Add let binding

build/fragments/source-references.js

@@ -35,6 +35,7 @@ knockoutDebugCallback([
     'src/binding/defaultBindings/hasfocus.js',
     'src/binding/defaultBindings/html.js',
     'src/binding/defaultBindings/ifIfnotWith.js',
+    'src/binding/defaultBindings/let.js',
     'src/binding/defaultBindings/options.js',
     'src/binding/defaultBindings/selectedOptions.js',
     'src/binding/defaultBindings/style.js',

spec/defaultBindings/letBehaviors.js

@@ -0,0 +1,35 @@
+describe('Binding: Let', function() {
+    beforeEach(jasmine.prepareTestNode);
+
+    it('Should be able to add custom properties that will be available to all child contexts', function() {
+        testNode.innerHTML = "<div data-bind=\"let: { '$customProp': 'my value' }\"><div data-bind='with: true'><div data-bind='text: $customProp'></div></div></div>";
+        ko.applyBindings(null, testNode);
+        expect(testNode).toContainText("my value");
+    });
+
+    it('Should update all child contexts when custom properties are updated', function() {
+        var observable = ko.observable(1);
+        testNode.innerHTML = "<div data-bind='let: { prop1 : prop()*2 }'><div data-bind='text: prop1'></div></div>";
+        ko.applyBindings({prop: observable}, testNode);
+        expect(testNode).toContainText("2");
+
+        // change observable
+        observable(2);
+        expect(testNode).toContainText("4");
+    });
+
+    it('Should update all custom properties when the parent context is updated', function() {
+        testNode.innerHTML = "<div data-bind='let: {obj1: $data}'><span data-bind='text:obj1.prop1'></span><span data-bind='text:prop2'></span></div>";
+        var vm = ko.observable({prop1: "First ", prop2: "view model"});
+        ko.applyBindings(vm, testNode);
+        expect(testNode).toContainText("First view model");
+
+        // change view model to new object
+        vm({prop1: "Second view ", prop2: "model"});
+        expect(testNode).toContainText("Second view model");
+
+        // change it again
+        vm({prop1: "Third view model", prop2: ""});
+        expect(testNode).toContainText("Third view model");
+    });
+});

spec/runner.html

@@ -62,6 +62,7 @@
         <script type="text/javascript" src="defaultBindings/htmlBehaviors.js"></script>
         <script type="text/javascript" src="defaultBindings/ifBehaviors.js"></script>
         <script type="text/javascript" src="defaultBindings/ifnotBehaviors.js"></script>
+        <script type="text/javascript" src="defaultBindings/letBehaviors.js"></script>
         <script type="text/javascript" src="defaultBindings/optionsBehaviors.js"></script>
         <script type="text/javascript" src="defaultBindings/selectedOptionsBehaviors.js"></script>
         <script type="text/javascript" src="defaultBindings/styleBehaviors.js"></script>

src/binding/defaultBindings/let.js

@@ -0,0 +1,10 @@
+ko.bindingHandlers['let'] = {
+    'init': function(element, valueAccessor, allBindings, viewModel, bindingContext) {
+        // Make a modified binding context, with extra properties, and apply it to descendant elements
+        var innerContext = bindingContext['extend'](valueAccessor);
+        ko.applyBindingsToDescendants(innerContext, element);
+
+        return { 'controlsDescendantBindings': true };
+    }
+};
+ko.virtualElements.allowedBindings['let'] = true;