appBar

AppBar is a top app bar that follows Material Design guidelines. In Flutter, it is a widget commonly supplied to the appBar property of a Scaffold. The AppBar provides a consistent location for a page title, navigation controls, and action icons, helping users identify the current screen and access common tasks.

Composition and behavior: A typical AppBar includes a leading widget for navigation (such as a back button or drawer button), a title, and an actions list containing interactive icons. It can also host a flexibleSpace for a custom background and a bottom area for components such as a tab bar. The title can be centered or left-aligned depending on platform conventions and the centerTitle setting.

Key properties: title defines the primary label, leading sets the widget at the start, and actions holds trailing interactive widgets. Styling options include backgroundColor and elevation. The AppBar offers centerTitle, automaticallyImplyLeading to control automatic back button behavior, toolbarHeight and leadingWidth for sizing, and shape for edge customization. It can attach a bottom widget through bottom and use flexibleSpace for advanced backgrounds. System visual aspects can be controlled with systemOverlayStyle or related settings to adjust status bar icon color.

Implementation and variants: AppBar implements the PreferredSizeWidget interface, which defines its preferred height. It is typically used within a Scaffold, and can be combined with SliverAppBar in scrollable layouts to achieve parallax or collapsing effects. SliverAppBar provides additional behaviors for scrolling content, while AppBar itself remains a fixed top bar in standard layouts.

See also: Flutter material library and Material Design guidelines.