Fix: utils.debounce timing corner case.

Invocations of the debounced method between when runNow is scheduled
and when it actually runs will schedule additional runNow invocations
to be executed in rapid succession.

Author: jamestalmage

src/utils.js

@@ -62,6 +62,7 @@
             var queue = [];
             var start;
             var cancelTimer;
+            var runScheduledForNextTick;
 
             // returns `fn` wrapped in a function that queues up each call event to be
             // invoked later inside fo runNow()
@@ -77,14 +78,17 @@
             }
 
             // clears the current wait timer and creates a new one
-            // however, if maxWait is exceeded, calls runNow() immediately
+            // however, if maxWait is exceeded, calls runNow() on the next tick.
             function resetTimer() {
               if( cancelTimer ) {
                 cancelTimer();
                 cancelTimer = null;
               }
               if( start && Date.now() - start > maxWait ) {
-                utils.compile(runNow);
+                if(!runScheduledForNextTick){
+                  runScheduledForNextTick = true;
+                  utils.compile(runNow);
+                }
               }
               else {
                 if( !start ) { start = Date.now(); }
@@ -96,6 +100,7 @@
             function runNow() {
               cancelTimer = null;
               start = null;
+              runScheduledForNextTick = false;
               var copyList = queue.slice(0);
               queue = [];
               angular.forEach(copyList, function(parts) {
@@ -114,7 +119,7 @@
            * @param {int} [maxWait] max milliseconds to wait before sending out, defaults to wait * 10 or 100
            */
           debounce: function(fn, ctx, wait, maxWait) {
-            var start, cancelTimer, args;
+            var start, cancelTimer, args, runScheduledForNextTick;
             if( typeof(ctx) === 'number' ) {
               maxWait = wait;
               wait = ctx;
@@ -130,25 +135,29 @@
             if( !maxWait ) { maxWait = wait*10 || 100; }
 
             // clears the current wait timer and creates a new one
-            // however, if maxWait is exceeded, calls runNow() immediately
+            // however, if maxWait is exceeded, calls runNow() on the next tick.
             function resetTimer() {
               if( cancelTimer ) {
                 cancelTimer();
                 cancelTimer = null;
               }
               if( start && Date.now() - start > maxWait ) {
-                utils.compile(runNow);
+                if(!runScheduledForNextTick){
+                  runScheduledForNextTick = true;
+                  utils.compile(runNow);
+                }
               }
               else {
                 if( !start ) { start = Date.now(); }
                 cancelTimer = utils.wait(runNow, wait);
               }
             }
 
-            // Clears the queue and invokes all of the functions awaiting notification
+            // Clears the queue and invokes the debounced function with the most recent arguments
             function runNow() {
               cancelTimer = null;
               start = null;
+              runScheduledForNextTick = false;
               fn.apply(ctx, args);
             }
 

tests/unit/utils.spec.js

@@ -51,6 +51,42 @@ describe('$firebaseUtils', function () {
     });
   });
 
+  describe('#debounce', function(){
+    it('should trigger function with arguments',function(){
+      var spy = jasmine.createSpy();
+      $utils.debounce(spy,10)('foo', 'bar');
+      $timeout.flush();
+      expect(spy).toHaveBeenCalledWith('foo', 'bar');
+    });
+
+    it('should only trigger once, with most recent arguments',function(){
+      var spy = jasmine.createSpy();
+      var fn =  $utils.debounce(spy,10);
+      fn('foo', 'bar');
+      fn('baz', 'biz');
+      $timeout.flush();
+      expect(spy.calls.count()).toBe(1);
+      expect(spy).toHaveBeenCalledWith('baz', 'biz');
+    });
+
+    it('should only trigger once (timing corner case)',function(){
+      var spy = jasmine.createSpy();
+      var fn =  $utils.debounce(spy, null, 1, 2);
+      fn('foo', 'bar');
+      var start = Date.now();
+
+      // block for 3ms without releasing
+      while(Date.now() - start < 3){ }
+
+      fn('bar', 'baz');
+      fn('baz', 'biz');
+      expect(spy).not.toHaveBeenCalled();
+      $timeout.flush();
+      expect(spy.calls.count()).toBe(1);
+      expect(spy).toHaveBeenCalledWith('baz', 'biz');
+    });
+  });
+
   describe('#updateRec', function() {
     it('should return true if changes applied', function() {
       var rec = {};