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joplin/packages/lib/EventDispatcher.ts

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type Listener<Value> = (data: Value)=> void;
type CallbackHandler<EventType> = (data: EventType)=> void;
// EventKeyType is used to distinguish events (e.g. a 'ClickEvent' vs a 'TouchEvent')
// while EventMessageType is the type of the data sent with an event (can be `void`)
export default class EventDispatcher<EventKeyType extends string|symbol|number, EventMessageType> {
// Partial marks all fields as optional. To initialize with an empty object, this is required.
// See https://stackoverflow.com/a/64526384
private listeners: Partial<Record<EventKeyType, Array<Listener<EventMessageType>>>>;
public constructor() {
this.listeners = {};
}
public dispatch(eventName: EventKeyType, event: EventMessageType = null) {
if (!this.listeners[eventName]) return;
const ls = this.listeners[eventName];
for (let i = 0; i < ls.length; i++) {
ls[i](event);
}
}
public on(eventName: EventKeyType, callback: CallbackHandler<EventMessageType>) {
if (!this.listeners[eventName]) this.listeners[eventName] = [];
this.listeners[eventName].push(callback);
return {
// Retuns false if the listener has already been removed, true otherwise.
remove: (): boolean => {
const originalListeners = this.listeners[eventName];
this.off(eventName, callback);
return originalListeners.length !== this.listeners[eventName].length;
},
};
}
// Equivalent to calling .remove() on the object returned by .on
public off(eventName: EventKeyType, callback: CallbackHandler<EventMessageType>) {
if (!this.listeners[eventName]) return;
// Replace the current list of listeners with a new, shortened list.
// This allows any iterators over this.listeners to continue iterating
// without skipping elements.
this.listeners[eventName] = this.listeners[eventName].filter(
otherCallback => otherCallback !== callback
);
}
}