Documentation
Stretch is available for Rust, Kotlin, Swift, and JavaScript. We will be adding more language bindings over time. For general help on using flexbox we suggest reading css-tricks.com/snippets/css/a-guide-to-flexbox and playing around with flexboxfroggy.com. Or you can always read the spec itself at www.w3.org/TR/css-flexbox-1
Installation
Start by creating a new project using npm
/ yarn
and adding the stretch-layout
dependency.
npm init wasm-app my-app
cd my-app
npm install --save stretch-layout
We are ready to use Stretch! Open up index.js
and replace its contents with the following. Once finished you should be able to run it with npm run start
to see the results of this basic layout calculation in the logs.
index.js
import { Allocator, Node } from 'stretch-layout';
const allocator = new Allocator();
const node = new Node(allocator, {width: 100, height: 100});
const layout = node.computeLayout();
console.log(layout.width, layout.height);
To integrate Stretch into an existing app you will need to use a bundler such as webpack which supports web assembly. Have a look at the code generated by the project setup above to get a sense of what configuration is needed.
Basics
Node
s are the core building blocks of a stretch layout tree. A node needs a Style
which describes the flexbox
properties for the node. By adding nodes as children to a parent node we create a tree which we can perform layout on. The result of a layout computation is a tree of Layout
nodes. This tree matches the structure of the Node
tree but contains the computed layout of each node.
index.js
import { Allocator, Node } from 'stretch-layout';
const allocator = new Allocator();
const node = new Node(allocator);
node.addChild(new Node(allocator,{width: 100, height: 100}));
const layout = node.computeLayout();
layout.width; // 100.0
layout.height; // 100.0
layout.childCount; // 1
Nodes can be be mutated and stretch will automatically recompute only the subtrees which have changed. This is incredibly powerful if you have a large node tree, perhaps representing the UI of an app, and only need to change the height of a single element.
index.js
import { Allocator, Node } from 'stretch-layout';
const allocator = new Allocator();
const node = new Node(allocator);
node.addChild(new Node(allocator, {width: 100, height: 100}));
node.computeLayout();
// Mutate node
node.setStyle({width: 100, height: 100})
// This call will return partially cached results
node.computeLayout();
Allocator
All stretch nodes are allocated within an allocator. All nodes in a tree should share the same allocator but can also be used across trees. Usually apps create a single Allocator.
constructor()
Node
Create a new node with a given style. Style may be undefined to create a node with default styles however this is not very useful.
constructor(allocator: Allocator, style: Style)
Set the measure function on a node. This function will be called on leaf nodes to resolve the intrinsic size of the node given a set of constraints.
setMeasure(measure: (width: number?, height: number?) => {
width: number,
height: number,
})
Adds a child to an existing node.
addChild(child: Node)
Removes a previously added child from an existing node.
removeChild(child: Node)
Removes the child at the given index of an existing node.
removeChildAtIndex(index: number)
Replaces the child at the given index of an existing node with the given new child.
replaceChildAtIndex(index: number, child: Node)
Returns the number of children associated with this node.
readonly childCount: number
Updates the style of an existing node.
setStyle(style: Style)
Returns the current style of a node.
getStyle(): Style
Marks the node as dirty and propagates this up the node tree. This function is called internally for any mutating function. The only time you have to ensure to call it manually is if some application state has changed which will effect the intrinsic size of a leaf node. For example if the text of a text node has changed.
markDirty()
Returns whether of not this node is marked as dirty and will need to be recalculated during the next layout pass.
isDirty(): boolean
Computes the layout of a node tree given a size. The size passed in is used to compute percentage sizes on the root node. If you don't need to do this then it is fine to pass {null, null}
however you will typically pass in the size of the viewport which will host this computed layout. The resulting tree of Layout
nodes mimics the structure of the computed Node
tree and includes position and size information..
computeLayout(size: {width?: number, height?: number})
Style
The Style
object contains all the properties associated with flexbox as well as some properties which we found useful outside of flexbox. For example positionType
can be set to PositionType.Absolute
which puts the node into a absolute layout context instead of a flexbox context. aspectRatio
is another property not part of the flexbox specification which when set ensures the node matches a certain aspect ratio.
type Dimension = string | number; // '100%', '100', 100, or 'auto'
type Style = {
display?: Display,
positionType?: PositionType,
direction?: Direction,
flexDirection?: FlexDirection,
flexWrap?: FlexWrap,
overflow?: Overflow,
alignItems?: AlignItems,
alignSelf?: AlignSelf,
alignContent?: AlignContent,
justifyContent?: JustifyContent,
start?: Dimension,
end?: Dimension,
top?: Dimension,
bottom?: Dimension,
marginStart?: Dimension,
marginEnd?: Dimension,
marginTop?: Dimension,
marginBottom?: Dimension,
paddingStart?: Dimension,
paddingEnd?: Dimension,
paddingTop?: Dimension,
paddingBottom?: Dimension,
boarderStart?: Dimension,
boarderEnd?: Dimension,
boarderTop?: Dimension,
boarderBottom?: Dimension,
flexGrow?: number,
flexShrink?: number,
flexBasis?: Dimension,
width?: Dimension,
height?: Dimension,
minWidth?: Dimension,
minHeight?: Dimension,
maxWidth?: Dimension,
maxHeight?: Dimension,
aspectRatio?: number,
}
AlignItems
describes how to align children along the cross axis of their parent. AlignItems
is very similar to JustifyContent
but instead of applying to the main axis, AlignItems
applies to the cross axis.
FlexStart
aligns children of a container to the start of the container's cross axis.FlexEnd
aligns children of a container to the end of the container's cross axis.Center
aligns children of a container in the center of the container's cross axis.Baseline
aligns children of a container to the baseline of that container's cross axis.Stretch
stretches children of a container to match the height of the container's cross axis.
Optional, if left undefined Stretch
is the default value.
enum AlignItems {
FlexStart,
FlexEnd,
Center,
Baseline,
Stretch,
}
AlignSelf
has the same options and effect as setting AlignItems
on the containing node but instead of affecting the children within the node you can apply this property to a single child to change its alignment within its parent. AlignSelf
overrides any option set on the parent with AlignItems
.
Auto
delegates to the parent'sAlignItems
property.FlexStart
aligns children of a container to the start of the container's cross axis.FlexEnd
aligns children of a container to the end of the container's cross axis.Center
aligns children of a container in the center of the container's cross axis.Baseline
aligns children of a container to the baseline of that container's cross axis.Stretch
stretches children of a container to match the height of the container's cross axis.
Optional, if left undefined Auto
is the default value.
enum AlignSelf {
Auto,
FlexStart,
FlexEnd,
Center,
Baseline,
Stretch,
}
AlignContent
defines the distribution of lines along the cross-axis. This only has effect when items are wrapped to multiple lines using FlexWrap
.
FlexStart
aligns wrapped lines to the start of the container's cross axis.FlexEnd
aligns wrapped lines to the end of the container's cross axis.Center
aligns wrapped lines in the center of the container's cross axis.Stretch
stretches wrapped lines to match the height of the container's cross axis.SpaceBetween
evenly spaces wrapped lines across the container's main axis, distributing remaining space between the lines.SpaceAround
evenly spaces wrapped lines across the container's main axis, distributing remaining space around the lines. Compared to space between using space around will result in space being distributed to the beginning of the first lines and end of the last line.
Optional, if left undefined Stretch
is the default value.
enum AlignContent {
FlexStart,
FlexEnd,
Center,
Stretch,
SpaceBetween,
SpaceAround,
}
The Direction
property specifies whether the component should be laid out in LTR
or RTL
writing mode.
Optional, Inherit
by default (root node will implicitly default to LTR
).
enum Direction {
Inherit,
LTR,
RTL,
}
The Display
property specifies whether the component should be laid out as Flex
or not at all. If None
is specified then the node will have a resulting size of zero. In the future this enum will be expanded to include more display options such as Grid
.
Optional, Flex
by default.
enum Display {
Flex,
None,
}
FlexDirection
controls the direction in which children of a node are laid out. This is also referred to as the main axis. The main axis is the direction in which children are laid out. The cross axis the axis perpendicular to the main axis, or the axis which wrapping lines are laid out in.
Row
aligns children from left to right. If wrapping is enabled then the next line will start under the first item on the left of the container.Column
aligns children from top to bottom. If wrapping is enabled then the next line will start to the left first item on the top of the container.RowReverse
aligns children from right to left. If wrapping is enabled then the next line will start under the first item on the right of the container.ColumnReverse
aligns children from bottom to top. If wrapping is enabled then the next line will start to the left first item on the bottom of the container.
Optional, if left undefined Row
is the default value.
enum FlexDirection {
Row,
Column,
RowReverse,
ColumnReverse,
}
JustifyContent
describes how to align children within the main axis of their container. For example, you can use this property to center a child horizontally within a container with FlexDirection
set to FlexDirection.Row
or vertically within a container with FlexDirection
set to FlexDirection.Column
.
FlexStart
aligns children of a container to the start of the container's main axis.FlexEnd
aligns children of a container to the end of the container's main axis.Center
aligns children of a container in the center of the container's main axis.SpaceBetween
evenly spaces of children across the container's main axis, distributing remaining space between the children.SpaceAround
evenly spaces of children across the container's main axis, distributing remaining space around the children. Compared to space between usingSpaceAround
will result in space being distributed to the beginning of the first child and end of the last child.SpaceEvenly
evenly spaces of children across the container's main axis, distributing remaining space around the children so that visually there is equal space around every child including at the edges.
Optional, if left undefined FlexStart
is the default value.
enum JustifyContent {
FlexStart,
FlexEnd,
Center,
SpaceBetween,
SpaceAround,
SpaceEvenly,
}
The Overflow
property specifies how the node should handle children which overflow their parent's size. This property is currently not used but may be used in the future.
Optional, Visible
by default.
enum Overflow {
Visible,
Hidden,
Scroll,
}
The PositionType
property specifies whether the component should be laid out relative to its sibling or absolutely positioned within its parent.
Optional, Relative
by default.
enum PositionType {
Relative,
Absolute,
}
The FlexWrap
property controls what happens when children overflow the size of the container along the main axis. By default children are forced into a single line (which can shrink components).
Wrap
enables wrapping of children if they overflow the main axis of the container.NoWrap
disables wrapping of children, children will instead shrink to fit the main axis or overflow depending on theirflexShrink
setting.WrapReverse
enables wrapping of children if they overflow the main axis of the container. UnlikeWrap
, lines are laid out in reverse order.
Optional, if left undefined NoWrap
is the default value.
enum FlexWrap {
NoWrap,
Wrap,
WrapReverse,
}
When PositionType
is set to PositionType.Relative
the start
, end
, top
, and bottom
properties specify the offset the node should have from its default position. In the case position is set to PositionType.Absolute
these properties indicate the distance this node's edge should be from the container's corresponding edge.
All position properties are optional and undefined
by default.
start: Dimension
end: Dimension
top: Dimension
bottom: Dimension
Margin effects the spacing around the outside of the node it is applied to. A node with margin will offset itself from the bounds of its parent but also offset the location of any siblings. If margin is set to auto then the margin is equal to the remaining space in that axis divided between all other auto margin values in that axis.
As apposed to layout engines Stretch does not support setting left/right margin but instead uses start/end to ensure equal support for right-to-left as well as left-to-right locales. In left-to-right locales start corresponds to left and end to right, the opposite is true in right-to-left locales.
All margin properties are optional and undefined
by default, resulting in no margin.
marginStart: Dimension
marginEnd: Dimension
marginTop: Dimension
marginBottom: Dimension
Padding affects the size of the node it is applied to. Padding will not add to the total size of a node if it has an explicit size set, instead it will inset the content. For auto-sized nodes padding will increase the size of the node as well as offset the location of any children.
As apposed to layout engines Stretch does not support setting left/right padding but instead uses start/end to ensure equal support for right-to-left as well as left-to-right locales. In left-to-right locales start corresponds to left and end to right, the opposite is true in right-to-left locales.
All padding properties are optional and undefined
by default, resulting in no padding.
paddingStart: Dimension
paddingEnd: Dimension
paddingTop: Dimension
paddingBottom: Dimension
Border affects the size of the node it is applied to. Border will not add to the total size of a node if it has an explicit size set, instead it will inset the content. For auto-sized nodes border will increase the size of the node as well as offset the location of any children.
As apposed to layout engines Stretch does not support setting left/right border but instead uses start/end to ensure equal support for right-to-left as well as left-to-right locales. In left-to-right locales start corresponds to left and end to right, the opposite is true in right-to-left locales.
All border properties are optional and undefined
by default, resulting in no border.
boarderStart: Dimension
boarderEnd: Dimension
boarderTop: Dimension
boarderBottom: Dimension
Flex grow describes how any space within a container should be distributed among its children along the main axis. After laying out its children, a container will distribute any remaining space according to the flex grow values specified by its children. Flex grow accepts any floating point value >= 0. A container will distribute any remaining space among its children weighted by the child’s flex grow value. If flexGrow
is set to 0 the node will not grow.
Optional, if left undefined 0.0
is the default value.
flexGrow: number
Flex shrink describes how to shrink children along the main axis in the case that the total size of the children overflow the size of the container on the main axis. flex shrink is very similar to flex grow and can be thought of in the same way if any overflowing size is considered to be negative remaining space. These two properties also work well together by allowing children to grow and shrink as needed. Flex shrink accepts any floating point value >= 0. A container will shrink its children weighted by the child’s flex shrink value. If flexShrink
is set to 0 the node will not shrink.
Optional, if left undefined 1.0
is the default value.
flexShrink: number
Flex basis is an axis-independent way of providing the default size of an item along the main axis. Setting the flex basis of a child is similar to setting the width of that child if its parent is a container with FlexDirection.Row
or setting the height of a child if its parent is a container with FlexDirection.Column
. The flex basis of an item is the default size of that item, the size of the item before any flex grow and flex shrink calculations are performed.
Optional, similar to width and height the default value is undefined which means that the flex basis is determined by the size of the node's content.
flexBasis: Dimension
The width
and height
properties specify the size of the node’s content area. Size can be specified in pixels, points, or a percentage of the parent size. If set to undefined
or 'auto'
the size is calculated based on the node’s content. What is meant by content is based on the kind of node, for example the size of a text node is the size of the text being displayed while the size of a flexbox node is the total size of its children.
Optional, 'auto'
by default.
width: Dimension
height: Dimension
The maximum size of a node, these properties take priority over all other properties. If left undefined the node has no maximum size.
Optional, undefined
by default.
minWidth: Dimension
minHeight: Dimension
The minimum size of a node, these properties take priority over all other properties. If left undefined the node has no minimum size.
Optional, undefined
by default.
maxWidth: Dimension
maxHeight: Dimension
Configures the aspect ratio of a node, very useful for media content.
- Accepts any floating point value > 0.
- Defined as the ratio between the width and the height of a node e.g. if a node has an aspect ratio of 2 then its width is twice the size of its height.
- Respects the min and max dimensions of an item.
- Has higher priority than flex grow
- If aspect ratio, width, and height are set then the cross axis dimension is overridden.
Optional, if left undefined no aspect ratio is enforced.
aspectRatio: number
Layout
Layout
nodes are returned from a layout calculation and mimic the structure of the computed Node
tree.
class Layout {
free(): void;
readonly width: number;
readonly height: number;
readonly x: number;
readonly y: number;
readonly childCount: number;
child(at: number): Layout;
}