Convert to Kotlin

2021-07-18 12:04:45 -07:00
@@ -1,56 +0,0 @@
-/*
- * Copyright 2016 - 2021 Anton Tananaev (anton@traccar.org)
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package org.traccar.manager;
-
-import android.app.Fragment;
-import android.os.Build;
-import android.os.Bundle;
-import android.preference.PreferenceManager;
-
-import androidx.annotation.NonNull;
-import androidx.annotation.RequiresApi;
-import androidx.appcompat.app.AppCompatActivity;
-
-public class MainActivity extends AppCompatActivity {
-
-    public static final String PREFERENCE_URL = "url";
-
-    @Override
-    protected void onCreate(Bundle savedInstanceState) {
-        super.onCreate(savedInstanceState);
-        setContentView(R.layout.activity_main);
-        if (savedInstanceState == null) {
-            initContent();
-        }
-    }
-
-    private void initContent() {
-        if (PreferenceManager.getDefaultSharedPreferences(this).contains(PREFERENCE_URL)) {
-            getFragmentManager().beginTransaction().add(android.R.id.content, new MainFragment()).commit();
-        } else {
-            getFragmentManager().beginTransaction().add(android.R.id.content, new StartFragment()).commit();
-        }
-    }
-
-    @RequiresApi(api = Build.VERSION_CODES.M)
-    @Override
-    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
-        Fragment fragment = getFragmentManager().findFragmentById(android.R.id.content);
-        if (fragment != null) {
-            fragment.onRequestPermissionsResult(requestCode, permissions, grantResults);
-        }
-    }
-}
@@ -0,0 +1,52 @@
+/*
+ * Copyright 2016 - 2021 Anton Tananaev (anton@traccar.org)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+@file:Suppress("DEPRECATION")
+package org.traccar.manager
+
+import android.os.Build
+import android.os.Bundle
+import androidx.annotation.RequiresApi
+import androidx.appcompat.app.AppCompatActivity
+import androidx.preference.PreferenceManager
+
+class MainActivity : AppCompatActivity() {
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        setContentView(R.layout.activity_main)
+        if (savedInstanceState == null) {
+            initContent()
+        }
+    }
+
+    private fun initContent() {
+        if (PreferenceManager.getDefaultSharedPreferences(this).contains(PREFERENCE_URL)) {
+            fragmentManager.beginTransaction().add(android.R.id.content, MainFragment()).commit()
+        } else {
+            fragmentManager.beginTransaction().add(android.R.id.content, StartFragment()).commit()
+        }
+    }
+
+    @RequiresApi(api = Build.VERSION_CODES.M)
+    override fun onRequestPermissionsResult(requestCode: Int, permissions: Array<String>, grantResults: IntArray) {
+        val fragment = fragmentManager.findFragmentById(android.R.id.content)
+        fragment?.onRequestPermissionsResult(requestCode, permissions, grantResults)
+    }
+
+    companion object {
+        const val PREFERENCE_URL = "url"
+    }
+}
@@ -13,206 +13,194 @@
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
-package org.traccar.manager;
+@file:Suppress("DEPRECATION")
+package org.traccar.manager

-import android.Manifest;
-import android.annotation.SuppressLint;
-import android.app.Activity;
-import android.content.ActivityNotFoundException;
-import android.content.BroadcastReceiver;
-import android.content.Context;
-import android.content.Intent;
-import android.content.IntentFilter;
-import android.content.pm.ApplicationInfo;
-import android.content.pm.PackageManager;
-import android.net.Uri;
-import android.os.Build;
-import android.os.Bundle;
-import android.preference.PreferenceManager;
-import android.view.View;
-import android.webkit.GeolocationPermissions;
-import android.webkit.JavascriptInterface;
-import android.webkit.ValueCallback;
-import android.webkit.WebChromeClient;
-import android.webkit.WebSettings;
-import android.webkit.WebView;
-import android.webkit.WebViewFragment;
+import android.Manifest
+import android.annotation.SuppressLint
+import android.app.Activity
+import android.content.ActivityNotFoundException
+import android.content.BroadcastReceiver
+import android.content.Context
+import android.content.DialogInterface
+import android.content.Intent
+import android.content.IntentFilter
+import android.content.pm.ApplicationInfo
+import android.content.pm.PackageManager
+import android.net.Uri
+import android.os.Build
+import android.os.Bundle
+import android.view.View
+import android.webkit.GeolocationPermissions
+import android.webkit.JavascriptInterface
+import android.webkit.ValueCallback
+import android.webkit.WebChromeClient
+import android.webkit.WebView
+import android.webkit.WebViewFragment
+import androidx.appcompat.app.AlertDialog
+import androidx.core.app.ActivityCompat
+import androidx.core.content.ContextCompat
+import androidx.localbroadcastmanager.content.LocalBroadcastManager
+import androidx.preference.PreferenceManager

-import androidx.annotation.NonNull;
-import androidx.annotation.Nullable;
-import androidx.appcompat.app.AlertDialog;
-import androidx.core.app.ActivityCompat;
-import androidx.core.content.ContextCompat;
-import androidx.localbroadcastmanager.content.LocalBroadcastManager;
+class MainFragment : WebViewFragment() {

-public class MainFragment extends WebViewFragment {
-
-    public final static String EVENT_LOGIN = "eventLogin";
-    public final static String EVENT_TOKEN = "eventToken";
-    public final static String KEY_TOKEN = "keyToken";
-
-    private static final int REQUEST_PERMISSIONS_LOCATION = 1;
-    private final static int REQUEST_FILE_CHOOSER = 1;
-
-    private LocalBroadcastManager broadcastManager;
-
-    public class AppInterface {
+    private lateinit var broadcastManager: LocalBroadcastManager

+    inner class AppInterface {
        @JavascriptInterface
-        public void postMessage(String message) {
+        fun postMessage(message: String) {
            if (message.contains("login")) {
-                broadcastManager.sendBroadcast(new Intent(EVENT_LOGIN));
+                broadcastManager.sendBroadcast(Intent(EVENT_LOGIN))
            }
        }
-
    }

-    @Override
-    public void onCreate(@Nullable Bundle savedInstanceState) {
-        super.onCreate(savedInstanceState);
-        broadcastManager = LocalBroadcastManager.getInstance(getActivity());
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        broadcastManager = LocalBroadcastManager.getInstance(activity)
    }

    @SuppressLint("SetJavaScriptEnabled")
-    @Override
-    public void onViewCreated(View view, Bundle savedInstanceState) {
-        super.onViewCreated(view, savedInstanceState);
+    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
+        super.onViewCreated(view, savedInstanceState)

-        if ((getActivity().getApplicationInfo().flags &= ApplicationInfo.FLAG_DEBUGGABLE) != 0) {
-            WebView.setWebContentsDebuggingEnabled(true);
+        if ((activity.applicationInfo.flags and ApplicationInfo.FLAG_DEBUGGABLE) != 0) {
+            WebView.setWebContentsDebuggingEnabled(true)
        }
-
-        getWebView().setWebChromeClient(webChromeClient);
-        getWebView().addJavascriptInterface(new AppInterface(), "appInterface");
-
-        WebSettings webSettings = getWebView().getSettings();
-        webSettings.setJavaScriptEnabled(true);
-        webSettings.setDomStorageEnabled(true);
-        webSettings.setDatabaseEnabled(true);
-        webSettings.setMediaPlaybackRequiresUserGesture(false);
-
-        String url = PreferenceManager.getDefaultSharedPreferences(
-                getActivity()).getString(MainActivity.PREFERENCE_URL, null);
-
-        getWebView().loadUrl(url);
+        webView.webChromeClient = webChromeClient
+        webView.addJavascriptInterface(AppInterface(), "appInterface")
+        val webSettings = webView.settings
+        webSettings.javaScriptEnabled = true
+        webSettings.domStorageEnabled = true
+        webSettings.databaseEnabled = true
+        webSettings.mediaPlaybackRequiresUserGesture = false
+        val url = PreferenceManager.getDefaultSharedPreferences(activity)
+            .getString(MainActivity.PREFERENCE_URL, null)
+        url?.let { webView.loadUrl(it) }
    }

-    private final BroadcastReceiver broadcastReceiver = new BroadcastReceiver() {
-        @Override
-        public void onReceive(Context context, Intent intent) {
-            String token = intent.getStringExtra(KEY_TOKEN);
-            String code = "updateNotificationToken && updateNotificationToken('" + token + "')";
-            getWebView().evaluateJavascript(code, null);
+    private val broadcastReceiver: BroadcastReceiver = object : BroadcastReceiver() {
+        override fun onReceive(context: Context, intent: Intent) {
+            val token = intent.getStringExtra(KEY_TOKEN)
+            val code = "updateNotificationToken && updateNotificationToken('$token')"
+            webView.evaluateJavascript(code, null)
        }
-    };
-
-    @Override
-    public void onStart() {
-        super.onStart();
-        IntentFilter intentFilter = new IntentFilter(EVENT_TOKEN);
-        broadcastManager.registerReceiver(broadcastReceiver, intentFilter);
    }

-    @Override
-    public void onStop() {
-        super.onStop();
-        broadcastManager.unregisterReceiver(broadcastReceiver);
+    override fun onStart() {
+        super.onStart()
+        val intentFilter = IntentFilter(EVENT_TOKEN)
+        broadcastManager.registerReceiver(broadcastReceiver, intentFilter)
    }

-    private ValueCallback<Uri> openFileCallback;
-    private ValueCallback<Uri[]> openFileCallback2;
+    override fun onStop() {
+        super.onStop()
+        broadcastManager.unregisterReceiver(broadcastReceiver)
+    }

-    @Override
-    public void onActivityResult(int requestCode, int resultCode, Intent data) {
+    private var openFileCallback: ValueCallback<Uri?>? = null
+    private var openFileCallback2: ValueCallback<Array<Uri>>? = null
+
+    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent) {
        if (requestCode == REQUEST_FILE_CHOOSER) {
-            Uri result = data == null || resultCode != Activity.RESULT_OK ? null : data.getData();
+            val result = if (resultCode != Activity.RESULT_OK) null else data.data
            if (openFileCallback != null) {
-                openFileCallback.onReceiveValue(result);
-                openFileCallback = null;
+                openFileCallback?.onReceiveValue(result)
+                openFileCallback = null
            }
            if (openFileCallback2 != null) {
-                openFileCallback2.onReceiveValue(result != null ? new Uri[] { result } : new Uri[0]);
-                openFileCallback2 = null;
+                openFileCallback2?.onReceiveValue(if (result != null) arrayOf(result) else arrayOf())
+                openFileCallback2 = null
            }
        }
    }

-    @Override
-    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
+    override fun onRequestPermissionsResult(requestCode: Int, permissions: Array<String>, grantResults: IntArray) {
        if (requestCode == REQUEST_PERMISSIONS_LOCATION) {
-            boolean granted = grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED;
+            val granted = grantResults.isNotEmpty() && grantResults[0] == PackageManager.PERMISSION_GRANTED
            if (geolocationCallback != null) {
-                geolocationCallback.invoke(geolocationRequestOrigin, granted, false);
-                geolocationRequestOrigin = null;
-                geolocationCallback = null;
+                geolocationCallback?.invoke(geolocationRequestOrigin, granted, false)
+                geolocationRequestOrigin = null
+                geolocationCallback = null
            }
        }
    }

-    private String geolocationRequestOrigin;
-    private GeolocationPermissions.Callback geolocationCallback;
+    private var geolocationRequestOrigin: String? = null
+    private var geolocationCallback: GeolocationPermissions.Callback? = null

-    private final WebChromeClient webChromeClient = new WebChromeClient() {
+    private val webChromeClient: WebChromeClient = object : WebChromeClient() {

-        @Override
-        public void onGeolocationPermissionsShowPrompt(String origin, GeolocationPermissions.Callback callback) {
-            geolocationRequestOrigin = null;
-            geolocationCallback = null;
-            if (ContextCompat.checkSelfPermission(getActivity(), Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
-                if (ActivityCompat.shouldShowRequestPermissionRationale(getActivity(), Manifest.permission.ACCESS_FINE_LOCATION)) {
-                    new AlertDialog.Builder(getActivity())
-                            .setMessage(R.string.permission_location_rationale)
-                            .setNeutralButton(android.R.string.ok, (dialog, which) -> {
-                                geolocationRequestOrigin = origin;
-                                geolocationCallback = callback;
-                                ActivityCompat.requestPermissions(
-                                        getActivity(), new String[] { Manifest.permission.ACCESS_FINE_LOCATION }, REQUEST_PERMISSIONS_LOCATION);
-                            })
-                            .show();
+        override fun onGeolocationPermissionsShowPrompt(origin: String, callback: GeolocationPermissions.Callback) {
+            geolocationRequestOrigin = null
+            geolocationCallback = null
+            if (ContextCompat.checkSelfPermission(activity, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
+                if (ActivityCompat.shouldShowRequestPermissionRationale(activity, Manifest.permission.ACCESS_FINE_LOCATION)) {
+                    AlertDialog.Builder(activity)
+                        .setMessage(R.string.permission_location_rationale)
+                        .setNeutralButton(android.R.string.ok) { dialog: DialogInterface?, which: Int ->
+                            geolocationRequestOrigin = origin
+                            geolocationCallback = callback
+                            ActivityCompat.requestPermissions(
+                                activity,
+                                arrayOf(Manifest.permission.ACCESS_FINE_LOCATION),
+                                REQUEST_PERMISSIONS_LOCATION
+                            )
+                        }
+                        .show()
                } else {
-                    geolocationRequestOrigin = origin;
-                    geolocationCallback = callback;
+                    geolocationRequestOrigin = origin
+                    geolocationCallback = callback
                    ActivityCompat.requestPermissions(
-                            getActivity(), new String[] { Manifest.permission.ACCESS_FINE_LOCATION }, REQUEST_PERMISSIONS_LOCATION);
+                        activity,
+                        arrayOf(Manifest.permission.ACCESS_FINE_LOCATION),
+                        REQUEST_PERMISSIONS_LOCATION
+                    )
                }
            } else {
-                callback.invoke(origin, true, false);
+                callback.invoke(origin, true, false)
            }
        }

        // Android 4.1+
-        protected void openFileChooser(ValueCallback<Uri> uploadMessage, String acceptType, String capture) {
-            openFileChooser(uploadMessage);
-        }
-
-        protected void openFileChooser(ValueCallback<Uri> uploadMessage) {
-            MainFragment.this.openFileCallback = uploadMessage;
-            Intent intent = new Intent(Intent.ACTION_GET_CONTENT);
-            intent.addCategory(Intent.CATEGORY_OPENABLE);
-            intent.setType("*/*");
-            startActivityForResult(Intent.createChooser(intent, getString(R.string.file_browser)), REQUEST_FILE_CHOOSER);
+        fun openFileChooser(uploadMessage: ValueCallback<Uri?>?, acceptType: String?, capture: String?) {
+            openFileCallback = uploadMessage
+            val intent = Intent(Intent.ACTION_GET_CONTENT)
+            intent.addCategory(Intent.CATEGORY_OPENABLE)
+            intent.type = "*/*"
+            startActivityForResult(
+                Intent.createChooser(intent, getString(R.string.file_browser)),
+                REQUEST_FILE_CHOOSER
+            )
        }

        // Android 5.0+
-        public boolean onShowFileChooser(WebView mWebView, ValueCallback<Uri[]> filePathCallback, WebChromeClient.FileChooserParams fileChooserParams) {
-            if (openFileCallback2 != null) {
-                openFileCallback2.onReceiveValue(null);
-                openFileCallback2 = null;
-            }
-
-            openFileCallback2 = filePathCallback;
+        override fun onShowFileChooser(
+            mWebView: WebView,
+            filePathCallback: ValueCallback<Array<Uri>>,
+            fileChooserParams: FileChooserParams
+        ): Boolean {
+            openFileCallback2?.onReceiveValue(null)
+            openFileCallback2 = filePathCallback
            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
-                Intent intent = fileChooserParams.createIntent();
+                val intent = fileChooserParams.createIntent()
                try {
-                    startActivityForResult(intent, REQUEST_FILE_CHOOSER);
-                } catch (ActivityNotFoundException e) {
-                    openFileCallback2 = null;
-                    return false;
+                    startActivityForResult(intent, REQUEST_FILE_CHOOSER)
+                } catch (e: ActivityNotFoundException) {
+                    openFileCallback2 = null
+                    return false
                }
            }
-            return true;
+            return true
        }
+    }

-    };
-
+    companion object {
+        const val EVENT_LOGIN = "eventLogin"
+        const val EVENT_TOKEN = "eventToken"
+        const val KEY_TOKEN = "keyToken"
+        private const val REQUEST_PERMISSIONS_LOCATION = 1
+        private const val REQUEST_FILE_CHOOSER = 1
+    }
 }
@@ -1,143 +0,0 @@
-/*
- * Copyright 2016 - 2021 Anton Tananaev (anton@traccar.org)
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package org.traccar.manager;
-
-import android.annotation.SuppressLint;
-import android.app.AlertDialog;
-import android.app.Fragment;
-import android.content.DialogInterface;
-import android.net.Uri;
-import android.os.AsyncTask;
-import android.os.Bundle;
-import android.preference.PreferenceManager;
-import android.util.Log;
-import android.view.LayoutInflater;
-import android.view.View;
-import android.view.ViewGroup;
-import android.widget.Button;
-import android.widget.EditText;
-
-import org.json.JSONException;
-import org.json.JSONObject;
-
-import java.io.BufferedReader;
-import java.io.IOException;
-import java.io.InputStreamReader;
-import java.net.HttpURLConnection;
-import java.net.URL;
-
-public class StartFragment extends Fragment implements View.OnClickListener {
-
-    private static final String TAG = StartFragment.class.getSimpleName();
-
-    private static final int MAX_REDIRECTS = 5;
-
-    private EditText serverField;
-    private Button startButton;
-
-    @Override
-    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
-        View view = inflater.inflate(R.layout.fragment_start, container, false);
-        serverField = (EditText) view.findViewById(R.id.field_server);
-        startButton = (Button) view.findViewById(R.id.button_start);
-        startButton.setOnClickListener(this);
-        return view;
-    }
-
-    @SuppressLint("StaticFieldLeak")
-    @Override
-    public void onClick(View view) {
-        startButton.setEnabled(false);
-
-        new AsyncTask<String, Void, Boolean>() {
-
-            @Override
-            protected Boolean doInBackground(String... urls) {
-                try {
-
-                    Uri uri = Uri.parse(urls[0]).buildUpon().appendEncodedPath("api/server").build();
-                    String url = uri.toString();
-                    HttpURLConnection urlConnection = null;
-
-                    for (int i = 0; i < MAX_REDIRECTS; i++) {
-                        URL resourceUrl = new URL(url);
-                        urlConnection = (HttpURLConnection) resourceUrl.openConnection();
-                        urlConnection.setInstanceFollowRedirects(false);
-
-                        switch (urlConnection.getResponseCode()) {
-                            case HttpURLConnection.HTTP_MOVED_PERM:
-                            case HttpURLConnection.HTTP_MOVED_TEMP:
-                                url = urlConnection.getHeaderField("Location");
-                                continue;
-                        }
-
-                        break;
-                    }
-
-                    BufferedReader reader = new BufferedReader(new InputStreamReader(urlConnection.getInputStream()));
-
-                    String line;
-                    StringBuilder responseBuilder = new StringBuilder();
-                    while ((line = reader.readLine()) != null) {
-                        responseBuilder.append(line);
-                    }
-
-                    new JSONObject(responseBuilder.toString());
-
-                    return true;
-
-                } catch (IOException | JSONException e) {
-                    Log.w(TAG, e);
-                }
-                return false;
-            }
-
-            @Override
-            protected void onPostExecute(Boolean result) {
-                if (getActivity() != null) {
-                    if (result) {
-                        onSuccess();
-                    } else {
-                        onError();
-                    }
-                }
-            }
-
-        }.execute(serverField.getText().toString());
-    }
-
-    private void onSuccess() {
-        PreferenceManager.getDefaultSharedPreferences(getActivity())
-                .edit().putString(MainActivity.PREFERENCE_URL, serverField.getText().toString()).apply();
-        getActivity().getFragmentManager()
-                .beginTransaction().replace(android.R.id.content, new MainFragment()).commitAllowingStateLoss();
-    }
-
-    private void onError() {
-        startButton.setEnabled(true);
-
-        AlertDialog alertDialog = new AlertDialog.Builder(getActivity()).create();
-        alertDialog.setMessage(getString(R.string.error_connection));
-        alertDialog.setButton(AlertDialog.BUTTON_NEUTRAL, getString(android.R.string.ok),
-                new DialogInterface.OnClickListener() {
-                    public void onClick(DialogInterface dialog, int which) {
-                        dialog.dismiss();
-                    }
-                });
-        alertDialog.show();
-    }
-
-}
@@ -0,0 +1,123 @@
+/*
+ * Copyright 2016 - 2021 Anton Tananaev (anton@traccar.org)
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+@file:Suppress("DEPRECATION")
+package org.traccar.manager
+
+import android.annotation.SuppressLint
+import android.app.AlertDialog
+import android.app.Fragment
+import android.net.Uri
+import android.os.AsyncTask
+import android.os.Bundle
+import android.util.Log
+import android.view.LayoutInflater
+import android.view.View
+import android.view.ViewGroup
+import android.widget.Button
+import android.widget.EditText
+import androidx.preference.PreferenceManager
+import org.json.JSONException
+import org.json.JSONObject
+import org.traccar.manager.StartFragment
+import java.io.BufferedReader
+import java.io.IOException
+import java.io.InputStreamReader
+import java.net.HttpURLConnection
+import java.net.URL
+
+class StartFragment : Fragment(), View.OnClickListener {
+
+    private lateinit var serverField: EditText
+    private lateinit var startButton: Button
+
+    override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? {
+        val view = inflater.inflate(R.layout.fragment_start, container, false)
+        serverField = view.findViewById(R.id.field_server)
+        startButton = view.findViewById(R.id.button_start)
+        startButton.setOnClickListener(this)
+        return view
+    }
+
+    @SuppressLint("StaticFieldLeak")
+    override fun onClick(view: View) {
+        startButton.isEnabled = false
+        object : AsyncTask<String, Unit, Boolean>() {
+            override fun doInBackground(vararg urls: String): Boolean {
+                try {
+                    val uri = Uri.parse(urls[0]).buildUpon().appendEncodedPath("api/server").build()
+                    var url = uri.toString()
+                    var urlConnection: HttpURLConnection? = null
+                    for (i in 0 until MAX_REDIRECTS) {
+                        val resourceUrl = URL(url)
+                        urlConnection = resourceUrl.openConnection() as HttpURLConnection
+                        urlConnection.instanceFollowRedirects = false
+                        when (urlConnection.responseCode) {
+                            HttpURLConnection.HTTP_MOVED_PERM, HttpURLConnection.HTTP_MOVED_TEMP -> {
+                                url = urlConnection.getHeaderField("Location")
+                                continue
+                            }
+                        }
+                        break
+                    }
+                    val reader = BufferedReader(InputStreamReader(urlConnection?.inputStream))
+                    var line: String?
+                    val responseBuilder = StringBuilder()
+                    while (reader.readLine().also { line = it } != null) {
+                        responseBuilder.append(line)
+                    }
+                    JSONObject(responseBuilder.toString())
+                    return true
+                } catch (e: IOException) {
+                    Log.w(TAG, e)
+                } catch (e: JSONException) {
+                    Log.w(TAG, e)
+                }
+                return false
+            }
+
+            override fun onPostExecute(result: Boolean) {
+                if (activity != null) {
+                    if (result) {
+                        onSuccess()
+                    } else {
+                        onError()
+                    }
+                }
+            }
+        }.execute(serverField.text.toString())
+    }
+
+    private fun onSuccess() {
+        PreferenceManager.getDefaultSharedPreferences(activity)
+            .edit().putString(MainActivity.PREFERENCE_URL, serverField.text.toString()).apply()
+        activity.fragmentManager
+            .beginTransaction().replace(android.R.id.content, MainFragment())
+            .commitAllowingStateLoss()
+    }
+
+    private fun onError() {
+        startButton.isEnabled = true
+        val alertDialog = AlertDialog.Builder(activity).create()
+        alertDialog.setMessage(getString(R.string.error_connection))
+        alertDialog.setButton(AlertDialog.BUTTON_NEUTRAL, getString(android.R.string.ok)) { dialog, _ -> dialog.dismiss() }
+        alertDialog.show()
+    }
+
+    companion object {
+        private val TAG = StartFragment::class.java.simpleName
+        private const val MAX_REDIRECTS = 5
+    }
+}