DropdownButton2<T> class

A Material Design button for selecting from a list of items.

A dropdown button lets the user select from a number of items. The button shows the currently selected item as well as an arrow that opens a menu for selecting another item.

One ancestor must be a Material widget and typically this is provided by the app's Scaffold.

The type T is the type of the value that each dropdown item represents. All the entries in a given menu must represent values with consistent types. Typically, an enum is used. Each DropdownMenuItem in items must be specialized with that same type argument.

The onChanged callback should update a state variable that defines the dropdown's value. It should also call State.setState to rebuild the dropdown with the new value.

{@tool dartpad} This sample shows a DropdownButton with a large arrow icon, purple text style, and bold purple underline, whose value is one of "One", "Two", "Free", or "Four".

** See code in examples/api/lib/material/dropdown/dropdown_button.0.dart ** {@end-tool}

If the onChanged callback is null or the list of items is null then the dropdown button will be disabled, i.e. its arrow will be displayed in grey and it will not respond to input. A disabled button will display the disabledHint widget if it is non-null. However, if disabledHint is null and hint is non-null, the hint widget will instead be displayed.

Requires one of its ancestors to be a Material widget.

See also:

• DropdownButtonFormField2, which integrates with the Form widget.
• DropdownMenuItem, the class used to represent the items.
• DropdownButtonHideUnderline, which prevents its descendant dropdown buttons from displaying their underlines.
• ElevatedButton, TextButton, ordinary buttons that trigger a single action.
• material.io/design/components/menus.html#dropdown-menu