var hasfocusUpdatingProperty = '__ko_hasfocusUpdating';
var hasfocusLastValue = '__ko_hasfocusLastValue';
ko.bindingHandlers['hasfocus'] = {
    'init': function(element, valueAccessor, allBindings) {
        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) {
                var active;
                try {
                    active = ownerDoc.activeElement;
                } catch(e) {
                    // IE9 throws if you access activeElement during page load (see issue #703)
                    active = ownerDoc.body;
                }
                isFocused = (active === element);
            }
            var modelValue = valueAccessor();
            ko.expressionRewriting.writeValueToProperty(modelValue, allBindings, 'hasfocus', isFocused, true);

            //cache the latest value, so we can avoid unnecessarily calling focus/blur in the update function
            element[hasfocusLastValue] = isFocused;
            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) {
        var value = !!ko.utils.unwrapObservable(valueAccessor());

        if (!element[hasfocusUpdatingProperty] && element[hasfocusLastValue] !== value) {
            value ? element.focus() : element.blur();

            // In IE, the blur method doesn't always cause the element to lose focus (for example, if the window is not in focus).
            // Setting focus to the body element does seem to be reliable in IE, but should only be used if we know that the current
            // element was focused already.
            if (!value && element[hasfocusLastValue]) {
                element.ownerDocument.body.focus();
            }

            // For IE, which doesn't reliably fire "focus" or "blur" events synchronously
            ko.dependencyDetection.ignore(ko.utils.triggerEvent, null, [element, value ? "focusin" : "focusout"]);
        }
    }
};
ko.expressionRewriting.twoWayBindings['hasfocus'] = true;

ko.bindingHandlers['hasFocus'] = ko.bindingHandlers['hasfocus']; // Make "hasFocus" an alias
ko.expressionRewriting.twoWayBindings['hasFocus'] = true;
