// 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/foundation.dart'; 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; @override final Size preferredSize; @override Widget build(BuildContext context) => child; }