Introduce BlockViewport.

sky/sdk/example/widgets/block_viewport.dart

Index: sky/sdk/example/widgets/block_viewport.dart
diff --git a/sky/sdk/example/widgets/block_viewport.dart b/sky/sdk/example/widgets/block_viewport.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c1e3d07a0e6ba96676da593a1b9225cefb9a7747
--- /dev/null
+++ b/sky/sdk/example/widgets/block_viewport.dart
@@ -0,0 +1,138 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:math' as math;
+
+import 'package:sky/rendering/sky_binding.dart';
+import 'package:sky/widgets/basic.dart';
+import 'package:sky/widgets/block_viewport.dart';
+import 'package:sky/widgets/material.dart';
+import 'package:sky/widgets/raised_button.dart';
+import 'package:sky/widgets/scaffold.dart';
+import 'package:sky/widgets/theme.dart';
+import 'package:sky/widgets/tool_bar.dart';
+import 'package:sky/widgets/widget.dart';
+
+class BlockViewportApp extends App {
+
+  List<double> lengths = <double>[];
+  double offset = 0.0;
+
+  static const double kMaxLength = 100.0;
+
+  static math.Random rand = new math.Random();
+
+  void addBox() {
+    lengths.add(rand.nextDouble() * kMaxLength);
+    updateEnabledState();
+  }
+
+  void removeBox(int index) {
+    lengths.removeAt(index);
+    updateEnabledState();
+  }
+
+  void goUp() {
+    offset -= 9.9;
+    updateEnabledState();
+  }
+
+  void goDown() {
+    offset += 20.45;
+    updateEnabledState();
+  }
+
+  bool enabledDown = true;
+  bool enabledUp = true;
+  bool enabledAdd = true;
+  bool enabledRemove = false;
+  void updateEnabledState() {
+    setState(() {
+      enabledUp = offset > -100.0;
+      enabledDown = offset < lengths.fold(0.0, (double result, double len) => result + len) + 100.0;
+      enabledAdd = true;
+      enabledRemove = lengths.length > 0;
+    });
+  }
+
+  Widget build() {
+    return new Theme(
+      data: new ThemeData.light(),
+      child: new Scaffold(
+        toolbar: new ToolBar(
+            center: new Text('Block Viewport Demo')),
+        body: new Material(
+          type: MaterialType.canvas,
+          child: new Flex([
+              new Container(
+                padding: new EdgeDims.symmetric(horizontal: 8.0, vertical: 25.0),
+                child: new Flex([
+                    new RaisedButton(
+                      enabled: enabledAdd,
+                      child: new Text('ADD'),
+                      onPressed: addBox
+                    ),
+                    new RaisedButton(
+                      enabled: enabledUp,
+                      child: new Text('UP'),
+                      onPressed: goUp
+                    ),
+                    new RaisedButton(
+                      enabled: enabledDown,
+                      child: new Text('DOWN'),
+                      onPressed: goDown
+                    )
+                  ],
+                  justifyContent: FlexJustifyContent.spaceAround
+                )
+              ),
+              new Flexible(
+                child: new Container(
+                  margin: new EdgeDims.all(8.0),
+                  decoration: new BoxDecoration(
+                    border: new Border.all(new BorderSide(color: new Color(0xFF000000)))
+                  ),
+                  padding: new EdgeDims.all(16.0),
+                  child: new BlockViewport(
+                    builder: builder,
+                    startOffset: offset,
+                    token: lengths.length
+                  )
+                )
+              ),
+            ],
+            direction: FlexDirection.vertical,
+            justifyContent: FlexJustifyContent.spaceBetween
+          )
+        )
+      )
+    );
+  }
+
+  Widget builder(int index) {
+    if (index >= lengths.length)
+      return null;
+    return new Listener(
+      key: lengths[index].toString(),
+      child: new Container(
+        decoration: new BoxDecoration(
+          backgroundColor: new Color((0xFF000000 + 0xFFFFFF * lengths[index] / kMaxLength).round())
+        ),
+        height: lengths[index] + 12.0
+      ),
+      onGestureTap: (_) {
+        removeBox(index);
+      }
+    );
+  }
+
+}
+
+void main() {
+  runApp(new BlockViewportApp());
+  SkyBinding.instance.onFrame = () {
+    // uncomment this for debugging:
+    // SkyBinding.instance.debugDumpRenderTree();
+  };
+}