⏺ Add callback-based start function with PayPalWebCheckoutCallback interface

  - Create PayPalWebCheckoutCallback fun interface following CardApproveOrderCallback pattern
  - Add overloaded start() function accepting PayPalWebCheckoutCallback

Author: kgangineni

CorePayments/src/main/java/com/paypal/android/corepayments/APIClientError.kt

@@ -9,9 +9,10 @@ import androidx.annotation.RestrictTo
 object APIClientError {
 
     // 0. An unknown error occurred.
-    fun unknownError(correlationId: String?) = PayPalSDKError(
+    fun unknownError(correlationId: String? = null, throwable: Throwable? = null) = PayPalSDKError(
         code = PayPalSDKErrorCode.UNKNOWN.ordinal,
         errorDescription = "An unknown error occurred. Contact developer.paypal.com/support.",
+        reason = throwable,
         correlationId = correlationId
     )
 
@@ -91,4 +92,16 @@ object APIClientError {
         )
         return error
     }
+
+    fun payPalSDKError(
+        code: Int,
+        errorDescription: String?,
+        correlationId: String? = null,
+        reason: Throwable? = null
+    ): PayPalSDKError = PayPalSDKError(
+        code = code,
+        errorDescription = errorDescription.orEmpty(),
+        correlationId = correlationId,
+        reason = reason
+    )
 }

CorePayments/src/main/java/com/paypal/android/corepayments/PayPalSDKErrorCode.kt

@@ -15,5 +15,6 @@ enum class PayPalSDKErrorCode {
     SERVER_RESPONSE_ERROR,
     CHECKOUT_ERROR,
     NATIVE_CHECKOUT_ERROR,
-    GRAPHQL_JSON_INVALID_ERROR
+    GRAPHQL_JSON_INVALID_ERROR,
+    NO_ACCESS_TOKEN_ERROR,
 }

CorePayments/src/main/java/com/paypal/android/corepayments/api/AuthenticationSecureTokenServiceAPI.kt

@@ -0,0 +1,66 @@
+package com.paypal.android.corepayments.api
+
+import androidx.annotation.RestrictTo
+import com.paypal.android.corepayments.APIClientError
+import com.paypal.android.corepayments.APIClientError.payPalSDKError
+import com.paypal.android.corepayments.APIRequest
+import com.paypal.android.corepayments.CoreConfig
+import com.paypal.android.corepayments.HttpMethod
+import com.paypal.android.corepayments.PayPalSDKErrorCode
+import com.paypal.android.corepayments.RestClient
+import com.paypal.android.corepayments.base64encoded
+import org.json.JSONObject
+
+@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
+class AuthenticationSecureTokenServiceAPI(
+    private val coreConfig: CoreConfig,
+    private val restClient: RestClient = RestClient(configuration = coreConfig),
+) {
+    suspend fun getClientToken(): GetClientTokenResult {
+        val requestBody = "grant_type=client_credentials&response_type=token"
+
+        val credentials = "${coreConfig.clientId}:"
+        val headers = mutableMapOf(
+            "Authorization" to "Basic ${credentials.base64encoded()}",
+            "Content-Type" to "application/x-www-form-urlencoded"
+        )
+
+        val apiRequest = APIRequest(
+            path = "v1/oauth2/token",
+            method = HttpMethod.POST,
+            body = requestBody,
+            headers = headers
+        )
+
+        return runCatching {
+            val httpResponse = restClient.send(apiRequest)
+            val correlationId = httpResponse.headers["paypal-debug-id"]
+            if (httpResponse.isSuccessful && httpResponse.body != null) {
+                val jsonObject = JSONObject(httpResponse.body)
+                val token = jsonObject.optString("access_token")
+                if (token.isNotEmpty()) {
+                    GetClientTokenResult.Success(token)
+                } else {
+                    val error = payPalSDKError(
+                        code = PayPalSDKErrorCode.NO_ACCESS_TOKEN_ERROR.ordinal,
+                        errorDescription = "Missing access_token in response",
+                        correlationId = correlationId
+                    )
+                    GetClientTokenResult.Failure(error)
+                }
+            } else {
+                val error = httpResponse.run {
+                    payPalSDKError(
+                        code = status,
+                        errorDescription = body,
+                        correlationId = correlationId
+                    )
+                }
+                GetClientTokenResult.Failure(error)
+            }
+        }.getOrElse { throwable ->
+            val error = APIClientError.unknownError(throwable = throwable)
+            GetClientTokenResult.Failure(error)
+        }
+    }
+}

CorePayments/src/main/java/com/paypal/android/corepayments/api/FetchClientToken.kt

@@ -0,0 +1,21 @@
+package com.paypal.android.corepayments.api
+
+import androidx.annotation.RestrictTo
+import com.paypal.android.corepayments.PayPalSDKError
+
+@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
+sealed class GetClientTokenResult {
+    /**
+     * The request to get client token was successful.
+     *
+     * @property token the access token from the response
+     */
+    data class Success(val token: String) : GetClientTokenResult()
+
+    /**
+     * There was an error with the request to get client token.
+     *
+     * @property error the error that occurred
+     */
+    data class Failure(val error: PayPalSDKError) : GetClientTokenResult()
+}