ESL Event Listeners New

Starting from the 4th release ESL has a built-in mechanism to work with DOM events. ESL event listeners have more control and advanced features than native DOM API. Besides, the ESlBaseElement and the ESLMixinElement have even more pre-built syntax sugar to make the consumer's code briefer.

One of the main advantages of ESL listeners is the extended control of subscriptions. All ESL listeners and their declarations are saved and associated with the host element. It means that ESL listeners can be subscribed or unsubscribed at any time in various ways. And most importantly, you do not need the original callback handler to do this.

Basic concepts

The ESL event listener module is based on the following major terms and classes:

The `host`

Everything that happens with ESL event listeners should be associated with a host object. The host is an object that "owns" (registers / deletes) the subscription.

By default, the host is used as an EventTarget (or an object implementing EventTarget interface) to subscribe. But the host object is not necessarily related to an EventTarget. We have at least two options to change the target. First of all, you can define the target explicitly. Another way is to specify the default DOM target of the host object by providing a special $host key (see ESLMixinElement implementation).

The host object is also used as a context to call the handler function of the subscription.

The `handler`

The handler is a function that is used to process the subscription event. ESL declares a generic type to describe such functions - ESLListenerHandler;

export type ESLListenerHandler<EType extends Event = Event> = (
  event: EType
) => void;

The `ESLEventListener` class and subscription

The subscriptions created by the ESL event listener module are instances of ESLEventListener class. All active subscriptions are stored in a hidden property of the host object.

ESLEventListener has the following basic properties:

event - event type that the subscription is listening to;
handler - reference for the function to call to handle the event (see The handler);
host - reference for the object that holds the subscription (see The host);
target - definition of EventTarget element (or string TraversingQuery to find it, see details in ESLEventDesriptor);
selector - CSS selector to use built-in event delegation;
capture - marker to use the capture phase of the DOM event life-cycle;
passive - marker to use passive (non-blocking) subscription of the native event (if supported);
once - marker to destroy the subscription after the first event catch.

All of the ESLEventListener instance fields are read-only; the subscription can't be changed once created.

The ESLEventListener, as a class, describes the subscription behavior and contains static methods to create and manage subscriptions.

Descriptors (`ESLEventDesriptor`, `ESLEventDesriptorFn`)

The event listener Descriptor is an object to describe future subscriptions. The ESL event listeners module has a few special details regarding such objects.

A simple descriptor is an object that is passed to ESL event listener API to create a subscription. It contains almost the same set of keys as the ESLEventListener instance.

In addition to that, ESL allows you to combine the ESLEventDesriptor data with the handler function. ESLEventDesriptorFn is a function handler that is decorated with the ESLEventDesriptor properties.

Here is the list of supported keys of ESLEventDesriptor:

event key

Type: string | PropertyProvider<string>
Description: the event type for subscription. Can be provided as a string or via provider function that will be called right before the subscription.

The event string (as a literal, or returned by PropertyProvider) can declare multiple event types separated by space. ESL will create a subscription (ESLEventListener object) for each event type in this case.
target key

Type: string | EventTarget | EventTarget[] | PropertyProvider<string | EventTarget | EventTarget[]>
Default Value: host object itself or $host key of the host object
Description: the key to declare exact EventTarget for the subscription. In case the target key is a string it is considered as a TraversingQuery. The query finds a target relatively to host | host.$host object, or in bounds of the DOM tree if it is absolute. The target key also supports an exact reference for EventTarget(s).

⚠ Any EventTarget or even ESL SynteticEventTarget (including ESLMediaQuery) can be a target for listener API.

⚠ See OOTB Extended Event Targets of ESL to know how to optimize handling of frequent events.

The target property can be declared via PropertyProvider as well.
selector key

Type: string | PropertyProvider<string>
Default Value: null
Description: the CSS selector to filter event targets for event delegation mechanism.

Supports PropertyProvider to declare the computed value as well.
capture key

Type: boolean
Default Value: false
Description: marker to use capturing phase of the DOM event to handle.
passive key

Type: boolean
Default Value: true if the event type is wheel, mousewheel, touchstart or touchmove Description: marker to use passive subscription to the native event.

⚠ ESL uses passive subscription by default for wheel, mousewheel, touchstart, touchmove events. You need to declare passive key explicitly to override this behavior.
once key

Type: boolean
Default Value: false
Description: marker to unsubscribe the listener after the first successful handling of the event.
auto key (for ESLEventDesriptorFn declaration only)

Type: boolean
Default Value: false for ESLEventUtils.initDescriptor, true for @listen decorator Description: marker to make an auto-subscribable descriptor. See Automatic (collectable) descriptors.
inherit key (for ESLEventDesriptorExt only)

Type: boolean
Description: available in extended version of ESLEventDesriptor that is used in the descriptor declaration API. Allows to inherit ESLEventDesriptor data from the ESLEventDesriptorFn from the prototype chain. See initDescriptor usages example.

Automatic (collectable) descriptors

Auto-collectable (or auto-subscribable) descriptors can be subscribed at once during the initialization of the host object.

To make an ESLEventDesriptorFn auto-collectable, the consumer should declare it with the auto marker using ESLEventUtils.initDescriptor or @listen decorator.

⚠ ESLEventUtils.initDescriptor (or @listen) stores the auto-collectable descriptors in the internal collection on the host.

The ESLBaseElment and the ESLMixinElement subscribes all auto-collectable descriptors in the connectedCallback. See the usage of ESLEventUtils.subscibe for more details.

`PropertyProvider` for `event`, `selector`, or `target`

The descriptor declaration usually happens with the class declaration when the instance and its subscription do not exist. We might have a problem if we want to pass subscription parameters that depend on the instance.

To resolve such a case, the event, selector, and target keys of ESL event listener API support PropertyProvider mechanism:

type PropertyProvider<T> = (this: unknown, that: unknown) => T;

See examples in the ESLEventUtils.initDescriptor section.

Public API (`ESLEventUtils`)

The units mentioned earlier are mostly implementation details of the module.

ESLEventUtils is a facade for all ESL event listener module capabilities.

Here is the module Public API:

⚡ `ESLEventUtils.subscribe`

Creates and subscribes an ESLEventListener.

Subscribes all auto-collectable (subscribable) descriptors of the host object:
```
ESLEventUtils.subscribe(host: object)
```

Subscribes handler function to the DOM event declared by eventType string:

ESLEventUtils.subscribe(host: object, eventType: string, handler: ESLListenerHandler)

Subscribes handler instance of ESLEventDescriptorFn using embedded meta-information:
```
ESLEventUtils.subscribe(host: object, handler: ESLEventDescriptorFn)
```

Subscribes handler function using ESLEventDescriptor:

ESLEventUtils.subscribe(host: object, descriptor: ESLEventDescriptor, handler: ESLListenerHandler)

Subscribes handler instance of ESLEventDescriptorFn with ESLEventDescriptor overriding meta-data:

ESLEventUtils.subscribe(host: object, descriptor: ESLEventDescriptor, handler: ESLEventDescriptorFn)

Parameters:

host - host element to store subscription (event target by default);
eventType - string DOM event type;
descriptor - event description data (ESLEventDescriptor);
handler - function callback handler or instance of ESLEventDescriptorFn

Examples:

ESLEventUtils.subscribe(host); - subscribes all auto-subscriptions of the host;
ESLEventUtils.subscribe(host, handlerFn); - subscribes handlerFn method (decorated as an ESLEventDescriptorFn) to the handlerFn.target;
ESLEventUtils.subscribe(host, 'click', handlerFn); - subscribes handlerFn function with the passed event type;
ESLEventUtils.subscribe(host, {event: 'scroll', target: window}, handlerFn); - subscribes handlerFn function with the passed additional descriptor data.

⚡ `ESLEventUtils.unsubscribe`

Allows unsubscribing existing subscriptions.

unsubscribe(host: HTMLElement, ...criteria: ESLListenerCriteria[]): ESLEventListener[];

Parameters:

host - host element to find subscriptions;
criteria - optional set of criteria to filter listeners to remove.

Examples:

ESLEventUtils.unsubscribe(host); - unsubscribes everything bound to the host
ESLEventUtils.unsubscribe(host, handlerFn); - unsubscribes everything that is bound to the host and is handled by the handlerFn
ESLEventUtils.unsubscribe(host, 'click'); - unsubscribes everything bound to the host and processing click event
ESLEventUtils.unsubscribe(host, 'click', handlerFn); - unsubscribes everything that is bound to the host, processing click event and is handled by the handlerFn
There can be any number of criteria.

⚡ `ESLEventUtils.isEventDescriptor`

Predicate to check if the passed argument is a type of ESLListenerDescriptorFn = ESLEventHandler & Partial<ESLEventDescriptor>.

ESLEventUtils.isEventDescriptor(obj: any): obj is ESLListenerDescriptorFn;

⚡ `ESLEventUtils.getAutoDescriptors`

Gathers auto-subscribable (collectable) descriptors from the passed object.

ESLEventUtils.descriptors(host?: any): ESLListenerDescriptorFn[]

Parameters:

host - object to get auto-collectable descriptors from;

⚡ `ESLEventUtils.descriptors`

Deprecated alias for ESLEventUtils.getAutoDescriptors

⚡ `ESLEventUtils.initDescriptor`

Decorates the passed key of the host object as ESLEventDescriptorFn

ESLEventUtils.initDescriptor<T extends object>(
  host: T,
  key: keyof T & string,
  desc: ESLEventDescriptorExt
): ESLEventDescriptorFn;

Parameters:

host - host object holder of decorated function;
key - key of the host object that contains a function to decorate;
desc - ESLEventDescriptor (extended) meta information to describe future subscriptions.

The extended ESLEventDescriptor information allows passing inherit marker to create a new descriptor instance based on the descriptor declared in the prototype chain for the same key.

⚠ If such a key is not found, a ReferenceError will be thrown.

Example:

class MyElement {
  onEvent() {} 
}
ESLEventUtils.initDescriptor(MyElement.prototype, 'onEvent', {event: 'event'});

`@listen` decorator

The @listen decorator (available under esl-utils/decorators) is syntax sugar above ESLEventUtils.initDescriptor method. It allows you to declare class methods as an ESLEventDescriptorFn using TS experimentalDecorators feature.

Listeners described by @listen are auto-subscribable if they are not inherited and not declared as manual explicitly. In case of inheritance the auto marker will be inherited from the parent descriptor.

Example:

class MyEl extends ESLBaseElement {
  private event: string;
  private selector: string;
  // Shortcut with just an event type
  @listen('click')
  onClick() {}
  // Shortcut with event type declared by PropertyProvider
  @listen((that: MyEl) => that.event)
  onEventProvided() {}
  // Full list of options is available
  @listen({event: 'click', target: 'body', capture: true})
  onBodyClick(e) {}
  // Property Providers example
  @listen({
    event: (that: MyEl) => that.event,
    seletor: (that: MyEl) => that.selector
  })
  onEventProvidedExt(e) {}
  // Will not subscribe authomatically
  @listen({event: 'click', auto: false})
  onClickManual(e) {}
}

⚡ `ESLEventUtils.listeners`

Gathers listeners currently subscribed to the passed host object.

ESLEventUtils.listeners(host: object, ...criteria: ESLListenerCriteria[]): ESLEventListener[];

Parameters:

host - object that stores and relates to the handlers;
criteria - optional set of criteria to filter the listeners list.

⚡ `ESLEventUtils.dispatch`

Dispatches custom DOM events. The dispatched event is bubbling and cancelable by default.

ESLEventUtils.dispatch(
  el: EventTarget,
  eventName: string,
  eventInit?: CustomEventInit
): boolean;

Parameters:

el - EventTarget to dispatch event;
eventName - name of the event to dispatch;
eventInit - object that specifies characteristics of the event.

Listeners Full Showcase Example

class TestCases {
  bind() {
    // Subcribes all auto descriptors (onEventAutoDescSugar and onEventAutoDesc)
    ESLEventUtils.subscribe(this);

    // Subscribes onEventManualFn on click
    ESLEventUtils.subscribe(this, 'click', this.onEventManualFn);

    // Subscribes onEventManualFn on window resize
    ESLEventUtils.subscribe(this, {event: 'resize', target: window}, this.onEventManualFn);

    // Subscribes onEventManualDesc using embeded information
    ESLEventUtils.subscribe(this, this.onEventManualDesc);

    // Subscribes onEventManualDesc using merged embeded and passed information
    ESLEventUtils.subscribe(this, {target: window}, this.onEventManualDesc);
  }

  unbind() {
    // Unsubcribes all subscriptions
    ESLEventUtils.unsubscribe(this);

    // Unsubcribes just onEventAutoDesc
    ESLEventUtils.unsubscribe(this, this.onEventAutoDesc);
  }

  @listen('event')
  onEventAutoDescSugar() {}

  onEventAutoDesc() {}

  onEventManualFn() {}

  onEventManualDesc() {}
}

ESLEventUtils.initDescriptor(TestCases.prototype, 'onEventAutoDesc', {event: 'event', auto: true});
ESLEventUtils.initDescriptor(TestCases.prototype, 'onEventManualDesc', {event: 'event'});

Extended `EventTarget`s and standard optimizations beta

⚡ `ESLEventUtils.decorate` and `ESLEventTargetDecorator`

In cases where the original event of the target happens too frequently to be handled every time, it might be helpful to limit its processing. In purpose to do that ESL allows the creation of decorated EventTargets. The decorated target will process the original target events dispatching with the passed async call decoration function (such as debounce or throttle).

The ESLEventUtils.decorate creates an instance of ESLEventTargetDecorator that decorates passed original EventTarget event emitting. The instances of ESLEventTargetDecorator are lazy and do not subscribe to the original event until they have their own subscriptions of the same event type.

⚠ Note ESLEventUtils.decorate method is cached, so created instances will be reused if the inner cache does not refuse additional arguments of the decorator. The cache does not handle multiple and non-primitive arguments.

ESLEventUtils.decorate(
  target: EventTarget,
  decorator: (fn: EventListener, ...args: any[]) => EventListener,
  ...args: any[]
): ESLEventTargetDecorator;

Parameters:

target - original EventTarget to consume events;
decorator - decoration function to decorate original target EventListeners;
args - optional arguments to pass to decorator.

Sharing of the decorated targets

As was mentioned above, the method ESLEventUtils.decorate (alias for ESLEventTargetDecorator.cached) works with a cache for simple cases. But in some cases, we might be interested in creating wrappers with a complex param, or we want to limit params usage across the project.

It might sound obvious, but there are no restrictions on sharing exact instances instead of using the method cache.

// shared-event-targets.ts
export const DEBOUNCED_WINDOW = ESLEventUtils.decorate(window, debounce, 1000);

// module.ts
class Component {
  @listen({event: 'resize', target: DEBOUNCED_WINDOW})
  onResize() {}
}

Optimize `window.resize` handling with debouncing

import {debounce} from '.../debounce';

ESLEventUtils.subscribe(host, {
  event: 'resize',
  target: /* instead just window */ ESLEventUtils.decorate(window, debounce, 250)
}, onResizeDebounced);

The sample above allows you to reuse debounced by 250 milliseconds version of the window, to receive fewer resize events (same as any other event types observed on debounced window version)

Optimize `window.scroll` handling with throttling

import {throttle} from '.../throttle';

ESLEventUtils.subscribe(host, {
  event: 'scroll',
  target: /* instead just window */ ESLEventUtils.decorate(window, throttle, 250)
}, onScrollThrottled);

The sample above allows you to reuse throttled by 250 milliseconds version of the window, to receive no more than one event per 250 milliseconds scroll events (same as any other event types observed on debounced window version)

⚡ `ESLEventUtils.resize` and `ESLResizeObserverTarget`

When you deal with responsive interfaces, you might need to observe an element resizes instead of responding to the whole window change. There is a tool for this in the native DOM API - `ResizeObserver'. The only problem is that it does not use events, while in practice, we work with it in the same way.

ESLEventUtils.resize creates cached ResizeObserver adaptation to EventTarget (ESLResizeObserverTarget) that allows you to get resize events when the observed element changes its size.

ESLEventUtils.resize(el: Element): ESLResizeObserverTarget;

Parameters:

el - Element to observe size changes.

ESLResizeObserverTarget creates itself once for an observed object with a weak reference-based cache. So any way of creating ESLResizeObserverTarget will always produce the same instance.

ESLEventUtils.resize(el) /**always*/ === ESLEventUtils.resize(el) So there is no reason to cache it manually.

Usage example:

ESLEventUtils.subscribe(host, {
  event: 'resize',
  target: ESLEventUtils.resize(el)
}, onResize);
// or
ESLEventUtils.subscribe(host, {
  event: 'resize',
  target: (host) => ESLEventUtils.resize(host.el)
}, onResize);

Embedded behavior of `ESLBaseElement` / `ESLMixinElement`

Shortcuts

All the inheritors of ESLBaseElement and ESLMixinElement contain the short aliases for some ESLEventUtils methods. The host parameter of the shortcut methods is always targeting the current element/mixin.

$$on ~ ESLEventUtils.subscribe(this, ...)
$$off ~ ESLEventUtils.unsubscribe(this, ...)
$$fire ~ ESLEventUtils.dispatch(this, ...)

Example:

this.$$on('click', this.onClick); // ESLEventUtils.subscribe(this, 'click', this.onClick)
this.$$off('click'); // ESLEventUtils.unsubscribe(this, 'click')

Auto-subscription / Auto-unbinding

All the inheritors of ESLBaseElement and ESLMixinElement automatically subscribe to all declared auto-subscribable descriptors of their prototype chain.

They also unsubscribe all own listeners attached via ESL automatically on disconnectedCallback.

The following short snippet of code describes a listener that will automatically subscribe and unsubscribe on connected/disconnected callback inside ESLBaseElement:

class MyEl extends ESLBaseElement {
  // connectedCallback() {
  //   super.connectedCallback(); // - already contains ESLEventUtils.subscribe(this) call
  // }

  // Will be subscribed automatically on connectedCallback and unsubscribed on disconnectedCallback
  @listen('click')
  onClick(e) {
    //...
  }
}

You can manage the subscription manually and link the whole meta information or part of it with the handler itself

class MyEl extends ESLBaseElement {
  @listen({event: 'click', auto: false}) // Won`t be subscribed automatically
  onClick(e) {
    // ...
  }

  myMethod() {
    this.$$on(this.onClick); // Manual subscription
    this.$$on({target: 'body'}, this.onClick); // Manual subscription with parameters (will be merged)
    this.$$off(this.onClick); // Unsubscribes this.onClick method
  }
}

ESL Event Listeners New

Basic concepts

The `host`

The `handler`

The `ESLEventListener` class and subscription

Descriptors (`ESLEventDesriptor`, `ESLEventDesriptorFn`)

`event` key

`target` key

`selector` key

`capture` key

`passive` key

`once` key

`auto` key (for `ESLEventDesriptorFn` declaration only)

`inherit` key (for `ESLEventDesriptorExt` only)

Automatic (collectable) descriptors

`PropertyProvider` for `event`, `selector`, or `target`

Public API (`ESLEventUtils`)

⚡ `ESLEventUtils.subscribe`

⚡ `ESLEventUtils.unsubscribe`

⚡ `ESLEventUtils.isEventDescriptor`

⚡ `ESLEventUtils.getAutoDescriptors`

⚡ `ESLEventUtils.descriptors`

⚡ `ESLEventUtils.initDescriptor`

`@listen` decorator

⚡ `ESLEventUtils.listeners`

⚡ `ESLEventUtils.dispatch`

Listeners Full Showcase Example

Extended `EventTarget`s and standard optimizations beta

⚡ `ESLEventUtils.decorate` and `ESLEventTargetDecorator`

Sharing of the decorated targets

Optimize `window.resize` handling with debouncing

Optimize `window.scroll` handling with throttling

⚡ `ESLEventUtils.resize` and `ESLResizeObserverTarget`

Embedded behavior of `ESLBaseElement` / `ESLMixinElement`

Shortcuts

Auto-subscription / Auto-unbinding

ESL Event Listeners New

Basic concepts

The host

The handler

The ESLEventListener class and subscription

Descriptors (ESLEventDesriptor, ESLEventDesriptorFn)

event key

target key

selector key

capture key

passive key

once key

auto key (for ESLEventDesriptorFn declaration only)

inherit key (for ESLEventDesriptorExt only)

Automatic (collectable) descriptors

PropertyProvider for event, selector, or target

Public API (ESLEventUtils)

⚡ ESLEventUtils.subscribe

⚡ ESLEventUtils.unsubscribe

⚡ ESLEventUtils.isEventDescriptor

⚡ ESLEventUtils.getAutoDescriptors

⚡ ESLEventUtils.descriptors

⚡ ESLEventUtils.initDescriptor

@listen decorator

⚡ ESLEventUtils.listeners

⚡ ESLEventUtils.dispatch

Listeners Full Showcase Example

Extended EventTargets and standard optimizations beta

⚡ ESLEventUtils.decorate and ESLEventTargetDecorator

Sharing of the decorated targets

Optimize window.resize handling with debouncing

Optimize window.scroll handling with throttling

⚡ ESLEventUtils.resize and ESLResizeObserverTarget

Embedded behavior of ESLBaseElement / ESLMixinElement

Shortcuts

Auto-subscription / Auto-unbinding

The `host`

The `handler`

The `ESLEventListener` class and subscription

Descriptors (`ESLEventDesriptor`, `ESLEventDesriptorFn`)

`event` key

`target` key

`selector` key

`capture` key

`passive` key

`once` key

`auto` key (for `ESLEventDesriptorFn` declaration only)

`inherit` key (for `ESLEventDesriptorExt` only)

`PropertyProvider` for `event`, `selector`, or `target`

Public API (`ESLEventUtils`)

⚡ `ESLEventUtils.subscribe`

⚡ `ESLEventUtils.unsubscribe`

⚡ `ESLEventUtils.isEventDescriptor`

⚡ `ESLEventUtils.getAutoDescriptors`

⚡ `ESLEventUtils.descriptors`

⚡ `ESLEventUtils.initDescriptor`

`@listen` decorator

⚡ `ESLEventUtils.listeners`

⚡ `ESLEventUtils.dispatch`

Extended `EventTarget`s and standard optimizations beta

⚡ `ESLEventUtils.decorate` and `ESLEventTargetDecorator`

Optimize `window.resize` handling with debouncing

Optimize `window.scroll` handling with throttling

⚡ `ESLEventUtils.resize` and `ESLResizeObserverTarget`

Embedded behavior of `ESLBaseElement` / `ESLMixinElement`