Resize Observer

Editor’s Draft,

Specification Metadata
This version:
https://drafts.csswg.org/resize-observer/index.html
Issue Tracking:
Inline In Spec
GitHub Issues
Editor:
(Microsoft)
Former Editor:
(Google)
Suggest an Edit for this Spec:
GitHub Editor

Abstract

This specification describes an API for observing changes to Element’s size.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “resize-observer” in the title, preferably like this: “[resize-observer] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 February 2018 W3C Process Document.

1. Introduction

This section is non-normative.

Responsive Web Components need to respond to content rect size changes. An example is an Element that displays a map:

Responsive Web Applications can already respond to viewport size changes. This is done with CSS media queries, or window.resize event.

The ResizeObserver API is an interface for observing changes to Element's content rect’s width and height. It is an Element's counterpart to window.resize event.

ResizeObserver’s notifications can be used to respond to changes in Element's size. Some interesting facts about these observations:

<canvas id="elipse" style="display:block"></canvas>
<div id="menu" style="display:block;width:100px">
    <img src="hamburger.jpg" style="width:24px;height:24px">
    <p class="title">menu title</p>
</div>
// In response to resize, elipse paints an elipse inside a canvas
document.querySelector('#elipse').handleResize = entry => {
    entry.target.width = entry.contentRect.width;
    entry.target.height = entry.contentRect.height;
    let rx = Math.floor(entry.contentRect.width / 2);
    let ry = Math.floor(entry.contentRect.height / 2);
    let ctx = entry.target.getContext('2d');
    ctx.beginPath();
    ctx.ellipse(rx, ry, rx, ry, 0, 0, 2 * Math.PI);
    ctx.stroke();
}
// In response to resize, change title visibility depending on width
document.querySelector('#menu').handleResize = entry => {
    let title = entry.target.querySelector(".title")
    if (entry.contentRect.width < 40)
        title.style.display = "none";
    else
        title.style.display = "inline-block";
}

var ro = new ResizeObserver( entries => {
  for (let entry of entries) {
    let cs = window.getComputedStyle(entry.target);
    console.log('watching element:', entry.target);
    console.log(entry.contentRect.width,' is ', cs.width);
    console.log(entry.contentRect.height,' is ', cs.height);
    console.log(entry.contentRect.top,' is ', cs.paddingTop);
    console.log(entry.contentRect.left,' is ', cs.paddingLeft);
    if (entry.target.handleResize)
        entry.target.handleResize(entry);
  }
});
ro.observe(document.querySelector('#elipse'));
ro.observe(document.querySelector('#menu'));

2. Resize Observer API

2.1. ResizeObserver interface

The ResizeObserver interface is used to observe changes to Element's content rect.

It is modeled after MutationObserver and IntersectionObserver.

enum ResizeObserverSizeOptions {
    "border-box", "content-box", "scroll-box"
};

In order to enable flexibility for authors to observe more boxes than solely the content-box ResizeObserver takes one or more of the following enums to observe

dictionary ResizeObserverOptions {
    (ResizeObserverSizeOptions or sequence<ResizeObserverSizeOptions>) size = "content-box";
};

Is size the right name here? It’s actually denoting which layout box we want to observe, possibly `box`?

This section is non-normative. An author may desire to observe more than one CSS box, as such the author can provide one or many boxes to be observed. For example:

// Observe the content-box
ro.observe(document.querySelector('#menu'));

// Observe the content and the border box
ro.observe(document.querySelector('#menu'), ['content-box', 'border-box']);

// Observe just the border box
ro.observe(document.querySelector('#menu'), 'border-box');

This does not have any impact on which box dimensions are returned to the defined callback when the event is fired, it solely defines which boxes the author wishes to observe layout changes on.

[Exposed=(Window),
 Constructor(ResizeObserverCallback callback)]
interface ResizeObserver {
    void observe(Element target, optional ResizeObserverOptions options);
    void unobserve(Element target);
    void disconnect();
};
new ResizeObserver(callback)
  1. Let this be a new ResizeObserver object.

  2. Set this internal callback slot to callback.

  3. Add this to Document's resizeObservers slot.

observe(target, options)

Adds target to the list of observed elements.

  1. If target is in observationTargets slot, return.

  2. Let resizeObservation be new ResizeObservation(target).

  3. Add the resizeObservation to the observationTargets slot.

unobserve(target)

Removes target from the list of observed elements.

  1. Let observation be ResizeObservation in observationTargets whose target slot is target.

  2. If observation is not found, return.

  3. Remove observation from observationTargets

disconnect()

1) Clear the observationTargets list.

2) Clear the activeTargets list.

2.2. ResizeObserverCallback

callback ResizeObserverCallback = void (sequence<ResizeObserverEntry> entries, ResizeObserver observer);

This callback delivers ResizeObserver's notifications. It is invoked by a broadcast active observations algorithm.

2.3. ResizeObserverEntry

[Constructor(Element target)
]
interface ResizeObserverEntry {
    readonly attribute Element target;
    readonly attribute DOMRectReadOnly contentRect;
    readonly attribute ResizeObserverSize borderBoxSize;
    readonly attribute ResizeObserverSize contentSize;
    readonly attribute ResizeObserverSize scrollSize;
};

Do we only want to return the box(es) that were requested to be observed, or all?

target, of type Element, readonly

The Element whose size has changed.

borderBoxSize, of type ResizeObserverSize, readonly

Element's border box rect when ResizeObserverCallback is invoked.

contentRect, of type DOMRectReadOnly, readonly

Element's content rect when ResizeObserverCallback is invoked.

scrollSize, of type ResizeObserverSize, readonly

Element's scrollport when ResizeObserverCallback is invoked.

new ResizeObserverEntry(target)
  1. Let this be a new ResizeObserverEntry.

  2. Set this target slot to target.

  3. If target is not an SVG element do these steps:

    1. Set this.contentRect.width to target.content width.

    2. Set this.contentRect.height to target.content height.

    3. Set this.contentRect.top to target.padding top.

    4. Set this.contentRect.left to target.padding left.

    5. To set the borderBoxSize, contentSize, and scrollSize of the current target do the following:

      1. Set this.inlineSize to the box’s inline edge size

      2. Set this.blockSize to the box’s block edge size

  4. If target is an SVG element do these steps:

    1. Set this.contentRect.top and this.contentRect.left to 0.

    2. Set this.contentRect.width to target.bounding box.width.

    3. Set this.contentRect.height to target.bounding box.height.

    4. To set the borderBoxSize, contentSize, and scrollSize of the current target do the following:

      1. Set this.inlineSize to the box’s inline bounding box size

      2. Set this.blockSize to the box’s block bounding box size

interface ResizeObserverSize {
    readonly attribute unrestricted double inlineSize;
    readonly attribute unrestricted double blockSize;
};

We’re covering the dimensions of the extra boxes, do we want to also cover their positions similiar to that of the initial contentRect?

2.4. ResizeObservation

ResizeObservation holds observation information for a single Element. This interface is not visible to Javascript.
[Constructor(Element target)
]
interface ResizeObservation {
    readonly attribute Element target;
    readonly attribute float broadcastWidth;
    readonly attribute float broadcastHeight;
    boolean isActive();
};
target, of type Element, readonly

The observed Element.

broadcastWidth, of type float, readonly

Width of last broadcast content width.

broadcastHeight, of type float, readonly

Width of last broadcast content height.

new ResizeObservation(target)
  1. Let this be a new ResizeObservation object

  2. Set this internal target slot to target

  3. Set this broadcastWidth slot to 0.

  4. Set this broadcastHeight slot to 0.

isActive()
  1. If target is an HTML element do these steps:

    1. If target.content width != broadcastWidth return true.

    2. If target.content height != broadcastHeight return true.

  2. If target is an SVGGraphicsElement do these steps:

    1. If target.bounding box width != broadcastWidth return true.

    2. If target.bounding box height != broadcastHeight return true.

  3. return false.

3. Processing Model

3.1. Internal Slot Definitions

3.1.1. Document

Document has a resizeObservers slot that is a list of ResizeObservers in this document. It is initialized to empty.

3.1.2. ResizeObserver

ResizeObserver has a callback slot, initialized by constructor.

ResizeObserver has an observationTargets slot, which is a list of ResizeObservations. It represents all Elements being observed.

ResizeObserver has a activeTargets slot, which is a list of ResizeObservations. It represents all Elements whose size has changed since last observation broadcast that are eligible for broadcast.

ResizeObserver has a skippedTargets slot, which is a list of ResizeObservations. It represents all Elements whose size has changed since last observation broadcast that are **not** eligible for broadcast

3.2. CSS Definitions

3.2.1. content rect

DOM content rect is a rect whose:

content width spec does not mention how multi-column layout affects content box. In this spec, content width of an Element inside multi-column is the result of getComputedStyle(Element).width. This currently evaluates to width of the first column.

Having content rect position be padding-top/left is useful for absolute positioning of target’s children. Absolute position coordinate space origin is topLeft of the padding rect.

Watching content rect means that:

Web content can also contain SVG elements. SVG Elements define bounding box instead of a content box. Content rect for SVGGraphicsElements is a rect whose:

3.3. Algorithms

3.3.1. Gather active observations at depth

It computes all active observations for a document. To gather active observations at depth, run these steps:

  1. Let depth be the depth passed in.

  2. For each observer in resizeObservers run these steps:

    1. Clear observer’s activeTargets, and skippedTargets

    2. For each observation in observer.observationTargets run this step:

      1. If observation.isActive() is true

        1. Let targetDepth be result of calculate depth for node for observation.target.

        2. If targetDepth is greater than depth then add observation to activeTargets.

        3. Else add observation to skippedTargets.

3.3.2. Has active observations

To determine if Document has active observations run these steps:

  1. For each observer in resizeObservers run this step:

    1. If observer.activeTargets is not empty, return true.

  2. return false.

3.3.3. Has skipped observations

To determine if Document has skipped observations run these steps:

  1. For each observer in resizeObservers run this step:

    1. If observer.skippedTargets is not empty, return true.

  2. return false.

3.3.4. Broadcast active observations

broadcast active observations delivers all active observations in a document, and returns the depth of the shallowest broadcast target depth.

To broadcast active observations for a document, run these steps:

  1. Let shallowestTargetDepth be ∞

  2. For each observer in document.resizeObservers run these steps:

    1. If observer.activeTargets slot is empty, continue.

    2. Let entries be an empty list of ResizeObserverEntryies.

    3. For each observation in activeTargets perform these steps:

      1. Let entry be new ResizeObserverEntry(observation.target)

      2. Add entry to entries

      3. Set observation.broadcastWidth to entry.contentRect.width.

      4. Set observation.broadcastHeight to entry.contentRect.height.

      5. Set targetDepth to the result of calculate depth for node for observation.target.

      6. Set shallowestTargetDepth to targetDepth if targetDepth < shallowestTargetDepth

    4. Invoke observer.callback with entries.

    5. Clear observer.activeTargets.

  3. Return shallowestTargetDepth.

3.3.5. Deliver Resize Loop Error

To deliver resize loop error notification run these steps:

  1. Create a new ErrorEvent.

  2. Initialize event’s message slot to "ResizeObserver loop completed with undelivered notifications.".

  3. Dispach the event to document’s window.

3.3.6. Calculate depth for node

To calculate depth for node node run these steps:

  1. Let p be the parent-traversal path from node to a root Element of this element’s DOM tree.

  2. Return number of nodes in p.

3.4. ResizeObserver Lifetime

A ResizeObserver will remain alive until both of these conditions are met:

3.5. External Spec Integrations

3.5.1. HTML Processing Model: Event Loop

ResizeObserver processing happens inside the step 7.12 of the HTML Processing Model event loop.

Step 12 is currently underspecified as:

For each fully active Document in docs, update the rendering or user interface of that Document and its browsing context to reflect the current state..

Existing step 12 can be fully specified as:

For each fully active Document in docs, run the following steps for that Document and its browsing contents:

  1. recalc styles

  2. update layout

  3. paint

ResizeObserver extends step 12 with resize notifications. It tries to deliver all pending notifications by looping until no pending notifications are available. This can cause an infinite loop.

Infinite loop is prevented by shrinking the set of nodes that can notify at every iteration. In each iteration, only nodes deeper than the shallowest node in previous iteration can notify.

An error is generated if notification loop completes, and there are undelivered notifications. Elements with undelivered notifications will be considered for delivery in the next loop.

Step 12 with ResizeObserver notifications is:

For each fully active Document in docs, run the following steps for that Document and its browsing contentx:

  1. recalc styles

  2. update layout

  3. set depth to 0

  4. gather active observations at depth depth for Document

  5. repeat while (document has active observations)

    1. set depth to broadcast active observations

    2. recalc styles

    3. update layout

    4. gather active observations at depth depth for Document

  6. if Document has skipped observations then deliver resize loop error notification

  7. update the rendering or user interface of Document and its browsing context to reflect the current state.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-BOX-3]
Elika Etemad. CSS Box Model Module Level 3. 9 August 2018. WD. URL: https://www.w3.org/TR/css-box-3/
[CSSOM-VIEW-1]
Simon Pieters. CSSOM View Module. 17 March 2016. WD. URL: https://www.w3.org/TR/cssom-view-1/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[GEOMETRY-1]
Simon Pieters; Dirk Schulze; Rik Cabanier. Geometry Interfaces Module Level 1. 25 November 2014. CR. URL: https://www.w3.org/TR/geometry-1/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[WebIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. 15 December 2016. ED. URL: https://heycam.github.io/webidl/

IDL Index

enum ResizeObserverSizeOptions {
    "border-box", "content-box", "scroll-box"
};

dictionary ResizeObserverOptions {
    (ResizeObserverSizeOptions or sequence<ResizeObserverSizeOptions>) size = "content-box";
};

[Exposed=(Window),
 Constructor(ResizeObserverCallback callback)]
interface ResizeObserver {
    void observe(Element target, optional ResizeObserverOptions options);
    void unobserve(Element target);
    void disconnect();
};

callback ResizeObserverCallback = void (sequence<ResizeObserverEntry> entries, ResizeObserver observer);

[Constructor(Element target)
]
interface ResizeObserverEntry {
    readonly attribute Element target;
    readonly attribute DOMRectReadOnly contentRect;
    readonly attribute ResizeObserverSize borderBoxSize;
    readonly attribute ResizeObserverSize contentSize;
    readonly attribute ResizeObserverSize scrollSize;
};

interface ResizeObserverSize {
    readonly attribute unrestricted double inlineSize;
    readonly attribute unrestricted double blockSize;
};

[Constructor(Element target)
]
interface ResizeObservation {
    readonly attribute Element target;
    readonly attribute float broadcastWidth;
    readonly attribute float broadcastHeight;
    boolean isActive();
};

Issues Index

Is size the right name here? It’s actually denoting which layout box we want to observe, possibly `box`?
Do we only want to return the box(es) that were requested to be observed, or all?
We’re covering the dimensions of the extra boxes, do we want to also cover their positions similiar to that of the initial contentRect?