Split hasfocus binding.

src/binding/defaultBindings/hasfocusBinding.js

@@ -0,0 +1,35 @@
+var hasfocusUpdatingProperty = '__ko_hasfocusUpdating';
+ko.bindingHandlers['hasfocus'] = {
+    'init': function(element, valueAccessor, allBindingsAccessor) {
+        var handleElementFocusChange = function(isFocused) {
+            // Where possible, ignore which event was raised and determine focus state using activeElement,
+            // as this avoids phantom focus/blur events raised when changing tabs in modern browsers.
+            // However, not all KO-targeted browsers (Firefox 2) support activeElement. For those browsers,
+            // prevent a loss of focus when changing tabs/windows by setting a flag that prevents hasfocus
+            // from calling 'blur()' on the element when it loses focus.
+            // Discussion at https://github.com/SteveSanderson/knockout/pull/352
+            element[hasfocusUpdatingProperty] = true;
+            var ownerDoc = element.ownerDocument;
+            if ("activeElement" in ownerDoc) {
+                isFocused = (ownerDoc.activeElement === element);
+            }
+            var modelValue = valueAccessor();
+            ko.expressionRewriting.writeValueToProperty(modelValue, allBindingsAccessor, 'hasfocus', isFocused, true);
+            element[hasfocusUpdatingProperty] = false;
+        };
+        var handleElementFocusIn = handleElementFocusChange.bind(null, true);
+        var handleElementFocusOut = handleElementFocusChange.bind(null, false);
+
+        ko.utils.registerEventHandler(element, "focus", handleElementFocusIn);
+        ko.utils.registerEventHandler(element, "focusin", handleElementFocusIn); // For IE
+        ko.utils.registerEventHandler(element, "blur",  handleElementFocusOut);
+        ko.utils.registerEventHandler(element, "focusout",  handleElementFocusOut); // For IE
+    },
+    'update': function(element, valueAccessor) {
+        if (!element[hasfocusUpdatingProperty]) {
+            var value = ko.utils.unwrapObservable(valueAccessor());
+            value ? element.focus() : element.blur();
+            ko.utils.triggerEvent(element, value ? "focusin" : "focusout"); // For IE, which doesn't reliably fire "focus" or "blur" events synchronously
+        }
+    }
+};