fix(ngRepeat): correctly track elements even when the collection is initially undefined

Previously if the collection model was set to undefined on the first digest,
the repeater would get confused and not use the correct tracking function
for associating model with dom elements in the repeater.

Closes #4145
Closes #3964

Author: jankuca

src/ng/directive/ngRepeat.js

@@ -218,7 +218,8 @@ var ngRepeatDirective = ['$parse', '$animate', function($parse, $animate) {
       return function($scope, $element, $attr){
         var expression = $attr.ngRepeat;
         var match = expression.match(/^\s*(.+)\s+in\s+(.*?)\s*(\s+track\s+by\s+(.+)\s*)?$/),
-          trackByExp, trackByExpGetter, trackByIdFn, trackByIdArrayFn, trackByIdObjFn, lhs, rhs, valueIdentifier, keyIdentifier,
+          trackByExp, trackByExpGetter, trackByIdExpFn, trackByIdArrayFn, trackByIdObjFn,
+          lhs, rhs, valueIdentifier, keyIdentifier,
           hashFnLocals = {$id: hashKey};
 
         if (!match) {
@@ -232,7 +233,7 @@ var ngRepeatDirective = ['$parse', '$animate', function($parse, $animate) {
 
         if (trackByExp) {
           trackByExpGetter = $parse(trackByExp);
-          trackByIdFn = function(key, value, index) {
+          trackByIdExpFn = function(key, value, index) {
             // assign key, value, and $index to the locals so that they can be used in hash functions
             if (keyIdentifier) hashFnLocals[keyIdentifier] = key;
             hashFnLocals[valueIdentifier] = value;
@@ -275,16 +276,17 @@ var ngRepeatDirective = ['$parse', '$animate', function($parse, $animate) {
               childScope,
               key, value, // key/value of iteration
               trackById,
+              trackByIdFn,
               collectionKeys,
               block,       // last object information {scope, element, id}
               nextBlockOrder = [];
 
 
           if (isArrayLike(collection)) {
             collectionKeys = collection;
-            trackByIdFn = trackByIdFn || trackByIdArrayFn;
+            trackByIdFn = trackByIdExpFn || trackByIdArrayFn;
           } else {
-            trackByIdFn = trackByIdFn || trackByIdObjFn;
+            trackByIdFn = trackByIdExpFn || trackByIdObjFn;
             // if object, extract keys, sort them and use to determine order of iteration over obj props
             collectionKeys = [];
             for (key in collection) {

test/ng/directive/ngRepeatSpec.js

@@ -841,6 +841,26 @@ describe('ngRepeat', function() {
       expect(newLis[1]).toEqual(lis[0]);
       expect(newLis[2]).toEqual(lis[1]);
     });
+
+    it('should be stable even if the collection is initially undefined', function () {
+      scope.items = undefined;
+      scope.$digest();
+
+      scope.items = [
+        { name: 'A' },
+        { name: 'B' },
+        { name: 'C' }
+      ];
+      scope.$digest();
+
+      lis = element.find('li');
+      scope.items.shift();
+      scope.$digest();
+
+      var newLis = element.find('li');
+      expect(newLis.length).toBe(2);
+      expect(newLis[0]).toBe(lis[1]);
+    });
   });
 
   it('should grow multi-node repeater', inject(function($compile, $rootScope) {
@@ -861,8 +881,6 @@ describe('ngRepeat', function() {
     $rootScope.$digest();
     expect(element.text()).toEqual('T1:D1;T2:D2;T3:D3;');
   }));
-
-
 });
 
 describe('ngRepeat animations', function() {