Stack class Null safety

A widget that positions its children relative to the edges of its box.

This class is useful if you want to overlap several children in a simple way, for example having some text and an image, overlaid with a gradient and a button attached to the bottom.

Each child of a Stack widget is either positioned or non-positioned. Positioned children are those wrapped in a Positioned widget that has at least one non-null property. The stack sizes itself to contain all the non-positioned children, which are positioned according to alignment (which defaults to the top-left corner in left-to-right environments and the top-right corner in right-to-left environments). The positioned children are then placed relative to the stack according to their top, right, bottom, and left properties.

The stack paints its children in order with the first child being at the bottom. If you want to change the order in which the children paint, you can rebuild the stack with the children in the new order. If you reorder the children in this way, consider giving the children non-null keys. These keys will cause the framework to move the underlying objects for the children to their new locations rather than recreate them at their new location.

For more details about the stack layout algorithm, see RenderStack.

If you want to lay a number of children out in a particular pattern, or if you want to make a custom layout manager, you probably want to use CustomMultiChildLayout instead. In particular, when using a Stack you can't position children relative to their size or the stack's own size.

link
Using a Stack you can position widgets over one another.

The sample creates a blue box that overlaps a larger green box, which itself overlaps an even larger red box. 

Stack(
  children: <Widget>[
    Container(
      width: 100,
      height: 100,
      color: Colors.red,
    ),
    Container(
      width: 90,
      height: 90,
      color: Colors.green,
    ),
    Container(
      width: 80,
      height: 80,
      color: Colors.blue,
    ),
  ],
)

link
This example shows how Stack can be used to enhance text visibility by adding gradient backdrops.

The gradient fades from transparent to dark grey at the bottom, with white text on top of the darker portion. 

SizedBox(
  width: 250,
  height: 250,
  child: Stack(
    children: <Widget>[
      Container(
        width: 250,
        height: 250,
        color: Colors.white,
      ),
      Container(
        padding: const EdgeInsets.all(5.0),
        alignment: Alignment.bottomCenter,
        decoration: BoxDecoration(
          gradient: LinearGradient(
            begin: Alignment.topCenter,
            end: Alignment.bottomCenter,
            colors: <Color>[
              Colors.black.withAlpha(0),
              Colors.black12,
              Colors.black45
            ],
          ),
        ),
        child: const Text(
          'Foreground Text',
          style: TextStyle(color: Colors.white, fontSize: 20.0),
        ),
      ),
    ],
  ),
)

See also:

Inheritance
Implementers

Constructors

Stack({Key? key, AlignmentGeometry alignment = AlignmentDirectional.topStart, TextDirection? textDirection, StackFit fit = StackFit.loose, Clip clipBehavior = Clip.hardEdge, List<Widget> children = const <Widget>[]})
Creates a stack layout widget.

Properties

alignment → AlignmentGeometry
How to align the non-positioned and partially-positioned children in the stack.
final
children → List<Widget>
The widgets below this widget in the tree.
finalinherited
clipBehavior → Clip
The content will be clipped (or not) according to this option.
final
fit → StackFit
How to size the non-positioned children in the stack.
final
hashCode → int
The hash code for this object.
nonVirtual">@nonVirtualread-onlyinherited
key → Key?
Controls how one widget replaces another widget in the tree.
finalinherited
runtimeType → Type
A representation of the runtime type of the object.
read-onlyinherited
textDirection → TextDirection?
The text direction with which to resolve alignment.
final

Methods

createElement() → MultiChildRenderObjectElement
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
inherited
createRenderObject(BuildContext context) → RenderStack
Creates an instance of the RenderObject class that this RenderObjectWidget represents, using the configuration described by this RenderObjectWidget.
override
debugDescribeChildren() → List<DiagnosticsNode>
Returns a list of DiagnosticsNode objects describing this node's children.
protected">@protectedinherited
debugFillProperties(DiagnosticPropertiesBuilder properties) → void
Add additional properties associated with the node.
override
didUnmountRenderObject(covariant RenderObject renderObject) → void
A render object previously associated with this widget has been removed from the tree. The given RenderObject will be of the same type as returned by this object's createRenderObject.
protected">@protectedinherited
noSuchMethod(Invocation invocation) → dynamic
Invoked when a non-existent 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}) → 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
updateRenderObject(BuildContext context, covariant RenderStack renderObject) → void
Copies the configuration described by this RenderObjectWidget to the given RenderObject, which will be of the same type as returned by this object's createRenderObject.
override

Operators

operator ==(Object other) → bool
The equality operator.
nonVirtual">@nonVirtualinherited