Add BackHandler compatibility layer.

app/src/main/java/org/thoughtcrime/securesms/compose/FragmentBackPressedState.kt

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2025 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.thoughtcrime.securesms.compose
+
+import androidx.activity.compose.BackHandler
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.Stable
+import androidx.compose.runtime.annotation.RememberInComposition
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.setValue
+import androidx.fragment.app.Fragment
+import androidx.lifecycle.Lifecycle
+import androidx.lifecycle.lifecycleScope
+import androidx.lifecycle.repeatOnLifecycle
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.launch
+
+/**
+ * Allows us to support view based fragments hosted in a compose-based activity having their
+ * own back handling.
+ */
+@Stable
+class FragmentBackPressedState @RememberInComposition constructor() {
+  var info by mutableStateOf<FragmentBackPressedInfo?>(null)
+
+  fun attach(fragment: Fragment) {
+    if (fragment is FragmentBackPressedInfoProvider) {
+      with(fragment) {
+        viewLifecycleOwner.lifecycleScope.launch {
+          repeatOnLifecycle(state = Lifecycle.State.CREATED) {
+            getFragmentBackPressedInfo().collect {
+              info = it
+            }
+          }
+        }
+      }
+    }
+  }
+}
+
+/**
+ * Describes the current back-pressed state, produced by a [Fragment]
+ */
+sealed interface FragmentBackPressedInfo {
+  object Disabled : FragmentBackPressedInfo
+  data class Enabled(val callback: () -> Unit) : FragmentBackPressedInfo
+}
+
+/**
+ * Fragment should implement this interface.
+ */
+interface FragmentBackPressedInfoProvider {
+  fun getFragmentBackPressedInfo(): Flow<FragmentBackPressedInfo>
+}
+
+/**
+ * BackHandler for interop with legacy style fragments.
+ * Don't forget to call [FragmentBackPressedState.attach]!
+ */
+@Composable
+fun FragmentBackHandler(state: FragmentBackPressedState) {
+  val info = state.info
+  val enabled = when (info) {
+    FragmentBackPressedInfo.Disabled -> false
+    is FragmentBackPressedInfo.Enabled -> true
+    null -> false
+  }
+
+  BackHandler(enabled = enabled) {
+    if (info is FragmentBackPressedInfo.Enabled) {
+      info.callback()
+    }
+  }
+}