Resize Observer

1. Introduction

This section is non-normative.

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

• it displays a map by tiling its content box with Element tiles.

• when resized, it must redo the tiling.

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

The ResizeObserver API is an interface for observing changes to Element’s size. It is an Element's counterpart to Window's resize event.

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

• Observation will fire when watched Element is inserted/removed from DOM.

• Observation will fire when watched Element display gets set to none.

• Observations do not fire for non-replaced inline Elements.

• Observations will not be triggered by CSS transforms.

• Observation will fire when observation starts if Element is being rendered, and Element’s size is not 0,0.

<canvas id="ellipse" 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, paint an ellipse inside a canvas
document.querySelector('#ellipse').handleResize = entry => {
    entry.target.width = entry.borderBoxSize[0].inlineSize;
    entry.target.height = entry.borderBoxSize[0].blockSize;
    let rx = Math.floor(entry.target.width / 2);
    let ry = Math.floor(entry.target.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.borderBoxSize[0].inlineSize < 40)
        title.style.display = 'none';
    else
        title.style.display = 'inline-block';
}

let 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.top, ' is ', cs.paddingTop);
    console.log(entry.contentRect.left, ' is ', cs.paddingLeft);
    console.log(entry.borderBoxSize[0].inlineSize, ' is ', cs.width);
    console.log(entry.borderBoxSize[0].blockSize, ' is ', cs.height);
    if (entry.target.handleResize)
        entry.target.handleResize(entry);
  }
});

ro.observe(document.querySelector('#ellipse'));
ro.observe(document.querySelector('#menu'));

2. Resize Observer API

2.1. ResizeObserver interface

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

It is modeled after MutationObserver and IntersectionObserver.

enum ResizeObserverBoxOptions {
    "border-box", "content-box", "device-pixel-content-box"
};

ResizeObserver can observe different kinds of CSS sizes:

• border-box : size of box border area as defined in CSS2.

• content-box : size of content area as defined in CSS2.

• device-pixel-content-box : size of content area as defined in CSS2, in device pixels, before applying any CSS transforms on the element or its ancestors. This size must contain integer values.

The device-pixel-content-box can be approximated by multiplying devicePixelRatio by the content-box size. However, due to browser-specific subpixel snapping behavior, authors cannot determine the correct way to round this scaled content-box size. How a UA computes the device pixel box for an element is implementation-dependent. One possible implementation could be to multiply the box size and position by the device pixel ratio, then round both the resulting floating-point size and position of the box to integer values, in a way that maximizes the quality of the rendered output.

Note that this size can be affected by position changes to the target, and thus is typically more expensive to observe than the other sizes.

dictionary ResizeObserverOptions {
    ResizeObserverBoxOptions box = "content-box";
};

This section is non-normative. An author may desire to observe more than one CSS box. In this case, author will need to use multiple ResizeObservers.

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

// Observe just the border box. Replaces previous observation.
ro1.observe(document.querySelector('#menu'), { box: '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 box the author wishes to observe layout changes on.

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

2.2. ResizeObserverCallback

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

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

2.3. ResizeObserverEntry

[Exposed=Window]
interface ResizeObserverEntry {
    readonly attribute Element target;
    readonly attribute DOMRectReadOnly contentRect;
    readonly attribute FrozenArray<ResizeObserverSize> borderBoxSize;
    readonly attribute FrozenArray<ResizeObserverSize> contentBoxSize;
    readonly attribute FrozenArray<ResizeObserverSize> devicePixelContentBoxSize;
};

contentRect is from the incubation phase of ResizeObserver and is only included for current web compat reasons. It may be deprecated in future levels.

The box size properties are exposed as FrozenArray in order to support elements that have multiple fragments, which occur in multi-column scenarios. However the current definitions of content rect and border box do not mention how those boxes are affected by multi-column layout. In this spec, there will only be a single ResizeObserverSize returned in the FrozenArray, which will correspond to the dimensions of the first column. A future version of this spec will extend the returned FrozenArray to contain the per-fragment size information.

[Exposed=Window]
interface ResizeObserverSize {
    readonly attribute unrestricted double inlineSize;
    readonly attribute unrestricted double blockSize;
};

3. Processing Model

3.1. ResizeObservation example struct

This section is non-normative. ResizeObservation is an example struct that can be used in implementation of Resize Observer. It is being included here in order to help provide clarity during the processing model. It effectively holds observation information for a single Element. This interface is not visible to Javascript.

interface ResizeObservation {
    constructor(Element target, ResizeObserverBoxOptions observedBox);
    readonly attribute Element target;
    readonly attribute ResizeObserverBoxOptions observedBox;
    readonly attribute FrozenArray<ResizeObserverSize> lastReportedSizes;
};

3.2. Internal Slot Definitions

3.2.1. Document

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

3.2.2. 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

3.3. CSS Definitions

3.3.1. content rect

• width is content width

• height is content height

• top is padding top

• left is padding left

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:

• observation will fire when watched Element is inserted/removed from DOM.

• observation will fire when watched Element display gets set to none.

• non-replaced inline Elements will always have an empty content rect.

• observations will not be triggered by CSS transforms.

Web content can also contain SVG elements. SVG elements that do not have associated CSS layout boxes define bounding box instead of a content box. Content rect for the SVGGraphicsElements without CSS layout boxes is a rect whose:

• width is bounding box width

• height is bounding box height

• top and left are 0

3.4. Algorithms

3.4.1. Gather active resize observations at depth

It computes all active resize observations for a document. To gather active resize 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]].

3.4.2. Has active resize observations

To determine if Document has active resize 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.

3.4.3. Has skipped resize observations

To determine if Document has skipped resize 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.

3.4.4. Create and populate a ResizeObserverEntry

1. Let this be a new ResizeObserverEntry.

2. Set this.target slot to target.

3. Set this.borderBoxSize slot to result of calculating box size given target and observedBox of "border-box".

4. Set this.contentBoxSize slot to result of calculating box size given target and observedBox of "content-box".

5. Set this.devicePixelContentBoxSize slot to result of calculating box size given target and observedBox of "device-pixel-content-box".

6. Set this.contentRect to logical this.contentBoxSize given target and observedBox of "content-box".

7. If target is not an SVG element or target is an SVG element with an associated CSS layout box do these steps:

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

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

8. If target is an SVG element without an associated CSS layout box do these steps:

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

3.4.5. Broadcast active resize observations

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

To broadcast active resize 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 the result of running create and populate a ResizeObserverEntry given observation.target.

      2. Add entry to entries.

      3. Set observation.lastReportedSizes to matching entry sizes.

         1. Matching sizes are entry.borderBoxSize if observation.observedBox is "border-box"

         2. Matching sizes are entry.contentBoxSize if observation.observedBox is "content-box"

         3. Matching sizes are entry.devicePixelContentBoxSize if observation.observedBox is "device-pixel-content-box"

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

      5. Set shallowestTargetDepth to targetDepth if targetDepth < shallowestTargetDepth

   4. Invoke observer.[[callback]] with entries.

   5. Clear observer.[[activeTargets]].

3. Return shallowestTargetDepth.

3.4.6. 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. Report the exception event.

3.4.7. Calculate depth for node

To calculate depth for node, given a node, run these steps:

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

2. Return number of nodes in p.

3.4.8. Calculate box size, given target and observed box

This algorithm computes target Element's observed box size. Type of box is described by ResizeObserverBoxOptions. The SVG elements which don’t have associated CSS layout boxes are an exception. The sizes of these elements are always their bounding box sizes, because these elements do not use standard CSS box model.

To calculate box size, given target and observedBox, run these steps:

1. Let computedSize be a new ResizeObserverSize object.

2. If target is an SVGGraphicsElement that does not have an associated CSS layout box:

   If observedBox is "border-box"

   If observedBox is "content-box"

   1. Set computedSize’s inlineSize attribute to target’s bounding box inline length.

   2. Set computedSize’s blockSize attribute to target’s bounding box block length.

   If observedBox is "device-pixel-content-box"

   1. Set computedSize’s inlineSize attribute to target’s bounding box inline length, in integral device pixels.

   2. Set computedSize’s blockSize attribute to target’s bounding box block length, in integral device pixels.

3. Otherwise:

   If observedBox is "border-box"

   1. Set computedSize’s inlineSize attribute to target’s border area inline length.

   2. Set computedSize’s blockSize attribute to target’s border area block length.

   If observedBox is "content-box"

   1. Set computedSize’s inlineSize attribute to target’s content area inline length.

   2. Set computedSize’s blockSize attribute to target’s content area block length.

   If observedBox is "device-pixel-content-box"

   1. Set computedSize’s inlineSize attribute to target’s content area inline length, in integral device pixels.

   2. Set computedSize’s blockSize attribute to target’s content area block length, in integral device pixels.

4. Return computedSize.

3.5. ResizeObserver Lifetime

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

• there are no scripting references to the observer.

• the observer is not observing any targets.