Implement a portable keypath observer.

@@ -20,6 +20,7 @@ module.exports = (grunt) ->
             'src/view.coffee'
             'src/bindings.coffee'
             'src/parsers.coffee'
+            'src/keypath_observer.coffee'
             'src/binders.coffee'
             'src/adapters.coffee'
             'src/export.coffee'

@@ -27,35 +27,15 @@ class Rivets.Binding
     @binder = {routine: @binder} if @binder instanceof Function
 
   setModel: =>
-    interfaces = (k for k, v of @view.adapters)
-    tokens = Rivets.KeypathParser.parse @keypath, interfaces, @view.config.rootInterface
-
-    @rootKey = tokens.shift()
-    @key = tokens.pop()
-    @objectPath ?= []
-
-    model = @view.adapters[@rootKey.interface].read @view.models, @rootKey.path
-
-    for token, index in tokens
-      current = @view.adapters[token.interface].read model, token.path
-
-      if @objectPath[index]?
-        if current isnt @objectPath[index]
-          @view.adapters[token.interface].unsubscribe model, token.path, @setModel
-          @view.adapters[token.interface].subscribe current, token.path, @setModel
-      else
-        @view.adapters[token.interface].subscribe model, token.path, @setModel
-
-      model = current
-
-    if @model
-      if @model isnt model
-        @unbind true if @key
-        @model = model
-        @bind true if @key
-        @sync()
-    else
-      @model = model
+    observer = new KeypathObserver @view, @view.models, @keypath, (target) =>
+      @unbind true if @key
+      @model = target
+      @bind true if @key
+      @sync()
+
+    @rootKey = observer.root
+    @key = observer.key
+    @model = observer.target
 
   # Applies all the current formatters to the supplied value and returns the
   # formatted value.

+class KeypathObserver
+  constructor: (@view, @model, @keypath, @callback) ->
+    @interfaces = (k for k, v of @view.adapters)
+    @objectPath = []
+    @tokens = Rivets.KeypathParser.parse @keypath, @interfaces, @view.config.rootInterface
+    @root = @tokens.shift()
+    @key = @tokens.pop()
+    @target = @realize()
+
+  update: =>
+    unless (next = @realize()) is @target
+      @callback @target = next
+
+  realize: =>
+    current = @view.adapters[@root.interface].read @view.models, @root.path
+
+    for token, index in @tokens
+      next = @view.adapters[token.interface].read current, token.path
+
+      if @objectPath[index]?
+        if next isnt @objectPath[index]
+          @view.adapters[token.interface].unsubscribe current, token.path, @update
+          @view.adapters[token.interface].subscribe next, token.path, @update
+      else
+        @view.adapters[token.interface].subscribe current, token.path, @update
+
+      current = next
+
+    current