leading property

A widget to display before the title.

If this is null and automaticallyImplyLeading is set to true, the AppBar will imply an appropriate widget. For example, if the AppBar is in a Scaffold that also has a Drawer, the Scaffold will fill this widget with an IconButton that opens the drawer (using Icons.menu). If there's no Drawer and the parent Navigator can go back, the AppBar will use a BackButton that calls Navigator.maybePop.

AppBar(
  leading: Builder(
    builder: (BuildContext context) {
      return IconButton(
        icon: const Icon(Icons.menu),
        onPressed: () { Scaffold.of(context).openDrawer(); },
        tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
      );
    },
  ),
)

The Builder is used in this example to ensure that the context refers to that part of the subtree. That way this code snippet can be used even inside the very code that is creating the Scaffold (in which case, without the Builder, the context wouldn't be able to see the Scaffold, since it would refer to an ancestor of that widget).

See also:

• Scaffold.appBar, in which an AppBar is usually placed.
• Scaffold.drawer, in which the Drawer is usually placed.