_observeEvent(context, event) {
        let key = this._join(context);
        if (!(key in this._data)) {
            this._data[key] = {};
        }
        let data = this._data[key];
        if (!(event in data)) {
            data[event] = 0;
        }
        data[event]++;
    }
@param {string[]} context
@param {string} event