Flutter Widget - DropDownButton (Flutter Series)
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.
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.