DetectScroll class

The DetectScroll can detect if the content of a Scrollable is larger than the Scrollable itself, which means that the content can be scrolled, and that a scrollbar is likely visible. It can also tell you the probable width of that scrollbar.

This is useful for positioning widgets relative to the scrollbar, so that the scrollbar doesn't overlap them. This can be important when the scrollbar is permanently visible, usually on the Web and desktop.

Note that DetectScroll will only detect the scrolling of its closest scrollable descendant (a scrollable is a SingleChildScrollView, ListView, GridView etc). Usually, you'd wrap the scrollable you care about directly with a DetectScroll. For example:

DetectScroll(
 child: SingleChildScrollView(
    child: Column( ...
 ...
);

To get the current scroll state and the scrollbar width, descendants can call:

bool canScroll = DetectScroll.of(context).canScroll;
double width = DetectScroll.of(context).scrollbarWidth;

For example, suppose you want to add a help button to the top-right corner of a scrollable, and account for the scrollbar width only if it's visible:

bool canScroll = DetectScroll.of(context).canScroll;
double width = DetectScroll.of(context).scrollbarWidth;

return Stack(
  children: [
     child,
     Positioned(
        right: canScroll ? width : 0,
        top: 0,
        child: HelpButton(),
     ),
  ],
);

Another alternative is using the optional onChange callback:

DetectScroll(
   onChange: ({
      required bool canScroll,
      required double width,
   }) {
      // Do something.
   }
   child: ...
),

In more detail:

The DetectScroll actually only detects if its subtree can scroll, in other words, that its closest descendant Scrollable has enough content so that not all of it fits the available visible space, and then informs its descendants about this fact.

Note this doesn't mean there is actually a scrollbar visible, but only that the content can be scrolled. For this reason, you should use it to detect scrollbars only when a scrollbar is always shown when the content doesn't fit (like on the web or desktop), or when you're using a custom scrollbar that is always visible.

Regarding the width of the scrollbar provided by DetectScroll, this information is calculated from the current theme at the DetectScroll. For this reason, if you use this width, make sure the scrollbar is actually using this same theme.

Inheritance

Available extensions

Constructors

DetectScroll({Key? key, required Widget child, void onChange({required bool canScroll, required double scrollbarWidth})?, bool cancelNotificationBubbling = false}): The DetectScroll can detect if the content of a Scrollable is larger than the Scrollable itself, which means that the content can be scrolled, and that a scrollbar is likely visible. It can also tell you the probable width of that scrollbar.
const

Properties

cancelNotificationBubbling → bool: If true, the DetectScroll will cancel the ScrollMetricsNotification bubbling. If false, the notification will be allowed to continue to be dispatched to further ancestors. The default is false.
final
child → Widget: final
hashCode → int: The hash code for this object.
no setterinherited
key → Key?: Controls how one widget replaces another widget in the tree.
finalinherited
makeRefreshable → Widget: Available on Widget?, provided by the WidgetExtension extension
Make your any widget refreshable with RefreshIndicator on top
no setter
onChange → void Function({required bool canScroll, required double scrollbarWidth})?: final
runtimeType → Type: A representation of the runtime type of the object.
no setterinherited

Methods

addMaterialWidget() → Material: Available on Widget, provided by the GenericExtensions extension
addTooltipWidget(String toolTip) → Tooltip: Available on Widget, provided by the GenericExtensions extension
animate({Key? key, List<Effect>? effects, AnimateCallback? onInit, AnimateCallback? onPlay, AnimateCallback? onComplete, bool? autoPlay, Duration? delay, AnimationController? controller, Adapter? adapter, double? target, double? value}) → Animate: Available on Widget, provided by the AnimateWidgetExtensions extension
Wraps the target Widget in an Animate instance, and returns the instance for chaining calls. Ex. myWidget.animate() is equivalent to Animate(child: myWidget).
borderRadius([BorderRadiusGeometry? borderRadius]) → Widget: Available on Widget, provided by the GenericExtensions extension
boxDecoration([BoxDecoration? boxDecoration]) → Widget: Available on Widget, provided by the GenericExtensions extension
center({double? heightFactor, double? widthFactor}) → Widget: Available on Widget?, provided by the WidgetExtension extension
set parent widget in center
colorFilter([ColorFilter? colorFilter]) → Widget: Available on Widget, provided by the GenericExtensions extension
set parent widget in center
cornerRadiusWithClipRRect(double radius) → ClipRRect: Available on Widget?, provided by the WidgetExtension extension
add corner radius
cornerRadiusWithClipRRectOnly({int bottomLeft = 0, int bottomRight = 0, int topLeft = 0, int topRight = 0}) → ClipRRect: Available on Widget?, provided by the WidgetExtension extension
add custom corner radius each side
createElement() → StatefulElement: Creates a StatefulElement to manage this widget's location in the tree.
inherited
createState() → State<DetectScroll>: 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
expand({int flex = 1}) → Widget: Available on Widget?, provided by the WidgetExtension extension
add Expanded to parent widget
fit({BoxFit? fit, AlignmentGeometry? alignment}) → Widget: Available on Widget?, provided by the WidgetExtension extension
add FittedBox to parent widget
flexible({int flex = 1, FlexFit? fit}) → Widget: Available on Widget?, provided by the WidgetExtension extension
add Flexible to parent widget
launch<T>(BuildContext context, {bool isNewTask = false, PageRouteAnimation? pageRouteAnimation, Duration? duration, String? routeName, Object? routeArguments}) → Future<T?>: Available on Widget?, provided by the WidgetExtension extension
Launch a new screen
noSuchMethod(Invocation invocation) → dynamic: Invoked when a nonexistent method or property is accessed.
inherited
onTap(Function? function, {BorderRadius? borderRadius, Color? splashColor, Color? hoverColor, Color? highlightColor, Color? focusColor, WidgetStateProperty<Color?>? overlayColor}) → Widget: Available on Widget?, provided by the WidgetExtension extension
add tap to parent widget
opacity({required double opacity, int durationInSecond = 1, Duration? duration}) → Widget: Available on Widget?, provided by the WidgetExtension extension
add opacity to parent widget
paddingAll(double padding) → Padding: Available on Widget?, provided by the WidgetExtension extension
return padding all
paddingBottom(double bottom) → Padding: Available on Widget?, provided by the WidgetExtension extension
return padding bottom
paddingDirectional({double start = 0.0, double top = 0.0, double end = 0.0, double bottom = 0.0}) → Widget: Available on Widget?, provided by the WidgetExtension extension
paddingLeft(double left) → Padding: Available on Widget?, provided by the WidgetExtension extension
return padding left
paddingOnly({double top = 0.0, double left = 0.0, double bottom = 0.0, double right = 0.0}) → Padding: Available on Widget?, provided by the WidgetExtension extension
return custom padding from each side
paddingRight(double right) → Padding: Available on Widget?, provided by the WidgetExtension extension
return padding right
paddingSymmetric({double vertical = 0.0, double horizontal = 0.0}) → Padding: Available on Widget?, provided by the WidgetExtension extension
return padding symmetric
paddingTop(double top) → Padding: Available on Widget?, provided by the WidgetExtension extension
return padding top
rotate({required double angle, bool transformHitTests = true, Offset? origin}) → Widget: Available on Widget?, provided by the WidgetExtension extension
add rotation to parent widget
scale({required double scale, Offset? origin, AlignmentGeometry? alignment, bool transformHitTests = true}) → Widget: Available on Widget?, provided by the WidgetExtension extension
add scaling to parent widget
toDiagnosticsNode({String? name, DiagnosticsTreeStyle? style}) → DiagnosticsNode: Returns a debug representation of the object that is used by debugging tools and by DiagnosticsNode.toStringDeep.
inherited
tooltip({required String msg}) → Widget: Available on Widget?, provided by the WidgetExtension extension
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
translate({required Offset offset, bool transformHitTests = true, Key? key}) → Widget: Available on Widget?, provided by the WidgetExtension extension
add translate to parent widget
validate({Widget value = const SizedBox()}) → Widget: Available on Widget?, provided by the WidgetExtension extension
Validate given widget is not null and returns given value if null.
visible(bool visible, {Widget? defaultWidget}) → Widget: Available on Widget?, provided by the WidgetExtension extension
set visibility
withHeight(double height) → SizedBox: Available on Widget?, provided by the WidgetExtension extension
With custom height
withRoundedCorners({Color backgroundColor = whiteColor, BorderRadius borderRadius = const BorderRadius.all(Radius.circular(8.0)), LinearGradient? gradient, BoxBorder? border, List<BoxShadow>? boxShadow, DecorationImage? decorationImage, BoxShape boxShape = BoxShape.rectangle}) → Container: Available on Widget?, provided by the WidgetExtension extension
withScroll({ScrollPhysics? physics, EdgeInsetsGeometry? padding, Axis scrollDirection = Axis.vertical, ScrollController? controller, DragStartBehavior dragStartBehavior = DragStartBehavior.start, bool? primary, required bool reverse}) → Widget: Available on Widget?, provided by the WidgetExtension extension
withShaderMask(List<Color> colors, {BlendMode blendMode = BlendMode.srcATop}) → Widget: Available on Widget?, provided by the WidgetExtension extension
Wrap with ShaderMask widget
withShaderMaskGradient(Gradient gradient, {BlendMode blendMode = BlendMode.srcATop}) → Widget: Available on Widget?, provided by the WidgetExtension extension
Wrap with ShaderMask widget Gradient
withShadow({Color bgColor = whiteColor, Color shadowColor = Colors.black12, dynamic blurRadius = 10.0, dynamic spreadRadius = 0.0, Offset offset = const Offset(0.0, 0.0), LinearGradient? gradient, BoxBorder? border, DecorationImage? decorationImage, BoxShape boxShape = BoxShape.rectangle}) → Container: Available on Widget?, provided by the WidgetExtension extension
withSize({double width = 0.0, double height = 0.0}) → SizedBox: Available on Widget?, provided by the WidgetExtension extension
With custom height and width
withTooltip({required String msg}) → Widget: Available on Widget?, provided by the WidgetExtension extension
Validate given widget is not null and returns given value if null.
withVisibility(bool visible, {Widget? replacement, bool maintainAnimation = false, bool maintainState = false, bool maintainSize = false, bool maintainSemantics = false, bool maintainInteractivity = false}) → Visibility: Available on Widget?, provided by the WidgetExtension extension
set widget visibility
withWidth(double width) → SizedBox: Available on Widget?, provided by the WidgetExtension extension
With custom width

Operators

operator ==(Object other) → bool: The equality operator.
inherited

Static Methods

of(BuildContext context) → _DetectScrollInherited: Allows descendants to get the scroll state. Example: