Resize Observer 1

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:

• 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.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:

• 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.

<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.

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

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;
};

2.4. ResizeObservation

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

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

• 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 define bounding box instead of a content box. Content rect for SVGGraphicsElements is a rect whose:

• width is bounding box width

• height is bounding box height

• top and left are 0

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:

• there are no scripting references to the observer.

• the observer is not observing any targets.

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)

   2. set depth to broadcast active observations

   3. recalc styles

   4. update layout

   5. 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.