Avoid accessing observable value twice during setup of CompoundObserver

This was affecting polymer startup. The basic problem is that Observer.open must read the underlying value, and was called during addObserver, only shortly after to have check_ call discardChanges() -- which read the underlying value again.

The solution is to delay opening the Observable until the CompoundObserver is opened

R=arv
BUG=

Review URL: https://codereview.appspot.com/85570043

src/observe.js

@@ -535,8 +535,8 @@
       addToAll(this);
       this.callback_ = callback;
       this.target_ = target;
-      this.state_ = OPENED;
       this.connect_();
+      this.state_ = OPENED;
       return this.value_;
     },
 
@@ -545,11 +545,11 @@
         return;
 
       removeFromAll(this);
-      this.state_ = CLOSED;
       this.disconnect_();
       this.value_ = undefined;
       this.callback_ = undefined;
       this.target_ = undefined;
+      this.state_ = CLOSED;
     },
 
     deliver: function() {
@@ -906,7 +906,6 @@
       if (this.state_ != UNOPENED && this.state_ != RESETTING)
         throw Error('Cannot add observers once started.');
 
-      observer.open(this.deliver, this);
       this.observed_.push(observerSentinel, observer);
     },
 
@@ -939,11 +938,17 @@
     check_: function(changeRecords, skipChanges) {
       var oldValues;
       for (var i = 0; i < this.observed_.length; i += 2) {
-        var pathOrObserver = this.observed_[i+1];
         var object = this.observed_[i];
-        var value = object === observerSentinel ?
-            pathOrObserver.discardChanges() :
-            pathOrObserver.getValueFrom(object)
+        var path = this.observed_[i+1];
+        var value;
+        if (object === observerSentinel) {
+          var observable = path;
+          value = this.state_ === UNOPENED ?
+              observable.open(this.deliver, this) :
+              observable.discardChanges();
+        } else {
+          value = path.getValueFrom(object);
+        }
 
         if (skipChanges) {
           this.value_[i / 2] = value;

tests/test.js

@@ -1174,10 +1174,6 @@ suite('CompoundObserver Tests', function() {
     function setValueFn(value) { return value / 2; }
 
     var compound = new CompoundObserver;
-    assert.throws(function () {
-      compound.addObserver(1);
-    });
-
     compound.addPath(model, 'a');
     compound.addObserver(new ObserverTransform(new PathObserver(model, 'b'),
                                                valueFn, setValueFn));