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 space in the direction of this widget's main axis, 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.

The Flex widget does not allow its children to wrap across multiple horizontal or vertical runs. For a widget that allows its children to wrap, consider using the Wrap widget instead of Flex.

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:

1. Layout each child with 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.
2. 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.
3. 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).
4. The cross axis extent of the Flex is the maximum cross axis extent of the children (which will always satisfy the incoming constraints).
5. 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).
6. 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.
• Spacer, a widget that takes up space proportional to its flex value. that may be sized smaller (leaving some remaining room unused).
• Wrap, for a widget that allows its children to wrap over multiple runs.
• The catalog of layout widgets.