# Apple Design

How Apple builds interfaces that stop feeling like a computer and start feeling like an extension of you. This knowledge comes from Apple's WWDC design talks — chiefly *Designing Fluid Interfaces* (WWDC 2018) — distilled and translated into the web platform (CSS, Pointer Events, `requestAnimationFrame`, spring libraries like Motion/Framer Motion).

The through-line: **an interface feels alive when motion starts from the current on-screen value, inherits the user's velocity, projects momentum forward, and can be grabbed and reversed at any instant.** Springs are the tool that makes all of this natural, because they are inherently interruptible and velocity-aware.

## The Core Idea

> "When we align the interface to the way we think and move, something magical happens — it stops feeling like a computer and starts feeling like a seamless extension of us."

An interface is fluid when it behaves like the physical world: things respond instantly, move continuously, carry momentum, resist at boundaries, and can be redirected mid-motion. Everything below is a way to get closer to that.

Apple frames design as serving four human needs: **safety/predictability, understanding, achievement, and joy.** Every rule here serves one of them.

## 1. Response — kill latency

The moment lag appears, the feeling of directness "falls off a cliff." Response is the foundation everything else is built on.

- **Respond on pointer-down, not on release.** Highlight a button the instant it's pressed. Waiting for `click`/touch-up to show feedback feels dead.
- **Be vigilant about every latency.** Audit debounces, artificial timers, transition waits, and the ~300ms tap delay. Anything on the input path that isn't essential is a regression.
- **Feedback must be continuous *during* the interaction, not just at the end.** For a drag, slider, or drawer, update the UI 1:1 with the pointer the whole way through — never animate only when the gesture completes.

```css
/* Feedback lives on the press, and it's instant */
.button:active {
  transform: scale(0.97);
  transition: transform 100ms ease-out;
}
```

## 2. Direct manipulation — 1:1 tracking

> "Touch and content should move together."

When the user drags something, it must stay glued to the finger — and respect the offset from *where they grabbed it*. Snapping to the element's center on grab breaks the illusion immediately.

- Use Pointer Events with `setPointerCapture` so tracking continues even when the pointer leaves the element's bounds.
- Track a short **velocity/position history** (last few `pointermove` events), not just the current point — you'll need velocity at release.

```js
el.addEventListener('pointerdown', (e) => {
  el.setPointerCapture(e.pointerId);
  const grabOffset = e.clientY - el.getBoundingClientRect().top; // respect where they grabbed
  // ...track position + timestamp history for velocity
});
```

## 3. Interruptibility — the single most important principle

> "The thought and the gesture happen in parallel."

Every animation must be interruptible and redirectable at any moment. A user must be able to grab a moving element mid-flight and reverse it without waiting for the animation to finish. A closing modal the user grabs again should follow the finger — not finish closing first, then reopen.

- **Never lock out input during a transition.**
- **Always animate from the *presentation* (current) value, never the target value.** On interrupt, read the element's live on-screen transform and start the new animation from there. Starting from the logical/target value causes a visible jump.
- **Avoid CSS transitions and `@keyframes` for anything gesture-driven** — they can't be smoothly grabbed and reversed mid-flight. Springs animate from the current value by default, which is exactly what interruption needs.
- **When a gesture reverses, blend velocity — don't hard-cut it.** Replacing one animation with another at a reversal creates a velocity discontinuity, a "brick wall." Spring libraries that carry velocity through a re-target avoid it. (This is what iOS's *additive animations* do natively; on the web, choose a spring library that re-targets from the current velocity.)
- **Decompose 2D motion into independent X and Y springs.** A single spring on a 2D distance desyncs when X and Y have different velocities.

## 4. Behavior over animation — use springs

> "Think of animation as a conversation between you and the object, not something prescribed by the interface."

A pre-scripted, fixed-duration animation can't respond to new input. A spring can — new input just changes the target, and the motion stays continuous. Reach for springs for anything a user can touch.

Apple deliberately replaced the physics triplet (mass/stiffness/damping) with two designer-friendly parameters. Think in these:

- **Damping ratio** — controls overshoot. `1.0` = critically damped, no bounce, smooth settle. `< 1.0` = overshoots and oscillates. Lower = bouncier.
- **Response**

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