Getting started with Svelte - Mo

Now that we've covered the basics, it's time to learn some advanced Svelte techniques, starting with motion.

The svelte/motion module exports two functions, tweened and spring, for creating writable stores whose values change over time after set and update, rather than immediately.

Tweens

Setting values and watching the DOM update automatically is cool. Know what's even cooler? Tweening those values. Svelte includes tools to help you build slick user interfaces that use animation to communicate changes.

<script>
    import { tweened } from 'svelte/motion';
    import { cubicOut } from 'svelte/easing';

    const progress = tweened(0, {
        duration: 400,
        easing: cubicOut
    });
</script>

<progress value={$progress} />

<button on:click={() => progress.set(0)}>
    0%
</button>

<button on:click={() => progress.set(0.25)}>
    25%
</button>

<button on:click={() => progress.set(0.5)}>
    50%
</button>

<button on:click={() => progress.set(0.75)}>
    75%
</button>

<button on:click={() => progress.set(1)}>
    100%
</button>

<style>
    progress {
        display: block;
        width: 100%;
    }
</style>

The full set of options available to tweened:

• delay — milliseconds before the tween starts
• duration — either the duration of the tween in milliseconds, or a (from, to) => milliseconds function allowing you to (e.g.) specify longer tweens for larger changes in value
• easing — a p => t function
• interpolate — a custom (from, to) => t => value function for interpolating between arbitrary values. By default, Svelte will interpolate between numbers, dates, and identically-shaped arrays and objects (as long as they only contain numbers and dates or other valid arrays and objects). If you want to interpolate (for example) colour strings or transformation matrices, supply a custom interpolator

You can also pass these options to progress.set and progress.update as a second argument, in which case they will override the defaults. The set and update methods both return a promise that resolves when the tween completes.

Spring

The spring function is an alternative to tweened that often works better for values that are frequently changing.

<script>
    import { spring } from 'svelte/motion';

    let coords = spring({ x: 50, y: 50 }, {
        stiffness: 0.1,
        damping: 0.25
    });

    let size = spring(10);
</script>

<svg
    on:mousemove={(e) => {
        coords.set({ x: e.clientX, y: e.clientY });
    }}
    on:mousedown={() => size.set(30)}
    on:mouseup={() => size.set(10)}
>
    <circle
        cx={$coords.x}
        cy={$coords.y}
        r={$size}
    />
</svg>

<div class="controls">
    <label>
        <h3>stiffness ({coords.stiffness})</h3>
        <input
            bind:value={coords.stiffness}
            type="range"
            min="0.01"
            max="1"
            step="0.01"
        />
    </label>

    <label>
        <h3>damping ({coords.damping})</h3>
        <input
            bind:value={coords.damping}
            type="range"
            min="0.01"
            max="1"
            step="0.01"
        />
    </label>
</div>

<style>
    svg {
        position: absolute;
        width: 100%;
        height: 100%;
        left: 0;
        top: 0;
    }

    circle {
        fill: #ff3e00;
    }

    .controls {
        position: absolute;
        top: 1em;
        right: 1em;
        width: 200px;
        user-select: none;
    }

    .controls input {
        width: 100%;
    }
</style>