merging in barrier ... soon to be after

test/functions.js

@@ -137,4 +137,22 @@ $(document).ready(function() {
     equals(composed('moe'), 'hi: moe!', 'in this case, the functions are also commutative');
   });
 
+  test("functions: barrier", function() {
+    var testBarrier = function(barrierAmount, timesCalled) {
+      var barrierCalled = 0;
+      var barrier = _.barrier(function(){ barrierCalled++; }, barrierAmount);
+
+
+      while (timesCalled--) {
+        barrier();
+      }
+
+      return barrierCalled;
+    }
+
+    equals(testBarrier(5, 5), 1, "barrier(N) should fire after being called N times");
+    equals(testBarrier(5, 4), 0, "barrier(N) should not fire unless called N times");
+    equals(testBarrier(5, 6), 1, "barrier(N) should fire only once even if called more than N times");
+  });
+
 });

underscore.js

@@ -510,6 +510,14 @@
     };
   };
 
+  // Returns a function that will only be executed after being called N times.
+  _.barrier = function(func, times) {
+    return function() {
+      if (--times === 0) { return func.apply(this, arguments); }
+    };
+  };
+
+
   // Object Functions
   // ----------------