Initial commit of Effen reactive framework experiment for Sky

sky/examples/fn/widgets/checkbox.dart

+part of widgets;
+
+class Checkbox extends ButtonBase {
+
+  bool checked;
+  ValueChanged onChanged;
+
+  static Style _style = new Style('''
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    -webkit-user-select: none;
+    cursor: pointer;
+    width: 30px;
+    height: 30px;'''
+  );
+
+  static Style _containerStyle = new Style('''
+    border: solid 2px;
+    border-color: rgba(90, 90, 90, 0.25);
+    width: 10px;
+    height: 10px;'''
+  );
+
+  static Style _containerHighlightStyle = new Style('''
+    border: solid 2px;
+    border-color: rgba(90, 90, 90, 0.25);
+    width: 10px;
+    height: 10px;
+    border-radius: 10px;
+    background-color: orange;
+    border-color: orange;'''
+  );
+
+  static Style _uncheckedStyle = new Style('''
+    top: 0px;
+    left: 0px;'''
+  );
+
+  static Style _checkedStyle = new Style('''
+    top: 0px;
+    left: 0px;
+    transform: translate(2px, -15px) rotate(45deg);
+    width: 10px;
+    height: 20px;
+    border-style: solid;
+    border-top: none;
+    border-left: none;
+    border-right-width: 2px;
+    border-bottom-width: 2px;
+    border-color: #0f9d58;'''
+  );
+
+  Checkbox({ Object key, this.onChanged, this.checked }) : super(key: key);
+
+  Node render() {
+    return new Container(
+      style: _style,
+      onClick: _handleClick,
+      onPointerDown: _handlePointerDown,
+      onPointerUp: _handlePointerUp,
+      onPointerCancel: _handlePointerCancel,
+      children: [
+        new Container(
+          style: _highlight ? _containerHighlightStyle : _containerStyle,
+          children: [
+            new Container(
+              style: checked ? _checkedStyle : _uncheckedStyle
+            )
+          ]
+        )
+      ]
+    );
+  }
+
+  void _handleClick(sky.Event e) {
+    onChanged(!checked);
+  }
+}