<script>
    import { onDestroy } from 'svelte'
    import { writable } from 'svelte/store';

    const count = writable(0, () => {
        console.log('Start subscribing')
                // Below function is called when execute unsubscribe function.
        return () => console.log('Stop subscribing');
    })

    const increment = () => count.update(value => {
        return value + 1
    })

    const unsubscribe = count.subscribe(value => {
        console.log(value)
    })

        onDestroy(unsubscribe)
</script>

<button on:click={increment}>
    Increment
</button>
<button on:click={unsubscribe}>
    Stop subscribe
</button>