feat($compile): expose transclusion $$slots on the $transclude function

Unfilled optional slots will exist as properties of `$$slots` but have a value of `null`.

Closes angular#13426

Author: petebacondarwin

src/ng/compile.js

@@ -337,14 +337,15 @@
  * The contents are compiled and provided to the directive as a **transclusion function**. See the
  * {@link $compile#transclusion Transclusion} section below.
  *
- * There are two kinds of transclusion depending upon whether you want to transclude just the contents of the
- * directive's element or the entire element:
+ * There are three kinds of transclusion depending upon whether you want to transclude just the contents of the
+ * directive's element, the entire element or parts of the element:
  *
  * * `true` - transclude the content (i.e. the child nodes) of the directive's element.
  * * `'element'` - transclude the whole of the directive's element including any directives on this
  *   element that defined at a lower priority than this directive. When used, the `template`
  *   property is ignored.
- *
+ * * **`{...}` (an object hash):** - map elements of the content onto transclusion "slots" in the template.
+ *   See {@link ngTransclude} for more information.
  *
  * #### `compile`
  *
@@ -1511,7 +1512,11 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
       // so that they are available inside the `controllersBoundTransclude` function
       var boundSlots = boundTranscludeFn.$$slots = createMap();
       for (var slotName in transcludeFn.$$slots) {
-        boundSlots[slotName] = createBoundTranscludeFn(scope, transcludeFn.$$slots[slotName], previousBoundTranscludeFn);
+        if (transcludeFn.$$slots[slotName]) {
+          boundSlots[slotName] = createBoundTranscludeFn(scope, transcludeFn.$$slots[slotName], previousBoundTranscludeFn);
+        } else {
+          boundSlots[slotName] = null;
+        }
       }
 
       return boundTranscludeFn;
@@ -1870,7 +1875,10 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
                 var optional = (slotName.charAt(0) === '?');
                 slotName = optional ? slotName.substring(1) : slotName;
                 slotNames[key] = slotName;
-                slots[slotName] = [];
+                // We explicitly assign `null` since this implies that a slot was defined but not filled.
+                // Later when calling boundTransclusion functions with a slot name we only error if the
+                // slot is `undefined`
+                slots[slotName] = null;
                 // filledSlots contains `true` for all slots that are either optional or have been
                 // filled. This is used to check that we have not missed any required slots
                 filledSlots[slotName] = optional;
@@ -1881,6 +1889,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
                 var slotName = slotNames[directiveNormalize(nodeName_(node))];
                 if (slotName) {
                   filledSlots[slotName] = true;
+                  slots[slotName] = slots[slotName] || [];
                   slots[slotName].push(node);
                 } else {
                   $template.push(node);
@@ -1894,9 +1903,12 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
                 }
               });
 
-              forEach(Object.keys(slots), function(slotName) {
-                slots[slotName] = compilationGenerator(mightHaveMultipleTransclusionError, slots[slotName], transcludeFn);
-              });
+              for(var slotName in slots) {
+                if (slots[slotName]) {
+                  // Only define a transclusion function if the slot was filled
+                  slots[slotName] = compilationGenerator(mightHaveMultipleTransclusionError, slots[slotName], transcludeFn);
+                }
+              }
             }
 
             $compileNode.empty(); // clear contents
@@ -2125,6 +2137,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
           // is later passed as `parentBoundTranscludeFn` to `publicLinkFn`
           transcludeFn = controllersBoundTransclude;
           transcludeFn.$$boundTransclude = boundTranscludeFn;
+          transcludeFn.$$slots = boundTranscludeFn.$$slots;
         }
 
         if (controllerDirectives) {
@@ -2222,13 +2235,15 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
           }
           if (slotName) {
             var slotTranscludeFn = boundTranscludeFn.$$slots[slotName];
-            if (!slotTranscludeFn) {
+            if (slotTranscludeFn) {
+              return slotTranscludeFn(scope, cloneAttachFn, transcludeControllers, futureParentElement, scopeToChild);
+            }
+            if (isUndefined(slotTranscludeFn)) {
               throw $compileMinErr('noslot',
                'No parent directive that requires a transclusion with slot name "{0}". ' +
                'Element: {1}',
                slotName, startingTag($element));
             }
-            return slotTranscludeFn(scope, cloneAttachFn, transcludeControllers, futureParentElement, scopeToChild);
           }
           return boundTranscludeFn(scope, cloneAttachFn, transcludeControllers, futureParentElement, scopeToChild);
         }

test/ng/compileSpec.js

@@ -7949,7 +7949,7 @@ describe('$compile', function() {
           '</minion-component>')($rootScope);
         $rootScope.$apply();
 
-        var minionTranscludeFn = capturedTranscludeFn.$$boundTransclude.$$slots['minionSlot'];
+        var minionTranscludeFn = capturedTranscludeFn.$$slots['minionSlot'];
         var minions = minionTranscludeFn();
         expect(minions[0].outerHTML).toEqual('<minion class="ng-scope">stuart</minion>');
         expect(minions[1].outerHTML).toEqual('<minion class="ng-scope">bob</minion>');
@@ -7959,7 +7959,7 @@ describe('$compile', function() {
         var minionScope = jqLite(minions[0]).scope();
         expect(minionScope.$parent).toBe(scope);
 
-        var bossTranscludeFn = capturedTranscludeFn.$$boundTransclude.$$slots['bossSlot'];
+        var bossTranscludeFn = capturedTranscludeFn.$$slots['bossSlot'];
         var boss = bossTranscludeFn();
         expect(boss[0].outerHTML).toEqual('<boss class="ng-scope">gru</boss>');
 
@@ -7972,6 +7972,34 @@ describe('$compile', function() {
         dealoc(minions);
       });
     });
+
+    it('should set unfilled optional transclude slots to `null` in the $transclude.$$slots property', function() {
+      var capturedTranscludeFn;
+      module(function() {
+        directive('minionComponent', function() {
+          return {
+            restrict: 'E',
+            scope: {},
+            transclude: {
+              minion: 'minionSlot',
+              boss: '?bossSlot'
+            },
+            link: function(s, e, a, c, transcludeFn) {
+              capturedTranscludeFn = transcludeFn;
+            }
+          };
+        });
+      });
+      inject(function($rootScope, $compile) {
+        element = $compile(
+          '<minion-component>' +
+            '<minion>stuart</minion>' +
+            '<span>dorothy</span>' +
+          '</minion-component>')($rootScope);
+        expect(capturedTranscludeFn.$$slots.minionSlot).toEqual(jasmine.any(Function));
+        expect(capturedTranscludeFn.$$slots.bossSlot).toBe(null);
+      });
+    });
   });