Add PaneContainer::onDidChangeActivePane

spec/pane-spec.coffee

@@ -43,6 +43,16 @@ describe "Pane", ->
       pane2.activate()
       expect(container.getActivePane()).toBe pane2
 
+    it "invokes ::onDidChangeActivePane observers on the container", ->
+      observed = []
+      container.onDidChangeActivePane (activePane) -> observed.push(activePane)
+
+      pane1.activate()
+      pane1.activate()
+      pane2.activate()
+      pane1.activate()
+      expect(observed).toEqual [pane1, pane2, pane1]
+
     it "invokes ::onDidActivate() observers", ->
       eventCount = 0
       pane1.onDidActivate -> eventCount++

src/pane-container.coffee

@@ -1,5 +1,6 @@
 {find} = require 'underscore-plus'
 {Model} = require 'theorist'
+{Emitter} = require 'event-kit'
 Serializable = require 'serializable'
 Pane = require './pane'
 
@@ -23,6 +24,9 @@ class PaneContainer extends Model
 
   constructor: (params) ->
     super
+
+    @emitter = new Emitter
+
     @subscribe @$root, @onRootChanged
     @destroyEmptyPanes() if params?.destroyEmptyPanes
 
@@ -36,6 +40,9 @@ class PaneContainer extends Model
     root: @root?.serialize()
     activePaneId: @activePane.id
 
+  onDidChangeActivePane: (fn) ->
+    @emitter.on 'did-change-active-pane', fn
+
   getRoot: -> @root
 
   replaceChild: (oldChild, newChild) ->
@@ -48,6 +55,12 @@ class PaneContainer extends Model
   getActivePane: ->
     @activePane
 
+  setActivePane: (pane) ->
+    if pane isnt @activePane
+      @activePane = pane
+      @emitter.emit 'did-change-active-pane', @activePane
+    @activePane
+
   paneForUri: (uri) ->
     find @getPanes(), (pane) -> pane.itemForUri(uri)?
 
@@ -80,7 +93,7 @@ class PaneContainer extends Model
     @previousRoot = root
 
     unless root?
-      @activePane = null
+      @setActivePane(null)
       return
 
     root.parent = this

src/pane.coffee

@@ -127,7 +127,7 @@ class Pane extends Model
   # Public: Makes this pane the *active* pane, causing it to gain focus
   # immediately.
   activate: ->
-    @container?.activePane = this
+    @container?.setActivePane(this)
     @emit 'activated'
     @emitter.emit 'did-activate'