@lit-labs/observers
Reactive controllers that make it easy to use the web platform observer
classes with Lit.

@lit-labs/observers
includes reactive controllers for:
- MutationObserver
- ResizeObserver
- IntersectionObserver
- PerformanceObserver
[!WARNING]
This package is part of Lit Labs. It
is published in order to get feedback on the design and may receive breaking
changes or stop being supported.
Please read our Lit Labs documentation
before using this library in production.
Give feedback: https://github.com/lit/lit/discussions/3355
Overview
The modern web platform provides a number of observer helpers that can be used
to detect changes to which web applications may want to react. By managing
one of these observers with a reactive controller, changes can be easily
integrated into the Lit reactive update lifecycle. The controller can also help
manage observer cleanup and rendering in response to changes.
Installation
From inside your project folder, run:
$ npm install @lit-labs/observers
Usage
IntersectionController
IntersectionController attaches a IntersectionObserver to the host and requests
updates whenever the IntersectionObserver observes changes to the intersection
state of the targets.
The controller can also compute and store an arbitrary value each time changes
occur.
Import
import {IntersectionController} from '@lit-labs/observers/intersection-controller.js';
Constructor
constructor(
host: ReactiveControllerHost & Element,
{target, config, callback, skipInitial}: IntersectionControllerConfig<T>
)
Config
config: IntersectionObserverInit
: Configuration object for the
IntersectionObserver.
target?: Element | null
: The element to observe. In addition to configuring
the target here, the observe
method can be called to observe additional
targets. When not specified, the target defaults to the host
. If set to
null
, no target is automatically observed. Only the configured target will
be re-observed if the host connects again after unobserving via disconnection.
callback?: IntersectionValueCallback<T>
: The callback used to process detected
changes into a value stored in the controller's value
property.
skipInitial?: boolean
: By default the callback
is called without changes
when a target is observed. This is done to help manage initial state, but this
setup step can be skipped by setting this to true.
Properties and Methods
value?: T
: The result of processing the observer's changes via the
callback
function passed to the config.
observe(target: Element)
: Observe the target element. The controller's
target
is automatically observed when the host connects.
unobserve(target: Element)
: Unobserve the target element.
disconnect()
: Disconnects the observer. This is done automatically when the
host disconnects.
MutationController
MutationController attaches a MutationObserver to the host and requests updates
whenever the MutationObserver observes changes to the DOM.
The controller can also compute and store an arbitrary value each time changes
occur.
Example
import {MutationController} from '@lit-labs/observers/mutation-controller.js';
class MyElement extends LitElement {
private _observer = new MutationController(this, {
config: {attributes: true},
});
render() {
return html` ${this._observer.value ? `Attributes set!` : ``} `;
}
}
Import
import {MutationController} from '@lit-labs/observers/mutation-controller.js';
Constructor
new MutationController<T = unknown>(
host: ReactiveControllerHost & Element,
{target, config, callback, skipInitial}: MutationControllerConfig<T>
)
The type parameter <T>
is the type of the value
property and the return
type of the callback
option.
MutationControllerConfig
config: MutationObserverInit
: Configuration object for the MutationObserver.
target?: Element | null
: The element to observe. In addition to configuring
the target here, the observe
method can be called to observe additional
targets. When not specified, the target defaults to the host
. If set to
null
, no target is automatically observed. Only the configured target will
be re-observed if the host connects again after unobserving via disconnection.
callback?: MutationValueCallback<T>
: The callback used to process detected
changes into a value stored in the controller's value
property.
skipInitial?: boolean
: By default the callback
is called without changes
when a target is observed. This is done to help manage initial state, but this
setup step can be skipped by setting this to true.
Properties and Methods
value
: The result of processing the observer's changes via the callback
function passed to the config.
observe(target: Element)
: Observe the target element. The controller's
target
is automatically observed when the host connects.
disconnect()
: Disconnects the observer. This is done automatically when the
host disconnects.
PerformanceController
PerformanceController attaches a PerformanceObserver to the host and requests
updates whenever the PerformanceObserver observes receives new performance
metrics.
Import
import {PerformanceController} from '@lit-labs/observers/performance-controller.js';
Constructor
constructor(
host: ReactiveControllerHost,
{config, callback, skipInitial}: PerformanceControllerConfig<T>
)
PerformanceControllerConfig
config: PerformanceObserverInit
: Configuration object for the MutationObserver.
callback?: PerformanceValueCallback<T>
: The callback used to process detected
changes into a value stored in the controller's value
property.
skipInitial?: boolean
: By default the callback
is called without changes
when a target is observed. This is done to help manage initial state, but this
setup step can be skipped by setting this to true.
Properties and Methods
value
: The result of processing the observer's changes via the callback
function passed to the config.
observe(target: Element)
: Observe the target element. The controller's
target
is automatically observed when the host connects.
disconnect()
: Disconnects the observer. This is done automatically when the
host disconnects.
ResizeController
ResizeController attaches a ResizeObserver to the host and requests updates
whenever the ResizeObserver detects size changes to its targets. The controller
can also compute and store an arbitrary value each time changes occur.
Import
import {ResizeController} from '@lit-labs/observers/resize-controller.js';
Constructor
constructor(
host: ReactiveControllerHost & Element,
{target, config, callback, skipInitial}: ResizeControllerConfig<T>
)
ResizeControllerConfig
config?: ResizeObserverOptions
: Configuration object for the
ResizeController.
target?: Element | null
: The element to observe. In addition to configuring
the target here, the observe
method can be called to observe additional
targets. When not specified, the target defaults to the host
. If set to
null
, no target is automatically observed. Only the configured target will
be re-observed if the host connects again after unobserving via disconnection.
callback?: ResizeValueCallback<T>
: The callback used to process detected
changes into a value stored in the controller's value
property.
skipInitial?: boolean
: By default the callback
is called without changes
when a target is observed. This is done to help manage initial state, but this
setup step can be skipped by setting this to true. }
Properties and Methods
value?: T
: The result of processing the observer's changes via the
callback
function passed to the config.
observe(target: Element)
: Observe the target element. The controller's
target
is automatically observed when the host connects.
unobserve(target: Element)
: Unobserve the target element.
disconnect()
: Disconnects the observer. This is done automatically when the
host disconnects.
Contributing
Please see CONTRIBUTING.md.