Using HTML slots to render platform views in the web
Summary
#Flutter now renders all web platform views in a consistent location of the DOM,
as direct children of flt-glass-pane
(regardless of the rendering backend:
html
or canvaskit
). Platform views are then "slotted" into the correct
position of the App's DOM with standard HTML features.
Up until this change, Flutter web would change the styling of the rendered contents of a platform views to position/size it to the available space. This is no longer the case. Users can now decide how they want to utilize the space allocated to their platform view by the framework.
Context
#The Flutter framework frequently tweaks its render tree to optimize the paint operations that are ultimately made per frame. In the web, these render tree changes often result in DOM operations.
Flutter web used to render its platform views (HtmlElementView
widgets)
directly into its corresponding position of the DOM.
Using certain DOM elements as the "target" of some DOM operations causes those
elements to lose their internal state. In practice, this means that iframe
tags are going to reload, video
players might restart, or an editable form
might lose its edits.
Flutter now renders platform views using slot elements inside of a single, app-wide shadow root. Slot elements can be added/removed/moved around the Shadow DOM without affecting the underlying slotted content (which is rendered in a constant location)
This change was made to:
- Stabilize the behavior of platform views in Flutter web.
- Unify how platform views are rendered in the web for both rendering
backends (
html
andcanvaskit
). - Provide a predictable location in the DOM that allows developers to reliably
use CSS to style their platform views, and to use other standard DOM API,
such as
querySelector
, andgetElementById
.
Description of change
#A Flutter web app is now rendered inside a common shadow root in which slot elements represent platform views. The actual content of each platform view is rendered as a sibling of said shadow root.
Before
#...
<flt-glass-pane>
...
<div id="platform-view">Contents</div> <!-- canvaskit -->
<!-- OR -->
<flt-platform-view>
#shadow-root
| <div id="platform-view">Contents</div> <!-- html -->
</flt-platform-view>
...
</flt-glass-pane>
...
After
#...
<flt-glass-pane>
#shadow-root
| ...
| <flt-platform-view-slot>
| <slot name="platform-view-1" />
| </flt-platform-view-slot>
| ...
<flt-platform-view slot="platform-view-1">
<div id="platform-view">Contents</div>
</flt-platform-view>
...
</flt-glass-pane>
...
After this change, when the framework needs to move DOM nodes around, it
operates over flt-platform-view-slot
s, which only contain a slot
element.
The slot projects the contents defined in flt-platform-view
elements outside
the shadow root. flt-platform-view
elements are never the target of DOM
operations from the framework, thus preventing the reload issues.
From an app's perspective, this change is transparent. However, this is considered a breaking change because some tests make assumptions about the internal DOM of a Flutter web app, and break.
Migration guide
#Code
#The engine may print a warning message to the console similar to:
Height of Platform View type: [$viewType] may not be set. Defaulting to `height: 100%`.
Set `style.height` to any appropriate value to stop this message.
or:
Width of Platform View type: [$viewType] may not be set. Defaulting to `width: 100%`.
Set `style.width` to any appropriate value to stop this message.
Previously, the content returned by PlatformViewFactory
functions was
resized and positioned by the framework. Instead, Flutter now sizes and
positions <flt-platform-view-slot>
, which is the parent of the slot where the
content is projected.
To stop the warning above, platform views need to set the style.width
and
style.height
of their root element to any appropriate (non-null) value.
For example, to make the root html.Element
fill all the available space
allocated by the framework, set its style.width
and style.height
properties
to '100%'
:
ui.platformViewRegistry.registerViewFactory(viewType, (int viewId) {
final html.Element htmlElement = html.DivElement()
// ..other props
..style.width = '100%'
..style.height = '100%';
// ...
return htmlElement;
});
If other techniques are used to lay out the platform view (like inset: 0
) a
value of auto
for width
and height
is enough to stop the warning.
Read more about CSS width
and CSS height
.
Tests
#After this change, user's test code does not need to deeply inspect the
contents of the shadow root of the App. All of the platform view contents will
be placed as direct children of flt-glass-pane
, wrapped in a
flt-platform-view
element.
Avoid looking inside the flt-glass-pane
shadow root, it is considered a
"private implementation detail", and its markup can change at any time,
without notice.
(See Relevant PRs below for examples of the "migrations" described above).
Timeline
#Landed in version: 2.3.0-16.0.pre
In stable release: 2.5
References
#Design document:
Relevant issues:
Relevant PRs:
- flutter/engine#25747: Introduces the feature.
- flutter/flutter#82926: Tweaks
flutter
tests. - flutter/plugins#3964: Tweaks to
plugins
code. - flutter/packages#364: Tweaks to
packages
code.
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