                                                 
  var pending = functions.length;                                                                                        
  var results = [];                                                                                                      
  for (var i = 0; i < functions.length; i++) {                                                                           
    // we need a wrapper here because i changes and we need one index                                                    
    // variable per closure                                                                                              
    var wrapper = function(index) {                                                                                      
      var fn = functions[index];                                                                                         
      if (typeof fn === 'string') {                                                                                      
        fn = opt_this[fn];                                                                                               
      }                                                                                                                  
      fn.call(opt_this, function(result) {                                                                               
        results[index] = result;                                                                                         
        if (--pending === 0) {                                                                                           
          continuation(results);                                                                                         
        }                                                                                                                
      });                                                                                                                
    };                                                                                                                   
    wrapper(i);                                                                                                          
  }                                                                                                                      
};