Container class Null safety

A convenience widget that combines common painting, positioning, and sizing widgets.

A container first surrounds the child with padding (inflated by any borders present in the decoration) and then applies additional constraints to the padded extent (incorporating the width and height as constraints, if either is non-null). The container is then surrounded by additional empty space described from the margin.

During painting, the container first applies the given transform, then paints the decoration to fill the padded extent, then it paints the child, and finally paints the foregroundDecoration, also filling the padded extent.

Containers with no children try to be as big as possible unless the incoming constraints are unbounded, in which case they try to be as small as possible. Containers with children size themselves to their children. The width, height, and constraints arguments to the constructor override this.

By default, containers return false for all hit tests. If the color property is specified, the hit testing is handled by ColoredBox, which always returns true. If the decoration or foregroundDecoration properties are specified, hit testing is handled by Decoration.hitTest.

Layout behavior

See BoxConstraints for an introduction to box layout models.

Since Container combines a number of other widgets each with their own layout behavior, Container's layout behavior is somewhat complicated.

Summary: Container tries, in order: to honor alignment, to size itself to the child, to honor the width, height, and constraints, to expand to fit the parent, to be as small as possible.

More specifically:

If the widget has no child, no height, no width, no constraints, and the parent provides unbounded constraints, then Container tries to size as small as possible.

If the widget has no child and no alignment, but a height, width, or constraints are provided, then the Container tries to be as small as possible given the combination of those constraints and the parent's constraints.

If the widget has no child, no height, no width, no constraints, and no alignment, but the parent provides bounded constraints, then Container expands to fit the constraints provided by the parent.

If the widget has an alignment, and the parent provides unbounded constraints, then the Container tries to size itself around the child.

If the widget has an alignment, and the parent provides bounded constraints, then the Container tries to expand to fit the parent, and then positions the child within itself as per the alignment.

Otherwise, the widget has a child but no height, no width, no constraints, and no alignment, and the Container passes the constraints from the parent to the child and sizes itself to match the child.

The margin and padding properties also affect the layout, as described in the documentation for those properties. (Their effects merely augment the rules described above.) The decoration can implicitly increase the padding (e.g. borders in a BoxDecoration contribute to the padding); see Decoration.padding.

Example

link
This example shows a 48x48 amber square (placed inside a Center widget in case the parent widget has its own opinions regarding the size that the Container should take), with a margin so that it stays away from neighboring widgets:

An amber colored container with the dimensions of 48 square pixels. 

Center(
  child: Container(
    margin: const EdgeInsets.all(10.0),
    color: Colors.amber[600],
    width: 48.0,
    height: 48.0,
  ),
)

link
This example shows how to use many of the features of Container at once. The constraints are set to fit the font size plus ample headroom vertically, while expanding horizontally to fit the parent. The padding is used to make sure there is space between the contents and the text. The color makes the box blue. The alignment causes the child to be centered in the box. Finally, the transform applies a slight rotation to the entire contraption to complete the effect.

A blue rectangular container with 'Hello World' in the center, rotated slightly in the z axis. 

Container(
  constraints: BoxConstraints.expand(
    height: Theme.of(context).textTheme.headline4!.fontSize! * 1.1 + 200.0,
  ),
  padding: const EdgeInsets.all(8.0),
  color: Colors.blue[600],
  alignment: Alignment.center,
  transform: Matrix4.rotationZ(0.1),
  child: Text('Hello World',
    style: Theme.of(context)
        .textTheme
        .headline4!
        .copyWith(color: Colors.white)),
)

See also:

Inheritance

Constructors

Container({Key? key, AlignmentGeometry? alignment, EdgeInsetsGeometry? padding, Color? color, Decoration? decoration, Decoration? foregroundDecoration, double? width, double? height, BoxConstraints? constraints, EdgeInsetsGeometry? margin, Matrix4? transform, AlignmentGeometry? transformAlignment, Widget? child, Clip clipBehavior = Clip.none})
Creates a widget that combines common painting, positioning, and sizing widgets.

Properties

alignment → AlignmentGeometry?
Align the child within the container.
final
child → Widget?
The child contained by the container.
final
clipBehavior → Clip
The clip behavior when Container.decoration is not null.
final
color → Color?
The color to paint behind the child.
final
constraints → BoxConstraints?
Additional constraints to apply to the child.
final
decoration → Decoration?
The decoration to paint behind the child.
final
foregroundDecoration → Decoration?
The decoration to paint in front of the child.
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
margin → EdgeInsetsGeometry?
Empty space to surround the decoration and child.
final
padding → EdgeInsetsGeometry?
Empty space to inscribe inside the decoration. The child, if any, is placed inside this padding.
final
runtimeType → Type
A representation of the runtime type of the object.
read-onlyinherited
transform → Matrix4?
The transformation matrix to apply before painting the container.
final
transformAlignment → AlignmentGeometry?
The alignment of the origin, relative to the size of the container, if transform is specified.
final

Methods

build(BuildContext context) → Widget
Describes the part of the user interface represented by this widget.
override
createElement() → StatelessElement
Creates a StatelessElement to manage this widget's location in the tree.
inherited
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
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

Operators

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