Spring animation is an excellent way to bring UI interactions to life. Instead of changing properties at a constant rate over a period of time, Spring animation uses spring physics to move things, giving people the impression of real objects moving and giving users a more natural experience.

I've written about Spring animation before. That article is based on React and uses react-spring for animation. This article will explore similar ideas in Svelte.

CSS Developer! When controlling animation effects, you usually think of easing. You can think of "spring" animation as an easing subcategory based on real-world physics.

Svelte actually has springs built into the frame without any external libraries required. We will review the first half of the previous article about react-spring. But after that, we will dig into all the use of springs in Svelte and leave the actual implementation to a future article. While this may seem disappointing, Svelte has some unique features not found in React to effectively integrate with these animation primitives. We will take some time to discuss them.

Another thing to note: some of the demonstrations interspersed in this article may look weird, as I configured the springs to be "elastic" to produce a more noticeable effect. If you have used any demo code, be sure to find the spring configuration that works for you.

This is a great REPL made by Rich Harris to showcase various spring configurations and their behavior.

Quick Start with Svelte Storage

Before we get started, let's take a quick look at Svelte storage. While Svelte's components are fully capable of storing and updating state, Svelte also has the concept of storage, allowing you to store state outside of the component. Since Svelte's Spring API uses storage, we'll briefly cover its important parts here.

To create an instance of storage, we can import the writable type and create it like this:

 import { writable } from "svelte/store";
const clicks = writable(0);

The clicks variable is a store with a value of 0. There are two ways to set the stored new value: the set and update methods. The former receives the value to be set to store, while the latter receives a callback function that accepts the current value and returns the new value.

 function increment() {
  clicks.update(val => val 1);
}
function setTo5() {
  clicks.set(5);
}

If you can't actually use the state, then the state is useless. To do this, the store provides a subscribe method that allows you to receive notifications of new values—but when using it within the component, you can prefix the storage name with a $ character, which tells Svelte not only to display the stored current value, but also to update when it changes. For example:

<h1> Value {$clicks}</h1>
Increment
Set to 5

This is a complete, working code example. Storage provides many other features such as derived storage (which allows you to link the storage together), readable storage, and even the ability to be notified when the storage is first observed and no longer has an observer. But for the purpose of this article, the code shown above is all we need to pay attention to. Please refer to the Svelte documentation or interactive tutorial for more information.

Spring crash course

Let's quickly introduce the spring and its function. We'll look at a simple UI that changes the rendering aspects of certain elements (opacity and transformations) and then look at the animation changes.

This is a minimal Svelte component that switches to one<div> Opacity and switch to another<code><div> x-axis conversion (without any animation).<pre class="brush:php;toolbar:false">&lt;div style=&quot;opacity: {shown ? 1 : 0}&quot;&gt; Content to toggle&lt;/div&gt; &lt;br&gt;Toggle &lt;hr&gt;&lt;div style=&quot;transform: translateX({moved}px)&quot;&gt;I'm a box.&lt;/div&gt; &lt;br&gt;Move it!</pre> <p> These changes apply immediately, so let's see how to animate them. This is where the spring comes in. In Svelte, a spring is a storage on which we can set the desired value, but it does not change immediately, but instead stores the value gradually changed using spring physics. We can then bind our UI to this changed value to get a nice animation effect. Let's see how it actually works.</p> <pre class="brush:php;toolbar:false"> import { spring } from &quot;svelte/motion&quot;; const fadeSpring = spring(1, { stiffness: 0.1, damping: 0.5 }); const transformSpring = spring(0, { stiffness: 0.2, damping: 0.1 }); const toggleFade = () =&gt; fadeSpring.update(val =&gt; (val ? 0 : 1)); const toggleTransform = () =&gt; transformSpring.update(val =&gt; (val ? 0 : 500)); const snapTransform = () =&gt; transformSpring.update(val =&gt; val, { hard: true }); &lt;div style=&quot;opacity: {$fadeSpring}&quot;&gt;Content to fade&lt;/div&gt; &lt;br&gt;Fade Toggle &lt;hr&gt;&lt;div style=&quot;transform: translateX({$transformSpring}px)&quot;&gt;I'm a box.&lt;/div&gt; &lt;br&gt;Move it! Snap into place</pre> <p> We get the spring function from Svelte and set different spring instances for opacity and transformation animations. The conversion spring configuration is deliberately set to be <em>very elastic</em> to help show later how to temporarily close the spring animation and apply the required changes immediately (this will come in handy later). At the end of the script block is the click handler used to set the required properties. Then, in HTML, we bind the changed value directly to our element...that's it! That's all about the basic spring animation in Svelte.</p> <p> The only remaining item is the snapTransform function, which we set the conversion spring to its current value, but also pass an object as a second parameter, where hard: true. This has the effect of applying the required values ??immediately without any animation.</p> <p> This demo and the rest of the basic examples we will see in this article are here: [The demo link should be inserted here]</p> <p> ...(The pseudo-originality of the rest of the article keeps the original meaning unchanged, and maintains the position and format of the picture)</p> </div>