When expanding a long Thread, display Messages by batches so it doesn't freeze the UI anymore

app/src/main/java/com/infomaniak/mail/data/cache/mailboxContent/RefreshController.kt

@@ -596,7 +596,7 @@ class RefreshController @Inject constructor(
         // Add Sentry log and leave if the Message already exists
         if (existingMessage != null && !existingMessage.isOrphan()) {
             SentryLog.i(
-                TAG,
+                "Realm",
                 "Already existing message in folder ${folder.displayForSentry()} | threadMode = ${localSettings.threadMode}",
             )
             return true
@@ -817,8 +817,4 @@ class RefreshController @Inject constructor(
         val onStart: (() -> Unit),
         val onStop: (() -> Unit),
     )
-
-    companion object {
-        private val TAG = RefreshController::class.java.simpleName
-    }
 }

app/src/main/java/com/infomaniak/mail/ui/main/thread/ThreadFragment.kt

@@ -147,6 +147,7 @@ class ThreadFragment : Fragment() {
         observeLightThemeToggle()
         observeThreadLive()
         observeMessagesLive()
+        observeBatchedMessages()
         observeFailedMessages()
         observeQuickActionBarClicks()
         observeSubjectUpdateTriggers()
@@ -428,13 +429,22 @@ class ThreadFragment : Fragment() {
                 return@observe
             }
 
-            threadAdapter.submitList(items)
+            if (threadState.hasSuperCollapsedBlockBeenClicked) {
+                displayBatchedMessages(items)
+            } else {
+                threadAdapter.submitList(items)
+            }
+
             if (messagesToFetch.isNotEmpty()) fetchMessagesHeavyData(messagesToFetch)
 
             fetchCalendarEvents(items)
         }
     }
 
+    private fun observeBatchedMessages() {
+        threadViewModel.batchedMessages.observe(viewLifecycleOwner, threadAdapter::submitList)
+    }
+
     private fun observeFailedMessages() {
         threadViewModel.failedMessagesUids.observe(viewLifecycleOwner, threadAdapter::updateFailedMessages)
     }

app/src/main/java/com/infomaniak/mail/ui/main/thread/ThreadViewModel.kt

@@ -74,6 +74,7 @@ class ThreadViewModel @Inject constructor(
 
     val threadLive = MutableLiveData<Thread?>()
     val messagesLive = MutableLiveData<Pair<ThreadAdapterItems, MessagesWithoutHeavyData>>()
+    val batchedMessages = SingleLiveEvent<List<Any>>()
 
     val quickActionBarClicks = SingleLiveEvent<QuickActionBarResult>()
 
@@ -246,6 +247,25 @@ class ThreadViewModel @Inject constructor(
         return@withContext message
     }
 
+    fun displayBatchedMessages(items: List<Any>) = viewModelScope.launch(ioCoroutineContext) {
+
+        tailrec suspend fun sendBatchesRecursively(input: List<Any>, output: MutableList<Any>, batchSize: Int = 1) {
+
+            val batch = input.take(batchSize)
+            output.addAll(batch)
+
+            // We need to post a different list each time, because the `submitList` function in AsyncListDiffer
+            // won't trigger if we send the same list object (https://stackoverflow.com/questions/49726385).
+            batchedMessages.postValue(ArrayList(output))
+
+            if (batch.size < batchSize) return
+            delay(DELAY_BETWEEN_EACH_BATCHED_MESSAGES)
+            sendBatchesRecursively(input.subList(batchSize, input.size), output)
+        }
+
+        sendBatchesRecursively(input = items, output = mutableListOf(), batchSize = 2)
+    }
+
     fun openThread(threadUid: String) = liveData(ioCoroutineContext) {
 
         val thread = threadController.getThread(threadUid) ?: run {
@@ -451,5 +471,6 @@ class ThreadViewModel @Inject constructor(
     companion object {
         private const val SUPER_COLLAPSED_BLOCK_MINIMUM_MESSAGES_LIMIT = 5
         private const val SUPER_COLLAPSED_BLOCK_FIRST_INDEX_LIMIT = 3
+        private const val DELAY_BETWEEN_EACH_BATCHED_MESSAGES = 50L
     }
 }