Improve once() method.

github: #37

Author: javadev

src/main/java/com/github/underscore/$.java

@@ -1,7 +1,7 @@
 /*
  * The MIT License (MIT)
  *
- * Copyright 2015 Valentyn Kolesnikov
+ * Copyright 2016 Valentyn Kolesnikov
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -1344,13 +1344,14 @@ public E apply() {
     public static <T> Function<T> once(final Function<T> function) {
         return new Function<T>() {
             private volatile boolean executed;
+            private T result;
             @Override
             public T apply() {
                 if (!executed) {
                     executed = true;
-                    delay(function, 0);
+                    result = function.apply();
                 }
-                return null;
+                return result;
             }
         };
     }

src/test/java/com/github/underscore/FunctionsTest.java

@@ -146,13 +146,14 @@ public void defer() throws Exception {
     @Test
     public void once() throws Exception {
         final Integer[] counter = new Integer[] {0};
-        Function<Void> incr = new Function<Void>() { public Void apply() {
-            counter[0]++; return null; } };
-        Function<Void> onceIncr = $.once(incr);
+        Function<Integer> incr = new Function<Integer>() { public Integer apply() {
+            counter[0]++; return counter[0]; } };
+        Function<Integer> onceIncr = $.once(incr);
         onceIncr.apply();
         onceIncr.apply();
         Thread.sleep(60);
         assertEquals("incr was called only once", 1, counter[0].intValue());
+        assertEquals("stores a memo to the last value", 1, onceIncr.apply().intValue());
     }
 
 /*

src/test/javascript/functions_spec.js

@@ -62,3 +62,13 @@
     _.delay(debouncedIncr, 16);
     _.delay(function(){ equal(counter, 1, 'incr was debounced'); }, 96);
   });
+
+  describe('once', function() {
+    var num = 0;
+    var increment = _.once(function(){ return ++num; });
+    increment();
+    increment();
+    equal(num, 1);
+
+    equal(increment(), 1, 'stores a memo to the last value');
+  });