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// Copyright 2017 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 'basic.dart';
import 'framework.dart';
/// An interface for widgets that can return the size this widget would prefer
/// if it were otherwise unconstrained.
/// There are a few cases, notably [AppBar] and [TabBar], where it would be
/// undesirable for the widget to constrain its own size but where the widget
/// needs to expose a preferred or "default" size. For example a primary
/// [Scaffold] sets its app bar height to the app bar's preferred height
/// plus the height of the system status bar.
/// Use [PreferredSize] to give a preferred size to an arbitrary widget.
abstract class PreferredSizeWidget implements Widget {
/// The size this widget would prefer if it were otherwise unconstrained.
/// In many cases it's only necessary to define one preferred dimension.
/// For example the [Scaffold] only depends on its app bar's preferred
/// height. In that case implementations of this method can just return
/// `new Size.fromHeight(myAppBarHeight)`;
Size get preferredSize;
/// A widget with a preferred size.
/// This widget does not impose any constraints on its child, and it doesn't
/// affect the child's layout in any way. It just advertises a preferred size
/// which can be used by the parent.
/// See also:
/// * [AppBar.bottom] and [Scaffold.appBar], which require preferred size widgets.
/// * [PreferredSizeWidget], the interface which this widget implements to expose
/// its preferred size.
/// * [AppBar] and [TabBar], which implement PreferredSizeWidget.
class PreferredSize extends StatelessWidget implements PreferredSizeWidget {
/// Creates a widget that has a preferred size.
const PreferredSize({
Key key,
@required this.child,
@required this.preferredSize,
}) : super(key: key);
/// The widget below this widget in the tree.
/// {@macro flutter.widgets.child}
final Widget child;
final Size preferredSize;
Widget build(BuildContext context) => child;