One Scaffold layout to rule them all.

packages/flutter/lib/src/material/scaffold.dart

@@ -2,6 +2,7 @@
 // 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 'dart:ui' as ui;
 
 import 'package:flutter/rendering.dart';
@@ -11,16 +12,23 @@ import 'constants.dart';
 import 'material.dart';
 import 'tool_bar.dart';
 
+const double _kFloatingActionButtonMargin = 16.0; // TODO(hmuller): should be device dependent
+
 const int _kBodyIndex = 0;
 const int _kToolBarIndex = 1;
+const int _kBottomSheetIndex = 2;
+const int _kSnackBarIndex = 3;
+const int _kFloatingActionButtonIndex = 4;
 
-// This layout has the same effect as putting the toolbar and body in a column
-// and making the body flexible. What's different is that in this case the
-// toolbar appears -after- the body in the stacking order, so the toolbar's
-// shadow is drawn on top of the body.
-class _ToolBarAndBodyLayout extends MultiChildLayoutDelegate {
+class _ScaffoldLayout extends MultiChildLayoutDelegate {
   void performLayout(Size size, BoxConstraints constraints, int childCount) {
-    assert(childCount == 2);
+    assert(childCount == 5);
+
+    // This part of the layout has the same effect as putting the toolbar and
+    // body in a column and making the body flexible. What's different is that
+    // in this case the toolbar appears -after- the body in the stacking order,
+    // so the toolbar's shadow is drawn on top of the body.
+
     final BoxConstraints toolBarConstraints = constraints.loosen().tightenWidth(size.width);
     final Size toolBarSize = layoutChild(_kToolBarIndex, toolBarConstraints);
     final double bodyHeight = size.height - toolBarSize.height;
@@ -28,10 +36,34 @@ class _ToolBarAndBodyLayout extends MultiChildLayoutDelegate {
     layoutChild(_kBodyIndex, bodyConstraints);
     positionChild(_kToolBarIndex, Point.origin);
     positionChild(_kBodyIndex, new Point(0.0, toolBarSize.height));
+
+    // The BottomSheet and the SnackBar are anchored to the bottom of the parent,
+    // they're as wide as the parent and are given their intrinsic height.
+    // If all three elements are present then either the center of the FAB straddles
+    // the top edge of the BottomSheet or the bottom of the FAB is
+    // _kFloatingActionButtonMargin above the SnackBar, whichever puts the FAB
+    // the farthest above the bottom of the parent. If only the FAB is has a
+    // non-zero height then it's inset from the parent's right and bottom edges
+    // by _kFloatingActionButtonMargin.
+
+    final BoxConstraints fullWidthConstraints = constraints.loosen().tightenWidth(size.width);
+    final Size bottomSheetSize = layoutChild(_kBottomSheetIndex, fullWidthConstraints);
+    final Size snackBarSize = layoutChild(_kSnackBarIndex, fullWidthConstraints);
+    final Size fabSize = layoutChild(_kFloatingActionButtonIndex, constraints.loosen());
+    positionChild(_kBottomSheetIndex, new Point(0.0, size.height - bottomSheetSize.height));
+    positionChild(_kSnackBarIndex, new Point(0.0, size.height - snackBarSize.height));
+
+    final double fabX = size.width - fabSize.width - _kFloatingActionButtonMargin;
+    double fabY = size.height - fabSize.height - _kFloatingActionButtonMargin;
+    if (snackBarSize.height > 0.0)
+      fabY = math.min(fabY, size.height - snackBarSize.height - fabSize.height - _kFloatingActionButtonMargin);
+    if (bottomSheetSize.height > 0.0)
+      fabY = math.min(fabY, size.height - bottomSheetSize.height - fabSize.height / 2.0);
+    positionChild(_kFloatingActionButtonIndex, new Point(fabX, fabY));
   }
 }
 
-final _ToolBarAndBodyLayout _toolBarAndBodyLayout = new _ToolBarAndBodyLayout();
+final _ScaffoldLayout _scaffoldLayout = new _ScaffoldLayout();
 
 class Scaffold extends StatelessComponent {
   Scaffold({
@@ -39,53 +71,35 @@ class Scaffold extends StatelessComponent {
     this.body,
     this.toolBar,
     this.snackBar,
-    this.floatingActionButton
+    this.floatingActionButton,
+    this.bottomSheet
   }) : super(key: key);
 
   final Widget body;
   final ToolBar toolBar;
   final Widget snackBar;
   final Widget floatingActionButton;
+  final Widget bottomSheet;
 
   Widget build(BuildContext context) {
-    final ToolBar paddedToolBar = toolBar?.withPadding(new EdgeDims.only(top: ui.window.padding.top));
+    final Widget paddedToolBar = toolBar?.withPadding(new EdgeDims.only(top: ui.window.padding.top));
     final Widget materialBody = body != null ? new Material(child: body) : null;
-    Widget toolBarAndBody;
-    if (paddedToolBar != null && materialBody != null)
-      toolBarAndBody = new CustomMultiChildLayout(<Widget>[materialBody, paddedToolBar],
-        delegate: _toolBarAndBodyLayout
-      );
-    else
-      toolBarAndBody = paddedToolBar ?? materialBody;
-
-    final List<Widget> bottomColumnChildren = <Widget>[];
-
-    if (floatingActionButton != null)
-      bottomColumnChildren.add(new Padding(
-        // TODO(eseidel): These change based on device size!
-        padding: const EdgeDims.only(right: 16.0, bottom: 16.0),
-        child: floatingActionButton
-      ));
-
-    // TODO(jackson): On tablet/desktop, minWidth = 288, maxWidth = 568
+    Widget constrainedSnackBar;
     if (snackBar != null) {
-      bottomColumnChildren.add(new ConstrainedBox(
+      // TODO(jackson): On tablet/desktop, minWidth = 288, maxWidth = 568
+      constrainedSnackBar = new ConstrainedBox(
         constraints: const BoxConstraints(maxHeight: kSnackBarHeight),
         child: snackBar
-      ));
-    }
-
-    final List<Widget> stackChildren = <Widget>[toolBarAndBody];
-
-    if (bottomColumnChildren.length > 0) {
-      stackChildren.add(new Positioned(
-        right: 0.0,
-        left: 0.0,
-        bottom: 0.0,
-        child: new Column(bottomColumnChildren, alignItems: FlexAlignItems.end)
-      ));
+      );
     }
-
-    return new Stack(stackChildren);
+    return new CustomMultiChildLayout(<Widget>[
+      materialBody ?? new Container(height: 0.0),
+      paddedToolBar ?? new Container(height: 0.0),
+      bottomSheet ?? new Container(height: 0.0),
+      constrainedSnackBar ?? new Container(height: 0.0),
+      floatingActionButton ?? new Container(height: 0.0)
+    ],
+      delegate: _scaffoldLayout
+    );
   }
 }

packages/flutter/lib/src/rendering/custom_layout.dart

@@ -11,6 +11,9 @@ abstract class MultiChildLayoutDelegate {
   final List<RenderBox> _indexToChild = <RenderBox>[];
 
   /// Returns the size of this object given the incomming constraints.
+  /// The size cannot reflect the instrinsic sizes of the children.
+  /// If this layout has a fixed width or height the returned size
+  /// can reflect that.
   Size getSize(BoxConstraints constraints) => constraints.biggest;
 
   /// Ask the child to update its layout within the limits specified by