hydrogen-web/src/utils/EventEmitter.js

export class EventEmitter {
    constructor() {
        this._handlersByName = {};
    }

    emit(name, ...values) {
        const handlers = this._handlersByName[name];
        if (handlers) {
            for(const h of handlers) {
                h(...values);
            }
        }
    }

    disposableOn(name, callback) {
        this.on(name, callback);
        return () => {
            this.off(name, callback);
        }
    }

    on(name, callback) {
        let handlers = this._handlersByName[name];
        if (!handlers) {
            this.onFirstSubscriptionAdded(name);
            this._handlersByName[name] = handlers = new Set();
        }
        handlers.add(callback);
    }

    off(name, callback) {
        const handlers = this._handlersByName[name];
        if (handlers) {
            handlers.delete(callback);
            if (handlers.length === 0) {
                delete this._handlersByName[name];
                this.onLastSubscriptionRemoved(name);
            }
        }
    }

    onFirstSubscriptionAdded(name) {}

    onLastSubscriptionRemoved(name) {}
}

export function tests() {
    return {
        test_on_off(assert) {
            let counter = 0;
            const e = new EventEmitter();
            const callback = () => counter += 1;
            e.on("change", callback);
            e.emit("change");
            e.off("change", callback);
            e.emit("change");
            assert.equal(counter, 1);
        },

        test_emit_value(assert) {
            let value = 0;
            const e = new EventEmitter();
            const callback = (v) => value = v;
            e.on("change", callback);
            e.emit("change", 5);
            e.off("change", callback);
            assert.equal(value, 5);
        },

        test_double_on(assert) {
            let counter = 0;
            const e = new EventEmitter();
            const callback = () => counter += 1;
            e.on("change", callback);
            e.on("change", callback);
            e.emit("change");
            e.off("change", callback);
            assert.equal(counter, 1);
        }
    };
}