How *not* to destroy View when exiting a route in

问题:

Regarding the new Ember.js routing system (described here), if I understand correctly, views are destroyed when you exit a route.

Is there any way to bypass destruction of views upon exiting a route, so that the state of the view is preserved when the user re-enters the route?

---

Update: Looks like, views are not destroyed unless the outlet view is being replaced in the new route. For e.g., if you are in stateA with ViewA in some {{outlet master}} and you go to stateB with ViewB in {{outlet master}}, then ViewB will replace ViewA. A way around this is to define multiple outlets when you need to preserve views, e.g., {{outlet master1}}, {{outlet master2}}, ...

A nice feature would be the ability to pass an array of views to the outlet. And also be able to choose whether views will be destroyed or just become hidden, upon exiting a route.

回答1:

I have since figure out how to modify the routing system, so that views inserted into outlets are not destroyed. First I override the Handlebars outlet helper, so that it loads an Ember.OutletView into {{outlet}}:

Ember.Handlebars.registerHelper('outlet', function(property, options) {
  if (property && property.data && property.data.isRenderData) {
    options = property;
    property = 'view';
  }

  options.hash.currentViewBinding = "controller." + property;

  return Ember.Handlebars.helpers.view.call(this, Ember.OutletView, options);
});

Where Ember.OutletView extends Ember.ContainerView as follows:

Ember.OutletView = Ember.ContainerView.extend({
    childViews: [],

    _currentViewWillChange: Ember.beforeObserver( function() {
        var childViews = this.get('childViews');

            // Instead of removing currentView, just hide all childViews
            childViews.setEach('isVisible', false);

    }, 'currentView'),

    _currentViewDidChange: Ember.observer( function() {
        var childViews = this.get('childViews'),
            currentView = this.get('currentView');

        if (currentView) {
            // Check if currentView is already within childViews array
            // TODO: test
            var alreadyPresent = childViews.find( function(child) {
               if (Ember.View.isEqual(currentView, child, [])) {          
                   return true;
               } 
            });

            if (!!alreadyPresent) {
                alreadyPresent.set('isVisible', true);
            } else {
                childViews.pushObject(currentView);
            }
        }
    }, 'currentView')

});

Basically we override _currentViewWillChange() and just hide all childViews instead of removing the currentView. Then in _currentViewDidChange() we check if the currentView is already inside childViews and act accordingly. The Ember.View.isEqual is a modified version of Underscore isEqual:

Ember.View.reopenClass({ 
    isEqual: function(a, b, stack) {
        // Identical objects are equal. `0 === -0`, but they aren't identical.
        // See the Harmony `egal` proposal: http://wiki.ecmascript.org/doku.php?id=harmony:egal.
        if (a === b) return a !== 0 || 1 / a == 1 / b;
        // A strict comparison is necessary because `null == undefined`.
        if (a == null || b == null) return a === b;
        // Unwrap any wrapped objects.
        if (a._chain) a = a._wrapped;
        if (b._chain) b = b._wrapped;
        // Compare `[[Class]]` names.
        var className = toString.call(a);
        if (className != toString.call(b)) return false;

        if (typeof a != 'object' || typeof b != 'object') return false;
        // Assume equality for cyclic structures. The algorithm for detecting cyclic
        // structures is adapted from ES 5.1 section 15.12.3, abstract operation `JO`.
        var length = stack.length;
        while (length--) {
            // Linear search. Performance is inversely proportional to the number of
            // unique nested structures.
            if (stack[length] == a) return true;
        }
        // Add the first object to the stack of traversed objects.
        stack.push(a);
        var size = 0, result = true;
        // Recursively compare objects and arrays.
        if (className == '[object Array]') {
            // Compare array lengths to determine if a deep comparison is necessary.
            size = a.length;
            result = size == b.length;
            if (result) {
                // Deep compare the contents, ignoring non-numeric properties.
                while (size--) {
                    // Ensure commutative equality for sparse arrays.
                    if (!(result = size in a == size in b && this.isEqual(a[size], b[size], stack))) break;
                }
            }
        } else {
            // Objects with different constructors are not equivalent.
            if (a.get('constructor').toString() != b.get('constructor').toString()) {
                return false;
            }

            // Deep compare objects.
            for (var key in a) {
                if (a.hasOwnProperty(key)) {
                    // Count the expected number of properties.
                    size++;
                    // Deep compare each member.
                    if ( !(result = b.hasOwnProperty(key) )) break;
                }
            }
        }
        // Remove the first object from the stack of traversed objects.
        stack.pop();
        return result;
    }
});

回答2:

So that the state of the view is preserved when the user re-enters the route.

I would, instead, store that information in the controller (or the state manager) so that when the route is re-entered, the new view is initialized with the old state. Does that make sense? So, for example, if it's a list of posts, and one is selected, you would store the data about which post was selected in the controller (or the state manager). After visiting a specific post and then coming back to the list, that same post would be selected.

I can imagine a use case where this wouldn't be very useful (e.g. scrolling to a specific position in a long list) so maybe that doesn't answer your question.