class {
    onMount() {
        this.numberOfInvocations = 0;
    }

    handleAttach(color, event, node) {
        this.numberOfInvocations++;
    }
}

<div>
    <ul>
        <for|color| of=input.colors>
            <li key=color once-attach("handleAttach", color)>${color}</li>
        </for>
    </ul>
</div>