check() {
        this.elements.forEach(el => {
            let passes  = this.options.test(el, this.options);
            let index   = this.current.indexOf(el);
            let current = index > -1;
            let entered = passes && !current;
            let exited  = !passes && current;

            if (entered) {
                this.current.push(el);
                this.emit('enter', el);
            }

            if (exited) {
                this.current.splice(index, 1);
                this.emit('exit', el);
            }

        });
        return this;
    }
Check each element in the registry, if an element
changes states, fire an event and operate on current.