Add PaneContainer::onDidChangeActivePaneItem

spec/pane-container-spec.coffee

@@ -66,3 +66,24 @@ describe "PaneContainer", ->
       pane2.destroy()
       expect(container.getActivePane()).toBe pane1
       expect(pane1.isActive()).toBe true
+
+  describe "::onDidChangeActivePaneItem()", ->
+    [container, pane1, pane2, observed] = []
+
+    beforeEach ->
+      container = new PaneContainer(root: new Pane(items: [new Object, new Object]))
+      container.getRoot().splitRight(items: [new Object, new Object])
+      [pane1, pane2] = container.getPanes()
+
+      observed = []
+      container.onDidChangeActivePaneItem (item) -> observed.push(item)
+
+    it "invokes observers when the active item of the active pane changes", ->
+      pane2.activateNextItem()
+      pane2.activateNextItem()
+      expect(observed).toEqual [pane2.itemAtIndex(1), pane2.itemAtIndex(0)]
+
+    it "invokes observers when the active pane changes", ->
+      pane1.activate()
+      pane2.activate()
+      expect(observed).toEqual [pane1.itemAtIndex(0), pane2.itemAtIndex(0)]

src/pane-container.coffee

@@ -1,6 +1,6 @@
 {find} = require 'underscore-plus'
 {Model} = require 'theorist'
-{Emitter} = require 'event-kit'
+{Emitter, CompositeDisposable} = require 'event-kit'
 Serializable = require 'serializable'
 Pane = require './pane'
 
@@ -26,10 +26,13 @@ class PaneContainer extends Model
     super
 
     @emitter = new Emitter
+    @subscriptions = new CompositeDisposable
 
     @subscribe @$root, @onRootChanged
     @destroyEmptyPanes() if params?.destroyEmptyPanes
 
+    @monitorActivePaneItem()
+
   deserializeParams: (params) ->
     params.root = atom.deserializers.deserialize(params.root, container: this)
     params.destroyEmptyPanes = atom.config.get('core.destroyEmptyPanes')
@@ -43,6 +46,17 @@ class PaneContainer extends Model
   onDidChangeActivePane: (fn) ->
     @emitter.on 'did-change-active-pane', fn
 
+  observeActivePane: (fn) ->
+    fn(@getActivePane())
+    @onDidChangeActivePane(fn)
+
+  onDidChangeActivePaneItem: (fn) ->
+    @emitter.on 'did-change-active-pane-item', fn
+
+  observeActivePaneItem: (fn) ->
+    fn(@getActivePaneItem())
+    @onDidChangeActivePaneItem(fn)
+
   getRoot: -> @root
 
   replaceChild: (oldChild, newChild) ->
@@ -61,6 +75,9 @@ class PaneContainer extends Model
       @emitter.emit 'did-change-active-pane', @activePane
     @activePane
 
+  getActivePaneItem: ->
+    @getActivePane().getActiveItem()
+
   paneForUri: (uri) ->
     find @getPanes(), (pane) -> pane.itemForUri(uri)?
 
@@ -99,7 +116,6 @@ class PaneContainer extends Model
     root.parent = this
     root.container = this
 
-
     @activePane ?= root if root instanceof Pane
 
   destroyEmptyPanes: ->
@@ -111,3 +127,17 @@ class PaneContainer extends Model
   # Called by Model superclass when destroyed
   destroyed: ->
     pane.destroy() for pane in @getPanes()
+    @subscriptions.dispose()
+    @emitter.dispose()
+
+  monitorActivePaneItem: ->
+    childSubscription = null
+    @subscriptions.add @observeActivePane (activePane) =>
+      if childSubscription?
+        @subscriptions.remove(childSubscription)
+        childSubscription.dispose()
+
+      childSubscription = activePane.observeActiveItem (activeItem) =>
+        @emitter.emit 'did-change-active-pane-item', activeItem
+
+      @subscriptions.add(childSubscription)