01 / One subject
You share food. Nothing else.
No chat, no feed, no profile, no display name. For the length of the challenge you are both a user id and a plate of food.
Self-initiatedProduct Design · iOS · 2026
BUBU:
losing weight in pairs.
An iOS app for two people on the same eight weeks. You photograph what you ate, the day prints itself as a receipt, and seven receipts bind into a paper journal you both sign.
App Store
Build complete · preparing submission
- Role
- Product design, UI, illustration
- Scope
- 23 screens · paired and solo
- Platform
- iOS · iPhone 17
- Status
- Built · one round of interviews · TestFlight next
01 / Why it exists
A dazi is easy to find and easy to lose.
In China a dazi (搭子) is a partner for exactly one activity: a lunch dazi, a gym dazi. What makes it appealing is how little it asks, and that is also why it falls apart. Nothing holds the pairing together, so it quietly stops after a week and the search starts again. Dieting is the worst case, because it needs two months rather than one evening.
Three rules the whole product is built on
02 / A fixed end
It ends on a date, not on a decision.
You pick 4, 8 or 12 weeks at setup and both pages count down to the same day. Nobody has to be the one who quits.
03 / Visible absence
A missed day prints as a blank.
No nudge, no guilt notification. An empty dashed frame on the day's receipt, and a counter that reads MISSED 1 / 3.
02 / How it works
Photograph the meal. The rest prints itself.
There is no form to fill in. You shoot the plate, the app cuts it out of its background, and from there the product is three pieces of printed matter: a receipt a day, a spread a week, and a track that runs the length of the challenge.
The day
At 23:59 the day seals itself as a receipt.
A receipt is already read as a day that has been closed and totalled. Sealing it means yesterday cannot be edited to look better, which is the only reason the other person's copy is worth anything. It is also the most useful answer to what to eat tonight: a real meal, eaten by someone on the same plan.
The week
Seven receipts, bound and laid open.
The journal is the only place the two of you are shown as one object. Turn it and the cover, the title page, your week and their week come up in order; open it flat and both weeks read across the gutter at once.
The finish
One track, two runners, a date at the end.
The home screen keeps the two of you on one track. The challenge page keeps the rules and the count. When the date arrives, the last screen is a celebration and a receipt you can share.
03 / What testing changed
Two conversations, two changes to the product.
Before TestFlight I sat down with two people who would meet the app from opposite ends: someone who would use it, and someone who has shipped things like it.
- Who
- A fitness creator with 20,000 followers
- What they said
- She liked the interface enough to want it for herself, on her own. Her point was that people will come for how it looks and may not have a buddy to hand, and the app should not turn them away at the door.
- What changed
- A solo mode. The same home, journal and challenge, with one runner on the track and a journal that holds only your week. Pairing became something you can add later rather than the price of entry.
- Who
- A product designer with three years of shipped work
- What they said
- His concern was the cold start. Matching strangers online is a bad experience in a new app, because there are not yet enough strangers: you wait, or you are given someone nothing like you.
- What changed
- Matching that is one-to-one for you and one-to-many for the system. You are shown exactly one buddy, chosen for a similar goal, a similar starting weight and the same length of challenge. But the person you are shown may already be someone else's buddy, and that is allowed. When A and B are paired and C arrives closest to A, C is matched to A; A still sees B, and B still sees A. Nobody waits for an even number of people to turn up.
How a third person joins
- Asees B
- Bsees A
- Csees A
Both conversations were about the prototype, before any real pair had run a challenge.
04 / The look
Flat paper, hairlines, and photographs printed straight onto it.
The whole app is one material. Nothing floats, nothing has a shadow, and nothing is rounded except the phone it runs on.
Four surfaces, one ink
Paper
#F6F6F4
Every screen but the journal and the receipt
Board
#C8C5AD
The journal's cover cloth and the desk it lies on
Page
#F6F5EE
The journal's own paper, a shade warmer than the app
Ink
#0E0E0E
Type, rules, the black key and the receipt ground
Two typefaces, no third
0123456789 · MISSED 1 / 3 · 08:05
Geist Mono carries every number, label and timestamp, because a receipt is a printed object and a receipt's numbers line up. Noto Sans SC carries the sentences. Nothing is set in a script face; the only handwriting in the app is drawn, not typed.
What the system refuses
- No drop shadows and no elevation. A card is a rule, not a layer.
- No rounded corners. Right angles everywhere, including the buttons.
- No white border on a photograph. A meal is cut out of its background and printed onto the paper.
- One filled button per screen, in ink, as a plain rectangle. The receipt is the one place the system inverts: paper type on a black ground.
05 / The cast
One pen, five loops.
The app has no mascot and no illustration library. It has a cast of two, drawn with the same line as the interface rules, who appear only at the moments the product needs a person rather than a number.
Skipping
Empty states, and the pause between two challenges
Both of you
The screen that confirms a match
Solo
Home, when you are running the challenge alone
The finish
Crossing the line on the last day
Goal reached
The celebration screen, and the receipt you can share from it
The loops run while they are on screen and stop when they are not.
06 / Every screen
The whole build, in the order you meet it.
Nineteen screens captured on an iPhone 17 simulator, from sign-up to the settings page. Drag the strip.
01 · Sign up
02 · Solo or paired- 03 · Set the goal
04 · Invite a buddy- 05 · Matched
06 · Home · paired- 07 · Today's receipt · both
- 08 · Today's receipt · mine
09 · Journal · cover
10 · Journal · my week
11 · Journal · the spread
12 · Challenge · progress
13 · Challenge · rules- 14 · Goal reached
- 15 · Home · solo
- 16 · Journal · solo
- 17 · Challenge · solo
18 · Settings
19 · Rename
07 / Shipping it
What is built, and what is between here and the App Store.
The app is not a prototype. It is a build that runs, and the work left is the work of getting a build reviewed rather than designed.
Done
- 23 screens, designed and built
- Paired and solo modes
- The receipt, the journal and the share card
- One-to-many matching, out of the second interview
- The five drawn loops
Between here and the store
- TestFlight round with real pairs, which is also the first test of the matching
- Privacy labels and the data the matching actually needs
- App Store screenshots and the listing
- Review, and whatever review asks for
One thing is still untested: the whole product rests on the assumption that a stranger's receipt is motivating rather than uncomfortable. Two interviews moved the product, but two interviews are not eight weeks of use. That is what TestFlight is for.