Settings: Dynamically update Top App Bar title to match open Fragment

lawnchair/res/layout/settings_activity.xml

@@ -14,6 +14,7 @@
         android:layout_height="wrap_content">
 
         <com.google.android.material.appbar.MaterialToolbar
+            android:id="@+id/top_app_bar"
             android:layout_width="match_parent"
             android:layout_height="56dp"
             app:title="Settings" />

lawnchair/src/ch/deletescape/lawnchair/settings/BasePreferenceFragment.kt

@@ -5,7 +5,11 @@ import androidx.preference.PreferenceFragmentCompat
 import com.android.launcher3.LauncherFiles
 import com.google.android.material.transition.MaterialSharedAxis
 
-open class BasePreferenceFragment(var preferenceResource: Int, var isTopLevelFragment: Boolean = false) : PreferenceFragmentCompat() {
+interface TitledFragment {
+    val title: String
+}
+
+abstract class BasePreferenceFragment(var preferenceResource: Int, var isTopLevelFragment: Boolean = false) : PreferenceFragmentCompat(), TitledFragment {
 
     override fun onCreatePreferences(savedInstanceState: Bundle?, rootKey: String?) {
         setPreferencesFromResource(preferenceResource, rootKey)

lawnchair/src/ch/deletescape/lawnchair/settings/GeneralSettingsFragment.kt

@@ -2,4 +2,7 @@ package ch.deletescape.lawnchair.settings
 
 import com.android.launcher3.R
 
-class GeneralSettingsFragment : BasePreferenceFragment(R.xml.general_settings)
+class GeneralSettingsFragment : BasePreferenceFragment(R.xml.general_settings) {
+    override val title: String
+        get() = "General"
+}

lawnchair/src/ch/deletescape/lawnchair/settings/HomeScreenSettingsFragment.kt

@@ -2,4 +2,7 @@ package ch.deletescape.lawnchair.settings
 
 import com.android.launcher3.R
 
-class HomeScreenSettingsFragment : BasePreferenceFragment(R.xml.home_screen_settings)
+class HomeScreenSettingsFragment : BasePreferenceFragment(R.xml.home_screen_settings) {
+    override val title: String
+        get() = "Home Screen"
+}

lawnchair/src/ch/deletescape/lawnchair/settings/SettingsActivity.kt

@@ -1,20 +1,29 @@
 package ch.deletescape.lawnchair.settings
 
 import android.os.Bundle
+import android.view.View
 import androidx.appcompat.app.AppCompatActivity
+import androidx.fragment.app.Fragment
+import androidx.fragment.app.FragmentManager
 import androidx.preference.Preference
 import androidx.preference.PreferenceFragmentCompat
 import com.android.launcher3.R
 import com.google.android.material.appbar.MaterialToolbar
 
-// TODO: Migrate missing code from ch.deletescape.settings.ui.SettingsActivity.
-
 class SettingsActivity : AppCompatActivity(), PreferenceFragmentCompat.OnPreferenceStartFragmentCallback {
     private lateinit var topAppBar: MaterialToolbar
 
+    private val fragmentListener = object: FragmentManager.FragmentLifecycleCallbacks() {
+        override fun onFragmentViewCreated(fm: FragmentManager, fragment: Fragment, v: View, savedInstanceState: Bundle?) {
+            topAppBar.title = (fragment as? TitledFragment)?.title
+        }
+    }
+
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
         setContentView(R.layout.settings_activity)
+        topAppBar = findViewById(R.id.top_app_bar)
+        supportFragmentManager.registerFragmentLifecycleCallbacks(fragmentListener, false)
         supportFragmentManager
             .beginTransaction()
             .replace(R.id.content, SettingsFragment())

lawnchair/src/ch/deletescape/lawnchair/settings/SettingsFragment.kt

@@ -4,4 +4,7 @@ import com.android.launcher3.R
 
 // TODO: Migrate missing code from ch.deletescape.settings.ui.SettingsActivity.
 
-class SettingsFragment : BasePreferenceFragment(R.xml.settings, true)
+class SettingsFragment : BasePreferenceFragment(R.xml.settings, true) {
+    override val title: String
+        get() = "Settings"
+}