# Build an NFC Habit Tracker with React Native
# Code with Beto · https://codewithbeto.dev/blog/nfc-habit-verification-react-native
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## This document

- **Title:** Build an NFC Habit Tracker with React Native
- **Canonical URL:** https://codewithbeto.dev/blog/nfc-habit-verification-react-native
- **Agents export:** https://codewithbeto.dev/blog/nfc-habit-verification-react-native/agents.txt
- **Author:** Beto
- **Published:** July 17, 2026
- **Summary:** How my habit tracker app pairs habits with physical NFC tags using Expo, react-native-nfc-manager, NDEF records, tag UIDs, and Expo Router deep links.
- **Access tier:** public (full article in this file)

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---

# Build an NFC Habit Tracker with React Native

A habit tracker has one obvious loophole.

You can tap the checkbox without doing the habit.

That is fine for most habits. But for the ones where I really want accountability, I wanted the check-in to happen where the habit happens.

So I added NFC verification to **Kujun**, a local-first habit tracker I recently built with React Native and Expo.

You can pair a habit with an inexpensive NFC sticker, then place or hide that sticker near the action. Put one next to your running shoes, inside your guitar case, near your medication, or beside the equipment you use at the gym.

From that point on, you cannot mark the habit complete from the couch. You have to go to that place and scan the exact tag.

This is not a security system. If someone is determined to cheat, they will find a way. The goal is to add one small moment of honesty between the intention and the check-in.

In this post, I want to show you how NFC works, how we can use it from React Native, what we write to the tag, how we verify it, and the iOS caveats that shaped the feature.

**How to build an NFC habit tracker**

A complete video walkthrough

[Watch on YouTube](https://youtu.be/NXc1j8oSm1I)

## What we are building

The complete flow looks like this:

1. The user creates a habit.
2. Kujun pairs that habit with a writable NFC tag.
3. The app writes a small deep link to the tag and saves the tag's unique ID locally.
4. The user places the tag where the habit happens.
5. Tapping a locked habit opens the iOS NFC scanner.
6. Only the paired tag can complete it.

If the tag is wrong, the completion is rejected. If the user is sick, traveling, or blocked by the weather, they can skip the day with a reason. The skip is logged and does not break the streak.

Unpairing also has a 24-hour cooldown. Without that delay, you could disable the lock the second it became inconvenient, which would make the entire feature decorative.

## What NFC actually is

NFC stands for **Near Field Communication**. It is a short-range wireless technology that lets a phone exchange a small amount of data with a nearby tag.

The stickers we use are passive. They do not have a battery, Wi-Fi, or an internet connection. When the iPhone gets close, its NFC reader creates the field that powers the tag long enough to read or write its memory.

For this feature, three pieces matter:

| Piece | What it is                                    | How Kujun uses it                                     |
| ----- | --------------------------------------------- | ----------------------------------------------------- |
| NFC   | The short-range communication layer           | Starts the interaction between the iPhone and the tag |
| UID   | The tag's hardware identifier                 | Proves that the foreground scan found the paired tag  |
| NDEF  | A standard format for data stored on NFC tags | Stores one URI record that points back into Kujun     |

My app writes one NDEF URI record. That is it.

We use NTAG213 and NTAG216 stickers because they are inexpensive, rewritable NFC Forum Type 2 tags, and they are easy to find.

Our payload is around 55 bytes, so even the smaller NTAG213 has plenty of room.

## React Native NFC Manager

For this integration I use [`react-native-nfc-manager`](https://github.com/revtel/react-native-nfc-manager). It gives us one API for starting NFC sessions, requesting a tag technology, reading tag information, and writing NDEF messages.

I also used `expo-crypto` to generate the random value stored in each tag URL.

```bash
bun add react-native-nfc-manager@^3.17.2
bunx expo install expo-crypto
```

This is a native module, so it will not work inside Expo Go. You need a development build and a physical iPhone for the real NFC flow.

The nice part is that the package includes an Expo config plugin. Kujun uses Continuous Native Generation, so all of the native configuration stays in `app.json` instead of relying on manual changes inside the generated `ios` directory.

```json
{
  "expo": {
    "scheme": "kujun",
    "plugins": [
      [
        "react-native-nfc-manager",
        {
          "nfcPermission": "Allow $(PRODUCT_NAME) to pair NFC tags and verify habit check-ins.",
          "includeNdefEntitlement": false
        }
      ]
    ]
  }
}
```

The plugin adds `NFCReaderUsageDescription` to the generated Info.plist and configures the NFC tag-reader entitlement. We set `includeNdefEntitlement` to `false` because Kujun uses the `TAG` reader-session format.

After changing the native configuration re-compile your app.

## What we save on the tag

An NFC tag should not become a tiny copy of your database.

Kujun is local-first, and I did not want the habit name, schedule, history, or any personal data sitting on a world-readable sticker.

The only value written to the tag is a URL:

```text
kujun://h/<habitId>?s=<secret>

# example
kujun://h/abc123?s=4c237f9e8a1b2c3d4e5f6a7b8c9d0e1f
```

The `habitId` tells Expo Router which habit route to open. The `secret` is 16 random bytes encoded as a 32-character hexadecimal string.

```ts
import * as Crypto from "expo-crypto";

export function newSecret(): string {
  return Array.from(Crypto.getRandomBytes(16))
    .map((byte) => byte.toString(16).padStart(2, "0"))
    .join("");
}

export function tagUrl(habitId: string, secret: string): string {
  return `kujun://h/${habitId}?s=${secret}`;
}
```

I want to be precise about the word "secret" here. NDEF data can be read and copied, so this is not a cryptographic guarantee that the physical tag is authentic.

For a foreground scan, Kujun verifies the tag's UID. The random URL value is proof for the deep-link path we will use when universal links are added later.

That tradeoff is acceptable because this feature is a personal commitment device, not an access badge or payment system.

## What we save on the iPhone

The local SQLite database stores the other side of the pairing:

```sql
CREATE TABLE IF NOT EXISTS nfc_pairings (
  habit_id  TEXT PRIMARY KEY NOT NULL,
  tag_uid   TEXT NOT NULL,
  secret    TEXT NOT NULL,
  paired_at TEXT NOT NULL,
  unpair_at TEXT
);
```

Each habit has at most one paired tag. We save:

- The habit ID
- The normalized tag UID
- The same random value written into the URL
- When the tag was paired
- When a pending unpair becomes effective

Nothing leaves the device. The tag only knows the URL, and the database remains the source of truth for the relationship.

## Pairing and writing the NFC tag

Pairing does two jobs in one iOS NFC session:

1. Read the tag UID.
2. Write the NDEF URL.

The detail that took real hardware testing was the requested NFC technology.

NTAG213 and NTAG216 are MiFare Ultralight tags. A plain `NfcTech.Ndef` request can read and write their NDEF message, but on iOS it did not reliably give us the UID we needed for foreground verification.

Kujun requests `NfcTech.MifareIOS` instead. That starts a tag-reader session where `getTag()` exposes the UID, while `ndefHandler` can still write the NDEF message.

#### Kujun's NFC pairing and writing code

```ts
import NfcManager, { Ndef, NfcTech } from "react-native-nfc-manager";

let started = false;

async function ensureStarted() {
  if (started) return;
  await NfcManager.start();
  started = true;
}

export async function pairTagForHabit(
  habit: Habit,
  isUidAvailable: (uid: string) => boolean,
) {
  const secret = newSecret();

  try {
    await ensureStarted();
    try {
      await NfcManager.requestTechnology(NfcTech.MifareIOS, {
        alertMessage: `Hold your iPhone near a blank tag for “${habit.name}”.`,
      });
    } catch {
      return { status: "canceled" } as const;
    }

    const tag = await NfcManager.getTag();
    const uid = normalizeUid(tag?.id);

    if (!uid) {
      throw new Error("The tag UID was not available");
    }

    if (!isUidAvailable(uid)) {
      return { status: "uid-in-use", uid } as const;
    }

    const url = tagUrl(habit.id, secret);
    const bytes = Ndef.encodeMessage([Ndef.uriRecord(url)]);

    await NfcManager.ndefHandler.writeNdefMessage(bytes);
    await NfcManager.setAlertMessageIOS("Tag paired").catch(() => {});

    return { status: "paired", uid, secret } as const;
  } catch {
    return { status: "error" } as const;
  } finally {
    await NfcManager.cancelTechnologyRequest().catch(() => {});
  }
}
```

The important NFC line is:

```ts
const bytes = Ndef.encodeMessage([Ndef.uriRecord(url)]);
await NfcManager.ndefHandler.writeNdefMessage(bytes);
```

`Ndef.uriRecord` turns the Kujun URL into a URI record. `Ndef.encodeMessage` serializes that record into the bytes expected by the tag.

The `finally` block matters too. Every session should call `cancelTechnologyRequest()`, whether the scan succeeds, fails, or the user closes Apple's NFC sheet.

Before writing, Kujun also checks whether that UID is already paired with another habit. One physical tag should not be able to satisfy two different commitments.

{/* TODO: Add screenshots of Apple's NFC pairing sheet and Kujun's successful "Your check-in has a place" state. */}

## Reading and verifying the tag

When the user taps a locked habit, Kujun opens another foreground NFC session. This time we do not need to parse the URL from the tag. We only need to compare the scanned UID with the UID saved during pairing.

```ts
export async function verifyTagForHabit(
  pairing: NfcPairing,
  habitName: string,
) {
  try {
    await ensureStarted();
    try {
      await NfcManager.requestTechnology(NfcTech.MifareIOS, {
        alertMessage: `Hold your iPhone near the tag for “${habitName}”.`,
      });
    } catch {
      return "canceled";
    }

    const tag = await NfcManager.getTag();
    const uid = normalizeUid(tag?.id);

    return uid === pairing.tagUid.toUpperCase() ? "match" : "wrong-tag";
  } catch {
    return "error";
  } finally {
    await NfcManager.cancelTechnologyRequest().catch(() => {});
  }
}
```

If the UIDs match, the completion mutation receives `{ verified: true }`. If they do not, the app shows the wrong-tag state and offers another scan.

The iOS system NFC sheet is the scan UI. I did not build a fake scanner animation on top of it. The system sheet already handles the reader session, instructions, cancellation, and familiar platform behavior.

## The important guard

It would be easy to put an NFC check inside the Today screen and call the feature done.

That would also be easy to bypass.

Kujun can complete habits from the Today screen, habit details, history, notification actions, and deep links. If only one button knows about the lock, another path can accidentally mark the habit complete.

The store rejects every unverified attempt to mark a locked habit as done:

```ts
export function toggleCompletion(
  habitId: string,
  date: string,
  options?: { verified?: boolean },
) {
  const wasDone = state.completions[habitId]?.[date] === true;

  if (!wasDone && !options?.verified && isLockEnforced(state, habitId)) {
    return false;
  }

  // Insert or remove the completion, then emit the new state.
}
```

Only the NFC scan paths pass `verified: true`. Existing call sites are safe by default.

Undoing a completion never needs a scan. Removing a check-in is not cheating, so there is no reason to make it harder.

Notifications needed the same treatment. Unlocked habits still show **Mark as Done**, but locked habits use a separate category with **Scan to Complete** and open the app in the foreground.

```ts
categoryIdentifier: locked ? LOCKED_REMINDER_CATEGORY : HABIT_REMINDER_CATEGORY;
```

The store guard still checks stale notifications that may have been scheduled before the habit was paired.

This is a small example of a bigger rule: **enforce important product rules at the data boundary, not only in the button that starts the flow.**

Related: [Notification Actions in Expo](/blog/expo-notification-actions)

## Handling the deep link

Because the app config defines `"scheme": "kujun"`, Expo Router can map a URL back into the app.

This tag URL:

```text
kujun://h/abc123?s=4c237f...
```

maps to this file:

```text
src/app/h/[habitId].tsx
```

Expo Router gives the screen both the dynamic `habitId` and the `s` query parameter:

```tsx
const { habitId, s } = useLocalSearchParams<{
  habitId: string;
  s?: string;
}>();

const state = getAppState();
const pairing = state.pairings[habitId];

if (!pairing || !s || s !== pairing.secret) {
  return "unverified";
}

completeHabit(habitId, todayKey(), { verified: true });
return "completed";
```

The route also handles the less exciting states that production code needs:

- The habit was already completed today.
- The habit is not scheduled today.
- The pairing no longer exists.
- The habit was deleted.
- The random value does not match.

`completeHabit` is idempotent, so scanning twice cannot toggle the habit back to incomplete. A valid scan also replaces a same-day skip because physically showing up beats the excuse.

## Wrap up

The interesting part of this feature was not writing a few bytes to a sticker.

It was deciding what the tag proves, what stays on the phone, which completion paths are allowed, and what should happen when real life gets in the way.

If you want to learn how to build features like this in a real app, my [React Native course](/learn) goes deeper into Expo Router, deep linking, native modules, notifications, local data, and the decisions that turn a demo into something you can actually ship.

**Learn by building real applications**

Go from fundamentals to shipping with deep dives into Expo Router, native modules, etc.

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