Introduce BackPressInterceptor

app/src/main/java/org/jellyfin/mobile/MainActivity.kt

@@ -9,9 +9,12 @@ import android.os.IBinder
 import android.provider.Settings
 import android.view.OrientationEventListener
 import android.widget.Toast
+import androidx.activity.OnBackPressedCallback
+import androidx.activity.addCallback
 import androidx.appcompat.app.AlertDialog
 import androidx.appcompat.app.AppCompatActivity
 import androidx.core.splashscreen.SplashScreen.Companion.installSplashScreen
+import androidx.fragment.app.Fragment
 import androidx.lifecycle.Lifecycle
 import androidx.lifecycle.lifecycleScope
 import androidx.lifecycle.repeatOnLifecycle
@@ -22,6 +25,7 @@ import org.jellyfin.mobile.player.cast.IChromecast
 import org.jellyfin.mobile.player.ui.PlayerFragment
 import org.jellyfin.mobile.setup.ConnectFragment
 import org.jellyfin.mobile.utils.AndroidVersion
+import org.jellyfin.mobile.utils.BackPressInterceptor
 import org.jellyfin.mobile.utils.Constants
 import org.jellyfin.mobile.utils.PermissionRequestHelper
 import org.jellyfin.mobile.utils.SmartOrientationListener
@@ -54,6 +58,37 @@ class MainActivity : AppCompatActivity() {
 
     private val orientationListener: OrientationEventListener by lazy { SmartOrientationListener(this) }
 
+    /**
+     * Passes back press events onto the currently visible [Fragment] if it implements the [BackPressInterceptor] interface.
+     *
+     * If the current fragment does not implement [BackPressInterceptor] or has decided not to intercept the event
+     * (see result of [BackPressInterceptor.onInterceptBackPressed]), the topmost backstack entry will be popped.
+     *
+     * If there is no topmost backstack entry, the event will be passed onto the dispatcher's fallback handler.
+     */
+    private val onBackPressedCallback: OnBackPressedCallback.() -> Unit = callback@{
+        val currentFragment = supportFragmentManager.findFragmentById(R.id.fragment_container)
+        if (currentFragment is BackPressInterceptor && currentFragment.onInterceptBackPressed()) {
+            // Top fragment handled back press
+            return@callback
+        }
+
+        // This is the same default action as in Activity.onBackPressed
+        if (!supportFragmentManager.isStateSaved && supportFragmentManager.popBackStackImmediate()) {
+            // Removed fragment from back stack
+            return@callback
+        }
+
+        // Let the system handle the back press
+        isEnabled = false
+        // Make sure that we *really* call the fallback handler
+        assert(!onBackPressedDispatcher.hasEnabledCallbacks()) {
+            "MainActivity should be the lowest onBackPressCallback"
+        }
+        onBackPressedDispatcher.onBackPressed()
+        isEnabled = true // re-enable callback in case activity isn't finished
+    }
+
     override fun onCreate(savedInstanceState: Bundle?) {
         installSplashScreen()
         setupKoinFragmentFactory()
@@ -95,6 +130,9 @@ class MainActivity : AppCompatActivity() {
             }
         }
 
+        // Handle back presses
+        onBackPressedDispatcher.addCallback(this, onBackPressed = onBackPressedCallback)
+
         // Setup Chromecast
         chromecast.initializePlugin(this)
     }
@@ -140,7 +178,7 @@ class MainActivity : AppCompatActivity() {
     }
 
     override fun onSupportNavigateUp(): Boolean {
-        onBackPressed()
+        onBackPressedDispatcher.onBackPressed()
         return true
     }
 
@@ -157,14 +195,6 @@ class MainActivity : AppCompatActivity() {
         orientationListener.disable()
     }
 
-    override fun onBackPressed() {
-        if (supportFragmentManager.backStackEntryCount > 0) {
-            supportFragmentManager.popBackStack()
-        } else {
-            super.onBackPressed()
-        }
-    }
-
     override fun onDestroy() {
         unbindService(serviceConnection)
         chromecast.destroy()

app/src/main/java/org/jellyfin/mobile/settings/SettingsFragment.kt

@@ -23,6 +23,7 @@ import de.Maxr1998.modernpreferences.preferences.choice.SelectionItem
 import org.jellyfin.mobile.R
 import org.jellyfin.mobile.app.AppPreferences
 import org.jellyfin.mobile.databinding.FragmentSettingsBinding
+import org.jellyfin.mobile.utils.BackPressInterceptor
 import org.jellyfin.mobile.utils.Constants
 import org.jellyfin.mobile.utils.applyWindowInsetsAsMargins
 import org.jellyfin.mobile.utils.extensions.requireMainActivity
@@ -31,7 +32,7 @@ import org.jellyfin.mobile.utils.isPackageInstalled
 import org.jellyfin.mobile.utils.withThemedContext
 import org.koin.android.ext.android.inject
 
-class SettingsFragment : Fragment() {
+class SettingsFragment : Fragment(), BackPressInterceptor {
 
     private val appPreferences: AppPreferences by inject()
     private val settingsAdapter: PreferencesAdapter by lazy { PreferencesAdapter(buildSettingsScreen()) }
@@ -59,6 +60,10 @@ class SettingsFragment : Fragment() {
         return binding.root
     }
 
+    override fun onInterceptBackPressed(): Boolean {
+        return settingsAdapter.goBack()
+    }
+
     override fun onDestroyView() {
         super.onDestroyView()
         requireMainActivity().setSupportActionBar(null)

app/src/main/java/org/jellyfin/mobile/utils/BackPressInterceptor.kt

@@ -0,0 +1,41 @@
+package org.jellyfin.mobile.utils
+
+import androidx.activity.OnBackPressedDispatcher
+import androidx.fragment.app.Fragment
+import org.jellyfin.mobile.MainActivity
+import org.jellyfin.mobile.utils.extensions.addFragment
+
+/**
+ * Additional hook for handling back presses in [Fragments][Fragment] (see [onInterceptBackPressed]).
+ *
+ * This hook is introduced since the AndroidX onBackPressedDispatcher system does not play well with the way we handle fragments:
+ * The WebViewFragment always needs to be active, since it contains the state of the web interface.
+ * To achieve this, we only add fragments (see [addFragment]) instead of doing the more common way
+ * and replacing the current fragment.
+ *
+ * This keeps the WebViewFragment alive, but unless the new fragment registers its own onBackPressedCallback,
+ * this also means that the WebViewFragment's onBackPressedCallbacks would still be the topmost dispatcher and therefore
+ * would be called (see [OnBackPressedDispatcher.onBackPressed]).
+ *
+ * This wouldn't be a problem if there was some way for the WebViewFragment (or any other fragment that's active) to
+ * know if it is the currently displayed fragment, since then it could deactivate its own onBackPressedCallback
+ * and the next callback would be called instead.
+ * The [MainActivity's][MainActivity] callback would then default to popping the backstack.
+ *
+ * There might be a way to implement this by using the backstack to determine if the current fragment is the topmost fragment,
+ * but sadly it seems that this isn't possible in a non-hacky way (as in hardcoding names of backstack entries).
+ *
+ * Instead, the MainActivity determines the currently visible fragment,
+ * and passes the back press event to it via the [onInterceptBackPressed] method.
+ */
+interface BackPressInterceptor {
+    /**
+     * Called when a back press is performed while this fragment is currently visible.
+     *
+     * @return `true` if the event was intercepted by the fragment,
+     *         `false` if the back press was not handled by the fragment.
+     *         The latter will result in a default action that closes the fragment
+     * @see MainActivity.onBackPressedCallback
+     */
+    fun onInterceptBackPressed(): Boolean = false
+}

app/src/main/java/org/jellyfin/mobile/webapp/WebViewFragment.kt

@@ -10,7 +10,6 @@ import android.view.View
 import android.view.ViewGroup
 import android.webkit.WebView
 import android.widget.Toast
-import androidx.activity.addCallback
 import androidx.appcompat.app.AlertDialog
 import androidx.core.view.ViewCompat
 import androidx.core.view.doOnNextLayout
@@ -31,6 +30,7 @@ import org.jellyfin.mobile.data.entity.ServerEntity
 import org.jellyfin.mobile.databinding.FragmentWebviewBinding
 import org.jellyfin.mobile.setup.ConnectFragment
 import org.jellyfin.mobile.utils.AndroidVersion
+import org.jellyfin.mobile.utils.BackPressInterceptor
 import org.jellyfin.mobile.utils.Constants
 import org.jellyfin.mobile.utils.Constants.FRAGMENT_WEB_VIEW_EXTRA_SERVER
 import org.jellyfin.mobile.utils.applyDefault
@@ -45,7 +45,7 @@ import org.jellyfin.mobile.utils.requestNoBatteryOptimizations
 import org.jellyfin.mobile.utils.runOnUiThread
 import org.koin.android.ext.android.inject
 
-class WebViewFragment : Fragment() {
+class WebViewFragment : Fragment(), BackPressInterceptor {
     val appPreferences: AppPreferences by inject()
     private val apiClientController: ApiClientController by inject()
     private val webappFunctionChannel: WebappFunctionChannel by inject()
@@ -102,13 +102,6 @@ class WebViewFragment : Fragment() {
             }
         }
         externalPlayer = ExternalPlayer(requireContext(), this, requireActivity().activityResultRegistry)
-
-        requireActivity().onBackPressedDispatcher.addCallback(this) {
-            if (!connected || !webappFunctionChannel.goBack()) {
-                isEnabled = false
-                activity?.onBackPressed()
-            }
-        }
     }
 
     override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View {
@@ -167,6 +160,10 @@ class WebViewFragment : Fragment() {
         }
     }
 
+    override fun onInterceptBackPressed(): Boolean {
+        return connected && webappFunctionChannel.goBack()
+    }
+
     override fun onDestroyView() {
         super.onDestroyView()
         webViewBinding = null