/**
                            `Y.App` extension that provides view transitions in browsers which support
                            native CSS3 transitions.
                            
                            @module app
                            @submodule app-transitions
                            @since 3.5.0
                            **/
                            
                            /**
                            `Y.App` extension that provides view transitions in browsers which support
                            native CSS3 transitions.
                            
                            View transitions provide an nice way to move from one "page" to the next that is
                            both pleasant to the user and helps to communicate a hierarchy between sections
                            of an application.
                            
                            When the `"app-transitions"` module is used, it will automatically mix itself
                            into `Y.App` and transition between `activeView` changes using the following
                            effects:
                            
                              - **`fade`**: Cross-fades between the old an new active views.
                            
                              - **`slideLeft`**: The old and new active views are positioned next to each
                                other and both slide to the left.
                            
                              - **`slideRight`**: The old and new active views are positioned next to each
                                other and both slide to the right.
                            
                            **Note:** Transitions are an opt-in feature and are enabled via an app's
                            `transitions` attribute.
                            
                            @class App.Transitions
                            @uses App.TransitionsNative
                            @extensionfor App
                            @since 3.5.0
                            **/
                            function AppTransitions() {}
                            
                            AppTransitions.ATTRS = {
                                /**
                                Whether or not this application should use view transitions, and if so then
                                which ones or `true` for the defaults which are specified by the
                                `transitions` prototype property.
                            
                                **Note:** Transitions are an opt-in feature and will only be used in
                                browsers which support native CSS3 transitions.
                            
                                @attribute transitions
                                @type Boolean|Object
                                @default false
                                @since 3.5.0
                                **/
                                transitions: {
                                    setter: '_setTransitions',
                                    value : false
                                }
                            };
                            
                            /**
                            Collect of transitions -> fx.
                            
                            A transition (e.g. "fade") is a simple name given to a configuration of fx to
                            apply, consisting of `viewIn` and `viewOut` properties who's values are names of
                            fx registered on `Y.Transition.fx`.
                            
                            By default transitions: `fade`, `slideLeft`, and `slideRight` have fx defined.
                            
                            @property FX
                            @type Object
                            @static
                            @since 3.5.0
                            **/
                            AppTransitions.FX = {
                                fade: {
                                    viewIn : 'app:fadeIn',
                                    viewOut: 'app:fadeOut'
                                },
                            
                                slideLeft: {
                                    viewIn : 'app:slideLeft',
                                    viewOut: 'app:slideLeft'
                                },
                            
                                slideRight: {
                                    viewIn : 'app:slideRight',
                                    viewOut: 'app:slideRight'
                                }
                            };
                            
                            AppTransitions.prototype = {
                                // -- Public Properties ----------------------------------------------------
                            
                                /**
                                Default transitions to use when the `activeView` changes.
                            
                                The following are types of changes for which transitions can be defined that
                                correspond to the relationship between the new and previous `activeView`:
                            
                                  * `navigate`: The default transition to use when changing the `activeView`
                                    of the application.
                            
                                  * `toChild`: The transition to use when the new `activeView` is configured
                                    as a child of the previously active view via its `parent` property as
                                    defined in this app's `views`.
                            
                                  * `toParent`: The transition to use when the new `activeView` is
                                    configured as the `parent` of the previously active view as defined in
                                    this app's `views`.
                            
                                **Note:** Transitions are an opt-in feature and will only be used in
                                browsers which support native CSS3 transitions.
                            
                                @property transitions
                                @type Object
                                @default
                                    {
                                        navigate: 'fade',
                                        toChild : 'slideLeft',
                                        toParent: 'slideRight'
                                    }
                                @since 3.5.0
                                **/
                                transitions: {
                                    navigate: 'fade',
                                    toChild : 'slideLeft',
                                    toParent: 'slideRight'
                                },
                            
                                // -- Public Methods -------------------------------------------------------
                            
                                /**
                                Sets which view is active/visible for the application. This will set the
                                app's `activeView` attribute to the specified `view`.
                            
                                The `view` will be "attached" to this app, meaning it will be both rendered
                                into this app's `viewContainer` node and all of its events will bubble to
                                the app. The previous `activeView` will be "detached" from this app.
                            
                                When a string-name is provided for a view which has been registered on this
                                app's `views` object, the referenced metadata will be used and the
                                `activeView` will be set to either a preserved view instance, or a new
                                instance of the registered view will be created using the specified `config`
                                object passed-into this method.
                            
                                A callback function can be specified as either the third or fourth argument,
                                and this function will be called after the new `view` becomes the
                                `activeView`, is rendered to the `viewContainer`, and is ready to use.
                            
                                @example
                                    var app = new Y.App({
                                        views: {
                                            usersView: {
                                                // Imagine that `Y.UsersView` has been defined.
                                                type: Y.UsersView
                                            }
                                        },
                            
                                        transitions: true,
                                        users      : new Y.ModelList()
                                    });
                            
                                    app.route('/users/', function () {
                                        this.showView('usersView', {users: this.get('users')});
                                    });
                            
                                    app.render();
                                    app.navigate('/uses/');
                                    // => Creates a new `Y.UsersView` and transitions to it.
                            
                                @method showView
                                @param {String|View} view The name of a view defined in the `views` object,
                                    or a view instance which should become this app's `activeView`.
                                @param {Object} [config] Optional configuration to use when creating a new
                                    view instance. This config object can also be used to update an existing
                                    or preserved view's attributes when `options.update` is `true`.
                                @param {Object} [options] Optional object containing any of the following
                                    properties:
                                  @param {Function} [options.callback] Optional callback function to call
                                    after new `activeView` is ready to use, the function will be passed:
                                      @param {View} options.callback.view A reference to the new
                                        `activeView`.
                                  @param {Boolean} [options.prepend=false] Whether the `view` should be
                                    prepended instead of appended to the `viewContainer`.
                                  @param {Boolean} [options.render] Whether the `view` should be rendered.
                                    **Note:** If no value is specified, a view instance will only be
                                    rendered if it's newly created by this method.
                                  @param {Boolean|String} [options.transition] Optional transition override.
                                    A transition can be specified which will override the default, or
                                    `false` for no transition.
                                  @param {Boolean} [options.update=false] Whether an existing view should
                                    have its attributes updated by passing the `config` object to its
                                    `setAttrs()` method. **Note:** This option does not have an effect if
                                    the `view` instance is created as a result of calling this method.
                                @param {Function} [callback] Optional callback Function to call after the
                                    new `activeView` is ready to use. **Note:** this will override
                                    `options.callback` and it can be specified as either the third or fourth
                                    argument. The function will be passed the following:
                                  @param {View} callback.view A reference to the new `activeView`.
                                @chainable
                                @since 3.5.0
                                **/
                                // Does not override `showView()` but does use `options.transitions`.
                            
                                // -- Protected Methods ----------------------------------------------------
                            
                                /**
                                Setter for `transitions` attribute.
                            
                                When specified as `true`, the defaults will be use as specified by the
                                `transitions` prototype property.
                            
                                @method _setTransitions
                                @param {Boolean|Object} transitions The new `transitions` attribute value.
                                @return {Mixed} The processed value which represents the new state.
                                @protected
                                @see App.Base.showView()
                                @since 3.5.0
                                **/
                                _setTransitions: function (transitions) {
                                    var defTransitions = this.transitions;
                            
                                    if (transitions && transitions === true) {
                                        return Y.merge(defTransitions);
                                    }
                            
                                    return transitions;
                                }
                            };
                            
                            // -- Namespace ----------------------------------------------------------------
                            Y.App.Transitions = AppTransitions;
                            Y.Base.mix(Y.App, [AppTransitions]);
                            
                            Y.mix(Y.App.CLASS_NAMES, {
                                transitioning: Y.ClassNameManager.getClassName('app', 'transitioning')
                            });