allow override of rendering after a collection sort

docs/marionette.collectionview.md

@@ -48,6 +48,7 @@ will provide features such as `onShow` callbacks, etc. Please see
 * [CollectionView: Automatic Rendering](#collectionview-automatic-rendering)
 * [CollectionView: Re-render Collection](#collectionview-re-render-collection)
 * [CollectionView's attachHtml](#collectionviews-attachhtml)
+* [CollectionView's resortView](#collectionviews-resortview)
 * [CollectionView's children](#collectionviews-children)
 * [CollectionView destroy](#collectionview-destroy)
 
@@ -737,6 +738,24 @@ Overrides of `attachHtml` that don't take into account the element
 buffer will work fine, but won't take advantage of the 60x performance
 increase the buffer provides.
 
+## CollectionView's resortView
+
+By default the `CollectionView` will maintain the order of its `collection`
+in the DOM. However on occasions the view may need to re-render to make this
+possible, for example if you were to change the comparator on the collection.
+By default `CollectionView` will call `render` when this happens, but there are
+cases where this may not be suitable. For instance when sorting the `children`
+in a `CompositeView`, you want to only render the internal collection.
+
+```js
+var cv = new Marionette.CollectionView({
+  collection: someCollection,
+  resortView: function() {
+    // provide custom logic for rendering after sorting the collection
+  }
+});
+```
+
 ## CollectionView's children
 
 The CollectionView uses [Backbone.BabySitter](https://github.com/marionettejs/backbone.babysitter)

spec/javascripts/sortedViews.spec.js

@@ -32,6 +32,9 @@ describe('collection/composite view sorting', function() {
         collection: this.collection,
       });
 
+      this.sinon.spy(this.collectionView, 'resortView');
+      this.sinon.spy(this.compositeView, 'resortView');
+
       this.collectionView.render();
       this.compositeView.render();
     });
@@ -109,6 +112,11 @@ describe('collection/composite view sorting', function() {
           expect(this.collectionView.$el).to.have.$text('12345');
           expect(this.compositeView.$el).to.have.$text('12345');
         });
+
+        it('should call resortViews', function() {
+          expect(this.collectionView.resortView).to.have.been.calledOnce;
+          expect(this.compositeView.resortView).to.have.been.calledOnce;
+        });
       });
     });
 
@@ -155,6 +163,11 @@ describe('collection/composite view sorting', function() {
           expect(this.collectionView.$el).to.have.$text('321');
           expect(this.compositeView.$el).to.have.$text('321');
         });
+
+        it('should call resortViews', function() {
+          expect(this.collectionView.resortView).to.have.been.calledOnce;
+          expect(this.compositeView.resortView).to.have.been.calledOnce;
+        });
       });
 
       describe('and adding a new child', function() {

src/marionette.collectionview.js

@@ -118,17 +118,25 @@ Marionette.CollectionView = Marionette.View.extend({
     return this;
   },
 
+  // Render view after sorting. Override this method to
+  // change how the view renders after a `sort` on the collection.
+  // An example of this would be to only `renderChildren` in a `CompositeView`
+  // rather than the full view.
+  resortView: function() {
+    this.render();
+  },
+
   // Internal method. This checks for any changes in the order of the collection.
   // If the index of any view doesn't match, it will render.
-  _sortViews: function(){
+  _sortViews: function() {
     // check for any changes in sort order of views
     var orderChanged = this.collection.find(function(item, index){
       var view = this.children.findByModel(item);
       return !view || view._index !== index;
     }, this);
 
     if (orderChanged) {
-      this.render();
+      this.resortView();
     }
   },