<itemvalue="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"/>
<itemvalue="trivia"/>
<itemvalue="The build context in which the widget with this key builds. The current context is null if there is no widget in the tree that matches this global key."/>
