fix(utils): add the ability to prevent the default action of onEvent (onclick, onpaste,etc..) by returning false.

Closes #236

Author: poxrud

Diff for: lib/browser/utils.ts

@@ -61,8 +61,16 @@ export function patchProperty(obj, prop) {
     }
 
     if (typeof fn === 'function') {
-      this[_prop] = fn;
-      this.addEventListener(eventName, fn, false);
+      var wrapFn = (event) => {
+        var result;
+        result = fn.apply(this, arguments);
+
+        if (result != undefined && !result)
+          event.preventDefault();
+      };
+
+      this[_prop] = wrapFn;
+      this.addEventListener(eventName, wrapFn, false);
     } else {
       this[_prop] = null;
     }

Diff for: test/browser/element.spec.ts

@@ -266,4 +266,33 @@ describe('element', function () {
     });
   });
 
+  describe('onEvent default behavior', function() {
+    var checkbox;
+    beforeEach(function () {
+      checkbox = document.createElement('input');
+      checkbox.type = "checkbox";
+      document.body.appendChild(checkbox);
+    });
+
+    afterEach(function () {
+      document.body.removeChild(checkbox);
+    });
+    
+    it('should be possible to prevent default behavior by returning false', function() {
+      checkbox.onclick = function() {
+        return false;
+      };
+
+      checkbox.click();
+      expect(checkbox.checked).toBe(false);
+    });
+
+    it('should have no effect on default behavior when not returning anything', function() {
+      checkbox.onclick = function() {};
+
+      checkbox.click();
+      expect(checkbox.checked).toBe(true);
+    });
+  });
+
 });