index.bs

<pre class='metadata'>
Title: Resize Observer
Shortname: resize-observer
Level: 1
Status: ED
ED: https://wicg.github.io/ResizeObserver/index.html
Group: webperf
URL: https://github.com/WICG/ResizeObserver/
Editor: Aleks Totic, Google, atotic@google.com
Abstract: This specification describes an API for observing changes to Element's size.
</pre>
<pre class="anchors">
urlPrefix: https://www.w3.org/TR/CSS2/box.html
    url: #content-width; type: dfn; text: content width
urlPrefix: https://www.w3.org/TR/CSS2/box.html
    url: #content-height; type: dfn; text: content height
urlPrefix: https://drafts.csswg.org/css-box-3/
    url: #padding-top; type: dfn; text: padding top
urlPrefix: https://drafts.csswg.org/css-box-3/
    url: #padding-left; type: dfn; text: padding left
urlPrefix: https://www.w3.org/TR/css3-positioning/
    url: #viewport; type: dfn; text: viewport
urlPrefix: https://html.spec.whatwg.org/multipage/
    urlPrefix: webappapis.html;
        url: #processing-model-8; type: dfn; text: HTML Processing Model
urlPrefix: https://github.com/WICG/IntersectionObserver/
    urlPrefix: index.html;
        url: #intersection-observer-interface; type: dfn; text: IntersectionObserver
urlPrefix: https://www.w3.org/TR/SVG2/
    urlPrefix: coords.html
        url: #BoundingBoxes; type: dfn; text: bounding box
urlPrefix: https://www.w3.org/TR/SVG2/#InterfaceSVGGraphicsElement
    urlPrefix: types.html
        url: #InterfaceSVGGraphicsElement; type: dfn; text: SVGGraphicsElement
urlPrefix: https://www.w3.org/TR/css3-multicol/
    url: #; type: dfn; text: Multi-column
</pre>
<pre class=link-defaults>
spec:dom; type:interface; text:Document
</pre>

<h2 id="intro">Introduction</h2>

<em>This section is non-normative.</em>

Responsive Web Components need to respond to <a>content rect</a>
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 <a>viewport</a> 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 <a>content rect</a>'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:

* 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 has display, and Element's size is not 0,0.

<div class="example">
  <pre highlight="html">
    &lt;canvas id="elipse" style="display:block">&lt;/canvas>
    &lt;div id="menu" style="display:block;width:100px">
        &lt;img src="hamburger.jpg" style="width:24px;height:24px">
        &lt;p class="title">menu title&lt;/p>
    &lt;/div>
  </pre>
  <pre highlight="js">
    // 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'));
  </pre>
</div>

<h2 id="api">Resize Observer API</h2>

<h3 id="resize-observer-interface">ResizeObserver interface</h3>

The ResizeObserver interface is used to observe changes to {{Element}}'s
<a>content rect</a>.

It is modeled after {{MutationObserver}} and <a>IntersectionObserver</a>.

<pre class="idl">
[Constructor(ResizeObserverCallback callback),
    Exposed=Window]
interface ResizeObserver {
    void observe(Element target);
    void unobserve(Element target);
    void disconnect();
};
</pre>

<div dfn-type="method" dfn-for="ResizeObserver">
    : <dfn constructor lt="ResizeObserver(callback)">new ResizeObserver(callback)</dfn>
    ::
        1. Let |this| be a new {{ResizeObserver}} object.

        2. Set |this| internal {{ResizeObserver/callback}} slot to callback.

        3. Add |this| to {{Document}}'s {{Document/resizeObservers}} slot.

    : <dfn method>observe(target)</dfn>
    ::
        Adds target to the list of observed elements.

        1. If |target| is in {{ResizeObserver/observationTargets}} slot, return.

        2. Let |resizeObservation| be new {{ResizeObservation}}(|target|).

        3. Add the |resizeObservation| to the {{ResizeObserver/observationTargets}} slot.

    :  <dfn method for="ResizeObserver">unobserve(target)</dfn>
    ::
        Removes |target| from the list of observed elements.

        1. Let |observation| be {{ResizeObservation}} in {{ResizeObserver/observationTargets}} whose target slot is |target|.

        2. If |observation| is not found, return.

        3. Remove |observation| from {{ResizeObserver/observationTargets}}

    : <dfn method>disconnect()</dfn>
    ::
        1) Clear the {{ResizeObserver/observationTargets}} list.

        2) Clear the {{ResizeObserver/activeTargets}} list.

</div>

<h3 id="resize-observer-callback">ResizeObserverCallback</h3>

<pre class="idl">
callback ResizeObserverCallback = void (sequence&lt;ResizeObserverEntry> entries, ResizeObserver observer);
</pre>

This callback delivers {{ResizeObserver}}'s notifications. It is invoked by a
<a>broadcast active observations</a> algorithm.

<h3 id="resize-observer-entry-interface">ResizeObserverEntry</h3>

<pre class="idl">
[Constructor(Element target)
]
interface ResizeObserverEntry {
    readonly attribute Element target;
    readonly attribute DOMRectReadOnly contentRect;
};
</pre>

<div dfn-type="attribute" dfn-for="ResizeObserverEntry">
    : <dfn>target</dfn>
    ::
        The {{Element}} whose size has changed.
    : <dfn>contentRect</dfn>
    ::
        {{Element}}'s <a>content rect</a> when {{ResizeObserverCallback}} is invoked.
</div>

<div dfn-type="method" dfn-for="ResizeObserverEntry">
    : <dfn constructor lt="ResizeObserverEntry(target)">new ResizeObserverEntry(target)</dfn>
    ::
        1. Let |this| be a new {{ResizeObserverEntry}}.

        2. Set |this| {{ResizeObserverEntry/target}} slot to |target|.

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

            1. Set |this|.contentRect.width to |target|.<a>content width</a>.

            2. Set |this|.contentRect.height to |target|.<a>content height</a>.

            3. Set |this|.contentRect.top to |target|.<a>padding top</a>.

            4. Set |this|.contentRect.left to |target|.<a>padding left</a>.

        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|.<a>bounding box</a>.width.

            3. Set |this|.contentRect.height to |target|.<a>bounding box</a>.height.

</div>

<h3 id="resize-observation-interface">ResizeObservation</h3>
ResizeObservation holds observation information for a single {{Element}}. This
interface is not visible to Javascript.

<pre class="idl">
[Constructor(Element target)
]
interface ResizeObservation {
    readonly attribute Element target;
    readonly attribute float broadcastWidth;
    readonly attribute float broadcastHeight;
    boolean isActive();
};
</pre>
<div dfn-type="attribute" dfn-for="ResizeObservation">
    : <dfn>target</dfn>
    :: The observed {{Element}}.
    : <dfn>broadcastWidth</dfn>
    :: Width of last broadcast <a>content width</a>.
    : <dfn>broadcastHeight</dfn>
    :: Width of last broadcast <a>content height</a>.
</div>
<div dfn-type="method" dfn-for="ResizeObservation">
    : <dfn constructor lt="ResizeObservation(target)">new ResizeObservation(target)</dfn>
    ::
        1. Let |this| be a new {{ResizeObservation}} object

        2. Set |this| internal {{ResizeObservation/target}} slot to |target|

        3. Set |this| {{ResizeObservation/broadcastWidth}} slot to 0.

        4. Set |this| {{ResizeObservation/broadcastHeight}} slot to 0.


    : <dfn method lt="isActive()">isActive()</dfn>
    ::

        1. If {{ResizeObservation/target}} is an HTML element do these steps:

            1. If {{ResizeObservation/target}}.<a>content width</a> != {{ResizeObservation/broadcastWidth}} return true.

            2. If {{ResizeObservation/target}}.<a>content height</a> != {{ResizeObservation/broadcastHeight}} return true.

        2. If {{ResizeObservation/target}} is an SVGGraphicsElement do these steps:

            1. If {{ResizeObservation/target}}.bounding box width != {{ResizeObservation/broadcastWidth}} return true.

            2. If {{ResizeObservation/target}}.bounding box height != {{ResizeObservation/broadcastHeight}} return true.

        3. return false.

</div>

<h2 id="processing-model">Processing Model</h2>

<h3 id="internal-slot-definitions">Internal Slot Definitions</h3>

<h4 id="document-slots">Document</h4>

<a>Document</a> has a <dfn attribute for="Document">resizeObservers</dfn> slot that is a list of {{ResizeObserver}}s in this document. It is initialized to empty.

<h4 id="resize-observer-slots">ResizeObserver</h4>

{{ResizeObserver}} has a <dfn attribute for="ResizeObserver">callback</dfn> slot, initialized by constructor.

{{ResizeObserver}} has an <dfn attribute for="ResizeObserver">observationTargets</dfn> slot, which is a list of {{ResizeObservation}}s.
It represents all Elements being observed.

{{ResizeObserver}} has a <dfn attribute for="ResizeObserver">activeTargets</dfn> slot, which is a list of {{ResizeObservation}}s. It represents all Elements whose size has changed since last observation broadcast that are eligible for broadcast.

{{ResizeObserver}} has a <dfn attribute for="ResizeObserver">skippedTargets</dfn> slot, which is a list of {{ResizeObservation}}s. It represents all Elements whose size has changed since last observation broadcast that are **not** eligible for broadcast

<h3 id="css-definitions">CSS Definitions</h3>
<h4 id="content-rect-h">content rect</h4>
DOM <dfn>content rect</dfn> is a rect whose:

* width is <a>content width</a>
* height is <a>content height</a>
* top is <a>padding top</a>
* left is <a>padding left</a>

<a>content width</a> spec does not mention how <a>multi-column</a> layout affects content box. In this spec, content width of an {{Element}} inside <a>multi-column</a> 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 define <a>bounding box</a> instead of a content box.
Content rect for <a>SVGGraphicsElement</a>s is a rect whose:

* width is <a>bounding box</a> width
* height is <a>bounding box</a> height
* top and left are 0

<h3 id="algorithms">Algorithms</h3>

<h4 id="gather-active-observations-h">Gather active observations at depth</h4>

It computes all active observations for a |document|. To <dfn>gather active observations at depth</dfn>, run these steps:

1. Let |depth| be the depth passed in.

1. For each |observer| in {{Document/resizeObservers}} run these steps:

    1. Clear |observer|'s {{ResizeObserver/activeTargets}}, and {{ResizeObserver/skippedTargets}}

    2. For each |observation| in |observer|.{{ResizeObserver/observationTargets}} run this step:

        1. If |observation|.{{ResizeObservation/isActive()}} is true

            1. Let |targetDepth| be result of <a>calculate depth for node</a> for |observation|.{{ResizeObservation/target}}.

            2. If |targetDepth| is greater than |depth| then add |observation| to {{ResizeObserver/activeTargets}}.

            3. Else add |observation| to {{ResizeObserver/skippedTargets}}.

<h4 id="has-active-observations-h">Has active observations</h4>

To determine if {{Document}} <dfn>has active observations</dfn> run these steps:

1. For each |observer| in {{Document/resizeObservers}} run this step:

    1. If |observer|.{{ResizeObserver/activeTargets}} is not empty, return true.

2. return false.

<h4 id="has-skipped-observations-h">Has skipped observations</h4>

To determine if {{Document}} <dfn>has skipped observations</dfn> run these steps:

1. For each |observer| in {{Document/resizeObservers}} run this step:

    1. If |observer|.{{ResizeObserver/skippedTargets}} is not empty, return true.

2. return false.

<h4 id="broadcast-resize-notifications-h">Broadcast active observations</h4>

<dfn>broadcast active observations</dfn> 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|.{{Document/resizeObservers}} run these steps:

    1. If |observer|.{{ResizeObserver/activeTargets}} slot is empty, continue.

    2. Let |entries| be an empty list of {{ResizeObserverEntry}}ies.

    3. For each |observation| in {{ResizeObserver/activeTargets}} perform these steps:

        1. Let |entry| be new {{ResizeObserverEntry}}(|observation|.target)

        2. Add |entry| to |entries|

        3. Set |observation|.{{ResizeObservation/broadcastWidth}} to |entry|.contentRect.width.

        4. Set |observation|.{{ResizeObservation/broadcastHeight}} to |entry|.contentRect.height.

        5. Set |targetDepth| to the result of <a>calculate depth for node</a> for |observation|.{{ResizeObservation/target}}.

        6. Set |shallowestTargetDepth| to |targetDepth| if |targetDepth| < |shallowestTargetDepth|

    4. Invoke |observer|.{{ResizeObserver/callback}} with |entries|.

    5. Clear |observer|.{{ResizeObserver/activeTargets}}.

3. Return |shallowestTargetDepth|.

<h4 id="deliver-resize-error">Deliver Resize Loop Error</h4>

To <dfn>deliver resize loop error notification</dfn> 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.

<h4 id="calculate-depth-for-node-h">Calculate depth for node</h4>

To <dfn>calculate depth for node</dfn> |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|.

<h3 id="lifetime">ResizeObserver Lifetime</h3>

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.

<h3 id="integrations">External Spec Integrations</h3>

<h4 id="html-event-loop"> HTML Processing Model: Event Loop</h4>

{{ResizeObserver}} processing happens inside the step 7.12 of the <a>HTML Processing Model</a> event loop.

Step 12 is currently underspecified as:

<q>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.</q>.

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. <a>gather active observations at depth</a> |depth| for {{Document}}

5. repeat while (document <a>has active observations</a>)

    2. set |depth| to <a>broadcast active observations</a>

    3. recalc styles

    4. update layout

    5. <a>gather active observations at depth</a> |depth| for {{Document}}

6. if {{Document}} <a>has skipped observations</a> then <a>deliver resize loop error notification</a>

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