Document which widgets require a Material ancestor

Also, fill out other documentation for these widgets.

Fixes #967

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

@@ -35,7 +35,8 @@ class ButtonTheme extends InheritedWidget {
   bool updateShouldNotify(ButtonTheme old) => color != old.color;
 }
 
-/// Base class for buttons in the Material theme.
+/// Base class for material design buttons.
+///
 /// Rather than using this class directly, please use [FlatButton] or [RaisedButton].
 ///
 /// MaterialButtons whose [onPressed] handler is null will be disabled. To have

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

@@ -15,12 +15,19 @@ import 'toggleable.dart';
 /// A material design checkbox
 ///
 /// The checkbox itself does not maintain any state. Instead, when the state of
-/// the checkbox changes, the component calls the `onChange` callback. Most
+/// the checkbox changes, the widget calls the [onChanged] callback. Most
-/// components that use a checkbox will listen for the `onChange` callback and
+/// widgets that use a checkbox will listen for the [onChanged] callback and
-/// rebuild the checkbox with a new `value` to update the visual appearance of
+/// rebuild the checkbox with a new [value] to update the visual appearance of
 /// the checkbox.
 ///
-/// <https://www.google.com/design/spec/components/lists-controls.html#lists-controls-types-of-list-controls>
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [Radio]
+///  * [Switch]
+///  * [Slider]
+///  * <https://www.google.com/design/spec/components/selection-controls.html#selection-controls-checkbox>
+///  * <https://www.google.com/design/spec/components/lists-controls.html#lists-controls-types-of-list-controls>
 class Checkbox extends StatelessWidget {
   /// Constructs a checkbox
   ///
@@ -33,8 +40,15 @@ class Checkbox extends StatelessWidget {
     this.onChanged
   }) : super(key: key);
 
+  /// Whether this checkbox is checked or unchecked.
   final bool value;
+
+  /// The color to use when this checkbox is checked.
+  ///
+  /// Defaults to accent color of the current theme.
   final Color activeColor;
+
+  /// Called when the user checks or unchecks the checkbox.
   final ValueChanged<bool> onChanged;
 
   @override

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

@@ -21,6 +21,18 @@ const TextStyle _kLabelStyle = const TextStyle(
   textBaseline: TextBaseline.alphabetic
 );
 
+/// A material design chip.
+///
+/// Chips represent complex entities in small blocks, such as a contact.
+///
+/// Supplying a non-null [onDeleted] callback will cause the chip to include a
+/// button for deleting the chip.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [CircleAvatar]
+///  * https://www.google.com/design/spec/components/chips.html
 class Chip extends StatelessWidget {
   const Chip({
     Key key,
@@ -29,8 +41,19 @@ class Chip extends StatelessWidget {
     this.onDeleted
   }) : super(key: key);
 
+  /// A widget to display prior to the chip's label.
+  ///
+  /// Typically a [CircleAvatar] widget.
   final Widget avatar;
+
+  /// The primary content of the chip.
+  ///
+  /// Typically a [Text] widget.
   final Widget label;
+
+  /// Called when the user deletes the chip, e.g., by tapping the delete button.
+  ///
+  /// The delete button is included in the ship only if this callback is non-null.
   final VoidCallback onDeleted;
 
   @override

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

@@ -13,8 +13,10 @@ import 'theme.dart';
 /// such an image, the user's initials. A given user's initials should
 /// always be paired with the same background color, for consistency.
 ///
-/// This class is used by [Chip].
+/// See also:
-/// See also: <https://www.google.com/design/spec/components/chips.html#chips-contact-chips>
+///  * [Chip]
+///  * [ListItem]
+///  * <https://www.google.com/design/spec/components/chips.html#chips-contact-chips>
 class CircleAvatar extends StatelessWidget {
   CircleAvatar({
     Key key,

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

@@ -17,6 +17,16 @@ import 'typography.dart';
 
 enum _DatePickerMode { day, year }
 
+/// A material design date picker.
+///
+/// The date picker widget is rarely used directly. Instead, consider using
+/// [showDatePicker], which creates a date picker dialog.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [showDatePicker]
+///  * <https://www.google.com/design/spec/components/pickers.html#pickers-date-pickers>
 class DatePicker extends StatefulWidget {
   DatePicker({
     this.selectedDate,
@@ -29,9 +39,18 @@ class DatePicker extends StatefulWidget {
     assert(lastDate != null);
   }
 
+  /// The currently selected date.
+  ///
+  /// This date is highlighted in the picker.
   final DateTime selectedDate;
+
+  /// Called when the user picks a date.
   final ValueChanged<DateTime> onChanged;
+
+  /// The earliest date the user is permitted to pick.
   final DateTime firstDate;
+
+  /// The latest date the user is permitted to pick.
   final DateTime lastDate;
 
   @override
@@ -165,7 +184,16 @@ class _DatePickerHeader extends StatelessWidget {
   }
 }
 
-// Fixed height component shows a single month and allows choosing a day
+/// Displays the days of a given month and allows choosing a day.
+///
+/// The days are arranged in a rectangular grid with one column for each day of
+/// the week.
+///
+/// Part of the material design [DatePicker].
+///
+/// See also:
+///  * [DatePicker].
+///  * <https://www.google.com/design/spec/components/pickers.html#pickers-date-pickers>
 class DayPicker extends StatelessWidget {
   DayPicker({
     this.selectedDate,
@@ -179,9 +207,18 @@ class DayPicker extends StatelessWidget {
     assert(displayedMonth != null);
   }
 
+  /// The currently selected date.
+  ///
+  /// This date is highlighted in the picker.
   final DateTime selectedDate;
+
+  /// The current date at the time the picker is displayed.
   final DateTime currentDate;
+
+  /// Called when the user picks a day.
   final ValueChanged<DateTime> onChanged;
+
+  /// The month whose days are displayed by this picker.
   final DateTime displayedMonth;
 
   @override
@@ -275,6 +312,16 @@ class DayPicker extends StatelessWidget {
   }
 }
 
+/// A scrollable list of months to allow picking a month.
+///
+/// Shows the days of each month in a rectangular grid with one column for each
+/// day of the week.
+///
+/// Part of the material design [DatePicker].
+///
+/// See also:
+///  * [DatePicker]
+///  * <https://www.google.com/design/spec/components/pickers.html#pickers-date-pickers>
 class MonthPicker extends StatefulWidget {
   MonthPicker({
     Key key,
@@ -290,10 +337,21 @@ class MonthPicker extends StatefulWidget {
     assert(selectedDate.isAfter(firstDate) || selectedDate.isAtSameMomentAs(firstDate));
   }
 
+  /// The currently selected date.
+  ///
+  /// This date is highlighted in the picker.
   final DateTime selectedDate;
+
+  /// Called when the user picks a month.
   final ValueChanged<DateTime> onChanged;
+
+  /// The earliest date the user is permitted to pick.
   final DateTime firstDate;
+
+  /// The latest date the user is permitted to pick.
   final DateTime lastDate;
+
+  /// The amount of vertical space to use for each month in the picker.
   final double itemExtent;
 
   @override
@@ -367,7 +425,15 @@ class _MonthPickerState extends State<MonthPicker> {
   }
 }
 
-// Scrollable list of years to allow picking a year
+/// A scrollable list of years to allow picking a year.
+///
+/// Part of the material design [DatePicker].
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [DatePicker]
+///  * <https://www.google.com/design/spec/components/pickers.html#pickers-date-pickers>
 class YearPicker extends StatefulWidget {
   YearPicker({
     Key key,
@@ -381,9 +447,18 @@ class YearPicker extends StatefulWidget {
     assert(lastDate.isAfter(firstDate));
   }
 
+  /// The currently selected date.
+  ///
+  /// This date is highlighted in the picker.
   final DateTime selectedDate;
+
+  /// Called when the user picks a year.
   final ValueChanged<DateTime> onChanged;
+
+  /// The earliest date the user is permitted to pick.
   final DateTime firstDate;
+
+  /// The latest date the user is permitted to pick.
   final DateTime lastDate;
 
   @override

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

@@ -78,6 +78,16 @@ class _DatePickerDialogState extends State<_DatePickerDialog> {
   }
 }
 
+/// Shows a dialog containing a material design date picker.
+///
+/// The returned Future resolves to the date selected by the user when the user
+/// closes the dialog. If the user cancels the dialog, the Future resolves to
+/// the initialDate.
+///
+/// See also:
+///  * [DatePicker]
+///  * [showTimePicker]
+///  * <https://www.google.com/design/spec/components/pickers.html#pickers-date-pickers>
 Future<DateTime> showDatePicker({
   BuildContext context,
   DateTime initialDate,

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

@@ -26,6 +26,18 @@ const double _kEdgeDragWidth = 20.0;
 const double _kMinFlingVelocity = 365.0;
 const Duration _kBaseSettleDuration = const Duration(milliseconds: 246);
 
+/// A material design drawer.
+///
+/// Typically used in the [Scaffold.drawer] property, a drawer slides in from
+/// the side of the screen and displays a list of items that the user can
+/// interact with. The top-most item in a drawer is typically a [DrawerHeader]
+/// that displays status information about the current user.
+///
+/// See also:
+///  * [Scaffold.drawer]
+///  * [DrawerItem]
+///  * [DrawerHeader]
+///  * <https://www.google.com/design/spec/patterns/navigation-drawer.html>
 class Drawer extends StatelessWidget {
   Drawer({
     Key key,
@@ -33,6 +45,7 @@ class Drawer extends StatelessWidget {
     this.child
   }) : super(key: key);
 
+  /// The height at which to place this drawer.
   final int elevation;
 
   /// The widget below this widget in the tree.
@@ -50,6 +63,14 @@ class Drawer extends StatelessWidget {
   }
 }
 
+/// Provides interactive behavior for [Drawer] widgets.
+///
+/// Drawer controllers are typically created automatically by [Scaffold]
+/// widgets.
+///
+/// See also:
+/// * [Drawer]
+/// * [Scaffold.drawer]
 class DrawerController extends StatefulWidget {
   DrawerController({
     GlobalKey key,

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

@@ -8,9 +8,16 @@ import 'constants.dart';
 import 'debug.dart';
 import 'theme.dart';
 
-// TODO(jackson): This class should usually render the user's
+/// The top-most region of a material design drawer.
-// preferred banner image rather than a solid background
+///
-
+/// Part of the material design [Drawer].
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [Drawer]
+///  * [DrawerItem]
+///  * <https://www.google.com/design/spec/patterns/navigation-drawer.html>
 class DrawerHeader extends StatelessWidget {
   const DrawerHeader({ Key key, this.child }) : super(key: key);
 
@@ -24,6 +31,8 @@ class DrawerHeader extends StatelessWidget {
     return new Container(
       height: statusBarHeight + kMaterialDrawerHeight,
       decoration: new BoxDecoration(
+        // TODO(jackson): This class should usually render the user's
+        // preferred banner image rather than a solid background
         backgroundColor: Theme.of(context).cardColor,
         border: const Border(
           bottom: const BorderSide(

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

@@ -11,6 +11,16 @@ import 'icons.dart';
 import 'ink_well.dart';
 import 'theme.dart';
 
+/// An item in a material design drawer.
+///
+/// Part of the material design [Drawer].
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [Drawer]
+///  * [DrawerHeader]
+///  * <https://www.google.com/design/spec/patterns/navigation-drawer.html>
 class DrawerItem extends StatelessWidget {
   const DrawerItem({
     Key key,
@@ -26,8 +36,13 @@ class DrawerItem extends StatelessWidget {
   /// The widget below this widget in the tree.
   final Widget child;
 
+  /// Called when the user taps this drawer item.
   final VoidCallback onPressed;
 
+  /// Whether this drawer item is currently selected.
+  ///
+  /// The currently selected item is highlighted to distinguish it from other
+  /// drawer items.
   final bool selected;
 
   Color _getIconColor(ThemeData themeData) {

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

@@ -243,6 +243,18 @@ class DropDownMenuItem<T> extends StatelessWidget {
   }
 }
 
+/// 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.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [RaisedButton]
+///  * [FlatButton]
+///  * <https://www.google.com/design/spec/components/buttons.html#buttons-dropdown-buttons>
 class DropDownButton<T> extends StatefulWidget {
   DropDownButton({
     Key key,
@@ -254,9 +266,16 @@ class DropDownButton<T> extends StatefulWidget {
     assert(items.where((DropDownMenuItem<T> item) => item.value == value).length == 1);
   }
 
+  /// The list of possible items to select among.
   final List<DropDownMenuItem<T>> items;
+
+  /// The currently selected item.
   final T value;
+
+  /// Called when the user selects an item.
   final ValueChanged<T> onChanged;
+
+  /// The height at which to place the menu when open.
   final int elevation;
 
   @override

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

@@ -26,8 +26,11 @@ import 'theme.dart';
 /// trying to change the button's [color] and it is not having any effect, check
 /// that you are passing a non-null [onPressed] handler.
 ///
+/// Requires one of its ancestors to be a [Material] widget.
+///
 /// See also:
-///  * [RaisedButton] class
+///  * [RaisedButton]
+///  * [DropDownButton]
 ///  * https://www.google.com/design/spec/components/buttons.html
 class FlatButton extends MaterialButton {
   FlatButton({

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

@@ -59,6 +59,7 @@ class FloatingActionButton extends StatefulWidget {
   /// If this is set to null, the button will be disabled.
   final VoidCallback onPressed;
 
+  /// The height at which to place this button.
   final int elevation;
 
   final int highlightElevation;

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

@@ -14,6 +14,11 @@ import 'theme.dart';
 export 'package:sky_services/editing/editing.mojom.dart' show KeyboardType;
 
 /// A material design text input field.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * <https://www.google.com/design/spec/components/text-fields.html>
 class Input extends StatefulWidget {
   Input({
     Key key,

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

@@ -8,13 +8,22 @@ import 'debug.dart';
 import 'ink_well.dart';
 import 'theme.dart';
 
-/// Material List items are one to three lines of text optionally flanked by icons.
+/// An item in a material design list.
-/// Icons are defined with the [leading] and [trailing] parameters. The first line of text
+///
-/// is not optional and is specified with [title]. The value of [subtitle] will
+/// [MaterialList] items are one to three lines of text optionally flanked by
-/// occupy the space allocated for an aditional line of text, or two lines if
+/// icons. Icons are defined with the [leading] and [trailing] parameters. The
-/// isThreeLine: true is specified. If dense: true is specified then the overall
+/// first line of text is not optional and is specified with [title]. The value
-/// height of this list item and the size of the DefaultTextStyles that wrap
+/// of [subtitle] will occupy the space allocated for an aditional line of text,
+/// or two lines if [isThreeLine] is true. If [dense] is true then the overall
+/// height of this list item and the size of the [DefaultTextStyle]s that wrap
 /// the [title] and [subtitle] widget are reduced.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [MaterialList]
+///  * [CircleAvatar]
+///  * <https://www.google.com/design/spec/components/lists.html>
 class ListItem extends StatelessWidget {
   ListItem({
     Key key,
@@ -31,14 +40,41 @@ class ListItem extends StatelessWidget {
     assert(isThreeLine ? subtitle != null : true);
   }
 
+  /// A widget to display before the title.
+  ///
+  /// Typically a [CircleAvatar] widget.
   final Widget leading;
+
+  /// The primary content of the list item.
+  ///
+  /// Typically a [Text] widget.
   final Widget title;
+
+  /// Additional content displayed below the title.
+  ///
+  /// Typically a [Text] widget.
   final Widget subtitle;
+
+  /// A widget to display after the title.
+  ///
+  /// Typically an [Icon] widget.
   final Widget trailing;
+
+  /// Whether this list item is intended to display three lines of text.
   final bool isThreeLine;
+
+  /// Whether this list item is part of a vertically dense list.
   final bool dense;
+
+  /// Whether this list item should be styled with the disabled color from the theme.
+  ///
+  /// If true, prevents the [onTap] and [onLongPress] callbacks from being operative.
   final bool enabled;
+
+  /// Called when the user taps this list item.
   final GestureTapCallback onTap;
+
+  /// Called when the user long-presses on this list item.
   final GestureLongPressCallback onLongPress;
 
   /// Add a one pixel border in between each item. If color isn't specified the

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

@@ -91,6 +91,7 @@ class Material extends StatefulWidget {
 
   final MaterialType type;
 
+  /// The height at which to place this material.
   final int elevation;
 
   final Color color;

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

@@ -403,6 +403,7 @@ class PopupMenuButton<T> extends StatefulWidget {
 
   final String tooltip;
 
+  /// The height at which to place the menu when open.
   final int elevation;
 
   /// The widget below this widget in the tree.

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

@@ -14,6 +14,25 @@ const double _kDiameter = 16.0;
 const double _kOuterRadius = _kDiameter / 2.0;
 const double _kInnerRadius = 5.0;
 
+/// A material design radio button.
+///
+/// Used to select between a number of mutually exclusive values. When one
+/// radio button in a group is selected, the other radio buttons in the group
+/// cease to be selected.
+///
+/// The radio button itself does not maintain any state. Instead, when the state
+/// of the radio button changes, the widget calls the [onChanged] callback.
+/// Most widget that use a radio button will listen for the [onChanged]
+/// callback and rebuild the radio button with a new [groupValue] to update the
+/// visual appearance of the radio button.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [CheckBox]
+///  * [Slider]
+///  * [Switch]
+///  * <https://www.google.com/design/spec/components/selection-controls.html#selection-controls-radio-button>
 class Radio<T> extends StatelessWidget {
   Radio({
     Key key,
@@ -23,9 +42,24 @@ class Radio<T> extends StatelessWidget {
     this.onChanged
   }) : super(key: key);
 
+  /// The value represented by this radio button.
   final T value;
+
+  /// The currently selected value for this group of radio buttons.
+  ///
+  /// This radio button is considered selected if its [value] matches the
+  /// [groupValue].
   final T groupValue;
+
+  /// The color to use when this radio button is selected.
+  ///
+  /// Defaults to accent color of the current theme.
   final Color activeColor;
+
+  /// Called when the user selects this radio button.
+  ///
+  /// For convenence, the radio button passes [value] as a parameter to this
+  /// callback.
   final ValueChanged<T> onChanged;
 
   bool get _enabled => onChanged != null;

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

@@ -23,9 +23,11 @@ import 'theme.dart';
 /// not having any effect, check that you are passing a non-null [onPressed]
 /// handler.
 ///
-/// See also:
+/// Requires one of its ancestors to be a [Material] widget.
 ///
-///  * [FlatButton] class
+/// See also:
+///  * [FlatButton]
+///  * [DropDownButton]
 ///  * <https://www.google.com/design/spec/components/buttons.html>
 class RaisedButton extends MaterialButton {
   RaisedButton({
@@ -54,8 +56,13 @@ class RaisedButton extends MaterialButton {
   /// Controls the default text color if the text color isn't explicit set.
   final ThemeBrightness colorBrightness;
 
+  /// The height at which to place this button.
   final int elevation;
+
+  /// The height at which to place this button when highlighted.
   final int highlightElevation;
+
+  /// The height at which to place this button when disabled.
   final int disabledElevation;
 
   @override

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

@@ -11,6 +11,22 @@ import 'constants.dart';
 import 'debug.dart';
 import 'theme.dart';
 
+/// A material design slider.
+///
+/// Used to select from a continuous range of values.
+///
+/// The slider itself does not maintain any state. Instead, when the state of
+/// the slider changes, the widget calls the [onChanged] callback. Most widgets
+/// that use a slider will listen for the [onChanged] callback and rebuild the
+/// slider with a new [value] to update the visual appearance of the slider.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [CheckBox]
+///  * [Radio]
+///  * [Switch]
+///  * <https://www.google.com/design/spec/components/sliders.html>
 class Slider extends StatelessWidget {
   Slider({
     Key key,
@@ -26,10 +42,27 @@ class Slider extends StatelessWidget {
     assert(value >= min && value <= max);
   }
 
+  /// The currently selected value for this slider.
+  ///
+  /// The slider's thumb is drawn at a position that corresponds to this value.
   final double value;
+
+  /// The minium value the user can select.
+  ///
+  /// Defaults to 0.0.
   final double min;
+
+  /// The maximum value the user can select.
+  ///
+  /// Defaults to 1.0.
   final double max;
+
+  /// The color to use for the portion of the slider that has been selected.
+  ///
+  /// Defaults to accent color of the current theme.
   final Color activeColor;
+
+  /// Called when the user selects a new value for the slider.
   final ValueChanged<double> onChanged;
 
   void _handleChanged(double value) {

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

@@ -13,6 +13,22 @@ import 'shadows.dart';
 import 'theme.dart';
 import 'toggleable.dart';
 
+/// A material design switch.
+///
+/// Used to toggle the on/off state of a single setting.
+///
+/// The switch itself does not maintain any state. Instead, when the state of
+/// the switch changes, the widget calls the [onChanged] callback. Most widgets
+/// that use a switch will listen for the [onChanged] callback and rebuild the
+/// switch with a new [value] to update the visual appearance of the switch.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [CheckBox]
+///  * [Radio]
+///  * [Slider]
+///  * <https://www.google.com/design/spec/components/selection-controls.html#selection-controls-switch>
 class Switch extends StatelessWidget {
   Switch({
     Key key,
@@ -23,10 +39,29 @@ class Switch extends StatelessWidget {
     this.onChanged
   }) : super(key: key);
 
+  /// Whether this switch is on or off.
   final bool value;
+
+  /// The color to use when this switch is on.
+  ///
+  /// Defaults to accent color of the current theme.
   final Color activeColor;
+
+  /// A decoration to use for the thumb of this switch when the switch is on.
+  ///
+  /// Defaults to a circular piece of material.
   final Decoration activeThumbDecoration;
+
+  /// A decoration to use for the thumb of this switch when the switch is off.
+  ///
+  /// Defaults to a circular piece of material.
   final Decoration inactiveThumbDecoration;
+
+  /// Called when the user toggles with switch on or off.
+  ///
+  /// The switch passes the new value to the callback but does not actually
+  /// change state until the parent widget rebuilds the switch with the new
+  /// value.
   final ValueChanged<bool> onChanged;
 
   @override

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

@@ -574,13 +574,18 @@ class TabBarSelectionState<T> extends State<TabBarSelection<T>> {
   }
 }
 
-/// Displays a horizontal row of tabs, one per label. If isScrollable is
+/// Displays a horizontal row of tabs, one per label.
-/// true then each tab is as wide as needed for its label and the entire
-/// [TabBar] is scrollable. Otherwise each tab gets an equal share of the
-/// available space. A [TabBarSelection] widget ancestor must have been
-/// built to enable saving and monitoring the selected tab.
 ///
-/// Tabs must always have an ancestor Material object.
+/// Requires one of its ancestors to be a [TabBarSelection] widget to enable
+/// saving and monitoring the selected tab.
+///
+/// Requires one of its ancestors to be a [Material] widget.
+///
+/// See also:
+///  * [TabBarSelection]
+///  * [TabBarView]
+///  * [AppBar.tabBar]
+///  * <https://www.google.com/design/spec/components/tabs.html>
 class TabBar<T> extends Scrollable {
   TabBar({
     Key key,
@@ -588,7 +593,14 @@ class TabBar<T> extends Scrollable {
     this.isScrollable: false
   }) : super(key: key, scrollDirection: Axis.horizontal);
 
+  /// The labels to display in the tabs.
   final Map<T, TabLabel> labels;
+
+  /// Whether this tab bar can be scrolled horizontally.
+  ///
+  /// If [isScrollable] is true then each tab is as wide as needed for its label
+  /// and the entire [TabBar] is scrollable. Otherwise each tab gets an equal
+  /// share of the available space.
   final bool isScrollable;
 
   double get minimumHeight {

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

@@ -92,6 +92,14 @@ class TimeOfDay {
 
 enum _TimePickerMode { hour, minute }
 
+/// A material design time picker.
+///
+/// The time picker widget is rarely used directly. Instead, consider using
+/// [showTimePicker], which creates a time picker dialog.
+///
+/// See also:
+///  * [showTimePicker]
+///  * <https://www.google.com/design/spec/components/pickers.html#pickers-time-pickers>
 class TimePicker extends StatefulWidget {
   TimePicker({
     this.selectedTime,
@@ -100,7 +108,12 @@ class TimePicker extends StatefulWidget {
     assert(selectedTime != null);
   }
 
+  /// The currently selected time.
+  ///
+  /// This time is highlighted in the picker.
   final TimeOfDay selectedTime;
+
+  /// Called when the user picks a time.
   final ValueChanged<TimeOfDay> onChanged;
 
   @override

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

@@ -67,6 +67,16 @@ class _TimePickerDialogState extends State<_TimePickerDialog> {
   }
 }
 
+/// Shows a dialog containing a material design time picker.
+///
+/// The returned Future resolves to the time selected by the user when the user
+/// closes the dialog. If the user cancels the dialog, the Future resolves to
+/// the initialTime.
+///
+/// See also:
+///  * [TimePicker]
+///  * [showDatePicker]
+///  * <https://www.google.com/design/spec/components/pickers.html#pickers-time-pickers>
 Future<TimeOfDay> showTimePicker({
   BuildContext context,
   TimeOfDay initialTime