feat(parse): ability to bind once an expressions

docs/content/guide/expression.ngdoc

@@ -197,3 +197,98 @@ expose a `$event` object within the scope of that expression.
 
 Note in the example above how we can pass in `$event` to `clickMe`, but how it does not show up
 in `{{$event}}`. This is because `$event` is outside the scope of that binding.
+
+
+## One-time binding
+
+An expression that starts with `::` is considered a one-time expression. One-time expressions
+will take the first non-undefined value assigned to them within a $digest
+
+<example module="oneTimeBidingExampleApp">
+  <file name="index.html">
+    <div ng-controller="EventController">
+      <button ng-click="clickMe($event)">Click Me</button>
+      <p>One time binding {{::name}}</p>
+      <p>Normal binding {{name}}</p>
+    </div>
+  </file>
+  <file name="script.js">
+    angular.module('oneTimeBidingExampleApp', []).
+      controller('EventController', ['$scope', function($scope) {
+        var counter = 0;
+        var names = ['Igor', 'Misko', 'Chirayu', 'Lucas'];
+        /*
+         * expose the event object to the scope
+         */
+        $scope.clickMe = function(clickEvent) {
+          $scope.name = names[counter % names.length];
+          counter++;
+        };
+      }]);
+  </file>
+</example>
+
+
+### Why this feature
+
+The main purpose of one-time binding expression is to provide a way to create a binding
+that gets deregistered and frees up resources once the binding is initialized.
+Reducing the number of expressions being watched makes the digest
+loop faster and allows more information to be displayed at the same time.
+
+
+### Value stabilization algorithm
+
+One-time binding expressions will retain the value of the expression at the end of the
+digest cycle as long as that value is not undefined. If the value of the expression is set
+within the digest loop and later, within the same digest loop, it is set to undefined,
+then the expression is not fulfilled and will remain watched. 
+
+  1. Given an expression that starts with `::` when a digest loop is entered and expression
+     is dirty-checked store the value as V
+  2. If V is not undefined mark the result of the expression as stable and schedule a task
+     to deregister the watch for this expression when we exit the digest loop
+  3. Process the digest loop as normal
+  4. When digest loop is done and all the values have settled process the queue of watch
+     deregistration tasks. For each watch to be deregistered check if it still evaluates
+     to value that is not `undefined`. If that's the case, deregister the watch. Otherwise
+     keep dirty-checking the watch in the future digest loops by following the same
+     algorithm starting from step 1
+
+
+### How to benefit from one-time binding
+
+When interpolating text or attributes. If the expression, once set, will not change
+then it is a candidate for one-time expression.
+
+```html
+  <div name="attr: {{::color}}">text: {{::name}}</div>
+```
+
+When using a directive with bidirectional binding and the parameters will not change
+
+```js
+someModule.directive('someDirective', function() {
+  return {
+    scope: {
+      name: '=',
+      color: '@'
+    },
+    template: '{{name}}: {{color}}'
+  };
+});
+```
+
+```html
+  <div some-directive name=“::myName” color=“My color is {{::myColor}}”></div>
+```
+
+
+When using a directive that takes an expression
+
+```html
+<ul>
+  <li ng-repeat="item in ::items">{{item.name}};</li>
+</ul>
+```
+

src/ng/compile.js

@@ -1490,6 +1490,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
                       parentSet(scope, parentValue = isolateScope[scopeName]);
                     }
                   }
+                  parentValueWatch.$$unwatch = parentGet.$$unwatch;
                   return lastValue = parentValue;
                 }, null, parentGet.literal);
                 break;
@@ -1818,6 +1819,8 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
           compile: valueFn(function textInterpolateLinkFn(scope, node) {
             var parent = node.parent(),
                 bindings = parent.data('$binding') || [];
+            // Need to interpolate again in case this is using one-time bindings
+            interpolateFn = $interpolate(text);
             bindings.push(interpolateFn);
             safeAddClass(parent.data('$binding', bindings), 'ng-binding');
             scope.$watch(interpolateFn, function interpolateFnWatchAction(value) {

src/ng/directive/ngBind.js

@@ -174,7 +174,11 @@ var ngBindHtmlDirective = ['$sce', '$parse', function($sce, $parse) {
     element.addClass('ng-binding').data('$binding', attr.ngBindHtml);
 
     var parsed = $parse(attr.ngBindHtml);
-    function getStringValue() { return (parsed(scope) || '').toString(); }
+    function getStringValue() {
+      var value = parsed(scope);
+      getStringValue.$$unwatch = parsed.$$unwatch;
+      return (value || '').toString();
+    }
 
     scope.$watch(getStringValue, function ngBindHtmlWatchAction(value) {
       element.html($sce.getTrustedHtml(parsed(scope)) || '');

src/ng/interpolate.js

@@ -252,16 +252,19 @@ function $InterpolateProvider() {
 
 
             try {
+              interpolationFn.$$unwatch = true;
               for (; i < ii; i++) {
                 val = getValue(parseFns[i](context));
                 if (allOrNothing && isUndefined(val)) {
+                  interpolationFn.$$unwatch = undefined;
                   return;
                 }
                 val = stringify(val);
                 if (val !== lastValues[i]) {
                   inputsChanged = true;
                 }
                 values[i] = val;
+                interpolationFn.$$unwatch = interpolationFn.$$unwatch && parseFns[i].$$unwatch;
               }
 
               if (inputsChanged) {

src/ng/parse.js

@@ -1246,13 +1246,19 @@ function $ParseProvider() {
     };
 
     return function(exp) {
-      var parsedExpression;
+      var parsedExpression,
+          oneTime;
 
       switch (typeof exp) {
         case 'string':
 
+          if (exp.charAt(0) === ':' && exp.charAt(1) === ':') {
+            oneTime = true;
+            exp = exp.substring(2);
+          }
+
           if (cache.hasOwnProperty(exp)) {
-            return cache[exp];
+            return oneTime ? oneTimeWrapper(cache[exp]) : cache[exp];
           }
 
           var lexer = new Lexer($parseOptions);
@@ -1265,14 +1271,43 @@ function $ParseProvider() {
             cache[exp] = parsedExpression;
           }
 
-          return parsedExpression;
+          if (parsedExpression.constant) {
+            parsedExpression.$$unwatch = true;
+          }
+
+          return oneTime ? oneTimeWrapper(parsedExpression) : parsedExpression;
 
         case 'function':
           return exp;
 
         default:
           return noop;
       }
+
+      function oneTimeWrapper(expression) {
+        var stable = false,
+            lastValue;
+        oneTimeParseFn.literal = expression.literal;
+        oneTimeParseFn.constant = expression.constant;
+        oneTimeParseFn.assign = expression.assign;
+        return oneTimeParseFn;
+
+        function oneTimeParseFn(self, locals) {
+          if (!stable) {
+            lastValue = expression(self, locals);
+            oneTimeParseFn.$$unwatch = isDefined(lastValue);
+            if (oneTimeParseFn.$$unwatch && self && self.$$postDigestQueue) {
+              self.$$postDigestQueue.push(function () {
+                // create a copy if the value is defined and it is not a $sce value
+                if ((stable = isDefined(lastValue)) && !lastValue.$$unwrapTrustedValue) {
+                  lastValue = copy(lastValue);
+                }
+              });
+            }
+          }
+          return lastValue;
+        }
+      }
     };
   }];
 }

src/ng/rootScope.js

@@ -338,14 +338,6 @@ function $RootScopeProvider(){
           watcher.fn = function(newVal, oldVal, scope) {listenFn(scope);};
         }
 
-        if (typeof watchExp == 'string' && get.constant) {
-          var originalFn = watcher.fn;
-          watcher.fn = function(newVal, oldVal, scope) {
-            originalFn.call(this, newVal, oldVal, scope);
-            arrayRemove(array, watcher);
-          };
-        }
-
         if (!array) {
           array = scope.$$watchers = [];
         }
@@ -391,24 +383,37 @@ function $RootScopeProvider(){
         var deregisterFns = [];
         var changeCount = 0;
         var self = this;
+        var unwatchFlags = new Array(watchExpressions.length);
+        var unwatchCount = watchExpressions.length;
 
         forEach(watchExpressions, function (expr, i) {
-          deregisterFns.push(self.$watch(expr, function (value, oldValue) {
+          var exprFn = $parse(expr);
+          deregisterFns.push(self.$watch(exprFn, function (value, oldValue) {
             newValues[i] = value;
             oldValues[i] = oldValue;
             changeCount++;
+            if (unwatchFlags[i] && !exprFn.$$unwatch) unwatchCount++;
+            if (!unwatchFlags[i] && exprFn.$$unwatch) unwatchCount--;
+            unwatchFlags[i] = exprFn.$$unwatch;
           }));
         }, this);
 
-        deregisterFns.push(self.$watch(function () {return changeCount;}, function () {
+        deregisterFns.push(self.$watch(watchGroupFn, function () {
           listener(newValues, oldValues, self);
+          if (unwatchCount === 0) {
+            watchGroupFn.$$unwatch = true;
+          } else {
+            watchGroupFn.$$unwatch = false;
+          }
         }));
 
         return function deregisterWatchGroup() {
           forEach(deregisterFns, function (fn) {
             fn();
           });
         };
+
+        function watchGroupFn() {return changeCount;}
       },
 
 
@@ -553,6 +558,7 @@ function $RootScopeProvider(){
               }
             }
           }
+          $watchCollectionWatch.$$unwatch = objGetter.$$unwatch;
           return changeDetected;
         }
 
@@ -644,6 +650,7 @@ function $RootScopeProvider(){
             dirty, ttl = TTL,
             next, current, target = this,
             watchLog = [],
+            stableWatchesCandidates = [],
             logIdx, logMsg, asyncTask;
 
         beginPhase('$digest');
@@ -694,6 +701,7 @@ function $RootScopeProvider(){
                         logMsg += '; newVal: ' + toJson(value) + '; oldVal: ' + toJson(last);
                         watchLog[logIdx].push(logMsg);
                       }
+                      if (watch.get.$$unwatch) stableWatchesCandidates.push({watch: watch, array: watchers});
                     } else if (watch === lastDirtyWatch) {
                       // If the most recently dirty watcher is now clean, short circuit since the remaining watchers
                       // have already been tested.
@@ -740,6 +748,13 @@ function $RootScopeProvider(){
             $exceptionHandler(e);
           }
         }
+
+        for (length = stableWatchesCandidates.length - 1; length >= 0; --length) {
+          var candidate = stableWatchesCandidates[length];
+          if (candidate.watch.get.$$unwatch) {
+            arrayRemove(candidate.array, candidate.watch);
+          }
+        }
       },
 
 

src/ng/sce.js

@@ -787,7 +787,9 @@ function $SceProvider() {
         return parsed;
       } else {
         return function sceParseAsTrusted(self, locals) {
-          return sce.getTrusted(type, parsed(self, locals));
+          var result = sce.getTrusted(type, parsed(self, locals));
+          sceParseAsTrusted.$$unwatch = parsed.$$unwatch;
+          return result;
         };
       }
     };