WinkPay SDK — Native Android Integration Guide

WinkPay is a biometric (face + palm) payment SDK for Android POS applications. This guide is for partner POS developers integrating the SDK into an Android host app.

The SDK ships as a single Maven artifact:

com.wink:winkpay-sdk:1.5.0

The customer-facing capture and confirmation UI is implemented as native Android views .

Integration paths at a glance

Most integrators want the embedded SDK. Use the wireless / companion paths only when you specifically need cross-process or cross-device delivery.

Table of contents

1. Quick start
2. Prerequisites & build setup
3. AndroidManifest.xml contributions
4. Public API surface
5. Credentials & device registration
6. Lifecycle & threading
7. Customer-facing display (dual-display devices)
8. Palm capture details
9. Error handling
10. Build notes & APK size
11. Troubleshooting

1. Quick start

The minimum integration is roughly 30 lines of Kotlin: initialize the SDK, register a callback, launch the customer-display Activity.

1.1 settings.gradle.kts

The SDK is delivered as a local Maven repository (a directory tree). Unzip the deliverable under vendor/winkpay-sdk/winkpay-sdk-repo in your project and declare the repos:

dependencyResolutionManagement {
    repositoriesMode.set(RepositoriesMode.PREFER_SETTINGS)
    repositories {
        google()
        mavenCentral()

        // WinkPay SDK (vendored local Maven repo, shipped in the SDK zip).
        maven { url = uri("${rootDir}/vendor/winkpay-sdk/winkpay-sdk-repo") }
    }
}

1.2 app/build.gradle.kts

android {
    defaultConfig {
        minSdk = 24
        ndk { abiFilters += "arm64-v8a" }   // SDK ships only arm64-v8a
    }

    packaging {
        jniLibs {
            useLegacyPackaging = true       // bundled native libs must extract to lib/, see §2
        }
    }
}

dependencies {
    implementation("com.wink:winkpay-sdk:1.5.0")
}

That single line pulls in everything the SDK needs at runtime — the networking, camera, biometric, and UI components, plus the SDK's bundled native runtime — as transitive dependencies. You do not declare any of them yourself.

1.3 Initialize once and start a payment

import android.app.ActivityOptions
import android.content.Context
import android.hardware.display.DisplayManager
import android.os.Bundle
import android.os.Build
import android.view.Display
import androidx.appcompat.app.AppCompatActivity
import com.wink.winkpay.WinkPaySdk
import com.wink.winkpay.embedded.EmbeddedPaymentCallback
import com.wink.winkpay.embedded.EmbeddedPaymentError
import com.wink.winkpay.embedded.EmbeddedPaymentRequest
import com.wink.winkpay.embedded.EmbeddedPaymentResult
import com.wink.winkpay.embedded.WinkPayEmbedded
import java.util.UUID

class PosMainActivity : AppCompatActivity() {

    private lateinit var winkPay: WinkPayEmbedded

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        // 1. Initialize the SDK. Pass merchant credentials and the device
        //    serial number directly — no .env file required.
        winkPay = WinkPayEmbedded.init(
            context = this,
            config = WinkPaySdk.Config(
                clientId             = "YOUR_CLIENT_ID",
                merchantClientSecret = "YOUR_MERCHANT_CLIENT_SECRET",
                deviceSerialNumber   = @Suppress("DEPRECATION") Build.SERIAL,
                sdkEnvironment       = "stage",   // "stage" or "prod"
            ),
        )

        findViewById<View>(R.id.payBtn).setOnClickListener { startFacePayment() }
    }

    private fun startFacePayment() {
        // 3. Register the result callback for this payment.
        val request = EmbeddedPaymentRequest(
            requestId = UUID.randomUUID().toString(),
            requestType = "PAY_FACE",      // PAY | PAY_FACE | PAY_PALM
            amount = 1500L,                // minor units (cents)
            currency = "USD",
            orderId = "ORDER-${System.currentTimeMillis()}",
            tenderId = "TENDER-001",
            cameraIndex = 0,               // 0 = back, 1 = front
            rotationAngle = 0,
        )

        winkPay.startPayment(request, object : EmbeddedPaymentCallback {
            override fun onSuccess(result: EmbeddedPaymentResult) {
                // Payment processed by the SDK (returnExternalTokens=false default):
                //   result.transactionId, result.amount, result.winkTag
                // External-token mode (returnExternalTokens=true):
                //   result.winkCardToken, result.winkTag, result.amount
            }
            override fun onCancelled(requestId: String) { /* user cancelled */ }
            override fun onFailure(error: EmbeddedPaymentError) {
                // error.errorCode is "AUTH_FAILED" or "LIVENESS_FAILED" today.
                // See §9 for the full list.
            }
        })

        // 4. Launch the customer display. On single-display devices pass
        //    Display.DEFAULT_DISPLAY (or omit) and you are done.
        val displayId = pickCustomerDisplayId()
        val intent = winkPay.createNativeIntent(this, displayId)
        val opts = ActivityOptions.makeBasic().setLaunchDisplayId(displayId)
        startActivity(intent, opts.toBundle())
    }

    private fun pickCustomerDisplayId(): Int {
        val dm = getSystemService(Context.DISPLAY_SERVICE) as DisplayManager
        return dm.displays.firstOrNull {
            it.displayId != Display.DEFAULT_DISPLAY && it.isValid
        }?.displayId ?: Display.DEFAULT_DISPLAY
    }
}

That's the whole integration. Everything below is reference material.

2. Prerequisites & build setup

useLegacyPackaging = true is mandatory. Without it, the SDK's bundled native libraries stay inside base.apk instead of being extracted to lib/arm64-v8a/, and the SDK fails to initialize at runtime. AGP 8 makes legacy packaging opt-in.

3. AndroidManifest.xml contributions

The SDK manifest declares the permissions and Activities the customer display needs; manifest merger pulls them into your app automatically. You do not need to copy these into your own manifest.

Inherited from the SDK:

You only add what you need for your own POS UI — typically just your launcher Activity and any platform-specific intent-filter your processor or POS shell requires.

CAMERA is required at runtime, but the SDK does not request it for you. You must request android.permission.CAMERA from the user yourself before launching a payment — typically in your app's first-launch flow or right before calling startPayment. If the permission is not granted when the customer-display Activity starts, the camera preview shows black and the biometric session eventually fails (there is no "permission denied" callback — request the permission proactively).

4. Public API surface

These are the only types third-party integrators are expected to touch. Everything else under com.wink.winkpay.* is internal.

4.1 WinkPaySdk and WinkPaySdk.Config

com.wink.winkpay.WinkPaySdk is a singleton. You don't normally call it directly — WinkPayEmbedded.init(...) calls it for you. Configuration is the data class WinkPaySdk.Config:

WinkPaySdk.Config(
    clientId:             String? = null,
    merchantClientSecret: String? = null,
    deviceSerialNumber:   String? = null,
    sdkEnvironment:       String  = "stage",  // "qa" | "stage" | "prod"
    palmMfaEnabled:       Boolean = true,
    enableHttpsServer:    Boolean = false,
    serverPort:           Int     = 8443,
    readWinkConfig:       Boolean = false,    // on-device test override; see below
    debugLogging:         Boolean = false,    // verbose SDK logs (off in production)
    envFileContent:       String? = null,     // legacy fallback
)

4.2 WinkPayEmbedded

com.wink.winkpay.embedded.WinkPayEmbedded is the integration facade.

object Companion {
    fun init(
        context: Context,
        config: WinkPaySdk.Config = WinkPaySdk.Config(),
    ): WinkPayEmbedded

    fun getInstance(): WinkPayEmbedded   // throws if init() not called
}

fun startPayment(request: EmbeddedPaymentRequest, callback: EmbeddedPaymentCallback)

fun startCheckin(request: EmbeddedCheckinRequest, callback: EmbeddedPaymentCallback)

fun startLiteRegistration(request: EmbeddedLiteRegistrationRequest, callback: EmbeddedPaymentCallback)

fun createNativeIntent(context: Context, displayId: Int = -1): Intent
fun createCaptureFragment(): Fragment   // embed in your own Activity — see §4.12

fun setAgeRestrictionOverride()         // merchant-side ID override — see §4.13
fun cancelPayment()
fun shutdown()

4.3 EmbeddedPaymentRequest

data class EmbeddedPaymentRequest(
    val requestId: String,                        // your unique id; UUIDv4 recommended
    val requestType: String,                      // "PAY" | "PAY_FACE" | "PAY_PALM"
    val amount: Long,                             // minor units (cents). Must be > 0
    val currency: String = "USD",                 // ISO 4217
    val orderId: String,                          // your POS order id; non-blank
    val tenderId: String,                         // your tender id (free-form, non-blank)
    val ageRestrictedPurchase: Boolean = false,   // gates capture behind merchant override
    val rotationAngle: Int = 0,                   // 0/90/180/270 — pose hint for face detection
    val cameraIndex: Int = 0,                     // 0 = back, 1 = front
    val returnExternalTokens: Boolean = false,    // see §4.7
    val returnOnFailure: Boolean = false,         // see §4.8
    val maxRetries: Int = 3,                      // max face-capture attempts; see §4.9
    // — Tuning knobs (see §4.9) —
    val minFaceRatio: Double? = null,             // 0.0–1.0; null = device-class default
    val livenessEnabled: Boolean? = null,         // null = SDK default (ON unless local liveness ran)
    val previewRotation: Int? = null,             // 0/90/180/270; null = camera default
    val activityOrientation: Int? = null,         // ActivityInfo.SCREEN_ORIENTATION_* constant
    // — Palm-specific overrides (see §4.10) —
    val palmRotationAngle: Int = 0,               // -1 = fall back to rotationAngle
    val palmCameraIndex: Int = -1,                // -1 = fall back to cameraIndex
    val palmPreviewRotation: Int? = null,         // null = fall back to previewRotation
)

PAY shows the face/palm chooser; PAY_FACE and PAY_PALM jump straight into the corresponding capture screen.

4.4 EmbeddedPaymentResult

data class EmbeddedPaymentResult(
    val requestId: String,
    val transactionId: String?,         // null in external-token mode
    val amount: Long,
    val winkTag: String? = null,        // biometric-resolved customer id
    val winkToken: String? = null,      // payment token, when available
    val winkCardToken: String? = null,  // selected card token, populated in external-token mode
    val mfaUsed: Boolean = false,       // true if MFA was performed during this auth
    val appID: String? = null,          // processor app id — populated for processors that require it, see §4.7
)

4.5 EmbeddedPaymentError and EmbeddedPaymentCallback

data class EmbeddedPaymentError(
    val requestId: String,
    val errorCode: String,
    val errorMessage: String,
)

interface EmbeddedPaymentCallback {
    fun onSuccess(result: EmbeddedPaymentResult)
    fun onCancelled(requestId: String)
    fun onFailure(error: EmbeddedPaymentError)
    fun onCheckinSuccess(result: EmbeddedCheckinResult) { /* default delegates to onSuccess */ }
    fun onLiteRegistrationSuccess(result: EmbeddedLiteRegistrationResult) { /* default delegates to onSuccess */ }
}

All callback methods are dispatched on the main thread.

4.6 Check-in mode (identity-only, no payment)

Use this for kiosk check-in and similar flows where you want to identify a returning user but not charge a card. The SDK runs the biometric capture and returns the full login response.

data class EmbeddedCheckinRequest(
    val requestId: String,
    val currency: String = "USD",
    val rotationAngle: Int = 0,
    val cameraIndex: Int = 0,
    val biometricType: String = "face",           // "face" or "palm"
    val returnOnFailure: Boolean = false,          // see §4.8
    val maxRetries: Int = 3,                       // max face-capture attempts; see §4.9
    // — Tuning knobs (see §4.9) —
    val minFaceRatio: Double? = null,
    val livenessEnabled: Boolean? = null,
    val previewRotation: Int? = null,
    val activityOrientation: Int? = null,
    // — Palm-specific overrides (see §4.10) —
    val palmRotationAngle: Int = 0,
    val palmCameraIndex: Int = -1,
    val palmPreviewRotation: Int? = null,
)

data class EmbeddedCheckinResult(
    val requestId: String,
    val winkTag: String?,
    val accessToken: String?,
    val loginResponseJson: String,        // full server JSON: user, cards, addresses, faceResponse
)

winkPay.startCheckin(EmbeddedCheckinRequest(requestId = UUID.randomUUID().toString()),
    object : EmbeddedPaymentCallback {
        override fun onCheckinSuccess(result: EmbeddedCheckinResult) {
            // Parse result.loginResponseJson — userPaymentDetails.user, .cards, .shippingAddresses
        }
        override fun onSuccess(r: EmbeddedPaymentResult) {}
        override fun onCancelled(requestId: String) {}
        override fun onFailure(error: EmbeddedPaymentError) {}
    })

isProfileVerified is emitted at two locations inside loginResponseJson for host integrator convenience:

• userPaymentDetails.isProfileVerified — the canonical field on the outer UserPaymentDetails.
• userPaymentDetails.user.isProfileVerified — mirrored onto the inner UserShort so hosts that read profile fields from the user object don't have to reach back up.

Both fields carry the same boolean and are written from the same source on the server response. Read whichever is convenient — they are kept in sync.

4.7 returnExternalTokens — let the host process the charge

If your processor already handles the actual card charge and you only need WinkPay to identify the customer and pick a card:

When returnExternalTokens = true and the configured clientId is for a processor that requires an app id to route the charge, the SDK additionally populates result.appID with that value. For processors that do not need it, appID is null.

4.8 returnOnFailure — surface auth failures to the host

By default, when biometric authentication fails (face not recognized, liveness rejected, etc.) the native UI shows a Retry / See Options error screen. If you want control to return to your POS app instead:

EmbeddedPaymentRequest(..., returnOnFailure = true)

The native Activity finishes with onFailure(EmbeddedPaymentError(...)) on the registered callback (see §9 for the error codes).

4.9 Capture tuning knobs

These are all optional overrides on EmbeddedPaymentRequest / EmbeddedCheckinRequest / EmbeddedLiteRegistrationRequest. Leave them off and the SDK picks per-device defaults — same as before any of these existed.

Examples:

// Handheld with sensor mounted at 90° pose, display rotated 270° from sensor
EmbeddedPaymentRequest(
    ...,
    rotationAngle = 90,
    previewRotation = 270,
    activityOrientation = ActivityInfo.SCREEN_ORIENTATION_LANDSCAPE,
)

// Kiosk: customer ~50 cm from screen — relax the strict default
EmbeddedCheckinRequest(..., minFaceRatio = 0.30)

// Supervised registration: skip server liveness, rely on local checks
EmbeddedLiteRegistrationRequest(..., livenessEnabled = false)

4.10 Per-modality (face vs palm) capture overrides

When face and palm capture need different rotation/camera configs on the same device — common on hardware with a dedicated palm-side sensor, or when MFA's "Use palm" fallback expects a different grip than face capture — set these palm-specific fields. All three default to "fall back to face values," so existing integrations keep working untouched.

Always send both sets when they differ so the MFA palm fallback (when face misses and the customer taps Use palm on the MFA screen) picks up the right config without the host having to re-issue a fresh request.

EmbeddedPaymentRequest(
    ...,
    rotationAngle = 90,  cameraIndex = 0,  previewRotation = 270,         // face
    palmRotationAngle = 0,  palmCameraIndex = 1,  palmPreviewRotation = 0, // palm
)

4.11 Lite registration

Face-enrollment + minimal profile creation (first name + phone — lastName and email are auto-generated placeholders the user updates later). Used for low-friction onboarding flows where the customer doesn't yet have a Wink account.

data class EmbeddedLiteRegistrationRequest(
    val requestId: String,
    val rotationAngle: Int = 0,
    val cameraIndex: Int = 0,
    val minFaceRatio: Double? = null,       // see §4.9
    val livenessEnabled: Boolean? = null,   // null = OFF for enrollment by default
    val previewRotation: Int? = null,
    val activityOrientation: Int? = null,
    val maxRetries: Int = 3,                // see §4.9
)

data class EmbeddedLiteRegistrationResult(
    val requestId: String,
    val accessToken: String?,        // identity-provider access token (or platform fallback)
    val winkSeed: String?,
    val oAuthRequestId: String?,
    val firstName: String?,
    val lastName: String?,           // auto-generated placeholder
    val email: String?,              // auto-generated placeholder
    val contactNo: String?,
    val alreadyEnrolled: Boolean,    // true when the face was recognized; no form shown
    val refreshToken: String? = null,
    val idToken: String? = null,
    val expiresIn: Int? = null,
    val tokenType: String? = null,
    val isKeycloakToken: Boolean = false,  // false when the SDK fell back to the platform token
)

winkPay.startLiteRegistration(
    EmbeddedLiteRegistrationRequest(requestId = UUID.randomUUID().toString()),
    object : EmbeddedPaymentCallback {
        override fun onLiteRegistrationSuccess(result: EmbeddedLiteRegistrationResult) {
            // result.accessToken is the identity-provider token (when isKeycloakToken == true)
            // — use it as a Bearer token for downstream API calls.
        }
        override fun onSuccess(r: EmbeddedPaymentResult) {}
        override fun onCancelled(requestId: String) {}
        override fun onFailure(error: EmbeddedPaymentError) {}
    })

Flow:

1. Fresh face enrollment scan.
2. If the face is already known → returns alreadyEnrolled = true with the existing user's tokens. No form shown.
3. Otherwise → the SDK shows a compact form (first name + phone with a country picker). On submit it validates the phone number, creates the profile, then exchanges the result for an identity-provider access token. If that exchange fails it falls back to the platform token (isKeycloakToken = false).

4.12 Embedding the capture UI in your own Activity (WinkPayCaptureFragment)

createNativeIntent(...) launches the SDK's full-screen customer-display Activity. If you'd rather have the capture screen draw inside your own layout — next to your branded chrome, in a sidebar, in a tablet master/detail pane — call createCaptureFragment() instead and mount the returned Fragment yourself.

// 1. Register the result callback as usual.
winkPay.startCheckin(
    EmbeddedCheckinRequest(
        requestId = UUID.randomUUID().toString(),
        cameraIndex = 0,
        rotationAngle = 0,
    ),
    callback,
)

// 2. Mount the fragment in your container instead of launching the Activity.
supportFragmentManager.beginTransaction()
    .replace(R.id.host_capture_container, winkPay.createCaptureFragment())
    .commit()

The fragment reads its capture parameters from the same process-scoped state that startCheckin / startPayment populates, so always call one of those before mounting the fragment.

Supported flows: the check-in flow (face capture → API auth → onCheckinSuccess) works end-to-end inside the fragment today. Full payment flows (startPayment) are not yet exposed through this entry point because they involve multiple downstream screens (PaymentOptions, PaymentConfirmation, MFA/Consent dialogs) that aren't suited to an embedded view — keep using createNativeIntent(...) for those.

Cancellation: removing the fragment from its container at any time tears down the capture session. If you want the registered callback to receive onCancelled, call winkPay.cancelPayment() as well — by default a host-driven removal silently disposes of state.

4.13 Age-restricted override (setAgeRestrictionOverride)

When an order is age-restricted (EmbeddedPaymentRequest.ageRestrictedPurchase = true) and the matched customer's profile is not verified, the customer display blocks payment and shows an ID prompt. If the merchant visually checks the customer's ID and approves the sale, call:

winkPay.setAgeRestrictionOverride()

This clears the customer-side ID gate for the current order so payment can proceed without profile verification. Typically wired to an "Override" button on the merchant display. Safe to call from any thread.

5. Credentials & device registration

5.1 Merchant credentials

Pass the merchant credentials directly on WinkPaySdk.Config:

WinkPaySdk.Config(
    clientId             = "YOUR_CLIENT_ID",
    merchantClientSecret = "YOUR_MERCHANT_CLIENT_SECRET",
    deviceSerialNumber   = Build.SERIAL,
    sdkEnvironment       = "stage",
)

Both fields are required for any biometric flow. Treat the secret as exactly that — keep it out of source control, and out of strings.xml. Stage vs prod URLs are baked into the SDK and selected via Config.sdkEnvironment — there's nothing else to configure for a standard deployment.

5.2 Device registration (deviceSerialNumber)

The SDK requires the device to be registered with the WinkPay platform before it can run face or palm. Pass the POS serial number on every init — the SDK takes care of the rest:

1. First launch: a background thread registers the serial with the WinkPay platform. The platform returns a server-issued device identifier, which the SDK persists locally.
2. Subsequent launches: the cached device identifier is reused. The serial you pass is ignored unless the cache was cleared (e.g. app data wiped).
3. At biometric capture time: the SDK attaches the cached device identifier to every biometric login call.

If registration hasn't completed yet (or failed), the capture screens display a small red ! "Device not registered" badge in the top-right corner. Biometric attempts in that state surface a "Device not registered. Please contact support." failure to your callback.

What to pass as deviceSerialNumber. On POS hardware with API 24–25, Build.SERIAL typically works. On newer devices, fall back to a stable per-device synthetic ID (e.g. "$MANUFACTURER-$MODEL-$ID" — or your own provisioned ID). Whatever you pass on the first run is what gets registered with WinkPay; it is not mutated locally afterwards.

5.3 Legacy envFileContent path

If you have an existing integration that reads merchant credentials from a .env-style file, you can keep using Config.envFileContent — the SDK still parses KEY=VALUE lines for CLIENT_ID, MERCHANT_CLIENT_SECRET, and the optional URL-override keys below. New integrations should use the direct fields in §5.1. When both are present, the direct fields win.

6. Lifecycle & threading

What happens when you call startPayment and launch the Activity

1. startPayment stores the order data in process-scoped state and registers your callback.
2. createNativeIntent returns an Intent for the SDK's customer-display Activity.
3. The Activity reads the order data in onCreate. If requestType was PAY_FACE or PAY_PALM, it routes straight to the corresponding capture screen; otherwise it shows the face/palm chooser.
4. Capture → API auth → consent / MFA dialogs as required → card selection → either pay or return external tokens.
5. The Activity finishes. Your callback fires (on the main thread):
   • onSuccess — payment processed or external tokens returned
   • onCancelled — user tapped cancel, or you called cancelPayment()
   • onFailure — auth/payment failed and returnOnFailure=true, or a terminal error occurred

Threading

• All callback methods are dispatched on the main thread.
• API calls (auth, payment, palm) run on a single-threaded executor inside the Activity; the Activity tears it down in onDestroy.
• Camera analysis runs on a dedicated analyzer executor; results are posted back to the main thread before any UI / API call.

What gets torn down on Activity destroy

The SDK's customer-display Activity releases all biometric, network, and camera resources automatically when it finishes. You don't need to do anything in your own lifecycle to "clean up" the SDK between payments — just register a fresh callback before the next startPayment.

Calling startPayment twice

startPayment overwrites the previously-registered callback. It does not serialize concurrent calls — only one biometric session can be in flight at a time. If you need to start a second payment after the first completes, wait for the callback to fire, then call startPayment again. There is no explicit "session id" — the requestId you supply is what threads back to your callback.

Process death / re-entry

The order data and callback registration are process-scoped and do not survive process death. If the OS kills your app while the Activity is running, restart the flow from your POS — there is no resumable session.

7. Customer-facing display (dual-display devices)

On single-display devices you can ignore this section: pass Display.DEFAULT_DISPLAY (or omit displayId) and you're done.

On dual-display POS hardware (separate merchant + customer screens), the capture UI should land on the customer display. Two things must be true:

1. The Intent must include FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_MULTIPLE_TASK — createNativeIntent(ctx, displayId) adds these for you when displayId >= 0.
2. You must pass an ActivityOptions bundle. Flags alone do not target a display.

val displays = (getSystemService(Context.DISPLAY_SERVICE) as DisplayManager).displays
val customerDisplayId = displays.firstOrNull {
    it.displayId != Display.DEFAULT_DISPLAY && it.isValid
}?.displayId ?: Display.DEFAULT_DISPLAY

val intent = winkPay.createNativeIntent(this, customerDisplayId)
val opts   = ActivityOptions.makeBasic().setLaunchDisplayId(customerDisplayId)
startActivity(intent, opts.toBundle())

Without the options bundle you'll see Failed to put TaskRecord on display N in logcat and the Activity will land on the merchant display.

Relaunching your own POS Activity on the customer display

If your POS UI also belongs on the customer display:

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    if (relaunchOnCustomerDisplayIfNeeded()) return
    // ... normal onCreate
}

private fun relaunchOnCustomerDisplayIfNeeded(): Boolean {
    if (intent.getBooleanExtra("__relaunched", false)) return false
    val dm = getSystemService(DisplayManager::class.java)
    val customer = dm.displays.firstOrNull {
        it.displayId != Display.DEFAULT_DISPLAY && it.isValid
    } ?: return false

    val current = if (Build.VERSION.SDK_INT >= 30) display?.displayId
                  else windowManager.defaultDisplay.displayId
    if (current == customer.displayId) return false

    val relaunch = Intent(this, javaClass).apply {
        putExtra("__relaunched", true)
        addFlags(Intent.FLAG_ACTIVITY_NEW_TASK or Intent.FLAG_ACTIVITY_MULTIPLE_TASK)
    }
    startActivity(relaunch, ActivityOptions.makeBasic()
        .setLaunchDisplayId(customer.displayId).toBundle())
    finish()
    return true
}

Add android:taskAffinity=".your_customer_task" to your Activity in the manifest so Android grants a fresh task on the customer display.

The SDK's customer-display Activity is already configured to land on a fresh task — you do not need to configure it.

8. Palm capture details

The SDK runs palm capture on-device, including liveness verification. The capture screen positions the user's hand, prompts a randomized gesture, and verifies the gesture before delivering a palm model to the WinkPay backend for identification. The full flow — including model build and identify call — is internal to the SDK; your host code only sees the eventual onSuccess or onFailure callback.

The capture screen guides the user into the correct pose automatically — surfacing on-screen prompts to move lower / further back or to centre the hand in the bracket when it sits just outside the usable zone, and only committing to capture once the hand has held a good pose steadily. If the underlying engine accepts a capture but stalls before liveness, the SDK silently re-acquires rather than freezing. No host configuration is involved.

If liveness is rejected the errorMessage starts with "Liveness check…" (e.g. "Liveness check timed out", "Liveness check failed"). The SDK turns this into errorCode = "LIVENESS_FAILED" if returnOnFailure is set; otherwise it routes to the SDK's retry screen.

When debugging false-rejects, enable SDK logging — WinkPaySdk.Config(debugLogging = true) or adb shell setprop log.tag.PalmCaptureSession DEBUG — and capture logcat with tag PalmCaptureSession; every reject logs a one-line reason. Reach out to the Wink team if you believe these rejects require SDK tuning.(Debug/info logs are off by default.)

8.1 Lite palm path (preview — off by default)

The SDK also ships an optional server-assisted palm capture path that offloads palm-model construction to an external modeling server, lowering on-device memory use on resource-constrained POS hardware. From the host's perspective there is no behavioural difference — palm identification still resolves to the same onSuccess / onFailure callback.

The path is selected via an optional configuration key:

Pass it through Config.envFileContent (legacy env-file path) alongside your other keys:

WinkPaySdk.Config(
    clientId             = "…",
    merchantClientSecret = "…",
    deviceSerialNumber   = Build.SERIAL,
    sdkEnvironment       = "stage",
    envFileContent       = "PALM_MODELING_SERVER=http://10.0.0.42:9090\n",
)

The server-assisted path is currently preview / staged — the on-device path remains the default and the recommended option for production deployments. The host-facing API (startPayment with PAY_PALM, startCheckin with biometricType = "palm") is identical for both paths.

9. Error handling

9.1 errorCode values delivered via EmbeddedPaymentError

The native flow produces the following error codes:

onFailure is only called when:

• You set EmbeddedPaymentRequest.returnOnFailure = true, or
• A terminal error occurs that the SDK does not have a retry path for (e.g. network failure during a payment that already debited).

By default (returnOnFailure = false) the native UI shows a Retry / See Options screen and the user can re-attempt without your code being notified.

9.2 User-visible strings you may see in logcat / errorMessage

Not all of these surface to your callback (most route to the native retry screen unless returnOnFailure = true), but you may see them while debugging:

Humanized face-scan errors. When the auth server returns an HTTP error with no useful body — common for HTTP 400 on an unusable face image (occluded, blank, oversized, malformed) — the SDK substitutes a face-specific message instead of surfacing the raw HTTP reason phrase ("Bad Request", "Unauthorized", "Internal Server Error", or a bare HTTP NNN fallback). The replacements are keyed off the HTTP class:

If the server does return a real, descriptive error body, the SDK surfaces that verbatim — the substitution only kicks in when the response body would otherwise be useless (empty, or a bare reason phrase). The substituted text appears in EmbeddedPaymentError.errorMessage when returnOnFailure = true, and on the native retry screen otherwise.

9.3 Initialization-time exceptions

These come out as standard Kotlin exceptions, not via the callback:

9.4 Companion-app error codes

When integrating via the companion app (intent-based IPC) instead of the embedded SDK, you'll see an additional set of error codes — INVALID_REQUEST, UNAUTHORIZED_CALLER, IDEMPOTENT_REJECT, PAYMENT_FAILED, NOT_IMPLEMENTED. Those are documented in POS_INTEGRATION.md, which is the authoritative reference for that integration path.

10. Build notes & APK size

10.1 ProGuard / R8 — consumer rules are auto-propagated

The SDK ships consumer-rules.pro and declares it via consumerProguardFiles, so it is automatically merged into your app's R8 rules at build time. You do not need to copy any WinkPay-specific rules into your app — the bundled set covers the public API surface (WinkPaySdk, WinkPaySdk.Config, WinkPayEmbedded, the EmbeddedPayment* data classes) and the SDK's own serialization needs.

If you minify your app and see R8 warnings about other transitive dependencies that your own app pulls in (your POS vendor SDK, analytics libraries, etc.), add the relevant -keep / -dontwarn rules in your own app/proguard-rules.pro — those live outside the WinkPay SDK and are your app's responsibility.

10.2 APK size impact

The SDK adds roughly 10 MB compressed (~25 MB uncompressed, arm64-v8a only) to your APK. The face-detection model is delivered on-device at runtime by the platform — zero APK cost.

To keep the build lean, set ndk.abiFilters += "arm64-v8a" (the SDK only ships arm64-v8a anyway — including other ABIs just inflates your APK with native libs that won't run).

10.3 Versioning

Maven coordinates are com.wink:winkpay-sdk:<version>. The current recommended version is 1.5.0. The SDK versioning convention is:

• 1.x.0 — feature releases
• 1.x.y — bug-fix releases

When you upgrade the SDK, swap the version in app/build.gradle.kts and copy the new local Maven repo over vendor/winkpay-sdk/winkpay-sdk-repo. No code changes are required for patch-level upgrades within the same major.

11. Troubleshooting