Expose bare minimum to add effects dynamically.

src/lib/template/templatizer.html

@@ -282,12 +282,14 @@
     _createHostPropEffector: function(prop) {
       var prefix = this._parentPropPrefix;
       return function(source, value) {
+        if (source !== prop) return;
         this.dataHost._templatized[prefix + prop] = value;
       };
     },
 
     _createInstancePropEffector: function(prop) {
       return function(source, value, old, fromAbove) {
+        if (source !== prop) return;
         if (!fromAbove) {
           this.dataHost._forwardInstanceProp(this, prop, value);
         }

src/standard/effectBuilder.html

@@ -35,6 +35,33 @@
       var prop = Polymer.Bind.addPropertyEffect(this, property, kind, effect);
       // memoize path function for faster lookup.
       prop.pathFn = this['_' + prop.kind + 'PathEffect'];
+      return prop;
+    },
+
+    addCustomEffect: function(property, fn) {
+      // In case this is the first effect of the property, we MUST create the
+      // underlying trigger machinery by hand.
+      var effects = Polymer.Bind.ensurePropertyEffects(this, property);
+      if (effects.length === 0) {
+        // The current value will be masked by the descriptor, read it ...
+        var val = this[property];
+        Polymer.Bind._createAccessors(this, property, effects);
+        // ... and apply on our data store
+        this.__data__[property] = val;
+      }
+      // Theoretically the effects must be sorted again, but `function`-effects
+      // are executed last, so we can skip this for now.
+      return this._addPropertyEffect(property, 'function', fn);
+    },
+
+    removeCustomEffect: function(property, fx) {
+      var effects = this._propertyEffects && this._propertyEffects[property];
+      if (effects) {
+        var index = effects.indexOf(fx);
+        if (index !== -1) {
+          effects.splice(index, 1);
+        }
+      }
     },
 
     // prototyping

src/standard/notify-path.html

@@ -323,6 +323,10 @@
         }
       },
 
+      _functionPathEffect: function(path, value, effect) {
+        Polymer.Bind._functionEffect.call(this, path, value, effect);
+      },
+
       _pathMatchesEffect: function(path, effect) {
         var effectArg = effect.trigger.name;
         return (effectArg == path) ||

test/unit/bind-elements.html

@@ -662,3 +662,7 @@
     });
   </script>
 </dom-module>
+
+<script>
+  Polymer({is: 'x-custom-effect'});
+</script>

test/unit/bind.html

@@ -1000,6 +1000,81 @@
   });
 });
 
+suite('custom user effects', function() {
+
+  test('Add custom effect', function() {
+    var el = document.createElement('x-custom-effect');
+
+    var called = 0;
+    el.addCustomEffect('foo', function(path, value, old) {
+      called += 1;
+      assert.equal(path, 'foo');
+      assert.equal(value, 'bar');
+      assert.equal(old, undefined);
+    });
+
+    el.foo = 'bar';
+    assert.equal(called, 1);
+  });
+
+  test('Remove custom effect', function() {
+    var el = document.createElement('x-custom-effect');
+
+    var called = 0;
+    var fx = el.addCustomEffect('foo', function() {
+      called += 1;
+    });
+
+    el.removeCustomEffect('foo', fx);
+
+    el.foo = 'bar';
+    assert.equal(called, 0);
+  });
+
+  test('Ensure old values are sent', function() {
+    var el = document.createElement('x-custom-effect');
+    el.foo = 'bar';
+
+    var called = 0;
+    el.addCustomEffect('foo', function(path, value, old) {
+      called += 1;
+      assert.equal(old, 'bar');
+    });
+
+    el.foo = 'quux';
+    assert.equal(called, 1);
+  });
+
+  test('Ensure path effects can be seen', function() {
+    var el = document.createElement('x-custom-effect');
+    el.foo = {bar: 'quux'};
+
+    var called = 0;
+    el.addCustomEffect('foo', function(path, value, old) {
+      called += 1;
+      assert.equal(path, 'foo.bar');
+      assert.equal(value, 'quod');
+      assert.equal(old, undefined);  // always undefined for structured paths!
+    });
+
+    el.set('foo.bar', 'quod');
+    assert.equal(called, 1);
+  });
+
+  test('Ensure independence', function() {
+    var el1 = document.createElement('x-custom-effect');
+    var called = 0;
+    el1.addCustomEffect('foo', function() {
+      called += 1;
+    });
+
+    var el2 = document.createElement('x-custom-effect');
+
+    el2.foo = 'bar';
+    assert.equal(called, 0);
+  });
+});
+
 </script>
 
 </body>