Add Config#observe, which watches a key path for changes

spec/app/config-spec.coffee

@@ -0,0 +1,34 @@
+describe "Config", ->
+  describe "#observe(keyPath)", ->
+    observeHandler = null
+
+    beforeEach ->
+      observeHandler = jasmine.createSpy("observeHandler")
+      config.foo = { bar: { baz: "value 1" } }
+      config.observe "foo.bar.baz", observeHandler
+
+    it "fires the given callback with the current value at the keypath", ->
+      expect(observeHandler).toHaveBeenCalledWith("value 1")
+
+    it "fires the callback every time the observed value changes", ->
+      observeHandler.reset() # clear the initial call
+      config.foo.bar.baz = "value 2"
+      config.update()
+      expect(observeHandler).toHaveBeenCalledWith("value 2")
+      observeHandler.reset()
+
+      config.foo.bar.baz = "value 1"
+      config.update()
+      expect(observeHandler).toHaveBeenCalledWith("value 1")
+
+    it "fires the callback when the full key path goes into and out of existence", ->
+      observeHandler.reset() # clear the initial call
+      delete config.foo.bar
+      config.update()
+
+      expect(observeHandler).toHaveBeenCalledWith(undefined)
+      observeHandler.reset()
+
+      config.foo.bar = { baz: "i'm back" }
+      config.update()
+      expect(observeHandler).toHaveBeenCalledWith("i'm back")

src/app/config.coffee

@@ -6,7 +6,7 @@ module.exports =
 class Config
   configDirPath: fs.absolute("~/.atom")
   configJsonPath: fs.absolute("~/.atom/config.json")
-  userInitScriptPath: fs.absolute("~/.atom/atom.coffe")
+  userInitScriptPath: fs.absolute("~/.atom/atom.coffee")
 
   load: ->
     if fs.exists(@configJsonPath)
@@ -36,8 +36,25 @@ class Config
 
   requireUserInitScript: ->
     try
+      console.log @userInitScriptPath
       require @userInitScriptPath if fs.exists(@userInitScriptPath)
     catch error
       console.error "Failed to load `#{@userInitScriptPath}`", error.stack, error
 
+  valueAtKeyPath: (keyPath) ->
+    value = this
+    for key in keyPath
+      break unless value = value[key]
+    value
+
+  observe: (keyPathString, callback) ->
+    keyPath = keyPathString.split('.')
+    value = @valueAtKeyPath(keyPath)
+    @on 'update', =>
+      newValue = @valueAtKeyPath(keyPath)
+      unless newValue == value
+        value = newValue
+        callback(value)
+    callback(value)
+
 _.extend Config.prototype, EventEmitter