fix(scope): improve error msg on unstable model

Author: mhevery

lib/change_detection/watch_group.dart

@@ -8,7 +8,7 @@ part 'ast.dart';
 part 'prototype_map.dart';
 
 typedef ReactionFn(value, previousValue);
-typedef ChangeLog(expression);
+typedef ChangeLog(String expression, current, previous);
 
 /**
  * Extend this class if you wish to pretend to be a function, but you don't know
@@ -341,7 +341,9 @@ class RootWatchGroup extends WatchGroup {
         (_changeDetector as ChangeDetector<_Handler>)
             .collectChanges(exceptionHandler);
     while (changeRecord != null) {
-      if (changeLog != null) changeLog(changeRecord.handler.expression);
+      if (changeLog != null) changeLog(changeRecord.handler.expression,
+                                       changeRecord.currentValue,
+                                       changeRecord.previousValue);
       changeRecord.handler.onChange(changeRecord);
       changeRecord = changeRecord.nextChange;
     }
@@ -353,7 +355,9 @@ class RootWatchGroup extends WatchGroup {
       try {
         var change = evalRecord.check();
         if (change != null && changeLog != null) {
-          changeLog(evalRecord.handler.expression);
+          changeLog(evalRecord.handler.expression,
+                    evalRecord.currentValue,
+                    evalRecord.previousValue);
         }
       } catch (e, s) {
         if (exceptionHandler == null) rethrow; else exceptionHandler(e, s);

lib/core/scope.dart

@@ -352,7 +352,7 @@ class RootScope extends Scope {
           if (changeLog == null) {
             log = [];
             digestLog = [];
-            changeLog = (value) => digestLog.add(value);
+            changeLog = (e, c, p) => digestLog.add('$e: $c <= $p');
           } else {
             log.add(digestLog.join(', '));
             digestLog.clear();
@@ -398,8 +398,10 @@ class RootScope extends Scope {
       assert((() {
         var watchLog = [];
         var observeLog = [];
-        (watchGroup as RootWatchGroup).detectChanges(changeLog: watchLog.add);
-        (observeGroup as RootWatchGroup).detectChanges(changeLog: observeLog.add);
+        (watchGroup as RootWatchGroup).detectChanges(
+            changeLog: (s, c, p) => watchLog.add('$s: $c <= $p'));
+        (observeGroup as RootWatchGroup).detectChanges(
+            changeLog: (s, c, p) => watchLog.add('$s: $c <= $p'));
         if (watchLog.isNotEmpty || observeLog.isNotEmpty) {
           throw 'Observer reaction functions should not change model. \n'
                 'These watch changes were detected: ${watchLog.join('; ')}\n'

lib/core_dom/ng_mustache.dart

@@ -19,7 +19,7 @@ class NgTextMustacheDirective {
     List items = interpolation.expressions.map((exp) {
       return parser(exp, filters:filters);
     }).toList();
-    AST ast = new PureFunctionAST(markup, new ArrayFn(), items);
+    AST ast = new PureFunctionAST('[[$markup]]', new ArrayFn(), items);
     scope.observe(ast, interpolation.call);
   }
 
@@ -51,7 +51,7 @@ class NgAttrMustacheDirective {
     List items = interpolation.expressions.map((exp) {
       return parser(exp, filters:filters);
     }).toList();
-    AST ast = new PureFunctionAST(markup, new ArrayFn(), items);
+    AST ast = new PureFunctionAST('[[$markup]]', new ArrayFn(), items);
     /*
       Attribute bindings are tricky. They need to be resolved on digest
       inline with components so that any bindings to component can

test/core/scope_spec.dart

@@ -708,7 +708,7 @@ main() => describe('scope', () {
         retValue = 2;
         expect(rootScope.flush).
           toThrow('Observer reaction functions should not change model. \n'
-                  'These watch changes were detected: logger("watch")\n'
+                  'These watch changes were detected: logger("watch"): 2 <= 1\n'
                   'These observe changes were detected: ');
       }));
     });
@@ -949,9 +949,9 @@ main() => describe('scope', () {
         rootScope.digest();
       }).toThrow('Model did not stabilize in 5 digests. '
                  'Last 3 iterations:\n'
-                      'a, b\n'
-                      'a, b\n'
-                      'a, b');
+                 'a: 2 <= 1, b: 2 <= 1\n'
+                 'a: 3 <= 2, b: 3 <= 2\n'
+                 'a: 4 <= 3, b: 4 <= 3');
     }));