Updated DropdownButton docs (#27042)

packages/flutter/lib/src/material/dropdown.dart

@@ -476,11 +476,55 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 /// 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 values the dropdown menu represents. All the
-/// entries in a given menu must represent values with consistent types.
+/// 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 snippet --template=stateful_widget}
+///
+/// This sample shows a `DropdownButton` whose value is one of
+/// "One", "Two", "Free", or "Four".
+///
+/// ```dart preamble
+/// String dropdownValue = 'One';
+/// ```
+///
+/// ```dart
+/// Widget build(BuildContext context) {
+///   return Scaffold(
+///     body: Center(
+///       child: DropdownButton<String>(
+///         value: dropdownValue,
+///         onChanged: (String newValue) {
+///           setState(() {
+///             dropdownValue = newValue;
+///           });
+///         },
+///         items: <String>['One', 'Two', 'Free', 'Four']
+///           .map<DropdownMenuItem<String>>((String value) {
+///             return DropdownMenuItem<String>(
+///               value: value,
+///               child: Text(value),
+///             );
+///           })
+///           .toList(),
+///       ),
+///     ),
+///   );
+/// }
+/// ```
+/// {@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.
+///
 /// Requires one of its ancestors to be a [Material] widget.
 ///
 /// See also:
@@ -493,12 +537,14 @@ class DropdownButtonHideUnderline extends InheritedWidget {
 class DropdownButton<T> extends StatefulWidget {
   /// Creates a dropdown button.
   ///
-  /// The [items] must have distinct values and if [value] isn't null it must be among them.
-  /// If [items] or [onChanged] is null, the button will be disabled, the down arrow will be grayed out, and
-  /// the [disabledHint] will be shown (if provided).
+  /// The [items] must have distinct values. If [value] isn't null then it
+  /// must be equal to one of the [DropDownMenuItem] values. If [items] or
+  /// [onChanged] is null, the button will be disabled, the down arrow
+  /// will be greyed out, and the [disabledHint] will be shown (if provided).
   ///
   /// The [elevation] and [iconSize] arguments must not be null (they both have
-  /// defaults, so do not need to be specified).
+  /// defaults, so do not need to be specified). The boolean [isDense] and
+  /// [isExpanded] arguments must not be null.
   DropdownButton({
     Key key,
     @required this.items,
@@ -512,14 +558,23 @@ class DropdownButton<T> extends StatefulWidget {
     this.isDense = false,
     this.isExpanded = false,
   }) : assert(items == null || value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1),
-      super(key: key);
+       assert(elevation != null),
+       assert(iconSize != null),
+       assert(isDense != null),
+       assert(isExpanded != null),
+       super(key: key);
 
-  /// The list of possible items to select among.
+  /// The list of items the user can select.
+  ///
+  /// 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.
   final List<DropdownMenuItem<T>> items;
 
-  /// The currently selected item, or null if no item has been selected. If
-  /// value is null then the menu is popped up as if the first item was
-  /// selected.
+  /// The value of the currently selected [DropdownMenuItem], or null if no
+  /// item has been selected. If `value` is null then the menu is popped up as
+  /// if the first item were selected.
   final T value;
 
   /// Displayed if [value] is null.
@@ -531,11 +586,17 @@ class DropdownButton<T> extends StatefulWidget {
   final Widget disabledHint;
 
   /// Called when the user selects an item.
+  ///
+  /// 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.
   final ValueChanged<T> onChanged;
 
   /// The z-coordinate at which to place the menu when open.
   ///
-  /// The following elevations have defined shadows: 1, 2, 3, 4, 6, 8, 9, 12, 16, 24
+  /// The following elevations have defined shadows: 1, 2, 3, 4, 6, 8, 9, 12,
+  /// 16, and 24. See [kElevationToShadow].
   ///
   /// Defaults to 8, the appropriate elevation for dropdown buttons.
   final int elevation;