Sudoku Flick — flat design, timing animations, and the rules I won't break

A month after the first post, Sudoku Flick is playable. There's a real solver, a real generator, a real undo stack, and the flick gesture stopped fighting me weeks ago. I've put about two hundred commits into it since the new year.

But the part I want to write about today is design discipline: the constraints I wrote down on day one and how they've held up under contact with real features.

The rule sheet I started with

Day one rules:

1. Flat design only. No shadows, no gradients, no blur, no glassmorphism, no 3D.
2. Timing-based animations only. withTiming with cubic easing. Never withSpring. No bounce, elastic, or overshoot.
3. Zero hardcoded colors. Every color goes through a theme token. The same component renders correctly in any palette.
4. Strategic negative space. Decoration is a defect.
5. The grid is the interface. The Sudoku board is the hero. Everything else exists to serve solving.

Each one of these has been tested at least once.

Rule 2 has been the hardest

The rule I get tempted to break the most is "no springs."

Springs are fun. A button that gives a little bounce on tap, a cell that wiggles when you place a wrong digit, a pop on success. The vocabulary of mobile delight is, almost entirely, written in spring physics.

But springs do two things I don't want:

1. They overshoot. The whole point of Sudoku Flick's aesthetic is calm. A spring that overshoots and settles is the visual equivalent of a small percussive sound. Fine once, exhausting after the seventieth placement.
2. They have non-deterministic duration. A withSpring(target, { damping: 15, stiffness: 80 }) can take anywhere from 200ms to 1.2s depending on the velocity it starts with. If two animations need to land at the same time (say, a digit appearing in a cell and a conflict highlight in a peer cell), withTiming lines them up cleanly. withSpring doesn't.

So: withTiming with cubic easing, every time. The constraint forces me to design transitions in terms of duration and easing, which are properties I can reason about, instead of physics, which are properties that emerge.

In Reanimated terms (with Easing for the curve):

const CAMERA_EASING = Easing.bezier(0.25, 0.1, 0.25, 1);

opacity.value = withTiming(1, { duration: 200, easing: CAMERA_EASING });

That CAMERA_EASING curve is the one I use for almost everything. It's a slight ease-out: fast to start, gentle to settle. The first time it appears in the file it feels like a deliberate choice. By the seventieth time, you stop noticing animations are happening, which is the goal.

The wrong-digit shake

The hardest case I hit was "what happens when the player places a digit that conflicts with the rules?"

The instinct from mobile-game muscle memory is: shake the cell. A horizontal shake done with springs is a one-liner. I wrote it. It worked. It felt wrong inside the app.

What I ended up with instead:

• The cell flashes the conflict color (semantic.error) for 120ms.
• The digit is not placed.
• The peer cells that caused the conflict flash the same color for the same duration.

No movement at all, just a brief color event. The information transfer is the same (this digit doesn't fit here, and these are why), but the page feels still.

The spring impulse came from "movement = feedback". But color is also feedback, and color obeys the same rules as the rest of the UI.

Rule 3 forced me to build a real theme system

"No hardcoded colors" sounds boring in a rule list. In practice it shaped most of the app's structure.

The shape:

const colors = useTheme(); // hook returns the active theme's tokens

<Cell
  borderColor={colors.gridBorder}
  background={isSelected ? colors.cellSelected : colors.cellBackground}
  textColor={isGiven ? colors.textGiven : colors.textUser}
/>

Every theme is an object that fills in the same set of keys. The component doesn't know which theme it's drawing in. Switching the theme is a single state change at the root; every component re-renders against the new tokens with no per-component code.

Two things I got out of this:

1. Colorblind themes for free. Two of the themes use a blue/yellow palette instead of red/green. I didn't have to find every conflict-color use site and change it; I had to define the tokens once. Components don't care.
2. Dark mode for free, also. Each theme has a light and dark variant. The "follow system" toggle just picks one of the two at render time.

The first time I tried to do a quick visual hack and hardcoded #FF6B6B for an error flash, I had to back it out within a day because it didn't work in the dark variant of one of the themes. After that I just stopped trying. Every color goes through the tokens.

Rule 5 is the rule I forget

"The grid is the interface" is the easiest rule to write and the hardest to enforce.

Every "wouldn't it be cool if…" feature I think of starts with adding something to the board area. A hint badge in the corner of cells with hint-worthy peers, a timer at the top, a streak counter, a bonus animation when you complete a row.

Every one of those, when I sit with it for a day, gets cut. The principle is the player should be able to look at the board and only see what's needed to solve the puzzle. Hints belong on demand, not always-on. Time elapsed belongs in a screen the player navigates to, not always-on. Streaks don't belong at all.

The single visual element on the board that isn't the puzzle: a small indicator above the active cell showing which digit is about to be placed when the flick lands. It's there for a third of a second, it's a thin outline, and it disappears the moment the gesture ends. Anything more permanent gets cut.

Things I added that don't break the rules

A few features I shipped this month that survived contact with the rule sheet:

• Practice mode for new players. A four-cell exercise where the player learns the flick gesture in isolation, before the real puzzle. Uses timing-based fades for cell transitions, no springs. The "all correct!" message fades in over 200ms, and a single "one more round" button fades in 150ms later. The player can replay as many times as they want; the Next button stays enabled after the first success.

• Draggable bottom nav. The settings/themes/about row at the bottom can be dragged side-to-side. You don't have to drag, you can tap, and the drag uses withTiming snap-to-position rather than springs. It works because Sudoku Flick has a deliberately small set of top-level destinations. The drag is faster than the tap for me, but the tap is the obvious thing to do for anyone else.

• Animation toggle in settings. This one was required. The setting reads from the system's "Reduce Motion" preference (via AccessibilityInfo) on first launch and can be overridden by the player. Every withTiming call in the codebase checks this value:

const duration = animationsEnabled ? 200 : 0;
opacity.value = withTiming(1, { duration, easing: CAMERA_EASING });

duration: 0 means the transition is instant but the callback structure is the same. State machines that wait for an animation to finish still work; they just don't wait.

Animation as a layer, not a feature

Across all of these, animation is a layer in the architecture, not a feature.

Every component renders the same regardless of whether animations are on. The state model doesn't change with the animation toggle. The toggle changes one thing: the duration argument to every transition.

It makes "what does this look like with animations off?" a one-line check. I don't have a separate accessibility-rendering path. I have one rendering path, parameterized by duration, where duration can be zero.

People turn animation off for good reasons: vestibular issues, low-power mode, sensory sensitivity. For an app that wants to ship to those people, that's the only design that's honest.

Where it stands

The puzzle generator and solver are working. Difficulty levels are stubbed in. The flick gesture is at a place where I trust it on a moving train (literally, I commute with it every day).

What I'm building next month: an onboarding flow that doesn't suck. Sudoku Flick is asking the player to learn a new input method, and "tap-to-place" is the muscle memory we have to overwrite. The plan is a few-screen, in-context tutorial that only teaches the flick. No "welcome to Sudoku Flick" marketing slide. No "here are our features" carousel. Just: here's a cell, do this, you did it.

That's the topic for next time. Spring-free, of course.

References

• Reanimated withTiming: also where the Easing module is documented
• Reanimated withSpring
• easings.net: visualization of common easing curves
• React Native AccessibilityInfo: for reading the system Reduce Motion preference
• Part 1 — Designing Sudoku Flick