diff --git a/app/src/main/kotlin/com/github/gotify/MissedMessageUtil.kt b/app/src/main/kotlin/com/github/gotify/MissedMessageUtil.kt
index cf6719e..3a69759 100644
--- a/app/src/main/kotlin/com/github/gotify/MissedMessageUtil.kt
+++ b/app/src/main/kotlin/com/github/gotify/MissedMessageUtil.kt
@@ -3,20 +3,19 @@ package com.github.gotify
 import com.github.gotify.api.Api
 import com.github.gotify.api.ApiException
 import com.github.gotify.api.Callback
-import com.github.gotify.api.Callback.SuccessCallback
 import com.github.gotify.client.api.MessageApi
 import com.github.gotify.client.model.Message
 import com.github.gotify.log.Log
 
 internal class MissedMessageUtil(private val api: MessageApi) {
-    fun lastReceivedMessage(successCallback: SuccessCallback<Long>) {
+    fun lastReceivedMessage(acceptID: (Long) -> Unit) {
         api.getMessages(1, 0L).enqueue(
             Callback.call(
-                onSuccess = { messages ->
+                onSuccess = Callback.SuccessBody { messages ->
                     if (messages.messages.size == 1) {
-                        successCallback.onSuccess(messages.messages[0].id)
+                        acceptID(messages.messages[0].id)
                     } else {
-                        successCallback.onSuccess(NO_MESSAGES)
+                        acceptID(NO_MESSAGES)
                     }
                 },
                 onError = {}
diff --git a/app/src/main/kotlin/com/github/gotify/api/Api.kt b/app/src/main/kotlin/com/github/gotify/api/Api.kt
index ac8ebc3..7b09a5a 100644
--- a/app/src/main/kotlin/com/github/gotify/api/Api.kt
+++ b/app/src/main/kotlin/com/github/gotify/api/Api.kt
@@ -4,6 +4,19 @@ import retrofit2.Call
 import java.io.IOException
 
 internal object Api {
+    @Throws(ApiException::class)
+    fun execute(call: Call<Void>) {
+        try {
+            val response = call.execute()
+
+            if (!response.isSuccessful) {
+                throw ApiException(response)
+            }
+        } catch (e: IOException) {
+            throw ApiException(e)
+        }
+    }
+
     @Throws(ApiException::class)
     fun <T> execute(call: Call<T>): T {
         try {
diff --git a/app/src/main/kotlin/com/github/gotify/api/Callback.kt b/app/src/main/kotlin/com/github/gotify/api/Callback.kt
index 14f0785..8c1e1c8 100644
--- a/app/src/main/kotlin/com/github/gotify/api/Callback.kt
+++ b/app/src/main/kotlin/com/github/gotify/api/Callback.kt
@@ -10,7 +10,15 @@ internal class Callback<T> private constructor(
     private val onError: ErrorCallback
 ) {
     fun interface SuccessCallback<T> {
-        fun onSuccess(data: T)
+        fun onSuccess(response: Response<T>)
+    }
+
+    fun interface SuccessBody<T> : SuccessCallback<T> {
+        override fun onSuccess(response: Response<T>) {
+            onResultSuccess(response.body() ?: throw ApiException("null response", response))
+        }
+
+        fun onResultSuccess(data: T)
     }
 
     fun interface ErrorCallback {
@@ -20,9 +28,7 @@ internal class Callback<T> private constructor(
     private class RetrofitCallback<T>(private val callback: Callback<T>) : retrofit2.Callback<T> {
         override fun onResponse(call: Call<T>, response: Response<T>) {
             if (response.isSuccessful) {
-                callback.onSuccess.onSuccess(
-                    response.body() ?: throw ApiException("null response", response)
-                )
+                callback.onSuccess.onSuccess(response)
             } else {
                 callback.onError.onError(ApiException(response))
             }
@@ -40,19 +46,12 @@ internal class Callback<T> private constructor(
             onError: ErrorCallback
         ): retrofit2.Callback<T> {
             return call(
-                onSuccess = { data -> context.runOnUiThread { onSuccess.onSuccess(data) } },
+                onSuccess = { response -> context.runOnUiThread { onSuccess.onSuccess(response) } },
                 onError = { exception -> context.runOnUiThread { onError.onError(exception) } }
             )
         }
 
-        fun <T> call(): retrofit2.Callback<T> {
-            return call(
-                onSuccess = {},
-                onError = {}
-            )
-        }
-
-        fun <T> call(onSuccess: SuccessCallback<T>, onError: ErrorCallback): retrofit2.Callback<T> {
+        fun <T> call(onSuccess: SuccessCallback<T> = SuccessCallback {}, onError: ErrorCallback = ErrorCallback {}): retrofit2.Callback<T> {
             return RetrofitCallback(merge(of(onSuccess, onError), errorCallback()))
         }
 
diff --git a/app/src/main/kotlin/com/github/gotify/init/InitializationActivity.kt b/app/src/main/kotlin/com/github/gotify/init/InitializationActivity.kt
index 5083439..6e56ba7 100644
--- a/app/src/main/kotlin/com/github/gotify/init/InitializationActivity.kt
+++ b/app/src/main/kotlin/com/github/gotify/init/InitializationActivity.kt
@@ -63,7 +63,7 @@ internal class InitializationActivity : AppCompatActivity() {
             .enqueue(
                 Callback.callInUI(
                     this,
-                    onSuccess = { user -> authenticated(user) },
+                    onSuccess = Callback.SuccessBody { user -> authenticated(user) },
                     onError = { exception -> failed(exception) }
                 )
             )
@@ -113,7 +113,7 @@ internal class InitializationActivity : AppCompatActivity() {
 
     private fun requestVersion(runnable: Runnable) {
         requestVersion(
-            callback = { version: VersionInfo ->
+            callback = Callback.SuccessBody { version: VersionInfo ->
                 Log.i("Server version: ${version.version}@${version.buildDate}")
                 settings.serverVersion = version.version
                 runnable.run()
diff --git a/app/src/main/kotlin/com/github/gotify/login/LoginActivity.kt b/app/src/main/kotlin/com/github/gotify/login/LoginActivity.kt
index a86f75e..ed84f7b 100644
--- a/app/src/main/kotlin/com/github/gotify/login/LoginActivity.kt
+++ b/app/src/main/kotlin/com/github/gotify/login/LoginActivity.kt
@@ -192,7 +192,7 @@ internal class LoginActivity : AppCompatActivity() {
     }
 
     private fun onValidUrl(url: String): SuccessCallback<VersionInfo> {
-        return SuccessCallback { version ->
+        return Callback.SuccessBody { version ->
             settings.url = url
             binding.checkurlProgress.visibility = View.GONE
             binding.checkurl.visibility = View.VISIBLE
@@ -261,7 +261,7 @@ internal class LoginActivity : AppCompatActivity() {
                 .enqueue(
                     Callback.callInUI(
                         this,
-                        onSuccess = { client -> onCreatedClient(client) },
+                        onSuccess = Callback.SuccessBody { client -> onCreatedClient(client) },
                         onError = { onFailedToCreateClient() }
                     )
                 )
diff --git a/app/src/main/kotlin/com/github/gotify/messages/provider/ApplicationHolder.kt b/app/src/main/kotlin/com/github/gotify/messages/provider/ApplicationHolder.kt
index 0f03ab8..68e5c41 100644
--- a/app/src/main/kotlin/com/github/gotify/messages/provider/ApplicationHolder.kt
+++ b/app/src/main/kotlin/com/github/gotify/messages/provider/ApplicationHolder.kt
@@ -20,7 +20,7 @@ internal class ApplicationHolder(private val activity: Activity, private val cli
             .enqueue(
                 Callback.callInUI(
                     activity,
-                    onSuccess = { apps -> onReceiveApps(apps) },
+                    onSuccess = Callback.SuccessBody { apps -> onReceiveApps(apps) },
                     onError = { onFailedApps() }
                 )
             )
diff --git a/app/src/main/kotlin/com/github/gotify/picasso/PicassoHandler.kt b/app/src/main/kotlin/com/github/gotify/picasso/PicassoHandler.kt
index a1e356a..2ece693 100644
--- a/app/src/main/kotlin/com/github/gotify/picasso/PicassoHandler.kt
+++ b/app/src/main/kotlin/com/github/gotify/picasso/PicassoHandler.kt
@@ -68,7 +68,7 @@ internal class PicassoHandler(private val context: Context, private val settings
             .apps
             .enqueue(
                 Callback.call(
-                    onSuccess = { apps ->
+                    onSuccess = Callback.SuccessBody { apps ->
                         appIdToAppImage.clear()
                         appIdToAppImage.putAll(MessageImageCombiner.appIdToImage(apps))
                     },
diff --git a/app/src/main/kotlin/com/github/gotify/service/WebSocketConnection.kt b/app/src/main/kotlin/com/github/gotify/service/WebSocketConnection.kt
index dfac6cb..b76605e 100644
--- a/app/src/main/kotlin/com/github/gotify/service/WebSocketConnection.kt
+++ b/app/src/main/kotlin/com/github/gotify/service/WebSocketConnection.kt
@@ -7,7 +7,6 @@ import android.os.Handler
 import android.os.Looper
 import com.github.gotify.SSLSettings
 import com.github.gotify.Utils
-import com.github.gotify.api.Callback.SuccessCallback
 import com.github.gotify.api.CertUtils
 import com.github.gotify.client.model.Message
 import com.github.gotify.log.Log
@@ -38,7 +37,7 @@ internal class WebSocketConnection(
     private var errorCount = 0
 
     private var webSocket: WebSocket? = null
-    private lateinit var onMessage: SuccessCallback<Message>
+    private lateinit var onMessageCallback: (Message) -> Unit
     private lateinit var onClose: Runnable
     private lateinit var onOpen: Runnable
     private lateinit var onBadRequest: BadRequestRunnable
@@ -56,8 +55,8 @@ internal class WebSocketConnection(
     }
 
     @Synchronized
-    fun onMessage(onMessage: SuccessCallback<Message>): WebSocketConnection {
-        this.onMessage = onMessage
+    fun onMessage(onMessage: (Message) -> Unit): WebSocketConnection {
+        this.onMessageCallback = onMessage
         return this
     }
 
@@ -168,7 +167,7 @@ internal class WebSocketConnection(
             syncExec {
                 Log.i("WebSocket($id): received message $text")
                 val message = Utils.JSON.fromJson(text, Message::class.java)
-                onMessage.onSuccess(message)
+                onMessageCallback(message)
             }
             super.onMessage(webSocket, text)
         }