Align class Null safety

A widget that aligns its child within itself and optionally sizes itself based on the child's size.

For example, to align a box at the bottom right, you would pass this box a tight constraint that is bigger than the child's natural size, with an alignment of Alignment.bottomRight.

This widget will be as big as possible if its dimensions are constrained and widthFactor and heightFactor are null. If a dimension is unconstrained and the corresponding size factor is null then the widget will match its child's size in that dimension. If a size factor is non-null then the corresponding dimension of this widget will be the product of the child's dimension and the size factor. For example if widthFactor is 2.0 then the width of this widget will always be twice its child's width.

How it works

The alignment property describes a point in the child's coordinate system and a different point in the coordinate system of this widget. The Align widget positions the child such that both points are lined up on top of each other.

link
The Align widget in this example uses one of the defined constants from Alignment, Alignment.topRight. This places the FlutterLogo in the top right corner of the parent blue Container.

A blue square container with the Flutter logo in the top right corner. 

Center(
  child: Container(
    height: 120.0,
    width: 120.0,
    color: Colors.blue[50],
    child: const Align(
      alignment: Alignment.topRight,
      child: FlutterLogo(
        size: 60,
      ),
    ),
  ),
)

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The Alignment used in the following example defines a single point:

The Alignment class uses a coordinate system with an origin in the center of the Container, as shown with the Icon above. Align will place the FlutterLogo at (36.0, 48.0) according to this coordinate system.

A blue square container with the Flutter logo positioned according to the Alignment specified above. A point is marked at the center of the container for the origin of the Alignment coordinate system. 

Center(
  child: Container(
    height: 120.0,
    width: 120.0,
    color: Colors.blue[50],
    child: const Align(
      alignment: Alignment(0.2, 0.6),
      child: FlutterLogo(
        size: 60,
      ),
    ),
  ),
)

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The FractionalOffset used in the following example defines two points:

  • (0.2 * width of FlutterLogo, 0.6 * height of FlutterLogo) = (12.0, 36.0) in the coordinate system of the blue container.
  • (0.2 * width of Align, 0.6 * height of Align) = (24.0, 72.0) in the coordinate system of the Align widget.

The Align widget positions the FlutterLogo such that the two points are on top of each other. In this example, the top left of the FlutterLogo will be placed at (24.0, 72.0) - (12.0, 36.0) = (12.0, 36.0) from the top left of the Align widget.

The FractionalOffset class uses a coordinate system with an origin in the top-left corner of the Container in difference to the center-oriented system used in the example above with Alignment.

A blue square container with the Flutter logo positioned according to the FractionalOffset specified above. A point is marked at the top left corner of the container for the origin of the FractionalOffset coordinate system. 

Center(
  child: Container(
    height: 120.0,
    width: 120.0,
    color: Colors.blue[50],
    child: const Align(
      alignment: FractionalOffset(0.2, 0.6),
      child: FlutterLogo(
        size: 60,
      ),
    ),
  ),
)

See also:

Inheritance
Implementers

Constructors

Align({Key? key, AlignmentGeometry alignment = Alignment.center, double? widthFactor, double? heightFactor, Widget? child})
Creates an alignment widget.
const

Properties

alignment → AlignmentGeometry
How to align the child.
final
child → Widget?
The widget below this widget in the tree.
finalinherited
hashCode → int
The hash code for this object.
nonVirtual">@nonVirtualread-onlyinherited
heightFactor → double?
If non-null, sets its height to the child's height multiplied by this factor.
final
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
widthFactor → double?
If non-null, sets its width to the child's width multiplied by this factor.
final

Methods

createElement() → SingleChildRenderObjectElement
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
inherited
createRenderObject(BuildContext context) → RenderPositionedBox
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 RenderPositionedBox 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