Fix scroll flickering on update

Author: alllexey

app/src/main/java/me/alllexey123/itmowidgets/ui/schedule/DayScheduleAdapter.kt

@@ -14,7 +14,7 @@ import java.time.Duration
 import java.time.LocalTime
 import java.time.format.DateTimeFormatter
 
-class DayScheduleAdapter(private var daySchedules: List<Schedule>) :
+class DayScheduleAdapter(private var schedules: List<Schedule>) :
     RecyclerView.Adapter<DayScheduleAdapter.DayViewHolder>() {
 
     private val viewPool = RecyclerView.RecycledViewPool()
@@ -26,7 +26,7 @@ class DayScheduleAdapter(private var daySchedules: List<Schedule>) :
     }
 
     override fun onBindViewHolder(holder: DayViewHolder, position: Int) {
-        val daySchedule = daySchedules[position]
+        val daySchedule = schedules[position]
         holder.dayTitle.text = ScheduleUtils.getRuDayOfWeek(daySchedule.date.dayOfWeek)
         val dtf = DateTimeFormatter.ofPattern("dd.MM")
         holder.dayDate.text = dtf.format(daySchedule.date)
@@ -48,12 +48,12 @@ class DayScheduleAdapter(private var daySchedules: List<Schedule>) :
     }
 
     fun updateData(newDaySchedules: List<Schedule>) {
-        val oldDaySchedules = this.daySchedules
-        this.daySchedules = newDaySchedules
+        val oldDaySchedules = this.schedules
+        this.schedules = newDaySchedules
         if (oldDaySchedules != newDaySchedules)notifyDataSetChanged()
     }
 
-    override fun getItemCount(): Int = daySchedules.size
+    override fun getItemCount(): Int = schedules.size
 
     inner class DayViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
         val dayTitle: TextView = itemView.findViewById(R.id.dayTitleTextView)

app/src/main/java/me/alllexey123/itmowidgets/ui/schedule/ScheduleActivity.kt

@@ -1,6 +1,7 @@
 package me.alllexey123.itmowidgets.ui.schedule
 
 import android.os.Bundle
+import android.os.Parcelable
 import android.view.View
 import android.widget.ProgressBar
 import android.widget.Toast
@@ -18,6 +19,8 @@ class ScheduleActivity : AppCompatActivity() {
     private lateinit var dayScheduleAdapter: DayScheduleAdapter
     private lateinit var progressBar: ProgressBar
 
+    private val snapHelper = PagerSnapHelper()
+
     private val scheduleViewModel: ScheduleViewModel by viewModels {
         val appContainer = (application as ItmoWidgetsApp).appContainer
         ScheduleViewModelFactory(appContainer.scheduleRepository)
@@ -43,10 +46,13 @@ class ScheduleActivity : AppCompatActivity() {
         dayScheduleAdapter = DayScheduleAdapter(listOf())
         outerRecyclerView.adapter = dayScheduleAdapter
 
-        val snapHelper = PagerSnapHelper()
         snapHelper.attachToRecyclerView(outerRecyclerView)
     }
 
+    private var isInitialLoad = true
+
+    private var layoutManagerState: Parcelable? = null
+
     private fun observeUiState() {
         scheduleViewModel.uiState.observe(this) { state ->
             when (state) {
@@ -57,22 +63,35 @@ class ScheduleActivity : AppCompatActivity() {
 
                 is ScheduleUiState.Success -> {
                     val scheduleList = state.schedule
-                    dayScheduleAdapter.updateData(scheduleList)
+                    if (scheduleList.isEmpty()) {
+                        outerRecyclerView.visibility = View.GONE
+                        progressBar.visibility = View.GONE
+                        return@observe
+                    }
 
-                    if (state.isCached) {
-                        progressBar.visibility = View.VISIBLE
+                    val layoutManager = outerRecyclerView.layoutManager as LinearLayoutManager
+                    if (!isInitialLoad) {
+                        layoutManagerState = layoutManager.onSaveInstanceState()
+                    }
 
-                        // scroll if cached
+                    dayScheduleAdapter.updateData(scheduleList)
+
+                    if (isInitialLoad) {
                         val today = LocalDate.now()
                         val todayIndex = scheduleList.indexOfFirst { it.date == today }
                         if (todayIndex != -1) {
-                            val layoutManager =
-                                outerRecyclerView.layoutManager as LinearLayoutManager
                             layoutManager.scrollToPositionWithOffset(todayIndex, 0)
                         }
-                    } else {
-                        progressBar.visibility = View.GONE
+                        isInitialLoad = false
+                    } else if (layoutManagerState != null) {
+                        layoutManager.onRestoreInstanceState(layoutManagerState)
                     }
+
+                    if (scheduleList.isNotEmpty()) {
+                        isInitialLoad = false
+                    }
+
+                    progressBar.visibility = if (state.isCached) View.VISIBLE else View.GONE
                     outerRecyclerView.visibility = View.VISIBLE
                 }