Svelte's transition API offers a powerful way to animate components entering or leaving the DOM, including creating custom transitions. Leveraging CSS animations by default ensures optimal performance. The basic syntax is straightforward: <element transition:transitionfunction=""></element>. You can also use in or out directives for one-way transitions.

Svelte's svelte/transition package provides seven pre-built transition functions, readily customizable with svelte/easing for diverse animation effects without writing custom code. Experiment with these to grasp the possibilities.

Need a Svelte introduction? We have a comprehensive overview available.

Crafting Custom Svelte Transitions

For finer control beyond pre-built options, Svelte allows defining custom transition functions, subject to specific conventions. The API structure, as documented, is:

transition = (node: HTMLElement, params: any) => {
  delay?: number,
  duration?: number,
  easing?: (t: number) => number,
  css?: (t: number, u: number) => string,
  tick?: (t: number, u: number) => void
}

A transition function receives the DOM node and returns an object with animation parameters. Crucially, it includes a css or tick function.

The css function returns a CSS string defining the animation (e.g., transforms or opacity changes). The tick function offers complete JavaScript control, but at a performance cost as it bypasses CSS animations.

Both css and tick use parameters t (0.00 to 1.00 on entry, 1.00 to 0.00 on exit) and u (1 - t). For instance, transform: scale(${t}) smoothly scales from 0 to 1 on entry and reverses on exit.

Let's build a custom transition to illustrate.

Your First Custom Svelte Transition

We'll start with a simple toggle to control an element's DOM presence using a Svelte #if block (remember, transitions only occur on DOM entry/exit).

<script>
  let showing = true;
</script>

<label for="showing">
  Showing
</label>
<input type="checkbox" bind:checked="{showing}" id="showing">

{#if showing}
  <h1>Hello custom transition!</h1>
{/if}

Toggling the checkbox shows the stark appearance/disappearance. Now, let's add a custom transition function:

<script>
  let showing = true;
  function whoosh(node) {
    console.log(node);
  }
</script>

<label for="showing">
  Showing
</label>
<input type="checkbox" bind:checked="{showing}" id="showing">

{#if showing}
  <h1 transition:whoosh="">Hello custom transition!</h1>
{/if}

Toggling now logs the element to the console, confirming the connection. We'll enhance this with animation. Let's create a css function for scaling:

<script>
  function swoop() {
    return {
      duration: 1000,
      css: (t) => `transform: scale(${t})`
    }
  }
  let showing = true;
</script>

<label for="showing">
  Showing
</label>
<input type="checkbox" bind:checked="{showing}" id="showing">

{#if showing}
  <h1 transition:swoop="">Hello custom transition!</h1>
{/if}

The element now scales, but abruptly. Using t for smooth animation:

<script>
  function swoop() {
    return {
      duration: 1000,
      css: (t) => `transform: scale(${t})`
    }
  }
  let showing = true;
</script>

<label for="showing">
  Showing
</label>
<input type="checkbox" bind:checked="{showing}" id="showing">

{#if showing}
  <h1 transition:swoop="">Hello custom transition!</h1>
{/if}

For a "swooping" effect, let's add translateX, animating from the side:

<script>
  function swoop() {
    return {
      duration: 1000,
      css: (t, u) => `transform: scale(${t}) translateX(${u * 100}%)`
    }
  }
  let showing = true;
</script>

<label for="showing">
  Showing
</label>
<input type="checkbox" bind:checked="{showing}" id="showing">

{#if showing}
  <h1 transition:swoop="">Hello custom transition!</h1>
{/if}

Here, u (inverse of t) controls the translateX, ensuring smooth movement.

Finally, let's add an easing function:

<script>
  import { elasticOut } from 'svelte/easing';

  function swoop() {
    return {
      duration: 1000,
      easing: elasticOut,
      css: (t, u) => `transform: scale(${t}) translateX(${u * 100}%)`
    }
  }
  let showing = true;
</script>

<label for="showing">
  Showing
</label>
<input type="checkbox" bind:checked="{showing}" id="showing">

{#if showing}
  <h1 transition:swoop="">Hello custom transition!</h1>
{/if}

Conclusion

You've now created a custom Svelte transition! This is just a starting point; explore the documentation and tutorials for more advanced techniques. Understanding the interplay of t and u is key to creating dynamic animations.

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