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<pre class='metadata'>
Title: CSS Layout API Level 1
Status: ED
Group: houdini
TR: https://www.w3.org/TR/css-layout-api-1/
ED: https://drafts.css-houdini.org/css-layout-api-1/
Shortname: css-layout-api
Level: 1
Abstract:
An API for allowing web developers to define their own layout modes with javascript.
See <a href="https://github.com/w3c/css-houdini-drafts/blob/master/css-layout-api/EXPLAINER.md">EXPLAINER</a>.
Editor: Greg Whitworth, gwhit@microsoft.com, w3cid 69511
Editor: Ian Kilpatrick, ikilpatrick@chromium.org, w3cid 73001
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/, w3cid 42199
Former Editor: Shane Stephens, shanestephens@google.com, w3cid 47691
Editor: Robert O'Callahan, robert@ocallahan.org
Editor: Rossen Atanassov, rossen.atanassov@microsoft.com, w3cid 49885
Ignored Terms: LayoutWorklet
Ignored Terms: create a workletglobalscope
</pre>
<style>
/* Put nice boxes around each algorithm. */
[data-algorithm]:not(.heading) {
padding: .5em;
border: thin solid #ddd; border-radius: .5em;
margin: .5em calc(-0.5em - 1px);
}
[data-algorithm]:not(.heading) > :first-child {
margin-top: 0;
}
[data-algorithm]:not(.heading) > :last-child {
margin-bottom: 0;
}
</style>
<pre class="link-defaults">
spec:css-break-3; type:dfn; text:fragment
spec:css-display-3; type:dfn; text:box
spec:css-display-3; type:value; for:display; text:none
spec:css-display-3; type:value; for:<display-inside>; text:grid
spec:css-display-3; type:value; for:<display-outside>; text:inline
spec:css-pseudo-4; type:selector; text:::after
spec:css-pseudo-4; type:selector; text:::before
spec:css-pseudo-4; type:selector; text:::first-letter
spec:css-pseudo-4; type:selector; text:::first-line
spec:dom; type:dfn; for:/; text:element
spec:infra; type:dfn; text:list
spec:html; type:dfn; for:global object; text:realm
spec:css22; type:property;
text:max-height
text:min-height
text:min-width
spec:css2; type:property; text:max-width
</pre>
<pre class="anchors">
urlPrefix: https://heycam.github.io/webidl/; type: dfn;
urlPrefix: #dfn-;
url: throw; text: thrown
urlPrefix: https://tc39.github.io/ecma262/#sec-; type: dfn;
text: constructor
text: Construct
url: ecmascript-data-types-and-values; text: type
url: get-o-p; text: Get
url: terms-and-definitions-function; text: function
urlPrefix: native-error-types-used-in-this-standard-
text: TypeError
urlPrefix: https://drafts.csswg.org/css-sizing/#; type: dfn
text: intrinsic sizes
urlPrefix: https://drafts.csswg.org/css-break/#; type: dfn
text: fragmentation break
urlPrefix: https://www.w3.org/TR/CSS21/; type:dfn
urlPrefix: box.html#;
url: box-dimensions; text: box model edges
urlPrefix: visudet.html#;
text: static position
urlPrefix: https://html.spec.whatwg.org/#; type: dfn
text: structuredserializeforstorage
text: structureddeserialize
</pre>
Introduction {#intro}
=====================
<em>This section is not normative.</em>
The layout stage of CSS is responsible for generating and positioning [=fragments=] from the [=box
tree=].
This specification describes an API which allows developers to layout a [=box=] in response to
computed style and [=box tree=] changes.
For a high level overview of this API, see the <a
href="https://github.com/w3c/css-houdini-drafts/blob/master/css-layout-api/EXPLAINER.md">EXPLAINER</a>.
Layout API Containers {#layout-api-containers}
==============================================
A new <a href="https://www.w3.org/TR/css-values-3/#comb-one">alternative</a> value is added
to the <<display-inside>> production: <code>layout(<<ident>>)</code>.
<dl dfn-for="display" dfn-type=value>
<dt><dfn>layout()</dfn>
<dd>
This value causes an element to generate a [=layout API container=] box.
</dl>
A <dfn>layout API container</dfn> is the box generated by an element with a <<display-inside>>
[=computed value=] ''layout()''.
A [=layout API container=] establishes a new <dfn>layout API formatting context</dfn> for its
contents. This is the same as establishing a block formatting context, except that the layout
provided by the author is used instead of the block layout.
For example, floats do not intrude into the layout API container, and the layout API container's
margins do not collapse with the margins of its contents.
[=Layout API containers=] form a containing block for their contents <a
href="https://www.w3.org/TR/CSS2/visudet.html#containing-block-details">exactly like block
containers do</a>. [[!CSS21]]
Note: In a future level of the specification there may be a way to override the containing block
behaviour.
The 'overflow' property applies to [=layout API containers=]. This is discussed in
[[#interaction-overflow]].
As the layout is entirely up to the author, properties which are used in other layout modes (e.g.
flex, block) may not apply. For example an author may not respect the 'margin' property on children.
<div class="example">
The HTML below shows an example of setting the ''display'' to a ''layout()'' function, if the CSS
Layout API is supported.
<pre class="lang-html">
<!DOCTYPE html>
<style>
@supports (display: layout(centering)) {
.centering-layout { display: layout(centering); }
}
</style>
<div class="centering-layout"></div>
</pre>
</div>
Layout API Container Painting {#painting}
-----------------------------------------
[=Layout API Container=] children paint exactly the same as inline blocks [[!CSS21]], except that
the order in which they are returned from the layout method (via
{{FragmentResultOptions/childFragments}}) is used in place of raw document order, and 'z-index'
values other than ''z-index/auto'' create a stacking context even if 'position' is ''static''.
Box Tree Transformations {#layout-api-box-tree}
-----------------------------------------------
The inflow children of a [=layout API container=] can act in different ways depending on the value
of [=document layout definition/layout options'=] {{LayoutOptions/childDisplay}} (set by
<code>layoutOptions</code> on the class).
If the value of [=document layout definition/layout options'=] {{LayoutOptions/childDisplay}} is
<code>"block"</code> the 'display' value of that child is [=blockified=]. This is similar to
children of [=flex containers=] or [=grid containers=]. See [[!css3-display]].
If the value of [=document layout definition/layout options'=] {{LayoutOptions/childDisplay}} is
<code>"normal"</code>, no [=blockification=] occurs. Instead children with a <<display-outside>>
[=computed value=] of ''inline'' (a [=root inline box=]) will produce a single {{LayoutFragment}}
representing each line when {{LayoutChild/layoutNextFragment()}} is called.
Note: This allows authors to adjust the available inline size of each line, and position each line
separately.
Children of a {{LayoutChild}} which represents [=root inline box=] also have some additional
transformations.
- A [=block-level=] box inside a [=inline-level=] box is [=inlinified=] I.e. its
<<display-outside>> is set to ''inline''.
- A [=float=] inside a [=inline-level=] box is not taken out of flow. Instead it must be treated as
inflow, and be [=inlinified=].
In both of the above cases the children become [=atomic inlines=].
Note: User agents would not perform any "inline splitting" or fragmenting when they encounter a
[=block-level=] box.
<div class="note">
Note: In the example below "inline-span" would be represented as a single {{LayoutChild}} with
both "block" and "float" being [=atomic inlines=].
<pre class="lang-html">
<span id="inline-span">
Text
<div id="block"></div>
<div id="float"></div>
Text
</span>
</pre>
</div>
Layout Worklet {#layout-worklet}
================================
The {{layoutWorklet}} attribute allows access to the {{Worklet}} responsible for all the classes
which are related to layout.
The {{layoutWorklet}}'s [=worklet global scope type=] is {{LayoutWorkletGlobalScope}}.
<pre class='idl'>
partial namespace CSS {
[SameObject] readonly attribute Worklet layoutWorklet;
};
</pre>
The {{LayoutWorkletGlobalScope}} is the global execution context of the {{layoutWorklet}}.
<pre class='idl'>
[Global=(Worklet,LayoutWorklet),Exposed=LayoutWorklet]
interface LayoutWorkletGlobalScope : WorkletGlobalScope {
void registerLayout(DOMString name, VoidFunction layoutCtor);
};
</pre>
<div class='example'>
Web developers can feature detect by:
<pre class='lang-javascript'>
if ('layoutWorklet' in CSS) {
console.log('CSS Layout API available!');
}
</pre>
</div>
Concepts {#concepts}
--------------------
This section describes internal data-structures created when {{registerLayout(name, layoutCtor)}} is
called.
A <dfn>layout definition</dfn> is a [=struct=] which describes the information needed by the
{{LayoutWorkletGlobalScope}} about the author defined layout (which can be referenced by the
''layout()'' function). It consists of:
- <dfn for="layout definition">class constructor</dfn> which is the class [=constructor=].
- <dfn for="layout definition">layout function</dfn> which is the layout [=function=] callback.
- <dfn for="layout definition">intrinsic sizes function</dfn> which is the intrinsic sizes
[=function=] callback.
- <dfn for="layout definition">constructor valid flag</dfn>.
- <dfn for="layout definition">input properties</dfn> which is a [=list=] of
<code>DOMStrings</code>.
- <dfn for="layout definition">child input properties</dfn> which is a [=list=] of
<code>DOMStrings</code>.
- <dfn for="layout definition">layout options</dfn> a {{LayoutOptions}}.
A <dfn>document layout definition</dfn> is a [=struct=] which describes the information needed by
the [=document=] about the author defined layout (which can be referenced by the ''layout()''
function). It consists of:
- <dfn for="document layout definition">input properties</dfn> which is a [=list=] of
<code>DOMStrings</code>
- <dfn for="document layout definition">child input properties</dfn> which is a [=list=] of
<code>DOMStrings</code>.
- <dfn for="document layout definition">layout options</dfn> a {{LayoutOptions}}.
Registering A Layout {#registering-layout}
------------------------------------------
The section describes how a web developer uses {{registerLayout(name, layoutCtor)}} to register a
layout.
<pre class='idl'>
[Exposed=LayoutWorklet]
dictionary LayoutOptions {
ChildDisplayType childDisplay = "block";
LayoutSizingMode sizing = "block-like";
};
[Exposed=LayoutWorklet]
enum ChildDisplayType {
"block", // default - "blockifies" the child boxes.
"normal",
};
[Exposed=LayoutWorklet]
enum LayoutSizingMode {
"block-like", // default - Sizing behaves like block containers.
"manual", // Sizing is specified by the web developer.
};
</pre>
The [=document=] has a [=map=] of <dfn>document layout definitions</dfn>. Initially this map is
empty; it is populated when {{registerLayout(name, layoutCtor)}} is called.
The {{LayoutWorkletGlobalScope}} has a [=map=] of <dfn>layout definitions</dfn>. Initially this map
is empty; it is populated when {{registerLayout(name, layoutCtor)}} is called.
Each [=box=] representing a [=layout API container=] has a [=map=] of <dfn>layout class
instances</dfn>. Initially this map is empty; it is populated when the user agent calls either
[=determine the intrinsic sizes=] or [=generate a fragment=] for a [=box=].
Each [=box=] representing a [=layout API container=] has a <dfn>styleMap</dfn> internal slot.
This is a {{StylePropertyMapReadOnly}} which contains the properties listed in
<code>inputProperties</code>.
The user agent <em>clear</em> the [=styleMap=] internal slot for a [=box=] when:
- The [=computed values=] of [=document layout definition/input properties=] for the [=box=]
changes.
- When the [=box=] is removed from the [=box tree=].
- Every 1000 layout passes.
Note: The above rule exists to ensure that web developers do not rely on being able to store
non-regeneratable state on the {{StylePropertyMapReadOnly}} object.
The 1000 limit was picked as a high upper bound, this limit may improve (downwards) over
time.
<div class='note'>
Note: The shape of the class should be:
<pre class='lang-javascript'>
registerLayout('example', class {
static inputProperties = ['--foo'];
static childrenInputProperties = ['--bar'];
static layoutOptions = {
childDisplay: 'normal',
sizing: 'block-like'
};
async intrinsicSizes(children, edges, styleMap) {
// Intrinsic sizes code goes here.
}
async layout(children, edges, constraints, styleMap, breakToken) {
// Layout code goes here.
}
});
</pre>
</div>
</div>
The algorithm below is run when the {{registerLayout(name, layoutCtor)}} is called. It notifies the
user agent layout engine about the new user defined layout.
<div algorithm>
When the <dfn method for=LayoutWorkletGlobalScope>registerLayout(|name|, |layoutCtor|)</dfn> method
is called, the user agent <em>must</em> run the following steps:
1. If the |name| is an empty string, [=throw=] a [=TypeError=] and abort all these steps.
2. Let |layoutDefinitionMap| be {{LayoutWorkletGlobalScope}}'s [=layout definitions=] map.
3. If |layoutDefinitionMap|[|name|] [=map/exists=] [=throw=] a "{{InvalidModificationError}}"
{{DOMException}} and abort all these steps.
4. Let |inputProperties| be an empty <code>sequence<DOMString></code>.
5. Let |inputPropertiesIterable| be the result of [=Get=](|layoutCtor|, "inputProperties").
6. If |inputPropertiesIterable| is not undefined, then set |inputProperties| to the result of
[=converting=] |inputPropertiesIterable| to a <code>sequence<DOMString></code>. If an
exception is [=thrown=], rethrow the exception and abort all these steps.
7. Filter |inputProperties| so that it only contains [=supported CSS properties=] and [=custom
properties=].
Note: The list of CSS properties provided by the input properties getter can either be
custom or native CSS properties.
Note: The list of CSS properties may contain shorthands.
Note: In order for a layout class to be forwards compatible, the list of CSS properties can
also contains currently invalid properties for the user agent. For example
<code>margin-bikeshed-property</code>.
8. Let |childInputProperties| be an empty <code>sequence<DOMString></code>.
9. Let |childInputPropertiesIterable| be the result of [=Get=](|layoutCtor|,
"childInputProperties").
10. If |childInputPropertiesIterable| is not undefined, then set |childInputProperties| to the
result of [=converting=] |childInputPropertiesIterable| to a
<code>sequence<DOMString></code>. If an exception is [=thrown=], rethrow the exception
and abort all these steps.
11. Filter |childInputProperties| so that it only contains [=supported CSS properties=] and [=custom
properties=].
12. Let |layoutOptionsValue| be the result of [=Get=](|layoutCtor|, "layoutOptions").
13. Let |layoutOptions| be the result of [=converting=] |layoutOptionsValue| to a
{{LayoutOptions}}. If an exception is [=thrown=], rethrow the exception and abort all these
steps.
14. Let |prototype| be the result of [=Get=](|layoutCtor|, "prototype").
15. If the result of [=Type=](|prototype|) is not Object, [=throw=] a [=TypeError=] and abort
all these steps.
16. Let |intrinsicSizesValue| be the result of [=Get=](|prototype|, "intrinsicSizes").
17. Let |intrinsicSizes| be the result of [=converting=] |intrinsicSizesValue| to the
[=Function=] [=callback function=] type. Rethrow any exceptions from the conversion.
18. Let |layoutValue| be the result of [=Get=](|prototype|, <code>"layout"</code>).
19. Let |layout| be the result of [=converting=] |layoutValue| to the [=Function=] [=callback
function=] type. Rethrow any exceptions from the conversion.
20. Let |definition| be a new [=layout definition=] with:
- [=class constructor=] being |layoutCtor|.
- [=layout function=] being |layout|.
- [=intrinsic sizes function=] being |intrinsicSizes|.
- [=constructor valid flag=] being <b>true</b>.
- [=layout definition/child input properties=] being |childInputProperties|.
- [=layout definition/input properties=] being |inputProperties|.
- [=layout definition/layout options=] being |layoutOptions|.
21. [=map/Set=] |layoutDefinitionMap|[|name|] to |definition|.
22. [=Queue a task=] to run the following steps:
1. Let |documentLayoutDefinitionMap| be the associated [=document's=] [=document layout
definitions=] [=map=].
2. Let |documentDefinition| be a new [=document layout definition=] with:
- [=document layout definition/child input properties=] being |childInputProperties|.
- [=document layout definition/input properties=] being |inputProperties|.
- [=document layout definition/layout options=] being |layoutOptions|.
3. If |documentLayoutDefinitionMap|[|name|] [=map/exists=], run the following steps:
1. Let |existingDocumentDefinition| be the result of [=map/get=]
|documentLayoutDefinitionMap|[|name|].
2. If |existingDocumentDefinition| is <code>"invalid"</code>, abort all these steps.
3. If |existingDocumentDefinition| and |documentDefinition| are not equivalent, (that is
[=document layout definition/input properties=], [=document layout definition/child
input properties=], and [=document layout definition/layout options=] are
different), then:
[=map/Set=] |documentLayoutDefinitionMap|[|name|] to <code>"invalid"</code>.
Log an error to the debugging console stating that the same class was registered
with different <code>inputProperties</code>, <code>childInputProperties</code>, or
<code>layoutOptions</code>.
4. Otherwise, [=map/set=] |documentLayoutDefinitionMap|[|name|] to |documentDefinition|.
</div>
Terminology {#terminology}
--------------------------
We define the following terms to be clear about which layout algorithm (formatting context) we are
talking about.
The <dfn>current layout</dfn> is the layout algorithm for the [=box=] we are currently performing
layout for.
The <dfn>parent layout</dfn> is the layout algorithm for the [=box's=] direct parent, (the layout
algorithm which is requesting the [=current layout=] to be performed).
A <dfn>child layout</dfn> is the layout algorithm for a {{LayoutChild}} of the [=current layout=].
Layout API {#layout-api}
========================
This section describes the objects of the Layout API provided to web developers.
Layout Children {#layout-children}
----------------------------------
A {{LayoutChild}} represents a inflow CSS generated [=box=] before layout has occurred. (The box or
boxes will all have a computed value of 'display' that is not ''none'').
The {{LayoutChild}} does not contain any layout information itself (like inline or block size) but
can be used to generate {{LayoutFragment}}s which do contain layout information.
An author cannot construct a {{LayoutChild}} with this API, this happens at a separate stage of the
user agent rendering engine (post style resolution).
An array of {{LayoutChild}}ren is passed into the layout/intrinsicSizes methods which represents the
children of the current box which is being laid out.
<pre class='idl'>
[Exposed=LayoutWorklet]
interface LayoutChild {
readonly attribute StylePropertyMapReadOnly styleMap;
Promise<IntrinsicSizes> intrinsicSizes();
Promise<LayoutFragment> layoutNextFragment(LayoutConstraintsOptions constraints, ChildBreakToken breakToken);
};
</pre>
The {{LayoutChild}} has internal slot(s):
- <dfn attribute for=LayoutChild>\[[box]]</dfn> a CSS [=box=].
- <dfn attribute for=LayoutChild>\[[styleMap]]</dfn> a {{StylePropertyMapReadOnly}}, this is the
computed style for the child, it is populated with only the properties listed in
<code>childInputProperties</code>.
- <dfn attribute for=LayoutChild>[[unique id]]</dfn> the [=unique id=] of the current [=layout
api context=]. This slot is used so that a {{LayoutChild}} used outside the current layout
pass is invalid.
The {{LayoutChild/[[styleMap]]}} may be pre-populated when the [=computed value=] for properties
listed in the in [=layout definition/child input properties=] for the {{LayoutChild/[[box]]}}.
<div class=example>
The example below shows the basic usage of a {{LayoutChild}}.
<pre class='lang-javascript'>
registerLayout('example-layout-child', class {
static childInputProperties = ['--foo'];
async layout(children, edges, constraints, styleMap) {
// An array of LayoutChildren is passed into both the layout function,
// and intrinsic sizes function below.
const child = children[0];
// You can query the any properties listed in "childInputProperties".
const fooValue = child.styleMap.get('--foo');
// And perform layout!
const fragment = await child.layoutNextFragment({});
}
async intrinsicSizes(children, edges, styleMap) {
// Or request the intrinsic size!
const childIntrinsicSize = await children[0].intrinsicSizes();
}
});
</pre>
</div>
A {{LayoutChild}} could be generated by:
- An [=element=].
- A [=root inline box=].
- A <a>::before</a> or <a>::after</a> pseudo-element.
Note: Other pseudo-elements such as <a>::first-letter</a> or <a>::first-line</a> do not
generate a {{LayoutChild}} for layout purposes. They are additional
styling information for a text node.
- An [=anonymous box=]. For example an anonymous box may be inserted as a result of:
- A text node which has undergone [=blockification=]. (Or more generally a [=root inline box=]
which has undergone [=blockification=]).
- An element with ''display: table-cell'' which doesn't have a parent with ''display: table''.
<div class="note">
Note: As an example the following would be placed into three {{LayoutChild}}ren:
<pre class="lang-html">
<style>
#box::before { content: 'hello!'; }
</style>
<!-- A ::before pseudo-element is inserted here. -->
<div id="box">A block level box with text.</div>
<img src="..." />
</pre>
</div>
<div class="note">
Note: As an example the following would be placed into a single {{LayoutChild}} as they share a
[=root inline box=]:
<pre class="lang-html">
This is a next node, <span>with some additional styling,
that may</span> break over<br>multiple lines.
</pre>
</div>
Multiple non-[=atomic inlines=] are placed within the same {{LayoutChild}} to allow rendering
engines to perform text shaping across element boundaries.
<div class="note">
Note: As an example the following should produce one {{LayoutFragment}} but is from
three non-[=atomic inlines=]:
<pre class="lang-html">
ع<span style="color: blue">ع</span>ع
</pre>
</div>
Note: When accessing the {{LayoutChild/styleMap}} the user agent can create a new
{{StylePropertyMapReadOnly}} if none exists yet.
<div algorithm>
The <dfn attribute for=LayoutChild>styleMap</dfn>, on getting from a {{LayoutChild}} |this|, the
user agent must perform the following steps:
1. If |this|' {{[[styleMap]]}} is null, then:
1. Let |box| be |this|' {{LayoutChild/[[box]]}}.
2. Let |definition| be the result of [=get a layout definition=].
3. Let |childInputProperties| be |definition|'s [=layout definition/child input
properties=].
4. Let |styleMap| be a new {{StylePropertyMapReadOnly}} populated with <em>only</em> the
[=computed values=] for properties listed in |childInputProperties| for |box|.
5. Set |this|' {{LayoutChild/[[styleMap]]}} internal slot to |styleMap|.
Note: If the user agent always pre-populates {{LayoutChild/[[styleMap]]}} then this branch
of the algorithm won't be reached.
2. Return |this|' {{StylePropertyMapReadOnly}} contained in the {{LayoutChild/[[styleMap]]}}
internal slot.
</div>
Note: The {{intrinsicSizes()}} method allows the web developer to query the intrinsic sizes of the
{{LayoutChild}}.
<div algorithm>
When the <dfn method for=LayoutChild>intrinsicSizes()</dfn> method is called on a {{LayoutChild}}
|this|, the user agent must perform the following steps:
1. Let |p| be a new promise.
2. Let |context| be the [=current layout's=] [=layout API context=].
3. If |this|' {{LayoutChild/[[unique id]]}} is not equal to |context|'s [=unique id=], reject
|p| with a "{{InvalidStateError}}" {{DOMException}}, and abort all these steps.
Note: This is to ensure that only {{LayoutChild}}ren passed in as arguments to either the
layout or intrinsicSizes method are used.
4. Let |task| be a new [=layout API work task=] with:
- [=layout api work task/layout child=] being |this|.
- [=layout api work task/task type=] being <code>"intrinsic-sizes"</code>.
- [=layout api work task/promise=] being |p|.
5. [=list/Append=] |task| to |context|'s [=work queue=].
6. Return |p|.
</div>
Note: The {{layoutNextFragment()}} method allows the web developer to produce a {{LayoutFragment}}
for a given {{LayoutChild}} (the result of performing layout).
<div algorithm>
When the <dfn method for=LayoutChild>layoutNextFragment(|constraints|, |breakToken|)</dfn> method is
called on a {{LayoutChild}} |this|, the user agent must perform the following steps:
1. Let |p| be a new promise.
2. Let |context| be the [=current layout's=] [=layout API context=].
3. If |this|' {{LayoutChild/[[unique id]]}} is not equal to |context|'s [=unique id=], reject
|p| with a "{{InvalidStateError}}" {{DOMException}}, and abort all these steps.
Note: This is to ensure that only {{LayoutChild}}ren passed in as arguments to either the
layout or intrinsicSizes method are used.
4. If |breakToken|'s {{ChildBreakToken/[[unique id]]} is not equal to |context|'s [=unique id=],
reject |p| with a "{{InvalidStateError}}" {{DOMException}}, and abort all these steps.
5. If |context|'s [=layout API context/mode=] is <code>"intrinsic-sizes"</code>, reject |p| with
a "{{NotSupportedError}}" {{DOMException}}.
Note: This is to ensure that inside a <code>intrinsicSizes</code> callback,
{{LayoutChild/layoutNextFragment()}} cannot be called.
6. Let |task| be a new [=layout API work task=] with:
- [=layout api work task/layout constraints=] being |constraints|.
- [=layout api work task/layout child=] being |this|.
- [=layout api work task/child break token=] being |breakToken|.
- [=layout api work task/task type=] being <code>"layout"</code>.
- [=layout api work task/promise=] being |p|.
7. [=list/Append=] |task| to |context|'s [=work queue=].
8. Return |p|.
</div>
### LayoutChildren and the Box Tree ### {#layout-child-box-tree}
Each [=box=] has a <dfn attribute for=box>\[[layoutChildMap]]</dfn> internal slot, which is a
[=map=] of {{LayoutWorkletGlobalScope}}s to {{LayoutChild}}ren.
Note: [=Get a layout child=] returns a {{LayoutChild}} object for the correct
{{LayoutWorkletGlobalScope}} and creates one if it doesn't exist yet.
<div algorithm="get a layout child">
When the user agent wants to <dfn>get a layout child</dfn> given |workletGlobalScope|, |name|,
|box|, and |uniqueId|, it <em>must</em> run the following steps:
1. Assert that:
- |box| is currently attached to the [=box tree=].
- |box|'s [=containing block=] is a [=layout API container=].
- The [=containing block's=] ''layout()'' function's first argument is |name|.
2. Let |layoutChildMap| be |box|'s {{[[layoutChildMap]]}}.
3. If |layoutChildMap|[|workletGlobalScope|] does not <a for=map>exist</a>, run the following
steps:
1. Let |definition| be the result of [=get a layout definition=] given |name|, and
|workletGlobalScope|.
Assert that [=get a layout definition=] succeeded, and |definition| is not
<code>"invalid"</code>.
2. Let |childInputProperties| be |definition|'s <a for="layout definition">child input
properties</a>.
3. Let |layoutChild| be a new {{LayoutChild}} with internal slot(s):
- {{LayoutChild/[[box]]}} set to |box|.
- {{[[styleMap]]}} set to a new {{StylePropertyMapReadOnly}} populated with
<em>only</em> the [=computed values=] for properties listed in
|childInputProperties|.
4. <a for=map>Set</a> |layoutChildMap|[|workletGlobalScope|] to |layoutChild|.
4. Let |layoutChild| be the result of <a for=map>get</a> |layoutChildMap|[|workletGlobalScope|].
5. Set |layoutChild|'s {{LayoutChild/[[unique id]]}} internal slot to |uniqueId|.
6. Return |layoutChild|.
</div>
When a [=box=] is inserted into the [=box tree=] the user agent <em>may</em> pre-populate the
{{[[layoutChildMap]]}} for all {{LayoutWorkletGlobalScope}}s.
When a [=box=] is removed from the [=box tree=] the user agent <em>must</em> clear the
{{[[layoutChildMap]]}}.
The user agent <em>must</em> clear the {{[[layoutChildMap]]}} internal slot every 1000 layout
passes.
Note: The above rule exists to ensure that web developers do not rely on being able to store
non-regeneratable state on the {{LayoutChild}} object.
The 1000 limit was picked as a high upper bound, this limit may improve (downwards) over time.
<div algorithm="update a layout child style">
When the user agent wants to <dfn>update a layout child style</dfn> given |box|, it <em>must</em>
run the following steps:
1. Assert that:
- |box| is currently attached to the [=box tree=].
2. If |box|'s [=containing block=] is not a [=layout API container=], abort all these
steps.
3. Let |layoutChildMap| be |box|'s {{[[layoutChildMap]]}}.
4. <a for=map>For each</a> |layoutChild| in |layoutChildMap|:
1. |layoutChild|'s {{[[styleMap]]}} to null.
</div>
When the [=computed values=] of [=document layout definition/child input properties=] for a [=box=]
changes the user agent must run the [=update a layout child style=] algorithm.
Layout Fragments {#layout-fragments}
------------------------------------
A {{LayoutFragment}} represents a CSS [=fragment=] of a {{LayoutChild}} after layout has occurred on
that child. This is produced by the {{LayoutChild/layoutNextFragment()}} method.
<pre class='idl'>
[Exposed=LayoutWorklet]
interface LayoutFragment {
readonly attribute double inlineSize;
readonly attribute double blockSize;
attribute double inlineOffset;
attribute double blockOffset;
readonly attribute any data;
readonly attribute ChildBreakToken? breakToken;
};
</pre>
The {{LayoutFragment}} has internal slot(s):
- <dfn attribute for=LayoutFragment>[[unique id]]</dfn> the [=unique id=] of the [=layout api
context=] which produced this child fragment. This slot is used so that a {{LayoutFragment}}
from a previous layout pass is invalid.
<hr>
The {{LayoutFragment}} has {{LayoutFragment/inlineSize}} and {{LayoutFragment/blockSize}}
attributes, which are set by the respective child's layout algorithm. They represent the <b>border
box</b> size of the CSS [=fragment=], and are relative to the [=current layout's=] writing mode.
The {{LayoutFragment/inlineSize}} and {{LayoutFragment/blockSize}} attributes cannot be changed. If
the [=current layout=] requires a different {{LayoutFragment/inlineSize}} or
{{LayoutFragment/blockSize}} the author must perform {{LayoutChild/layoutNextFragment()}} again with
different arguments in order to get different results.
The author inside the current layout can position a resulting {{LayoutFragment}} by setting its
{{LayoutFragment/inlineOffset}} and {{LayoutFragment/blockOffset}} attributes. If not set by the
author they default to zero. The {{LayoutFragment/inlineOffset}} and {{LayoutFragment/blockOffset}}
attributes represent the position of the {{LayoutFragment}} relative to its parent's <b>border
box</b>, before transform or positioning (e.g. if a fragment is [=relatively positioned=]) has
been applied.
<figure>
<img src="img/layout-fragment-offsets.png" width="800"
alt="An example of position a fragment in different writing modes.">
<figcaption>
A simple visualization showing positioning a {{LayoutFragment}} using
{{LayoutFragment/inlineOffset}} and {{LayoutFragment/blockOffset}} in different writing
modes.
</figcaption>
</figure>
<div class=example>
The example below shows the basic usage of a {{LayoutFragment}}.
<pre class='lang-javascript'>
registerLayout('example-layout-fragment', class {
async layout(children, edges, constraints, styleMap) {
// You must perform layout to generate a fragment.
const fragment = await child.layoutNextFragment({});
// You can query the size of the fragment produced:
console.log(fragment.inlineSize);
console.log(fragment.blockSize);
// You can set the position of the fragment, e.g. this will set it to the
// top-left corner:
fragment.inlineOffset = edges.inlineStart;
fragment.blockOffset = edges.blockStart;
// Data may be passed from the child layout:
console.log(fragment.data);
// If the child fragmented, you can use the breakToken to produce the next
// fragment in the chain.
const nextFragment = await child.layoutNextFragment({}, fragment.breakToken);
}
});
</pre>
</div>
A [=layout API container=] can communicate with other [=layout API containers=] by using the
{{LayoutFragment/data}} attribute. This is set by the {{FragmentResultOptions/data}} member in the
{{FragmentResultOptions}} dictionary.
The {{LayoutFragment}}'s {{LayoutFragment/breakToken}} specifies where the {{LayoutChild}} last
fragmented. If the {{LayoutFragment/breakToken}} is null the {{LayoutChild}} wont produce any more
{{LayoutFragment}}s for that token chain. The {{LayoutFragment/breakToken}} can be passed to the
{{LayoutChild/layoutNextFragment()}} function to produce the next {{LayoutFragment}} for a
particular child. The {{LayoutFragment/breakToken}} cannot be changed.
If the [=current layout=] requires a different {{LayoutFragment/breakToken}} the author must perform
{{LayoutChild/layoutNextFragment()}} again with different arguments.
Intrinsic Sizes {#intrinsic-sizes}
----------------------------------
<pre class='idl'>
[Exposed=LayoutWorklet]
interface IntrinsicSizes {
readonly attribute double minContentSize;
readonly attribute double maxContentSize;
};
</pre>
A {{IntrinsicSizes}} object represents the [=min-content size=] and [=max-content size=] of a CSS
[=box=]. It has {{IntrinsicSizes/minContentSize}} and {{IntrinsicSizes/maxContentSize}} attributes
which represent the <b>border box</b> min/max-content contribution of the {{LayoutChild}} for the
[=current layout=]. The attributes are relative to the inline direction of the [=current layout's=]
writing mode.
The {{IntrinsicSizes/minContentSize}} and {{IntrinsicSizes/maxContentSize}} cannot be changed. They
must not change for a {{LayoutChild}} within the current layout pass.
<div class="example">
The example below shows the border-box intrinsic sizes of two children.
<pre class="lang-html">
<style>
.child-0 {
width: 380px;
border: solid 10px;
}
.child-1 {
border: solid 5px;
}
.box {
display: layout(intrinsic-sizes-example);
font: 25px/1 Ahem;
}
</style>
<div class="box">
<div class="child-0"></div>
<div class="child-1">XXX XXXX</div>
</div>
</pre>
<pre class="lang-javascript">
registerLayout('intrinsic-sizes-example', class {
async intrinsicSizes(children, edges, styleMap) {
const childrenSizes = await Promise.all(children.map((child) => {
return child.intrinsicSizes();
}));
childrenSizes[0].minContentSize; // 400, (380+10+10) child has a fixed size.
childrenSizes[0].maxContentSize; // 400, (380+10+10) child has a fixed size.
childrenSizes[1].minContentSize; // 100, size of "XXXX".
childrenSizes[1].maxContentSize; // 200, size of "XXX XXXX".
}
layout() {}
});
</pre>
</div>
Layout Constraints {#layout-constraints}
----------------------------------------
A {{LayoutConstraints}} object is passed into the layout method which represents the all the
constraints for the [=current layout=] to perform layout within.
<pre class='idl'>
[Exposed=LayoutWorklet]
interface LayoutConstraints {
readonly attribute double availableInlineSize;
readonly attribute double availableBlockSize;
readonly attribute double? fixedInlineSize;
readonly attribute double? fixedBlockSize;
readonly attribute double percentageInlineSize;
readonly attribute double percentageBlockSize;
readonly attribute double? blockFragmentationOffset;
readonly attribute BlockFragmentationType blockFragmentationType;
readonly attribute any data;
};
enum BlockFragmentationType { "none", "page", "column", "region" };
</pre>
The {{LayoutConstraints}} object has {{LayoutConstraints/availableInlineSize}} and
{{LayoutConstraints/availableBlockSize}} attributes. This represents the [=available space=] for the
[=current layout=] to respect.
Note: Some layouts may need to produce a {{LayoutFragment}} which exceed this size. For example a
[=replaced element=]. The [=parent layout=] should expect this to occur and deal with it
appropriately.
A [=parent layout=] may require the [=current layout=] to be exactly a particular size. If the
{{LayoutConstraints/fixedInlineSize}} or {{LayoutConstraints/fixedBlockSize}} are specified the
[=current layout=] should produce a {{LayoutFragment}} with a the specified size in the appropriate
direction.
The {{LayoutConstraints}} object has {{LayoutConstraints/percentageInlineSize}} and
{{LayoutConstraints/percentageBlockSize}} attributes. These represent the size that percentages
should be resolved against while performing layout.
The {{LayoutConstraints}} has a {{LayoutConstraints/blockFragmentationType}} attribute. The
[=current layout=] should produce a {{LayoutFragment}} which fragments at the
{{LayoutConstraints/blockFragmentationOffset}} if possible.
The [=current layout=] can choose not to fragment a {{LayoutChild}} based on the
{{LayoutConstraints/blockFragmentationType}}, for example if the child has a property like
''break-inside: avoid-page;''.
<div class="example">