Flex class
A widget that displays its children in a one-dimensional array.
The Flex widget allows you to control the axis along which the children are placed (horizontal or vertical). This is referred to as the main axis. If you know the main axis in advance, then consider using a Row (if it's horizontal) or Column (if it's vertical) instead, because that will be less verbose.
To cause a child to expand to fill the available vertical space, wrap the child in an Expanded widget.
The Flex widget does not scroll (and in general it is considered an error to have more children in a Flex than will fit in the available room). If you have some widgets and want them to be able to scroll if there is insufficient room, consider using a ListView.
If you only have one child, then rather than using Flex, Row, or Column, consider using Align or Center to position the child.
Layout algorithm
This section describes how a Flex is rendered by the framework. See BoxConstraints for an introduction to box layout models.
Layout for a Flex proceeds in six steps:
- Layout each child a null or zero flex factor (e.g., those that are not Expanded) with unbounded main axis constraints and the incoming cross axis constraints. If the crossAxisAlignment is CrossAxisAlignment.stretch, instead use tight cross axis constraints that match the incoming max extent in the cross axis.
- Divide the remaining main axis space among the children with non-zero flex factors (e.g., those that are Expanded) according to their flex factor. For example, a child with a flex factor of 2.0 will receive twice the amount of main axis space as a child with a flex factor of 1.0.
- Layout each of the remaining children with the same cross axis constraints as in step 1, but instead of using unbounded main axis constraints, use max axis constraints based on the amount of space allocated in step 2. Children with Flexible.fit properties that are FlexFit.tight are given tight constraints (i.e., forced to fill the allocated space), and children with Flexible.fit properties that are FlexFit.loose are given loose constraints (i.e., not forced to fill the allocated space).
- The cross axis extent of the Flex is the maximum cross axis extent of the children (which will always satisfy the incoming constraints).
- The main axis extent of the Flex is determined by the mainAxisSize property. If the mainAxisSize property is MainAxisSize.max, then the main axis extent of the Flex is the max extent of the incoming main axis constraints. If the mainAxisSize property is MainAxisSize.min, then the main axis extent of the Flex is the sum of the main axis extents of the children (subject to the incoming constraints).
- Determine the position for each child according to the mainAxisAlignment and the crossAxisAlignment. For example, if the mainAxisAlignment is MainAxisAlignment.spaceBetween, any main axis space that has not been allocated to children is divided evenly and placed between the children.
See also:
- Row, for a version of this widget that is always horizontal.
- Column, for a version of this widget that is always vertical.
- Expanded, to indicate children that should take all the remaining room.
- Flexible, to indicate children that should share the remaining room but
- Spacer, a widget that takes up space proportional to it's flex value. that may be sized smaller (leaving some remaining room unused).
- The catalog of layout widgets.
- Inheritance
- Implementers
Constructors
-
Flex({Key key, @required Axis direction, MainAxisAlignment mainAxisAlignment: MainAxisAlignment.start, MainAxisSize mainAxisSize: MainAxisSize.max, CrossAxisAlignment crossAxisAlignment: CrossAxisAlignment.center, TextDirection textDirection, VerticalDirection verticalDirection: VerticalDirection.down, TextBaseline textBaseline, List<
Widget> children: const [] }) - Creates a flex layout. [...]
Properties
- crossAxisAlignment → CrossAxisAlignment
-
How the children should be placed along the cross axis. [...]
final
- direction → Axis
-
The direction to use as the main axis. [...]
final
- mainAxisAlignment → MainAxisAlignment
-
How the children should be placed along the main axis. [...]
final
- mainAxisSize → MainAxisSize
-
How much space should be occupied in the main axis. [...]
final
- textBaseline → TextBaseline
-
If aligning items according to their baseline, which baseline to use.
final
- textDirection → TextDirection
-
Determines the order to lay children out horizontally and how to interpret
start
andend
in the horizontal direction. [...]final - verticalDirection → VerticalDirection
-
Determines the order to lay children out vertically and how to interpret
start
andend
in the vertical direction. [...]final -
children
→ List<
Widget> -
The widgets below this widget in the tree. [...]
final, inherited
- hashCode → int
-
The hash code for this object. [...]
read-only, inherited
- key → Key
-
Controls how one widget replaces another widget in the tree. [...]
final, inherited
- runtimeType → Type
-
A representation of the runtime type of the object.
read-only, inherited
Methods
-
createRenderObject(
BuildContext context) → RenderFlex -
Creates an instance of the RenderObject class that this
RenderObjectWidget represents, using the configuration described by this
RenderObjectWidget. [...]
override
-
debugFillProperties(
DiagnosticPropertiesBuilder properties) → void -
Add additional properties associated with the node. [...]
override
-
getEffectiveTextDirection(
BuildContext context) → TextDirection -
The value to pass to RenderFlex.textDirection. [...]
@protected
-
updateRenderObject(
BuildContext context, covariant RenderFlex 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
-
createElement(
) → MultiChildRenderObjectElement -
RenderObjectWidgets always inflate to a RenderObjectElement subclass.
inherited
-
debugDescribeChildren(
) → List< DiagnosticsNode> -
Returns a list of DiagnosticsNode objects describing this node's
children. [...]
@protected, inherited
-
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, inherited
-
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 toStringDeep. [...]
inherited
-
toString(
{DiagnosticLevel minLevel: DiagnosticLevel.debug }) → String -
Returns 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 ==(
dynamic other) → bool -
The equality operator. [...]
inherited