Add configurable behavior of the back button

This PR allows users to change how the back button functions. Closes #2196

Possible choices are following:

 - **Default** - back button functions how it currently functions (closes the app if there is nowhere to go back to)

 - **Open navigation drawer** - back button always opens the navigation drawer instead of closing the app

 - **Double tap to exit** - like default, but requires two taps to close the app

 - **Confirm to exit** - like default, but prompts user if they really want to exit

app/src/main/java/de/danoeh/antennapod/activity/MainActivity.java

@@ -27,6 +27,7 @@ import android.view.MenuItem;
 import android.view.View;
 import android.widget.AdapterView;
 import android.widget.ListView;
+import android.widget.Toast;
 
 import com.bumptech.glide.Glide;
 
@@ -123,6 +124,8 @@ public class MainActivity extends CastEnabledActivity implements NavDrawerActivi
 
     private Subscription subscription;
 
+    private long lastBackButtonPressTime = 0;
+
     @Override
     public void onCreate(Bundle savedInstanceState) {
         setTheme(UserPreferences.getNoTitleTheme());
@@ -645,9 +648,35 @@ public class MainActivity extends CastEnabledActivity implements NavDrawerActivi
     public void onBackPressed() {
         if(isDrawerOpen()) {
             drawerLayout.closeDrawer(navDrawer);
+        } else {
+            switch (UserPreferences.getBackButtonBehavior()) {
+                case OPEN_DRAWER:
+                    drawerLayout.openDrawer(navDrawer);
+                    break;
+                case SHOW_PROMPT:
+                    new AlertDialog.Builder(this)
+                        .setMessage(R.string.close_prompt)
+                        .setPositiveButton(R.string.yes, new DialogInterface.OnClickListener() {
+                            @Override
+                            public void onClick(DialogInterface dialogInterface, int i) {
+                                MainActivity.super.onBackPressed();
+                            }
+                        })
+                        .setNegativeButton(R.string.no, null)
+                        .setCancelable(false)
+                        .show();
+                    break;
+                case DOUBLE_TAP:
+                    if(lastBackButtonPressTime < System.currentTimeMillis() - 2000) {
+                        Toast.makeText(this, R.string.double_tap_toast, Toast.LENGTH_SHORT).show();
+                        lastBackButtonPressTime = System.currentTimeMillis();
                     } else {
                         super.onBackPressed();
                     }
+                    break;
+                default: super.onBackPressed();
+            }
+        }
     }
 
     private DBReader.NavDrawerData navDrawerData;

app/src/main/res/xml/preferences_user_interface.xml

@@ -57,4 +57,14 @@
                 android:summary="@string/pref_lockscreen_background_sum"
                 android:title="@string/pref_lockscreen_background_title"/>
     </PreferenceCategory>
+    <PreferenceCategory android:title="@string/behavior">
+        <ListPreference
+                android:entryValues="@array/back_button_behavior_values"
+                android:entries="@array/back_button_behavior_options"
+                android:key="prefBackButtonBehavior"
+                android:title="@string/pref_back_button_behavior_title"
+                android:summary="@string/pref_back_button_behavior_sum"
+                android:defaultValue="default"
+                app:useStockLayout="true"/>
+    </PreferenceCategory>
 </PreferenceScreen>

core/src/main/java/de/danoeh/antennapod/core/preferences/UserPreferences.java

@@ -52,6 +52,7 @@ public class UserPreferences {
     public static final String PREF_COMPACT_NOTIFICATION_BUTTONS = "prefCompactNotificationButtons";
     public static final String PREF_LOCKSCREEN_BACKGROUND = "prefLockscreenBackground";
     private static final String PREF_SHOW_DOWNLOAD_REPORT = "prefShowDownloadReport";
+    private static final String PREF_BACK_BUTTON_BEHAVIOR = "prefBackButtonBehavior";
 
     // Queue
     private static final String PREF_QUEUE_ADD_TO_FRONT = "prefQueueAddToFront";
@@ -809,4 +810,18 @@ public class UserPreferences {
     public enum VideoBackgroundBehavior {
         STOP, PICTURE_IN_PICTURE, CONTINUE_PLAYING
     }
+
+    public enum BackButtonBehavior {
+        DEFAULT, OPEN_DRAWER, DOUBLE_TAP, SHOW_PROMPT
+    }
+
+    public static BackButtonBehavior getBackButtonBehavior() {
+        switch (prefs.getString(PREF_BACK_BUTTON_BEHAVIOR, "default")) {
+            case "default": return BackButtonBehavior.DEFAULT;
+            case "drawer": return BackButtonBehavior.OPEN_DRAWER;
+            case "doubletap": return BackButtonBehavior.DOUBLE_TAP;
+            case "prompt": return BackButtonBehavior.SHOW_PROMPT;
+            default: return BackButtonBehavior.DEFAULT;
+        }
+    }
 }

core/src/main/res/values/arrays.xml

@@ -277,4 +277,18 @@
         <item>@string/select_all_above</item>
         <item>@string/select_all_below</item>
     </string-array>
+
+    <string-array name="back_button_behavior_options">
+        <item>@string/back_button_default</item>
+        <item>@string/back_button_open_drawer</item>
+        <item>@string/back_button_double_tap</item>
+        <item>@string/back_button_show_prompt</item>
+    </string-array>
+
+    <string-array name="back_button_behavior_values">
+        <item>default</item>
+        <item>drawer</item>
+        <item>doubletap</item>
+        <item>prompt</item>
+    </string-array>
 </resources>

core/src/main/res/values/strings.xml

@@ -470,6 +470,15 @@
     <string name="pref_videoBehavior_sum">Behavior when leaving video playback</string>
     <string name="stop_playback">Stop playback</string>
     <string name="continue_playback">Continue audio playback</string>
+    <string name="behavior">Behavior</string>
+    <string name="pref_back_button_behavior_title">Back button behavior</string>
+    <string name="pref_back_button_behavior_sum">Change behavior of the back button.</string>
+    <string name="back_button_default">Default</string>
+    <string name="back_button_open_drawer">Open navigation drawer</string>
+    <string name="back_button_double_tap">Double tap to exit</string>
+    <string name="back_button_show_prompt">Confirm to exit</string>
+    <string name="close_prompt">Are you sure you want to close AntennaPod?</string>
+    <string name="double_tap_toast">Tap back button again to exit</string>
 
     <!-- Auto-Flattr dialog -->
     <string name="auto_flattr_enable">Enable automatic flattring</string>