The core RenderSliver protocol. (#7370)

This implements a new RenderViewport2 class to replace the existing
RenderViewport class.

packages/flutter/lib/rendering.dart

@@ -44,6 +44,7 @@ export 'src/rendering/proxy_box.dart';
 export 'src/rendering/rotated_box.dart';
 export 'src/rendering/semantics.dart';
 export 'src/rendering/shifted_box.dart';
+export 'src/rendering/sliver.dart';
 export 'src/rendering/stack.dart';
 export 'src/rendering/table.dart';
 export 'src/rendering/tweens.dart';

packages/flutter/test/rendering/rendering_tester.dart

@@ -46,6 +46,13 @@ TestRenderingFlutterBinding get renderer {
 
 /// Place the box in the render tree, at the given size and with the given
 /// alignment on the screen.
+///
+/// If you've updated `box` and want to lay it out again, use [pumpFrame].
+///
+/// Once a particular [RenderBox] has been passed to [layout], it cannot easily
+/// be put in a different place in the tree or passed to [layout] again, because
+/// [layout] places the given object into another [RenderBox] which you would
+/// need to unparent it from (but that box isn't itself made available).
 void layout(RenderBox box, {
   BoxConstraints constraints,
   FractionalOffset alignment: FractionalOffset.center,

packages/flutter/test/rendering/slivers_layout_test.dart

+// 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 'package:flutter/rendering.dart';
+import 'package:test/test.dart';
+
+import 'rendering_tester.dart';
+
+int layouts = 0;
+
+class RenderLayoutWatcher extends RenderProxyBox {
+  RenderLayoutWatcher(RenderBox child) : super(child);
+  @override
+  void performLayout() {
+    layouts += 1;
+    super.performLayout();
+  }
+}
+
+void main() {
+  test('RenderViewport2 basic test - impact of layout', () {
+    RenderSliverToBoxAdapter sliver;
+    RenderViewport2 viewport;
+    RenderBox box;
+    RenderObject root = new RenderLayoutWatcher(
+      viewport = new RenderViewport2(
+        children: <RenderSliver>[
+          sliver = new RenderSliverToBoxAdapter(child: box = new RenderSizedBox(const Size(100.0, 400.0))),
+        ],
+      ),
+    );
+    expect(layouts, 0);
+    layout(root);
+    expect(layouts, 1);
+    expect(box.localToGlobal(box.size.center(Point.origin)), const Point(400.0, 200.0));
+
+    sliver.child = box = new RenderSizedBox(const Size(100.0, 300.0));
+    expect(layouts, 1);
+    pumpFrame();
+    expect(layouts, 1);
+    expect(box.localToGlobal(box.size.center(Point.origin)), const Point(400.0, 150.0));
+
+    viewport.offset = new ViewportOffset.fixed(20.0);
+    expect(layouts, 1);
+    pumpFrame();
+    expect(layouts, 1);
+    expect(box.localToGlobal(box.size.center(Point.origin)), const Point(400.0, 130.0));
+
+    viewport.offset = new ViewportOffset.fixed(-20.0);
+    expect(layouts, 1);
+    pumpFrame();
+    expect(layouts, 1);
+    expect(box.localToGlobal(box.size.center(Point.origin)), const Point(400.0, 170.0));
+
+    viewport.anchor = 20.0 / 600.0;
+    expect(layouts, 1);
+    pumpFrame();
+    expect(layouts, 1);
+    expect(box.localToGlobal(box.size.center(Point.origin)), const Point(400.0, 190.0));
+
+    viewport.axisDirection = AxisDirection.up;
+    expect(layouts, 1);
+    pumpFrame();
+    expect(layouts, 1);
+    expect(box.localToGlobal(box.size.center(Point.origin)), const Point(400.0, 600.0 - 190.0));
+  });
+
+}