Allow PreferenceScreens to have a key

Being able to tag PreferenceScreens with a key is useful for organizational reasons, even if they don't persist any data to the SharedPreferences.
For backwards compatibility and flexibility, the default key for PreferenceScreens is still an empty string.
The root screen, however, now gets a special reserved key, which may not be used for sub-screens (but could theoretically still be used for Preferences, this is highly discouraged though). With that root key, it will be possible to do better screen traversal in the future.

library/src/main/java/de/Maxr1998/modernpreferences/Preferences.kt

@@ -28,6 +28,7 @@ import androidx.annotation.LayoutRes
 import androidx.annotation.StringRes
 import androidx.core.content.edit
 import androidx.core.view.isVisible
+import de.Maxr1998.modernpreferences.helpers.KEY_ROOT_SCREEN
 import de.Maxr1998.modernpreferences.preferences.CollapsePreference
 import de.Maxr1998.modernpreferences.preferences.TwoStatePreference
 import java.util.concurrent.atomic.AtomicBoolean
@@ -286,7 +287,7 @@ open class Preference(key: String) : AbstractPreference(key) {
  * - Every [PreferenceScreen] can be bound to a different [SharedPreferences] file
  * - Even though you can change the [enabled] or the [persistent] state, it doesn't have any effect in this instance
  */
-class PreferenceScreen private constructor(builder: Builder) : Preference("") {
+class PreferenceScreen private constructor(builder: Builder) : Preference(builder.key) {
     internal val prefs = builder.prefs
     private val keyMap: Map<String, Preference> = builder.keyMap
     private val preferences: List<Preference> = builder.preferences
@@ -349,8 +350,9 @@ class PreferenceScreen private constructor(builder: Builder) : Preference("") {
         adapter?.notifyItemRangeChanged(position, itemCount)
     }
 
-    class Builder(private var context: Context?) : AbstractPreference("") {
-        constructor(builder: Builder) : this(builder.context)
+    class Builder private constructor(private var context: Context?, key: String) : AbstractPreference(key) {
+        constructor(context: Context?) : this(context, KEY_ROOT_SCREEN)
+        constructor(builder: Builder, key: String = "") : this(builder.context, key)
 
         /**
          * The filename to use for the [SharedPreferences] of this [PreferenceScreen]
@@ -379,10 +381,15 @@ class PreferenceScreen private constructor(builder: Builder) : Preference("") {
          * [subScreen][de.Maxr1998.modernpreferences.helpers.subScreen] for this.
          */
         fun addPreferenceItem(p: Preference) {
+            if (p.key == KEY_ROOT_SCREEN)
+                throw UnsupportedOperationException("" +
+                        "A screen with key '$KEY_ROOT_SCREEN' cannot be added as a sub-screen! " +
+                        "If you are trying to add a sub-screen to your preferences model, " +
+                        "use the `subScreen {}` function.")
             if (p.key.isEmpty() && p !is PreferenceScreen)
                 throw UnsupportedOperationException("Preference key may not be empty!")
 
-            if (p is PreferenceScreen || keyMap.put(p.key, p) == null)
+            if (p.key.isEmpty() || keyMap.put(p.key, p) == null)
                 preferences.add(p)
             else throw UnsupportedOperationException("A preference with this key is already in the screen!")
 

library/src/main/java/de/Maxr1998/modernpreferences/helpers/PreferencesDsl.kt

@@ -31,8 +31,8 @@ inline fun screen(context: Context?, block: PreferenceScreen.Builder.() -> Unit)
 
 val emptyScreen: PreferenceScreen by lazy { screen(null) {} }
 
-inline fun PreferenceScreen.Builder.subScreen(block: PreferenceScreen.Builder.() -> Unit) {
-    addPreferenceItem(PreferenceScreen.Builder(this).apply(block).build())
+inline fun PreferenceScreen.Builder.subScreen(key: String = "", block: PreferenceScreen.Builder.() -> Unit) {
+    addPreferenceItem(PreferenceScreen.Builder(this, key).apply(block).build())
 }
 
 // Preference DSL functions

library/src/main/java/de/Maxr1998/modernpreferences/helpers/Utils.kt

@@ -18,6 +18,8 @@ package de.Maxr1998.modernpreferences.helpers
 
 import android.widget.SeekBar
 
+const val KEY_ROOT_SCREEN = "root"
+
 internal fun SeekBar.onSeek(callback: (Int, Boolean) -> Unit) {
     setOnSeekBarChangeListener(object : SeekBar.OnSeekBarChangeListener {
         private var changed = false