ui: migrate to OnBackPressedDispatcher

This is compatible with Android 13's prediction-based back gesture
animation.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

ui/src/main/java/com/wireguard/android/activity/MainActivity.kt

@@ -9,6 +9,8 @@ import android.os.Bundle
 import android.view.Menu
 import android.view.MenuItem
 import android.view.View
+import androidx.activity.OnBackPressedCallback
+import androidx.activity.addCallback
 import androidx.appcompat.app.ActionBar
 import androidx.fragment.app.FragmentManager
 import androidx.fragment.app.FragmentTransaction
@@ -26,27 +28,29 @@ import com.wireguard.android.model.ObservableTunnel
 class MainActivity : BaseActivity(), FragmentManager.OnBackStackChangedListener {
     private var actionBar: ActionBar? = null
     private var isTwoPaneLayout = false
+    private var backPressedCallback: OnBackPressedCallback? = null
 
-    override fun onBackPressed() {
+    private fun handleBackPressed() {
         val backStackEntries = supportFragmentManager.backStackEntryCount
         // If the two-pane layout does not have an editor open, going back should exit the app.
         if (isTwoPaneLayout && backStackEntries <= 1) {
             finish()
             return
         }
-        // Deselect the current tunnel on navigating back from the detail pane to the one-pane list.
-        if (!isTwoPaneLayout && backStackEntries == 1) {
+
+        if (backStackEntries >= 1)
             supportFragmentManager.popBackStack()
+
+        // Deselect the current tunnel on navigating back from the detail pane to the one-pane list.
+        if (backStackEntries == 1)
             selectedTunnel = null
-            return
-        }
-        super.onBackPressed()
     }
 
     override fun onBackStackChanged() {
+        val backStackEntries = supportFragmentManager.backStackEntryCount
+        backPressedCallback?.isEnabled = backStackEntries >= 1
         if (actionBar == null) return
         // Do not show the home menu when the two-pane layout is at the detail view (see above).
-        val backStackEntries = supportFragmentManager.backStackEntryCount
         val minBackStackEntries = if (isTwoPaneLayout) 2 else 1
         actionBar!!.setDisplayHomeAsUpEnabled(backStackEntries >= minBackStackEntries)
     }
@@ -57,6 +61,7 @@ class MainActivity : BaseActivity(), FragmentManager.OnBackStackChangedListener
         actionBar = supportActionBar
         isTwoPaneLayout = findViewById<View?>(R.id.master_detail_wrapper) != null
         supportFragmentManager.addOnBackStackChangedListener(this)
+        backPressedCallback = onBackPressedDispatcher.addCallback(this) { handleBackPressed() }
         onBackStackChanged()
     }
 
@@ -69,7 +74,7 @@ class MainActivity : BaseActivity(), FragmentManager.OnBackStackChangedListener
         return when (item.itemId) {
             android.R.id.home -> {
                 // The back arrow in the action bar should act the same as the back button.
-                onBackPressed()
+                onBackPressedDispatcher.onBackPressed()
                 true
             }
             R.id.menu_action_edit -> {