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// @flow
// An event handler can take an optional event argument
// and should not return a value
type EventHandler = (event?: any) => void;
// An array of all currently registered event handlers for a type
type EventHandlerList = Array<EventHandler>;
// A map of event types and their corresponding event handlers.
type EventHandlerMap = {
[type: string]: EventHandlerList,
};
/** Mitt: Tiny (~200b) functional event emitter / pubsub.
* @name mitt
* @returns {Mitt}
*/
export default function mitt(all: EventHandlerMap) {
all = all || Object.create(null);
return {
/**
* Register an event handler for the given type.
*
* @param {String} type Type of event to listen for, or `"*"` for all events
* @param {Function} handler Function to call in response to given event
* @memberOf mitt
*/
on(type: string, handler: EventHandler) {
(all[type] || (all[type] = [])).push(handler);
},
/**
* Remove an event handler for the given type.
*
* @param {String} type Type of event to unregister `handler` from, or `"*"`
* @param {Function} handler Handler function to remove
* @memberOf mitt
*/
off(type: string, handler: EventHandler) {
let e = all[type] || (all[type] = []);
e.splice(e.indexOf(handler) >>> 0, 1);
},
/**
* Invoke all handlers for the given type.
* If present, `"*"` handlers are invoked after type-matched handlers.
*
* @param {String} type The event type to invoke
* @param {Any} [evt] Any value (object is recommended and powerful), passed to each handler
* @memberof mitt
*/
emit(type: string, evt: any) {
(all[type] || []).map((handler) => { handler(evt); });
(all['*'] || []).map((handler) => { handler(type, evt); });
}
};
}
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