Add computed def as function

README.md

@@ -38,6 +38,20 @@ computed: {
 }
 ```
 
+`computed` can also be a function returning an object.
+
+```js
+computed: function() {
+    return {
+        grossPrice: {
+            get: function () {
+                return 105;
+            }
+        }
+    };
+}
+```
+
 Each property that declares `get` or `set` method is treated as computed.
 
 Get the value of computed property,

src/backbone.computedfields.js

@@ -1,5 +1,14 @@
 Backbone.ComputedFields = (function(Backbone, _){
 
+    var _isFunction = function(obj) {
+        // == instead of === and || false are optimizations
+        // to go around nasty bugs in IE11, Safari 8 and old v8
+        // see underscore#isFunction
+        /* jshint eqeqeq: false */
+        return typeof obj == 'function' || false;
+        /* jshint eqeqeq: true */
+    };
+
     var ComputedFields = function (model) {
         this.model = model;
         this._computedFields = [];
@@ -29,8 +38,10 @@ Backbone.ComputedFields = (function(Backbone, _){
         },
 
         _lookUpComputedFields: function () {
-            for (var obj in this.model.computed) {
-                var field = this.model.computed[obj];
+            var computed = _isFunction(this.model.computed) ? this.model.computed() : this.model.computed;
+
+            for (var obj in computed) {
+                var field = computed[obj];
 
                 if (field && (field.set || field.get)) {
                     this._computedFields.push({name: obj, field: field});

test/spec/backbone.computedfields.spec.js

@@ -59,6 +59,69 @@ describe('Backbone.ComputedFields spec', function() {
 
     });
 
+    describe('when ComputedFields initialized and computed is a function', function () {
+
+        var model;
+
+        beforeEach(function () {
+            var Model = Backbone.Model.extend({
+                initialize: function () {
+                    this.computedFields = new Backbone.ComputedFields(this);
+                },
+
+                computed: function() {
+                    return {
+                        grossPrice: {
+                            get: function () {
+                                return 100;
+                            }
+                        }
+                    };
+                }
+            });
+
+            model = new Model({ netPrice: 100, vatRate: 5});
+        });
+
+        it ('should be initialized', function () {
+            expect(model.computedFields).to.exist;
+            expect(model.computedFields._computedFields.length).to.equal(1);
+        });
+
+        it ('should access model attributes', function () {
+            expect(model.get('netPrice')).to.equal(100);
+            expect(model.get('vatRate')).to.equal(5);
+        });
+
+        describe('when initialize with empty', function () {
+           beforeEach(function () {
+                var Model = Backbone.Model.extend({
+                    initialize: function () {
+                        this.getHasBeenCalled = false;
+                        this.computedFields = new Backbone.ComputedFields(this);
+                    },
+
+                    computed: function() {
+                        return {
+                            grossPrice: {
+                                get: function () {
+                                    this.getHasBeenCalled = true;
+                                }
+                            }
+                        };
+                    }
+                });
+
+                model = new Model();
+            });
+
+            it ('should not call computed field getter', function () {
+                expect(model.getHasBeenCalled).to.equal(false);
+            });
+        });
+
+    });
+
     describe('when ComputedFields are used', function () {
         beforeEach(function () {
             var Model = Backbone.Model.extend({
@@ -587,4 +650,4 @@ describe('Backbone.ComputedFields spec', function() {
 
     });
 
-});
+});