Serialize the items and activeItem via PaneModel

This necessitates setting the ::activeView based on the model's
::activeItem instead of doing it in ::showItem.

spec/pane-container-spec.coffee

@@ -16,7 +16,7 @@ describe "PaneContainer", ->
       serialize: -> { deserializer: 'TestView', @name }
       getUri: -> path.join(temp.dir, @name)
       save: -> @saved = true
-      isEqual: (other) -> @name is other.name
+      isEqual: (other) -> @name is other?.name
 
     container = new PaneContainer
     pane1 = new Pane(new TestView('1'))

spec/pane-spec.coffee

@@ -13,7 +13,7 @@ describe "Pane", ->
     initialize: ({@id, @text}) ->
     serialize: -> { deserializer: 'TestView', @id, @text }
     getUri: -> @id
-    isEqual: (other) -> @id == other.id and @text == other.text
+    isEqual: (other) -> other? and @id == other.id and @text == other.text
 
   beforeEach ->
     atom.deserializers.add(TestView)
@@ -558,12 +558,12 @@ describe "Pane", ->
         pane2 = pane1.splitRight()
         expect(container.find('.row .pane').toArray()).toEqual [pane1[0], pane2[0]]
         expect(pane2.items).toEqual []
-        expect(pane2.activeItem).toBe null
+        expect(pane2.activeItem).toBeUndefined()
 
         pane3 = pane2.splitRight()
         expect(container.find('.row .pane').toArray()).toEqual [pane1[0], pane2[0], pane3[0]]
         expect(pane3.items).toEqual []
-        expect(pane3.activeItem).toBe null
+        expect(pane3.activeItem).toBeUndefined()
 
     describe "splitLeft(items...)", ->
       it "builds a row if needed, then appends a new pane before itself", ->

src/pane-model.coffee

@@ -1,7 +1,24 @@
+{find, compact} = require 'underscore-plus'
 {Model} = require 'theorist'
+Serializable = require 'serializable'
 
 module.exports =
 class PaneModel extends Model
-  activeItem: null
+  Serializable.includeInto(this)
 
-  constructor: ({@items}) ->
+  @properties
+    activeItem: null
+
+  constructor: ({@items, @activeItem}) ->
+    @items ?= []
+    @activeItem ?= @items[0]
+
+  serializeParams: ->
+    items: compact(@items.map((item) -> item.serialize?()))
+    activeItemUri: @activeItem?.getUri?()
+
+  deserializeParams: (params) ->
+    {items, activeItemUri} = params
+    params.items = items.map (itemState) -> atom.deserializers.deserialize(itemState)
+    params.activeItem = find params.items, (item) -> item.getUri?() is activeItemUri
+    params

src/pane.coffee

@@ -27,11 +27,12 @@ class Pane extends View
 
   @delegatesProperties 'items', 'activeItem', toProperty: 'model'
 
+  previousActiveItem: null
+
   # Private:
   initialize: (args...) ->
-    if args[0]?.items # deserializing
-      {items, activeItemUri, @focusOnAttach} = args[0]
-      @model = new PaneModel({items})
+    if args[0]?.model?
+      {@model, @focusOnAttach} = args[0]
     else
       @model = new PaneModel(items: args)
 
@@ -39,8 +40,7 @@ class Pane extends View
 
     @viewsByItem = new WeakMap()
 
-    unless activeItemUri? and @showItemForUri(activeItemUri)
-      @showItem(@items[0]) if @items.length > 0
+    @subscribe @model.$activeItem, 'value', @onActiveItemChanged
 
     @command 'pane:save-items', @saveItems
     @command 'pane:show-next-item', @showNextItem
@@ -66,13 +66,12 @@ class Pane extends View
     @on 'focusin', => @makeActive()
 
   deserializeParams: (params) ->
-    params.items = _.compact(params.items.map (itemState) -> atom.deserializers.deserialize(itemState))
+    params.model = PaneModel.deserialize(params.model)
     params
 
   serializeParams: ->
-    items: _.compact(@items.map (item) -> item.serialize?())
+    model: @model.serialize()
     focusOnAttach: @is(':has(:focus)')
-    activeItemUri: @getActivePaneItem()?.getUri?()
 
   # Private:
   afterAttach: (onDom) ->
@@ -146,20 +145,23 @@ class Pane extends View
 
   # Public: Focuses the given item.
   showItem: (item) ->
-    return if !item? or item is @activeItem
+    if item?
+      @addItem(item)
+      @model.activeItem = item
 
-    if @activeItem
-      @activeItem.off? 'title-changed', @activeItemTitleChanged
+  onActiveItemChanged: (item) =>
+    @previousActiveItem?.off? 'title-changed', @activeItemTitleChanged
+    @previousActiveItem = item
+
+    return unless item?
 
     isFocused = @is(':has(:focus)')
-    @addItem(item)
     item.on? 'title-changed', @activeItemTitleChanged
     view = @viewForItem(item)
     @itemViews.children().not(view).hide()
     @itemViews.append(view) unless view.parent().is(@itemViews)
     view.show() if @attached
     view.focus() if isFocused
-    @activeItem = item
     @activeView = view
     @trigger 'pane:active-item-changed', [item]