diff --git a/IcarusAndroid/app/src/main/java/framework/everywaretechnologies/it/icarusandroid/MainActivity.java b/IcarusAndroid/app/src/main/java/framework/everywaretechnologies/it/icarusandroid/MainActivity.java
index e859de570cd17c0725e29bacb183ca4680e35c35..ed5d14de47aa20685df8b3b1307c59514f5bd67d 100644
--- a/IcarusAndroid/app/src/main/java/framework/everywaretechnologies/it/icarusandroid/MainActivity.java
+++ b/IcarusAndroid/app/src/main/java/framework/everywaretechnologies/it/icarusandroid/MainActivity.java
@@ -29,8 +29,8 @@ public class MainActivity extends AppCompatActivity {
     private Icarus instance;
 
     private EditText eventNameEditText;
-    private EditText eventCategoryEditText;
-    private EditText eventLabelEditText;
+    Switch eventCategorySwitch;
+    Switch eventLabelSwitch;
     private EditText mostProbableActivityEditText;
 
     @Override
@@ -61,8 +61,8 @@ public class MainActivity extends AppCompatActivity {
         else intPrefSwitch.setChecked(true);
 
         eventNameEditText=findViewById(R.id.editText_eventName);
-        eventCategoryEditText=findViewById(R.id.editText_eventCategory);
-        eventLabelEditText=findViewById(R.id.editText_eventLabel);
+        eventCategorySwitch=findViewById(R.id.event_category_switch);
+        eventLabelSwitch=findViewById(R.id.event_label_switch);
         mostProbableActivityEditText=findViewById(R.id.editText_mostProbableActivity);
     }
 
@@ -106,16 +106,18 @@ public class MainActivity extends AppCompatActivity {
 
         Log.d(CLASS_NAME,"on btn track event");
 
-        String eventName,eventCategory,eventLabel,mostProbableActivity;
+        String eventName,mostProbableActivity;
+        EventTrackResource.EventCategory eventCategory;
+        EventTrackResource.EventLabel eventLabel;
 
         if(eventNameEditText.getText().length() == 0) eventName="eventoProva";
         else eventName=eventNameEditText.getText().toString();
 
-        if(eventCategoryEditText.getText().length()==0) eventCategory=EventTrackResource.EventCategories.APPLICATION.name().toLowerCase();
-        else eventCategory=eventCategoryEditText.getText().toString();
+        if(!eventCategorySwitch.isChecked()) eventCategory=EventTrackResource.EventCategory.APPLICATION;
+        else eventCategory=EventTrackResource.EventCategory.ACTION;
 
-        if(eventLabelEditText.getText().length()==0) eventLabel = EventTrackResource.EventLabel.FOREGROUND.name().toLowerCase();
-        else eventLabel=eventLabelEditText.getText().toString();
+        if(!eventLabelSwitch.isChecked()) eventLabel = EventTrackResource.EventLabel.FOREGROUND;
+        else eventLabel=EventTrackResource.EventLabel.BACKGROUND;
 
         if(mostProbableActivityEditText.getText().length()==0) mostProbableActivity="running";
         else mostProbableActivity=mostProbableActivityEditText.getText().toString();
diff --git a/IcarusAndroid/app/src/main/res/layout/activity_main.xml b/IcarusAndroid/app/src/main/res/layout/activity_main.xml
index 4224fa3a485cbc500bd8deb7c455d5c04ecc62bc..3a4a0b09bb4e56ecb8ab6b3ca46942db7b874e6b 100644
--- a/IcarusAndroid/app/src/main/res/layout/activity_main.xml
+++ b/IcarusAndroid/app/src/main/res/layout/activity_main.xml
@@ -75,6 +75,14 @@
         android:text="Invia pref changes" />
 
 
+    <TextView
+        android:id="@+id/textView"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:text="Track event"
+        android:textColor="@android:color/holo_red_light"
+        android:textSize="24sp" />
+
     <EditText
         android:id="@+id/editText_eventName"
         android:layout_width="match_parent"
@@ -83,28 +91,35 @@
         android:hint="event name"
         android:inputType="textPersonName" />
 
-    <EditText
-        android:id="@+id/editText_eventCategory"
+    <Switch
+        android:id="@+id/event_category_switch"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:ems="18"
-        android:hint="event category"
-        android:inputType="textPersonName" />
+        android:showText="true"
+        android:splitTrack="false"
+        android:text="Event category"
+        android:textOff="application"
+        android:textOn="action"
+        android:textSize="18sp" />
 
-    <EditText
-        android:id="@+id/editText_eventLabel"
+    <Switch
+        android:id="@+id/event_label_switch"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        android:ems="18"
-        android:hint="event label"
-        android:inputType="textPersonName" />
+        android:showText="true"
+        android:splitTrack="false"
+        android:text="Event label"
+        android:textOff="foreground"
+        android:textOn="background"
+        android:textSize="18sp" />
+
 
     <EditText
         android:id="@+id/editText_mostProbableActivity"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
         android:ems="18"
-        android:hint="most probable activity"
+        android:hint="activity: walking or cycling. empty for running"
         android:inputType="textPersonName" />
 
     <Button
diff --git a/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/EventTrackResource.java b/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/EventTrackResource.java
index 8207ffc1f87155ff92af7a7e146c99172517c222..cfdec2851d7c6d4f52f6e77ac2ea4b7ab6566409 100644
--- a/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/EventTrackResource.java
+++ b/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/EventTrackResource.java
@@ -5,7 +5,22 @@ import org.json.JSONObject;
 
 public class EventTrackResource extends IcarusResource {
 
-    public enum EventCategories{ //categorie di evento
+    //------------ JSON KEYS --------------------
+    private final static String CONFIDENCE_KEY="confidence";
+    private final static String START_TIMESTAMP_KEY="startTimestamp";
+    private final static String HEADING_KEY="heading";
+    private final static String COURSE_KEY="course";
+    private final static String SPEED_KEY="speed";
+    private final static String ACTIVITY_KEY="activity";
+    private final static String DIRECTION_KEY="direction";
+    private final static String EVENT_NAME_KEY="EventName";
+    private final static String EVENT_CATEGORY_KEY="EventCategory";
+    private final static String EVENT_LABEL_KEY="EventLabel";
+    private final static String EVENT_CONTEXT_KEY="EventContext";
+
+    //-----------------------------------------------------
+
+    public enum EventCategory{ //categorie di evento
 
         APPLICATION,ACTION,NOTIFICATION;
     }
@@ -16,8 +31,8 @@ public class EventTrackResource extends IcarusResource {
     }
 
     private String eventName;
-    private String eventCategory;
-    private String eventLabel;
+    private EventCategory eventCategory;
+    private EventLabel eventLabel;
 
     private int heading;
     private int course;
@@ -25,7 +40,7 @@ public class EventTrackResource extends IcarusResource {
 
     //i due successivi attributi sono quelli ritornati dall'ActivityRecognition di Google che dev'essere implementata nell'app chiamante
     //convertiti da quelli di Google a quelli del server.. vedi iMoveAndroid per il metodo di mapping tra i valori ritornati da Google e quelli
-    //per il server di ewtech. Fare copia incolla del metodo in progetti futuri. Non lo metto qua perchè non ho accesso ai codici delle attività
+    //per il server di ewtech. Fare copia incolla del metodo in progetti futuri. Non lo metto qua perchè da qua non ho accesso ai codici delle attività
     //non avendo la dipendenza all'ActivityRecognition all'interno di questa libreria
     private String mostProbableUserActivityName;
     private int confidence;
@@ -34,7 +49,7 @@ public class EventTrackResource extends IcarusResource {
 
     private String[] userActivityToTrack;
 
-    public EventTrackResource(String _eventName, String _eventCategory, String _eventLabel, int _heading, int _course, int _speed,
+    public EventTrackResource(String _eventName, EventCategory _eventCategory, EventLabel _eventLabel, int _heading, int _course, int _speed,
                               String _mostProbableUserActivityName, int _confidence, boolean _accessibilityEnabled){
 
         super();
@@ -66,8 +81,8 @@ public class EventTrackResource extends IcarusResource {
                     userActivityJSONObject.put(activityType,false);
 
             }
-            userActivityJSONObject.put("confidence",confidence);
-            userActivityJSONObject.put("startTimestamp",System.currentTimeMillis());
+            userActivityJSONObject.put(CONFIDENCE_KEY,confidence);
+            userActivityJSONObject.put(START_TIMESTAMP_KEY,System.currentTimeMillis());
 
         } catch (JSONException jsonException){
 
@@ -78,8 +93,8 @@ public class EventTrackResource extends IcarusResource {
 
         try {
 
-            directionJSONObject.put("heading",heading); //0..360
-            directionJSONObject.put("course",course); //0..360
+            directionJSONObject.put(HEADING_KEY,heading); //0..360
+            directionJSONObject.put(COURSE_KEY,course); //0..360
 
         } catch (JSONException jsonException){
 
@@ -90,9 +105,9 @@ public class EventTrackResource extends IcarusResource {
 
         try {
 
-            eventContextJSONObject.put("speed",speed);
-            eventContextJSONObject.put("activity",userActivityJSONObject);
-            eventContextJSONObject.put("direction",directionJSONObject);
+            eventContextJSONObject.put(SPEED_KEY,speed);
+            eventContextJSONObject.put(ACTIVITY_KEY,userActivityJSONObject);
+            eventContextJSONObject.put(DIRECTION_KEY,directionJSONObject);
 
         } catch (JSONException jsonException){
 
@@ -103,10 +118,10 @@ public class EventTrackResource extends IcarusResource {
 
         try {
 
-            toSend.put("EventName",eventName);
-            toSend.put("EventCategory",eventCategory);
-            toSend.put("EventLabel",eventLabel+"_TalkBack"+(accessibilityEnabled?"_ON":"_OFF"));
-            toSend.put("EventContext",eventContextJSONObject);
+            toSend.put(EVENT_NAME_KEY,eventName);
+            toSend.put(EVENT_CATEGORY_KEY,eventCategory.name().toLowerCase());
+            toSend.put(EVENT_LABEL_KEY,eventLabel.name().toLowerCase()+"_TalkBack"+(accessibilityEnabled?"_ON":"_OFF"));
+            toSend.put(EVENT_CONTEXT_KEY,eventContextJSONObject);
 
         } catch (JSONException jsonException){
 
diff --git a/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/Icarus.java b/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/Icarus.java
index d54083ee1ca765922b84379e7b2214de8cd814da..64867546faa45fbdca5f86b654df171beb082392 100644
--- a/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/Icarus.java
+++ b/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/Icarus.java
@@ -40,6 +40,25 @@ public class Icarus {
 
     private final static String ICARUS_URL="https://webdev.ewlab.di.unimi.it/icarus/str";
 
+    //---------------- JSON KEYS ---------------------
+    private final static String UTC_KEY="utc";
+    private final static String USER_KEY="user";
+    private final static String LANG_KEY="lang";
+    private final static String ICARUS_VERSION_KEY="icarusversion";
+    private final static String APP_NAME_KEY="appname";
+    private final static String APP_VERSION_KEY="appversion";
+    private final static String OS_VERSION_KEY="osversion";
+    private final static String PLATFORM_KEY="platform";
+    private final static String DEVICE_KEY="device";
+    private final static String UUID_KEY="uuid";
+    private final static String TALKBACK_KEY="talkback";
+    private final static String DEBUG_KEY="debug";
+    private final static String TIMESTAMP_KEY="timestamp";
+    private final static String APP_DATA_KEY="appdata";
+    private final static String USER_DATA_KEY="userdata";
+
+    //------------------------------------------
+
     private static final Icarus ourInstance = new Icarus();
     private Queue<JSONObject> pendingResources; //FIFO
     private Lock pendingResourceLock;
@@ -190,8 +209,8 @@ public class Icarus {
             long userTime=utcTime+timeOffsetBetweenMeAndGMT;
 
             JSONObject timeJSON=new JSONObject();
-            timeJSON.put("utc",utcTime);
-            timeJSON.put("user",userTime);
+            timeJSON.put(UTC_KEY,utcTime);
+            timeJSON.put(USER_KEY,userTime);
 
             //---------------------- APP JSON ---------------------
 
@@ -238,22 +257,22 @@ public class Icarus {
             }
 
             JSONObject appInfoJSON=new JSONObject();
-            appInfoJSON.put("lang", Locale.getDefault().getLanguage());
-            appInfoJSON.put("icarusversion","Icarus:"+curContext.getResources().getInteger(R.integer.lib_version));
-            appInfoJSON.put("appname",appName.toString());
-            appInfoJSON.put("appversion",clientAppVersion);
-            appInfoJSON.put("osversion", Build.VERSION.SDK_INT);
-            appInfoJSON.put("platform","android");
-            appInfoJSON.put("device",getDeviceName());
-            appInfoJSON.put("uuid",uuid);
-            appInfoJSON.put("talkback",accessibilityEnabled);
+            appInfoJSON.put(LANG_KEY, Locale.getDefault().getLanguage());
+            appInfoJSON.put(ICARUS_VERSION_KEY,"Icarus:"+curContext.getResources().getInteger(R.integer.lib_version));
+            appInfoJSON.put(APP_NAME_KEY,appName.toString());
+            appInfoJSON.put(APP_VERSION_KEY,clientAppVersion);
+            appInfoJSON.put(OS_VERSION_KEY, Build.VERSION.SDK_INT);
+            appInfoJSON.put(PLATFORM_KEY,"android");
+            appInfoJSON.put(DEVICE_KEY,getDeviceName());
+            appInfoJSON.put(UUID_KEY,uuid);
+            appInfoJSON.put(TALKBACK_KEY,accessibilityEnabled);
 
             //---------------- GENERAL JSON --------------------
 
-            toSend.put("debug",debug);
-            toSend.put("timestamp",timeJSON);
-            toSend.put("appdata",appInfoJSON);
-            toSend.put("userdata",userData);
+            toSend.put(DEBUG_KEY,debug);
+            toSend.put(TIMESTAMP_KEY,timeJSON);
+            toSend.put(APP_DATA_KEY,appInfoJSON);
+            toSend.put(USER_DATA_KEY,userData);
 
         } catch (JSONException e) {
             e.printStackTrace();
diff --git a/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/UserSettingsResource.java b/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/UserSettingsResource.java
index 5746759bb85205ccc81eb488782abb4e9ccba495..c5d1ea62e0f8e9cbecdb33310608e8dc51486392 100644
--- a/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/UserSettingsResource.java
+++ b/IcarusAndroid/icarus/src/main/java/framework/everywaretechnologies/it/icarus/UserSettingsResource.java
@@ -15,6 +15,12 @@ public class UserSettingsResource extends IcarusResource { //classe da non seria
 
     private final static String USER_SETTINGS_RES_CLASS="UserSettingsResource";
 
+    //--------------- JSON KEYS -------------------------
+    private final static String OLD_KEY="old";
+    private final static String NEW_KEY="new";
+    private final static String ACCESSIBILITY_COLOR_INVERSION_ENABLED_KEY="accessibility_color_inversion_enabled";
+    private final static String FONT_SIZE_KEY="font_size";
+
     private Map<String,String> prefsOldNew; //il secondo String è in realtà un JSONObject convertito in String
     private SharedPreferences callerSharedPrefs;
     private Context c;
@@ -35,31 +41,31 @@ public class UserSettingsResource extends IcarusResource { //classe da non seria
         JSONObject jsonObject=new JSONObject();
         try {
 
-            if (callerSharedPrefs.contains(prefName + "_old")) { //se ho già toccato quella pref in passato
+            if (callerSharedPrefs.contains(prefName + "_"+OLD_KEY)) { //se ho già toccato quella pref in passato
 
-                boolean oldValue=callerSharedPrefs.getBoolean(prefName+"_old",false);
+                boolean oldValue=callerSharedPrefs.getBoolean(prefName+"_"+OLD_KEY,false);
 
                 if(oldValue!=newValue){
 
-                    jsonObject.put("old", oldValue);
-                    jsonObject.put("new",newValue);
+                    jsonObject.put(OLD_KEY, oldValue);
+                    jsonObject.put(NEW_KEY,newValue);
                     prefsOldNew.put(prefName,jsonObject.toString());
 
                     //update shared preferences dell'app chiamante
                     SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                    editor.putBoolean(prefName+"_old",newValue);
+                    editor.putBoolean(prefName+"_"+OLD_KEY,newValue);
                     editor.apply(); //apply aggiorna le prefs in bg
                 }
 
             } else { //se è la prima volta che l'utente tocca quella pref
 
-                jsonObject.put("old", null); //TODO: valutare se mettere null come "null" perchè se no nel JSON è come non mettere questo campo, non me lo mette proprio
-                jsonObject.put("new",newValue);
+                jsonObject.put(OLD_KEY, null); //TODO: valutare se mettere null come "null" perchè se no nel JSON è come non mettere questo campo, non me lo mette proprio
+                jsonObject.put(NEW_KEY,newValue);
                 prefsOldNew.put(prefName,jsonObject.toString());
 
                 //update shared preferences dell'app chiamante
                 SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                editor.putBoolean(prefName+"_old",newValue);
+                editor.putBoolean(prefName+"_"+OLD_KEY,newValue);
                 editor.apply(); //apply aggiorna le prefs in bg
             }
 
@@ -73,31 +79,31 @@ public class UserSettingsResource extends IcarusResource { //classe da non seria
         JSONObject jsonObject=new JSONObject();
         try {
 
-            if (callerSharedPrefs.contains(prefName + "_old")) {
+            if (callerSharedPrefs.contains(prefName + "_"+OLD_KEY)) {
 
-                int oldValue=callerSharedPrefs.getInt(prefName+"_old",-1);
+                int oldValue=callerSharedPrefs.getInt(prefName+"_"+OLD_KEY,-1);
 
                 if(oldValue!=newValue){
 
-                    jsonObject.put("old", oldValue);
-                    jsonObject.put("new",newValue);
+                    jsonObject.put(OLD_KEY, oldValue);
+                    jsonObject.put(NEW_KEY,newValue);
                     prefsOldNew.put(prefName,jsonObject.toString());
 
                     //update shared preferences dell'app chiamante
                     SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                    editor.putInt(prefName+"_old",newValue);
+                    editor.putInt(prefName+"_"+OLD_KEY,newValue);
                     editor.apply(); //apply aggiorna le prefs in bg
                 }
 
             } else { //se è la prima volta che l'utente tocca quella pref
 
-                jsonObject.put("old", null);
-                jsonObject.put("new",newValue);
+                jsonObject.put(OLD_KEY, null);
+                jsonObject.put(NEW_KEY,newValue);
                 prefsOldNew.put(prefName,jsonObject.toString());
 
                 //update shared preferences dell'app chiamante
                 SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                editor.putInt(prefName+"_old",newValue);
+                editor.putInt(prefName+"_"+OLD_KEY,newValue);
                 editor.apply(); //apply aggiorna le prefs in bg
             }
 
@@ -111,31 +117,31 @@ public class UserSettingsResource extends IcarusResource { //classe da non seria
         JSONObject jsonObject=new JSONObject();
         try {
 
-            if (callerSharedPrefs.contains(prefName + "_old")) {
+            if (callerSharedPrefs.contains(prefName + "_"+OLD_KEY)) {
 
-                float oldValue=callerSharedPrefs.getFloat(prefName+"_old",-1);
+                float oldValue=callerSharedPrefs.getFloat(prefName+"_"+OLD_KEY,-1);
 
                 if(oldValue!=newValue){
 
-                    jsonObject.put("old", oldValue);
-                    jsonObject.put("new",newValue);
+                    jsonObject.put(OLD_KEY, oldValue);
+                    jsonObject.put(NEW_KEY,newValue);
                     prefsOldNew.put(prefName,jsonObject.toString());
 
                     //update shared preferences dell'app chiamante
                     SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                    editor.putFloat(prefName+"_old",newValue);
+                    editor.putFloat(prefName+"_"+OLD_KEY,newValue);
                     editor.apply(); //apply aggiorna le prefs in bg
                 }
 
             } else { //se è la prima volta che l'utente tocca quella pref
 
-                jsonObject.put("old", null);
-                jsonObject.put("new",newValue);
+                jsonObject.put(OLD_KEY, null);
+                jsonObject.put(NEW_KEY,newValue);
                 prefsOldNew.put(prefName,jsonObject.toString());
 
                 //update shared preferences dell'app chiamante
                 SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                editor.putFloat(prefName+"_old",newValue);
+                editor.putFloat(prefName+"_"+OLD_KEY,newValue);
                 editor.apply(); //apply aggiorna le prefs in bg
             }
 
@@ -149,31 +155,31 @@ public class UserSettingsResource extends IcarusResource { //classe da non seria
         JSONObject jsonObject=new JSONObject();
         try {
 
-            if (callerSharedPrefs.contains(prefName + "_old")) {
+            if (callerSharedPrefs.contains(prefName + "_"+OLD_KEY)) {
 
-                long oldValue=callerSharedPrefs.getLong(prefName+"_old",-1);
+                long oldValue=callerSharedPrefs.getLong(prefName+"_"+OLD_KEY,-1);
 
                 if(oldValue!=newValue){
 
-                    jsonObject.put("old", oldValue);
-                    jsonObject.put("new",newValue);
+                    jsonObject.put(OLD_KEY, oldValue);
+                    jsonObject.put(NEW_KEY,newValue);
                     prefsOldNew.put(prefName,jsonObject.toString());
 
                     //update shared preferences dell'app chiamante
                     SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                    editor.putFloat(prefName+"_old",newValue);
+                    editor.putFloat(prefName+"_"+OLD_KEY,newValue);
                     editor.apply(); //apply aggiorna le prefs in bg
                 }
 
             } else { //se è la prima volta che l'utente tocca quella pref
 
-                jsonObject.put("old", null);
-                jsonObject.put("new",newValue);
+                jsonObject.put(OLD_KEY, null);
+                jsonObject.put(NEW_KEY,newValue);
                 prefsOldNew.put(prefName,jsonObject.toString());
 
                 //update shared preferences dell'app chiamante
                 SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                editor.putFloat(prefName+"_old",newValue);
+                editor.putFloat(prefName+"_"+OLD_KEY,newValue);
                 editor.apply(); //apply aggiorna le prefs in bg
             }
 
@@ -187,31 +193,31 @@ public class UserSettingsResource extends IcarusResource { //classe da non seria
         JSONObject jsonObject=new JSONObject();
         try {
 
-            if (callerSharedPrefs.contains(prefName + "_old")) {
+            if (callerSharedPrefs.contains(prefName + "_"+OLD_KEY)) {
 
-                String oldValue=callerSharedPrefs.getString(prefName+"_old","");
+                String oldValue=callerSharedPrefs.getString(prefName+"_"+OLD_KEY,"");
 
                 if(!oldValue.equals(newValue)){
 
-                    jsonObject.put("old", oldValue);
-                    jsonObject.put("new",newValue);
+                    jsonObject.put(OLD_KEY, oldValue);
+                    jsonObject.put(NEW_KEY,newValue);
                     prefsOldNew.put(prefName,jsonObject.toString());
 
                     //update shared preferences dell'app chiamante
                     SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                    editor.putString(prefName+"_old",newValue);
+                    editor.putString(prefName+"_"+OLD_KEY,newValue);
                     editor.apply(); //apply aggiorna le prefs in bg
                 }
 
             } else { //se è la prima volta che l'utente tocca quella pref
 
-                jsonObject.put("old", null);
-                jsonObject.put("new",newValue);
+                jsonObject.put(OLD_KEY, null);
+                jsonObject.put(NEW_KEY,newValue);
                 prefsOldNew.put(prefName,jsonObject.toString());
 
                 //update shared preferences dell'app chiamante
                 SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                editor.putString(prefName+"_old",newValue);
+                editor.putString(prefName+"_"+OLD_KEY,newValue);
                 editor.apply(); //apply aggiorna le prefs in bg
             }
 
@@ -237,17 +243,17 @@ public class UserSettingsResource extends IcarusResource { //classe da non seria
         try{
 
             accessibilityDisplayColorInversionEnabled=Settings.Secure.getInt(c.getContentResolver(), Settings.Secure.ACCESSIBILITY_DISPLAY_INVERSION_ENABLED);
-            int oldAccessibilityDisplayColorInversion=callerSharedPrefs.getInt("accessibility_color_inversion_enabled",-1);
+            int oldAccessibilityDisplayColorInversion=callerSharedPrefs.getInt(ACCESSIBILITY_COLOR_INVERSION_ENABLED_KEY,-1);
 
             if(oldAccessibilityDisplayColorInversion!=accessibilityDisplayColorInversionEnabled){
 
                 JSONObject jsonObject=new JSONObject();
-                jsonObject.put("old",oldAccessibilityDisplayColorInversion);
-                jsonObject.put("new",accessibilityDisplayColorInversionEnabled);
-                prefsOldNew.put("accessibility_color_inversion_enabled",jsonObject.toString());
+                jsonObject.put(OLD_KEY,oldAccessibilityDisplayColorInversion);
+                jsonObject.put(NEW_KEY,accessibilityDisplayColorInversionEnabled);
+                prefsOldNew.put(ACCESSIBILITY_COLOR_INVERSION_ENABLED_KEY,jsonObject.toString());
 
                 SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                editor.putInt("accessibility_color_inversion_enabled",accessibilityDisplayColorInversionEnabled);
+                editor.putInt(ACCESSIBILITY_COLOR_INVERSION_ENABLED_KEY,accessibilityDisplayColorInversionEnabled);
                 editor.apply();
             }
 
@@ -263,17 +269,17 @@ public class UserSettingsResource extends IcarusResource { //classe da non seria
         try {
 
             fontSize=Settings.System.getFloat(c.getContentResolver(), Settings.System.FONT_SCALE);
-            float oldFontSize=callerSharedPrefs.getFloat("font_size",-1);
+            float oldFontSize=callerSharedPrefs.getFloat(FONT_SIZE_KEY,-1);
 
             if(oldFontSize!=fontSize){ //se dall'ultimo aggiornamento delle pref è cambiato
 
                 JSONObject jsonObject=new JSONObject();
-                jsonObject.put("old",oldFontSize);
-                jsonObject.put("new",fontSize);
-                prefsOldNew.put("font_size",jsonObject.toString());
+                jsonObject.put(OLD_KEY,oldFontSize);
+                jsonObject.put(NEW_KEY,fontSize);
+                prefsOldNew.put(FONT_SIZE_KEY,jsonObject.toString());
 
                 SharedPreferences.Editor editor=callerSharedPrefs.edit();
-                editor.putFloat("font_size",fontSize);
+                editor.putFloat(FONT_SIZE_KEY,fontSize);
                 editor.apply();
             }
         } catch (Settings.SettingNotFoundException e){