Add optional valueFn to PathObserver

This is the first step towards adding a CompoundPathObserver to observe-js (and removing CompoundBinding from TemplateBind). The motivation for this is to reduce allocation and total number of observers when CompoundObservation is needed.

This step allows for code to special case when there is a conversion function, but only one dependent path.

R=arv
BUG=

Review URL: https://codereview.appspot.com/13208043

src/observe.js

@@ -587,25 +587,28 @@
     }
   };
 
-  function PathObserver(object, pathString, callback, target, token) {
-    this.value = undefined;
+  function PathObserver(object, pathString, callback, target, token, valueFn) {
+    this.valueFn = valueFn;
 
     var path = getPath(pathString);
     if (!path) {
+      // Invalid path.
       this.closed = true;
-      this.value = undefined;
+      this.value = this.valueFn ? this.valueFn() : undefined;
       return;
     }
 
     if (!path.length) {
+      // 0-length path.
       this.closed = true;
-      this.value = object;
+      this.value = this.valueFn ? this.valueFn(object) : object;
       return;
     }
 
     if (!isObject(object)) {
+      // non-object & non-0-length path.
       this.closed = true;
-      this.value = undefined;
+      this.value = this.valueFn ? this.valueFn() : undefined;
       return;
     }
 
@@ -631,7 +634,10 @@
     },
 
     check: function() {
-      this.value = this.path.getValueFrom(this.object, this.observedSet);
+      // Note: Extracting this to a member function for use here and below
+      // regresses dirty-checking path perf by about 25% =-(.
+      var newValue = this.path.getValueFrom(this.object, this.observedSet)
+      this.value = this.valueFn ? this.valueFn(newValue) : newValue;
 
       if (areSameValue(this.value, this.oldValue))
         return false;
@@ -641,8 +647,10 @@
     },
 
     sync: function(hard) {
-      if (hard)
-        this.value = this.path.getValueFrom(this.object, this.observedSet);
+      if (hard) {
+        var newValue = this.path.getValueFrom(this.object, this.observedSet)
+        this.value = this.valueFn ? this.valueFn(newValue) : newValue;
+      }
 
       this.oldValue = this.value;
     }

tests/test.js

@@ -345,6 +345,28 @@ suite('PathObserver Tests', function() {
     observer.close();
   });
 
+  test('valueFn', function() {
+    var model = { };
+
+    function valueFn(value) {
+      return isNaN(value) ? value : value * 3;
+    }
+
+    observer = new PathObserver(model, 'foo', callback, undefined, undefined,
+                                valueFn);
+
+    model.foo = 1;
+    assertPathChanges(3, undefined);
+
+    model.foo = 2;
+    assertPathChanges(6, 3);
+
+    delete model.foo;
+    assertPathChanges(undefined, 6);
+
+    observer.close();
+  });
+
   test('Path With Indices', function() {
     var model = [];