Retain scroll position in task list

app/src/googleplay/java/org/tasks/injection/ActivityComponent.java

@@ -1,5 +1,7 @@
 package org.tasks.injection;
 
+import android.arch.lifecycle.ViewModel;
+
 import com.todoroo.astrid.activity.BeastModePreferences;
 import com.todoroo.astrid.activity.ShareLinkActivity;
 import com.todoroo.astrid.activity.TaskListActivity;
@@ -37,6 +39,7 @@ import org.tasks.reminders.MissedCallActivity;
 import org.tasks.reminders.NotificationActivity;
 import org.tasks.reminders.SnoozeActivity;
 import org.tasks.themes.Theme;
+import org.tasks.ui.TaskListViewModel;
 import org.tasks.voice.VoiceCommandActivity;
 import org.tasks.widget.ShortcutConfigActivity;
 import org.tasks.widget.WidgetConfigActivity;
@@ -130,4 +133,6 @@ public interface ActivityComponent {
     void inject(GoogleTaskListSettingsActivity googleTaskListSettingsActivity);
 
     void inject(TaskerCreateTaskActivity taskerCreateTaskActivity);
+
+    void inject(TaskListViewModel taskListViewModel);
 }

app/src/main/java/com/todoroo/astrid/activity/TaskListActivity.java

@@ -6,6 +6,7 @@
 package com.todoroo.astrid.activity;
 
 import android.annotation.SuppressLint;
+import android.arch.lifecycle.ViewModelProviders;
 import android.content.Intent;
 import android.content.res.Configuration;
 import android.os.Bundle;
@@ -59,6 +60,7 @@ import org.tasks.ui.DeadlineControlSet;
 import org.tasks.ui.EmptyTaskEditFragment;
 import org.tasks.ui.NavigationDrawerFragment;
 import org.tasks.ui.PriorityControlSet;
+import org.tasks.ui.TaskListViewModel;
 
 import javax.inject.Inject;
 
@@ -102,6 +104,7 @@ public class TaskListActivity extends InjectingAppCompatActivity implements
     @BindView(R.id.detail) FrameLayout detail;
 
     private NavigationDrawerFragment navigationDrawer;
+    private TaskListViewModel viewModel;
 
     /** For indicating the new list screen should be launched at fragment setup time */
     public static final String TOKEN_CREATE_NEW_LIST_NAME = "newListName"; //$NON-NLS-1$
@@ -123,6 +126,10 @@ public class TaskListActivity extends InjectingAppCompatActivity implements
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
 
+        viewModel = ViewModelProviders.of(this).get(TaskListViewModel.class);
+
+        getComponent().inject(viewModel);
+
         currentNightMode = getNightMode();
 
         setContentView(R.layout.task_list_activity);
@@ -300,6 +307,7 @@ public class TaskListActivity extends InjectingAppCompatActivity implements
         }
 
         if(item instanceof Filter) {
+            viewModel.clear();
             startActivity(TaskIntents.getTaskListIntent(this, (Filter) item));
         }
     }

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

@@ -6,6 +6,7 @@
 package com.todoroo.astrid.activity;
 
 import android.app.Activity;
+import android.arch.lifecycle.ViewModelProviders;
 import android.content.BroadcastReceiver;
 import android.content.Context;
 import android.content.Intent;
@@ -47,7 +48,6 @@ import org.tasks.R;
 import org.tasks.activities.FilterSettingsActivity;
 import org.tasks.analytics.Tracker;
 import org.tasks.analytics.Tracking;
-import org.tasks.data.TaskListDataProvider;
 import org.tasks.dialogs.DialogBuilder;
 import org.tasks.dialogs.SortDialog;
 import org.tasks.gtasks.SyncAdapterHelper;
@@ -61,6 +61,7 @@ import org.tasks.tasklist.ViewHolderFactory;
 import org.tasks.ui.CheckBoxes;
 import org.tasks.ui.MenuColorizer;
 import org.tasks.ui.ProgressDialogAsyncTask;
+import org.tasks.ui.TaskListViewModel;
 
 import java.util.List;
 
@@ -105,7 +106,6 @@ public class TaskListFragment extends InjectingFragment implements
     @Inject DialogBuilder dialogBuilder;
     @Inject CheckBoxes checkBoxes;
     @Inject TaskCreator taskCreator;
-    @Inject protected TaskListDataProvider taskListDataProvider;
     @Inject TimerPlugin timerPlugin;
     @Inject ViewHolderFactory viewHolderFactory;
     @Inject protected Tracker tracker;
@@ -118,6 +118,8 @@ public class TaskListFragment extends InjectingFragment implements
     @BindView(R.id.task_list_coordinator) CoordinatorLayout coordinatorLayout;
     @BindView(R.id.recycler_view) RecyclerView recyclerView;
 
+    private TaskListViewModel taskListViewModel;
+
     private TaskAdapter taskAdapter = null;
     private TaskListRecyclerAdapter recyclerAdapter;
     private final RefreshReceiver refreshReceiver = new RefreshReceiver();
@@ -169,6 +171,8 @@ public class TaskListFragment extends InjectingFragment implements
 
     @Override
     public void onAttach(Activity activity) {
+        taskListViewModel = ViewModelProviders.of(getActivity()).get(TaskListViewModel.class);
+
         super.onAttach(activity);
 
         callbacks = (TaskListFragmentCallbackHandler) activity;
@@ -347,8 +351,7 @@ public class TaskListFragment extends InjectingFragment implements
     public void onActivityCreated(Bundle savedInstanceState) {
         super.onActivityCreated(savedInstanceState);
 
-        taskListDataProvider.getLiveData(filter, taskProperties())
+        taskListViewModel.getTasks(filter, taskProperties()).observe(getActivity(), list -> {
-                .observe(getActivity(), list -> {
             if (list.isEmpty()) {
                 swipeRefreshLayout.setVisibility(View.GONE);
                 emptyRefreshLayout.setVisibility(View.VISIBLE);
@@ -363,8 +366,7 @@ public class TaskListFragment extends InjectingFragment implements
             recyclerAdapter.setList(list);
 
             recyclerAdapter.restoreSaveState(saveState);
-                        }
+        });
-                );
 
         ((DefaultItemAnimator) recyclerView.getItemAnimator()).setSupportsChangeAnimations(false);
         recyclerAdapter.applyToRecyclerView(recyclerView);
@@ -431,7 +433,7 @@ public class TaskListFragment extends InjectingFragment implements
     public void loadTaskListContent(boolean animate) {
         recyclerAdapter.setAnimate(animate);
 
-        taskListDataProvider.invalidate();
+        taskListViewModel.invalidate();
     }
 
     protected TaskAdapter createTaskAdapter() {

app/src/main/java/org/tasks/ui/TaskListViewModel.java

@@ -1,6 +1,7 @@
-package org.tasks.data;
+package org.tasks.ui;
 
 import android.arch.lifecycle.LiveData;
+import android.arch.lifecycle.ViewModel;
 import android.arch.paging.LivePagedListBuilder;
 import android.arch.paging.LivePagedListProvider;
 import android.arch.paging.PagedList;
@@ -15,6 +16,9 @@ import com.todoroo.astrid.core.SortHelper;
 import com.todoroo.astrid.dao.TaskDao;
 import com.todoroo.astrid.data.Task;
 
+import org.tasks.data.LimitOffsetDataSource;
+import org.tasks.data.Tag;
+import org.tasks.data.TaskAttachment;
 import org.tasks.preferences.Preferences;
 
 import javax.inject.Inject;
@@ -22,19 +26,30 @@ import javax.inject.Inject;
 import static com.todoroo.astrid.activity.TaskListFragment.FILE_METADATA_JOIN;
 import static com.todoroo.astrid.activity.TaskListFragment.TAGS_METADATA_JOIN;
 
-public class TaskListDataProvider {
+public class TaskListViewModel extends ViewModel {
+
+    @Inject TaskDao taskDao;
+    @Inject Preferences preferences;
 
-    private final TaskDao taskDao;
-    private final Preferences preferences;
     private LimitOffsetDataSource latest;
+    private LiveData<PagedList<Task>> tasks;
+    private Filter filter;
+
+    public void clear() {
+        tasks = null;
+        latest = null;
+        filter = null;
+    }
 
-    @Inject
+    public LiveData<PagedList<Task>> getTasks(Filter filter, Property<?>[] properties) {
-    public TaskListDataProvider(TaskDao taskDao, Preferences preferences) {
+        if (tasks == null || !filter.equals(this.filter)) {
-        this.taskDao = taskDao;
+            this.filter = filter;
-        this.preferences = preferences;
+            tasks = getLiveData(filter, properties);
+        }
+        return tasks;
     }
 
-    public LiveData<PagedList<Task>> getLiveData(Filter filter, Property<?>[] properties) {
+    private LiveData<PagedList<Task>> getLiveData(Filter filter, Property<?>[] properties) {
         return new LivePagedListBuilder<>(
                 new LivePagedListProvider<Integer, Task>() {
                     @Override