Ensure DragAndDropDiffer works sequentially

app/src/main/java/com/todoroo/astrid/activity/TaskListFragment.kt

@@ -201,15 +201,13 @@ class TaskListFragment : Fragment(), OnRefreshListener, Toolbar.OnMenuItemClickL
         (recyclerView.itemAnimator as DefaultItemAnimator).supportsChangeAnimations = false
         recyclerView.layoutManager = LinearLayoutManager(context)
         listViewModel.observe(this) {
-            lifecycleScope.launch {
-                submitList(it)
-                if (it.isEmpty()) {
-                    swipeRefreshLayout.visibility = View.GONE
-                    emptyRefreshLayout.visibility = View.VISIBLE
-                } else {
-                    swipeRefreshLayout.visibility = View.VISIBLE
-                    emptyRefreshLayout.visibility = View.GONE
-                }
+            submitList(it)
+            if (it.isEmpty()) {
+                swipeRefreshLayout.visibility = View.GONE
+                emptyRefreshLayout.visibility = View.VISIBLE
+            } else {
+                swipeRefreshLayout.visibility = View.VISIBLE
+                emptyRefreshLayout.visibility = View.GONE
             }
         }
         setupRefresh(swipeRefreshLayout)
@@ -222,8 +220,8 @@ class TaskListFragment : Fragment(), OnRefreshListener, Toolbar.OnMenuItemClickL
         return parent
     }
 
-    private suspend fun submitList(tasks: List<TaskContainer>) {
-        if (tasks is PagedList<*>) {
+    private fun submitList(tasks: List<TaskContainer>) {
+        if (tasks is PagedList<TaskContainer>) {
             if (recyclerAdapter !is PagedListRecyclerAdapter) {
                 setAdapter(
                         PagedListRecyclerAdapter(
@@ -233,7 +231,7 @@ class TaskListFragment : Fragment(), OnRefreshListener, Toolbar.OnMenuItemClickL
         } else if (recyclerAdapter !is DragAndDropRecyclerAdapter) {
             setAdapter(
                     DragAndDropRecyclerAdapter(
-                            lifecycleScope, taskAdapter, recyclerView, viewHolderFactory, this, tasks, preferences))
+                            taskAdapter, recyclerView, viewHolderFactory, this, tasks, preferences))
             return
         }
         recyclerAdapter?.submitList(tasks)

app/src/main/java/com/todoroo/astrid/adapter/NavigationDrawerAdapter.kt

@@ -16,6 +16,9 @@ import com.todoroo.astrid.api.Filter
 import com.todoroo.astrid.api.FilterListItem
 import io.reactivex.disposables.CompositeDisposable
 import io.reactivex.subjects.PublishSubject
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.asCoroutineDispatcher
 import org.tasks.LocalBroadcastManager
 import org.tasks.activities.DragAndDropDiffer
 import org.tasks.billing.Inventory
@@ -26,6 +29,7 @@ import org.tasks.locale.Locale
 import org.tasks.preferences.Preferences
 import org.tasks.themes.ColorProvider
 import java.util.*
+import java.util.concurrent.Executors
 import javax.inject.Inject
 import kotlin.math.max
 
@@ -47,6 +51,8 @@ class NavigationDrawerAdapter @Inject constructor(
     override val updates: Queue<Pair<MutableList<FilterListItem>, DiffUtil.DiffResult?>> = LinkedList()
     override var items = initializeDiffer(ArrayList())
     override var dragging = false
+    override val scope: CoroutineScope =
+            CoroutineScope(Executors.newSingleThreadExecutor().asCoroutineDispatcher() + Job())
 
     fun setOnClick(onClick: (FilterListItem?) -> Unit) {
         this.onClick = onClick
@@ -60,6 +66,10 @@ class NavigationDrawerAdapter @Inject constructor(
         selected = savedInstanceState.getParcelable(TOKEN_SELECTED)
     }
 
+    override fun onDetachedFromRecyclerView(recyclerView: RecyclerView) {
+        super.dispose()
+    }
+
     override fun getItemId(position: Int) = position.toLong()
 
     override fun getItemCount() = items.size

app/src/main/java/org/tasks/activities/DragAndDropDiffer.kt

@@ -7,8 +7,9 @@ import io.reactivex.android.schedulers.AndroidSchedulers
 import io.reactivex.disposables.CompositeDisposable
 import io.reactivex.schedulers.Schedulers
 import io.reactivex.subjects.PublishSubject
-import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.withContext
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.cancel
+import kotlinx.coroutines.launch
 import java.util.*
 
 interface DragAndDropDiffer<T, R> : ListUpdateCallback {
@@ -17,12 +18,13 @@ interface DragAndDropDiffer<T, R> : ListUpdateCallback {
     val disposables: CompositeDisposable
     var items: R
     var dragging: Boolean
+    val scope: CoroutineScope
 
-    suspend fun submitList(list: List<T>) {
-        val transform = withContext(Dispatchers.Default) {
-            transform(list)
+    fun submitList(list: List<T>) {
+        scope.launch {
+            val transform = transform(list)
+            publishSubject.onNext(transform)
         }
-        publishSubject.onNext(transform)
     }
 
     fun calculateDiff(last: Pair<R, DiffUtil.DiffResult?>, next: R): Pair<R, DiffUtil.DiffResult?> {
@@ -51,7 +53,7 @@ interface DragAndDropDiffer<T, R> : ListUpdateCallback {
     fun initializeDiffer(list: List<T>): R {
         val initial = transform(list)
         disposables.add(publishSubject
-                .observeOn(Schedulers.computation())
+                .observeOn(Schedulers.single())
                 .scan(Pair(initial, null), { last: Pair<R, DiffUtil.DiffResult?>, next: R ->
                     calculateDiff(last, next)
                 })
@@ -64,4 +66,9 @@ interface DragAndDropDiffer<T, R> : ListUpdateCallback {
     fun transform(list: List<T>): R
 
     fun diff(last: R, next: R): DiffUtil.DiffResult
+
+    fun dispose() {
+        disposables.dispose()
+        scope.cancel()
+    }
 }

app/src/main/java/org/tasks/tasklist/DragAndDropRecyclerAdapter.kt

@@ -2,7 +2,6 @@ package org.tasks.tasklist
 
 import android.graphics.Canvas
 import android.view.ViewGroup
-import androidx.lifecycle.LifecycleCoroutineScope
 import androidx.recyclerview.widget.DiffUtil
 import androidx.recyclerview.widget.ItemTouchHelper
 import androidx.recyclerview.widget.ItemTouchHelper.*
@@ -13,16 +12,19 @@ import com.todoroo.astrid.adapter.TaskAdapter
 import com.todoroo.astrid.utility.Flags
 import io.reactivex.disposables.CompositeDisposable
 import io.reactivex.subjects.PublishSubject
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.asCoroutineDispatcher
 import kotlinx.coroutines.runBlocking
 import org.tasks.activities.DragAndDropDiffer
 import org.tasks.data.TaskContainer
 import org.tasks.preferences.Preferences
 import java.util.*
+import java.util.concurrent.Executors
 import kotlin.math.max
 import kotlin.math.min
 
 class DragAndDropRecyclerAdapter(
-        private val scope: LifecycleCoroutineScope,
         private val adapter: TaskAdapter,
         private val recyclerView: RecyclerView,
         viewHolderFactory: ViewHolderFactory,
@@ -43,6 +45,8 @@ class DragAndDropRecyclerAdapter(
     override val updates: Queue<Pair<SectionedDataSource, DiffUtil.DiffResult?>> = LinkedList()
     override var dragging = false
     override var items = initializeDiffer(tasks)
+    override val scope: CoroutineScope =
+            CoroutineScope(Executors.newSingleThreadExecutor().asCoroutineDispatcher() + Job())
 
     override fun onBindViewHolder(holder: RecyclerView.ViewHolder, position: Int) {
         val viewType = getItemViewType(position)
@@ -59,7 +63,7 @@ class DragAndDropRecyclerAdapter(
 
     override fun getItemViewType(position: Int) = if (items.isHeader(position)) 1 else 0
 
-    override suspend fun submitList(list: List<TaskContainer>) {
+    override fun submitList(list: List<TaskContainer>) {
         super.submitList(list)
     }
 
@@ -95,8 +99,8 @@ class DragAndDropRecyclerAdapter(
     }
 
     override fun onDetachedFromRecyclerView(recyclerView: RecyclerView) {
+        super.dispose()
         itemTouchHelper.attachToRecyclerView(null)
-        disposables.dispose()
     }
 
     override fun getTaskCount() = items.taskCount

app/src/main/java/org/tasks/tasklist/PagedListRecyclerAdapter.kt

@@ -14,15 +14,17 @@ class PagedListRecyclerAdapter(
         private val recyclerView: RecyclerView,
         viewHolderFactory: ViewHolderFactory,
         taskList: TaskListFragment,
-        list: List<TaskContainer>,
-        preferences: Preferences) : TaskListRecyclerAdapter(adapter, viewHolderFactory, taskList, preferences) {
+        list: PagedList<TaskContainer>,
+        preferences: Preferences
+) : TaskListRecyclerAdapter(adapter, viewHolderFactory, taskList, preferences) {
 
     private val differ: AsyncPagedListDiffer<TaskContainer> =
             AsyncPagedListDiffer(this, AsyncDifferConfig.Builder(ItemCallback()).build())
 
     override fun getItem(position: Int) = differ.getItem(position)
 
-    override suspend fun submitList(list: List<TaskContainer>) = differ.submitList(list as PagedList<TaskContainer>)
+    override fun submitList(list: List<TaskContainer>) =
+            differ.submitList(list as PagedList<TaskContainer>)
 
     override fun onMoved(fromPosition: Int, toPosition: Int) {
         val recyclerViewState = recyclerView.layoutManager!!.onSaveInstanceState()
@@ -37,8 +39,6 @@ class PagedListRecyclerAdapter(
     override fun getTaskCount() = itemCount
 
     init {
-        if (list is PagedList<*>) {
-            differ.submitList(list as PagedList<TaskContainer>?)
-        }
+        differ.submitList(list as PagedList<TaskContainer>?)
     }
 }

app/src/main/java/org/tasks/tasklist/TaskListRecyclerAdapter.kt

@@ -44,7 +44,7 @@ abstract class TaskListRecyclerAdapter internal constructor(
 
     abstract fun dragAndDropEnabled(): Boolean
 
-    abstract suspend fun submitList(list: List<TaskContainer>)
+    abstract fun submitList(list: List<TaskContainer>)
 
     override fun onInserted(position: Int, count: Int) {
         notifyItemRangeInserted(position, count)