About sizing and positioning
layout has two parts: component sizing and component positioning.
Flex controls the layout of components by using a set of rules.
These layout rules are a combination of sizing rules for individual
components, and sizing and positioning rules for containers.
Because Flex defines default layout rules, you often do not have
to set the size or position of components. Instead, you can concentrate
on building the logic of your application and let Flex control the
While Flex has default layout rules, you can use the component’s
properties and methods to customize the layout. For example, all
components have several properties, including height and width,
for specifying the component’s size in absolute or container-relative
Each container also has properties and styles that you can use
to configure aspects of layout. You can also use different positioning
techniques for laying out components in a container. For example,
some containers support absolute x- and y-coordinate–based
About layout with Spark and MX containers
While the sizing and layout rules for Spark and MX containers
are almost identical, there are a few differences between the two
types of containers:
Spark containers let you specify the layout class
assigned to the container, such as BasicLayout, HorizontalLayout,
VerticalLayout, and TileLayout.
MX containers have a predefined
layout built in to the class. Therefore, the MX VBox container always
lays out its children in a vertical column, and the MX Tile container
always lays out its children in rows and columns. The only exceptions
to this rule are the MX Application and MX Panel containers. These containers
let you switch among absolute, vertical, and horizontal layout.
Spark containers use properties on the layout class to control
the layout of the container. For example, use the TileLayout.verticalGap and TileLayout.horizontalGap properties
to set the gaps between children of a Spark container that uses
the TileLayout class.
MX containers use properties and styles
of the container class. Therefore, to set the gaps for a MX container,
set the verticalGap and horizontalGap styles
on the container.
About the Layout Manager
The Layout Manager controls
layout in Flex. The manager uses the following three-stage process
to determine the size and position of each component in an application:
- Stage 1 - Commitment pass
- Determines the property settings of the application’s components.
This pass allows components whose contents depend on property settings
to configure themselves before Flex determines their sizes and positions.
the commitment pass, the Layout Manager causes each component to run
its commitProperties() method, which determines
the property values.
- Stage 2 - Measurement pass
- Calculates the default size of every component in the application.
This pass starts from the most deeply nested components and works
out toward the application container.
The measurement pass
determines the measured, or default, size of each component.
The default size of each container is based on the default or explicit (if
specified) sizes of its children. For example, an SkinnableContainer
that uses the HorizontalLayout class has a default width equal to
the sum of the default or explicit widths of all its children, plus
the thickness of the borders, any padding, and the gaps between
During the measurement pass, the Layout Manager
causes each component to call its measure() method
to determine the component’s default size. All sizing properties
are calculated before Flex applies any transform to the component, such
as a rotation, skew, or other transform.
- Stage 3 - Layout pass
- Lays out your application, including moving and resizing any
components. This pass starts from the outermost container and works
in toward the innermost component. The layout pass determines the
actual size and placement of each component. It also does any programmatic
drawing, such as calls to the lineTo() or drawRect() methods.
Layout Manager causes each component to run its updateDisplayList() method
to lay out the component’s children; for this reason, this pass
is also referred to as the update pass.
When lay out occurs
lays out components when your application initializes. Flex also
performs a layout pass when the application or a user does something
that affects the sizes or positions of visual elements, including
The application changes properties that specify sizing
or positioning, such as x, y, width, height, scaleX, scaleY, rotationX, rotationY,
and the transform matrix.
A change affects the calculated width or height of a component,
such as when the label text for a Button control changes, or the
user resizes a component.
child is added or removed from a container, a child is resized,
or a child is moved. For example, if your application can change
the size of a component, Flex updates the layout of the container
to reposition its children, based on the new size of the child.
A property or style that requires measurement and drawing,
such as the fontFamily, changes.
Manually forcing layout
Sometimes, you might have to programmatically cause Flex
to update application layout. Situations where you must force a
layout include the following:
When printing multiple page data grids by using the PrintDataGrid
Before playing an effect, if the start values have just been
set on the target.
When capturing bitmap data after making property changes.
To force a layout, call the validateNow() method
of the component to lay out. This method causes Flex to validate
and update the properties, sizes, and layout of the object and all
its children, and to redraw them, if necessary. Because this method
is computation-intensive, be careful to call it only when it is necessary.
About component sizing
Flex provides several ways for you to control the size
of controls and containers:
- Default sizing
- Flex automatically determines the sizes of controls and containers.
- Explicit sizing
- You set the height and width properties
of the component to absolute values.
- Percentage-based sizing
- You specify the component size as a percentage of its parent
- Constraint-based layout
- You control size and position by anchoring component’s sides
to locations in their container.
For details on controlling
component sizes, see Sizing components.
About component positioning
Flex provides two mechanisms for positioning controls:
- Automatic positioning
automatically positions a container’s children according to a set
of container- and component-specific rules. Most containers use
automatic positioning. Automatic positioning is sometimes referred
to as automatic layout. One of the differences between Spark and
MX containers is that the layout algorithm for MX containers is
fixed, but for Spark containers it is selectable by changing the
container’s layout property.
- Absolute positioning
specify each child’s x and y properties,
or use a constraint-based layout that specifies the distance between
one or more of the container’s sides and the child’s sides, baseline,
or center. Absolute positioning is sometimes referred to as absolute
layout. Spark containers with a layout of BasicLayout and the MX
Canvas container use absolute layout. You can also choose absolute
layout for the MX Application and Panel containers.
For details on controlling the positions of controls, see Positioning components.
About basic layout properties and methods
While Flex provides many properties that you can use to
control and monitor component layout, there are a few that you use
frequently. The following figure shows these properties and methods:
This figure shows how Flex lays out a component in its parent
container. The x, y, height,
and width properties of the component determine
its size before any skins or transformations are applied to it.
These are properties that you can set when defining your application.
After Flex applies any skins or transformations to the component,
Flex uses the rectangular area, called the bounding box,
to position the component in its parent container. The bounding
box defines the rectangular are taken up by the component in its
parent container. In the absence of any skin or transformation, the
size of the bounding box is the same as the height and width of
A graphical component, such as Ellipse and Rect, can define a
stroke for their border. If the component defines a stroke, the
center of the stroke runs along the outer edge of the component.
For example, if you define a two-pixels wide border, the border
appears in the area one-pixel outside the component to one-pixel
inside the component. The border has to fit completely within the bounding
box. Therefore, the upper-left corner of the component might be offset
from the edge of the bounding box to accommodate the border.
The following table describes the basic component layout properties
Property or method
The x and y coordinate of the upper-left
corner of the component in the parent container. You can set these
properties for the children of a container that use absolute layout.
Otherwise, Flex calculates these values for you.
The height and width of the component. You
can set these properties as a pixel value or as a percent of the
size of the parent container. Otherwise, Flex calculates these values
top, bottom, left, right
The distances between the sides of the component
and sides of the parent container. Use these properties when performing
constraint-based layout. Only containers that use absolute layout
support constraint-based layout. For more information, see Using constraints to control component layout.
The x and y coordinates of the bounding
box of the component in the parent container after any skins and
transformations have been applied to the component.
The height hand width of the bounding box
of the component in the parent container after any skins and transformations
have been applied to the component. \
Basic layout rules and considerations
Flex performs layout according to the following basic rules.
If you remember these rules, you should be able to easily understand
the details of Flex layout. These rules help you determine why Flex
lays out your application as it does and to determine how to modify
your application appearance.
For a detailed description of how Flex sizes components, see Determining and controlling component sizes. For detailed information
on component positioning, see Positioning components.
Flex first determines all components’ measured (default)
or explicitly set sizes up, from the innermost child components
to the outermost (Application) component. This is done in the measurement
After the measurement pass, Flex determines all percentage-based
sizes and constraints applied to the components. Flex then lays
out components down, from the outermost container to the
innermost components. This is done in the layout pass.
Sizes that you set to a pixel value are mandatory and fixed,
and override any maximum or minimum size specifications that you
set for the component.
The default sizes determined in the measurement pass specify
the sizes of components that do not have explicit or percentage-based
sizes (or use constraint-based layout), and are fixed.
Percentage-based size specifications are advisory, and are
relative to the size of the parent container. The layout algorithms
satisfy the request if possible, and use the percentage values to
determine proportional sizes, but the actual sizes can be less than
the requested sizes. Percentage-based sizes are always within the
component’s maximum and minimum sizes, and, subject to those bounds,
don’t cause a container’s children to exceed the container size.
Component layout patterns
Flex uses different patterns to lay out
different containers and their children. These patterns generally
fit in the type categories listed in the following table. The table
describes the general layout behavior for each type, how the default size
of the container is determined, and how Flex sizes percentage-based children.
Default layout behavior
Spark container using BasicLayout
Canvas container or Application or Panel container with layout="absolute"
General layout: Children of the container
do not interact. That is, children can overlap and the position
of one child does not affect the position of any other child. You
specify the child positions explicitly or use constraints to anchor
the sides, baselines, or centers of the children relative to the
Default sizing: The measurement pass
finds the child with the lowest bottom edge and the child with the
rightmost edge, and uses these values to determine the container
Percentage-based children: Sizing uses different
rules depending on whether you use constraint-based layout or x- and y-
coordinate positioning. See Flex component sizing techniques.
Containers that arrange children linearly
Spark containers using HorizontalLayout
MX Box, DividedBox, Form, Panel, TitleWindow containers
General layout: All children of the
container are arranged in a single row or column. Each child’s height
and width can differ from all other children’s heights or widths.
Default sizing: The
container fits the default or explicit sizes of all children and
all gaps, borders, and padding.
Percentage-based children: If
children with percentage-based sizing request more than the available
space, the actual sizes are set to fit in the space, proportionate
to the requested percentages.
MX Grid container
General layout: A container with
a vertical layout of rows. Each row is a container with a horizontal
layout of child controls, where all items are constrained to align
with each other. The heights of all the cells in a single row are
the same, but each row can have a different height. The widths of
all cells in a single column are the same, but each column can have
a different width. You can define a different number of cells for
each row or each column of the Grid container, and individual cells
can span columns or rows.
Default sizing: The grid
fits the individual rows and children at their default sizes.
Percentage-based children: If
children use percentage-based sizing, the sizing rules fit the children
GridItem components within their rows, and GridRow components within the
grid size according to linear container sizing rules.
Spark container using TileLayout
General layout: The container is
a grid of equal-sized cells. The cells can be in row-first or column-first
For MX, if you do not specify explicit or percentage-based dimensions,
the container has as close as possible to an equal number of rows
and columns, with the direction property determining
the orientation with the larger number of items, if necessary.
Spark, if you do not specify explicit or percentage-based dimensions,
the container arranges the rows and columns so that the container
has an equal height and width. The orientation property
determining the orientation with the larger number of items, if
Default sizing: If you do not specify tileWidth and tileHeight properties
(MX Tile container) or columnWidth and rowHeight (Spark
Tile layout), the container uses the measured or explicit size of
the largest child cell for the size of each child cell.
Percentage-based children: The
percentage-based sizes of a child component specify a percentage
of the individual cell, not of the Tile container.
MX ViewStack, Accordion, and TabNavigator containers
General layout: The container displays
one child at a time.
Default sizing: The container
size is determined by the measured or explicit dimensions of the
initially selected child, and thereafter, all children are forced
to be that initial size. If you set the resizeToChild property
to true the container resizes to accommodate the
measured or explicit size of each child, as that child appears.
Percentage-based children: As
a general rule, you either use 100% for both height and width, which
causes the children to fill the navigator bounds, or do not use