DropdownButton<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.
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:
- DropdownButtonFormField, 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
- Inheritance
-
- Object
- DiagnosticableTree
- Widget
- StatefulWidget
- DropdownButton
Constructors
-
DropdownButton({Key? key, required List<
DropdownMenuItem< ? items, DropdownButtonBuilder? selectedItemBuilder, T? value, Widget? hint, Widget? disabledHint, required ValueChanged<T> >T?> ? onChanged, VoidCallback? onTap, int elevation = 8, TextStyle? style, Widget? underline, Widget? icon, Color? iconDisabledColor, Color? iconEnabledColor, double iconSize = 24.0, bool isDense = false, bool isExpanded = false, double? itemHeight = kMinInteractiveDimension, Color? focusColor, FocusNode? focusNode, bool autofocus = false, Color? dropdownColor, bool? enableFeedback, AlignmentGeometry alignment = AlignmentDirectional.centerStart, BorderRadius? borderRadius, bool isDropdown = false, bool forceWidth = false, bool? isPinnedSelected, bool isOutsideShadow = false, EdgeInsets? itemPadding, Color? itemBackgroundColor, Color? unselectItemBackgroundColor}) - Creates a dropdown button.
Properties
- alignment → AlignmentGeometry
-
Defines how the hint or the selected item is positioned within the button.
final
- autofocus → bool
-
True if this widget will be selected as the initial focus when no other
node in its scope is currently focused.
final
- borderRadius → BorderRadius?
-
Defines the corner radii of the menu's rounded rectangle shape.
final
- disabledHint → Widget?
-
A preferred placeholder widget that is displayed when the dropdown is disabled.
final
- dropdownColor → Color?
-
The background color of the dropdown.
final
- elevation → int
-
The z-coordinate at which to place the menu when open.
final
- enableFeedback → bool?
-
Whether detected gestures should provide acoustic and/or haptic feedback.
final
- focusColor → Color?
-
The color for the button's Material when it has the input focus.
final
- focusNode → FocusNode?
-
An optional focus node to use as the focus node for this widget.
final
- forceWidth → bool
-
新增字段:强制撑满宽度
final
- hashCode → int
-
The hash code for this object.
no setterinherited
- hint → Widget?
-
A placeholder widget that is displayed by the dropdown button.
final
- icon → Widget?
-
The widget to use for the drop-down button's icon.
final
- iconDisabledColor → Color?
-
The color of any Icon descendant of icon if this button is disabled,
i.e. if onChanged is null.
final
- iconEnabledColor → Color?
-
The color of any Icon descendant of icon if this button is enabled,
i.e. if onChanged is defined.
final
- iconSize → double
-
The size to use for the drop-down button's down arrow icon button.
final
- isDense → bool
-
Reduce the button's height.
final
- isDropdown → bool
-
新增字段:
默认为 false , flutter sdk 自带的效果
设置为 true ,菜单贴在锚点的下方
final
- isExpanded → bool
-
Set the dropdown's inner contents to horizontally fill its parent.
final
- isOutsideShadow → bool
-
新增字段:内容区域外,是否绘制背景层。
final
- isPinnedSelected → bool?
-
当选中了一个item后,再次显示时,是否优先固定展示此项,默认为是,Flutter SDK默认效果。
final
- itemBackgroundColor → Color?
-
新增字段:item选中背景色
final
- itemHeight → double?
-
If null, then the menu item heights will vary according to each menu item's
intrinsic height.
final
- itemPadding → EdgeInsets?
-
新增字段:item Padding
final
-
items
→ List<
DropdownMenuItem< ?T> > -
The list of items the user can select.
final
- key → Key?
-
Controls how one widget replaces another widget in the tree.
finalinherited
-
The maximum height of the menu.
final
-
onChanged
→ ValueChanged<
T?> ? -
Called when the user selects an item.
final
- onTap → VoidCallback?
-
Called when the dropdown button is tapped.
final
- runtimeType → Type
-
A representation of the runtime type of the object.
no setterinherited
- selectedItemBuilder → DropdownButtonBuilder?
-
A builder to customize the dropdown buttons corresponding to the
DropdownMenuItems in items.
final
- style → TextStyle?
-
The text style to use for text in the dropdown button and the dropdown
menu that appears when you tap the button.
final
- underline → Widget?
-
The widget to use for drawing the drop-down button's underline.
final
- unselectItemBackgroundColor → Color?
-
新增字段:item未选中背景色
final
- value → T?
-
The value of the currently selected DropdownMenuItem.
final
Methods
-
createElement(
) → StatefulElement -
Creates a StatefulElement to manage this widget's location in the tree.
inherited
-
createState(
) → State< DropdownButton< T> > -
Creates the mutable state for this widget at a given location in the tree.
override
-
debugDescribeChildren(
) → List< DiagnosticsNode> -
Returns a list of DiagnosticsNode objects describing this node's
children.
inherited
-
debugFillProperties(
DiagnosticPropertiesBuilder properties) → void -
Add additional properties associated with the node.
inherited
-
noSuchMethod(
Invocation invocation) → dynamic -
Invoked when a nonexistent method or property is accessed.
inherited
-
toDiagnosticsNode(
{String? name, DiagnosticsTreeStyle? style}) → DiagnosticsNode -
Returns a debug representation of the object that is used by debugging
tools and by DiagnosticsNode.toStringDeep.
inherited
-
toString(
{DiagnosticLevel minLevel = DiagnosticLevel.info}) → String -
A string representation of this object.
inherited
-
toStringDeep(
{String prefixLineOne = '', String? prefixOtherLines, DiagnosticLevel minLevel = DiagnosticLevel.debug, int wrapWidth = 65}) → String -
Returns a string representation of this node and its descendants.
inherited
-
toStringShallow(
{String joiner = ', ', DiagnosticLevel minLevel = DiagnosticLevel.debug}) → String -
Returns a one-line detailed description of the object.
inherited
-
toStringShort(
) → String -
A short, textual description of this widget.
inherited
Operators
-
operator ==(
Object other) → bool -
The equality operator.
inherited