Prevent Svelte transition on first render after onMount

I'm working with the following code in Svelte 3.55.0.

First, I have an empty webpage because I have no items to render. Then, onMount fires and I fetch some items from an API. Finally, the items get rendered.

<script>
import { slide } from "svelte/transition";
import { onMount } from "svelte";

// Initially we have no items.
let items = [];
let id = 0;

onMount(() => {
    // Fetch items from API.
    items = [
        {id: id, name: `item ${id++}`},
        {id: id, name: `item ${id++}`},
        {id: id, name: `item ${id++}`},
    ];
});

function addItem() {
    items = [
        ...items,
        {id: id, name: `item ${id++}`},
    ];
}
</script>

<div>
    <button on:click={addItem}>add</button>
    {#each items as it (it.id)}
        <div transition:slide>
            <p>{it.id} {it.name}</p>
        </div>
    {/each}
</div>

The problem is that fetch gets like 50 items and I don't want to play transitions for any of them. However, I do want transitions when individual items are added or removed only inside of the list.

Is there a way to achieve this effect?

Sandbox: https://codesandbox.io/s/blue-shape-njxx9o?file=/App.svelte

Solution

This could be achieved by adding an #if block around the main element so it's only rendered after the items are fetched and adding the |local flag to the transition - tutorial

REPL

(Like here the transition only seem to play correctly if the outer element has display:flex)

<script>
    import { slide } from "svelte/transition";
    import {onMount} from 'svelte'

    let items = [];
    let id = 0;
    let itemsFetched = false

    onMount(async() => {
        setTimeout(() => {
            items = [
                {id: id, name: `item ${id++}`},
                {id: id, name: `item ${id++}`},
                {id: id, name: `item ${id++}`},             
            ];
            console.log('fetched')
            itemsFetched = true
        },1000)
    })

    function addItem() {
        items = [
            ...items,
            {id: id, name: `item ${id++}`},
        ];
    }
</script>

<button on:click={addItem}>add</button>

{#if itemsFetched}
<div id="outer">
    {#each items as it (it.id)}
    <div transition:slide|local={{duration: 2000}}>
        <p>{it.id} {it.name}</p>
    </div>
    {/each}
</div>
{/if}

<style>
    #outer {
        display: flex;
        flex-direction: column;
    }
</style>

Alternative with #await REPL

<script>
    import { slide } from "svelte/transition";
    import {onMount} from 'svelte'

    let items = [];
    let id = 0;

    function fetchItems() {
        return new Promise(res => {
            setTimeout(() => {
                items = [
                    {id: id, name: `item ${id++}`},
                    {id: id, name: `item ${id++}`},
                    {id: id, name: `item ${id++}`},             
                ];
                console.log('fetched')
                res()
            },1000)
        })      
    }

    function addItem() {
        items = [
            ...items,
            {id: id, name: `item ${id++}`},
        ];
    }
</script>

<button on:click={addItem}>add</button>

{#await fetchItems() then _}
<div id="outer">
    {#each items as it (it.id)}
    <div transition:slide|local={{duration: 2000}}>
        <p>{it.id} {it.name}</p>
    </div>
    {/each}
</div>
{/await}

<style>
    #outer {
        display: flex;
        flex-direction: column;
    }
</style>