// Copyright 2014 The Flutter 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. /// /// Widgets that need to know the preferred size of their child can require /// that their child implement this interface by using this class rather /// than [Widget] as the type of their `child` property. /// /// Use [PreferredSize] to give a preferred size to an arbitrary widget. // (We ignore `avoid_implementing_value_types` here because the superclass // doesn't really implement `operator ==`, it just overrides it to _prevent_ it // from being implemented, which is the exact opposite of the spirit of the // `avoid_implementing_value_types` lint.) // ignore: avoid_implementing_value_types 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. /// /// Parents like [Scaffold] use [PreferredSizeWidget] to require that their /// children implement that interface. To give a preferred size to an arbitrary /// widget so that it can be used in a `child` property of that type, this /// widget, [PreferredSize], can be used. /// /// Widgets like [AppBar] implement a [PreferredSizeWidget], so that this /// [PreferredSize] widget is not necessary for them. /// /// {@tool dartpad} /// This sample shows a custom widget, similar to an [AppBar], which uses a /// [PreferredSize] widget, with its height set to 80 logical pixels. /// Changing the [PreferredSize] can be used to change the height /// of the custom app bar. /// /// ** See code in examples/api/lib/widgets/preferred_size/preferred_size.0.dart ** /// {@end-tool} /// /// 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 that the parent can query. const PreferredSize({ super.key, required this.child, required this.preferredSize, }); /// The widget below this widget in the tree. /// /// {@macro flutter.widgets.ProxyWidget.child} final Widget child; @override final Size preferredSize; @override Widget build(BuildContext context) => child; }