Fix: Implements double tap back in whiteboard

Author: ShaanNarendran

AnkiDroid/src/main/java/com/ichi2/anki/android/back/ExitViaDoubleTapBackBackPressCallback.kt

@@ -29,6 +29,31 @@ import com.ichi2.anki.snackbar.showSnackbar
 import com.ichi2.utils.HandlerUtils
 import timber.log.Timber
 
+/**
+ * Creates an [OnBackPressedCallback] that requires two back presses to exit.
+ * On first press, [onFirstBack] is called and the callback is temporarily disabled
+ * for 2s. If back is pressed again within that window,
+ * it goes through normally. After the timeout, re-enables if [shouldReEnable] returns true.
+ *
+ * @param enabled initial enabled state
+ * @param onFirstBack action on first back press (typically show a snackbar)
+ * @param shouldReEnable condition checked after timeout to decide whether to re-enable
+ */
+fun doubleBackPressCallback(
+    enabled: Boolean,
+    onFirstBack: () -> Unit,
+    shouldReEnable: () -> Boolean = { true },
+): OnBackPressedCallback =
+    object : OnBackPressedCallback(enabled) {
+        override fun handleOnBackPressed() {
+            onFirstBack()
+            isEnabled = false
+            HandlerUtils.executeFunctionWithDelay(Consts.SHORT_TOAST_DURATION) {
+                isEnabled = shouldReEnable()
+            }
+        }
+    }
+
 /**
  * Note: this uses sharedPreferences via AnkiDroidApp, so must be called after
  * [AnkiActivity.showedActivityFailedScreen]

AnkiDroid/src/main/java/com/ichi2/anki/ui/windows/reviewer/ReviewerFragment.kt

@@ -49,10 +49,12 @@ import androidx.lifecycle.flowWithLifecycle
 import androidx.lifecycle.lifecycleScope
 import androidx.lifecycle.repeatOnLifecycle
 import anki.scheduler.CardAnswer.Rating
+import com.google.android.material.snackbar.Snackbar
 import com.ichi2.anki.CollectionManager
 import com.ichi2.anki.DispatchKeyEventListener
 import com.ichi2.anki.Flag
 import com.ichi2.anki.R
+import com.ichi2.anki.android.back.doubleBackPressCallback
 import com.ichi2.anki.cardviewer.Gesture
 import com.ichi2.anki.common.annotations.NeedsTest
 import com.ichi2.anki.common.utils.android.isRobolectric
@@ -163,8 +165,22 @@ class ReviewerFragment :
     ) {
         super.onViewCreated(view, savedInstanceState)
 
+        val doubleBackCallback =
+            doubleBackPressCallback(
+                enabled = false,
+                onFirstBack = { showSnackbar(R.string.back_pressed_once, Snackbar.LENGTH_SHORT) },
+                shouldReEnable = { viewModel.whiteboardEnabledFlow.value },
+            )
+        requireActivity().onBackPressedDispatcher.addCallback(viewLifecycleOwner, doubleBackCallback)
+
+        viewModel.whiteboardEnabledFlow
+            .flowWithLifecycle(lifecycle)
+            .collectIn(lifecycleScope) { isWhiteboardEnabled ->
+                doubleBackCallback.isEnabled = isWhiteboardEnabled
+            }
+
         binding.backButton.setOnClickListener {
-            requireActivity().finish()
+            requireActivity().onBackPressedDispatcher.onBackPressed()
         }
 
         setupBindings()