feature: lazy load favorites

Pixiview/Illust/FavoritesPage.xaml

@@ -14,17 +14,20 @@
                      IconImageSource="{DynamicResource FontIconShare}"/>
     </ContentPage.ToolbarItems>
     <Grid>
-        <ScrollView x:Name="scrollView" HorizontalOptions="Fill" HorizontalScrollBarVisibility="Never">
+        <ScrollView x:Name="scrollView" Scrolled="ScrollView_Scrolled"
+                    HorizontalOptions="Fill" HorizontalScrollBarVisibility="Never">
             <StackLayout>
                 <ActivityIndicator x:Name="activityLoading" Margin="0, 16, 0, 0"
                                    HeightRequest="40"
                                    IsRunning="{Binding IsLoading}"
                                    IsVisible="{Binding IsLoading}"/>
-                <u:FlowLayout ItemsSource="{Binding Illusts}"
+                <u:FlowLayout ItemsSource="{Binding Illusts}" MaxHeightChanged="FlowLayout_MaxHeightChanged"
                               HorizontalOptions="Fill" Column="{Binding Columns}"
-                              Margin="16"
+                              Margin="16" RowSpacing="16" ColumnSpacing="16"
-                              RowSpacing="16" ColumnSpacing="16"
                               ItemTemplate="{StaticResource cardView}"/>
+                <ActivityIndicator x:Name="activityBottomLoading" Margin="0, -10, 0, 16"
+                                   IsRunning="{Binding IsBottomLoading}"
+                                   IsVisible="{Binding IsBottomLoading}"/>
             </StackLayout>
         </ScrollView>
     </Grid>

Pixiview/Illust/FavoritesPage.xaml.cs

@@ -12,22 +12,30 @@ namespace Pixiview.Illust
 {
     public partial class FavoritesPage : FavoriteIllustCollectionPage
     {
+        private const int STEP = 24;
+
+        private int startIndex;
+        private int nextIndex;
         private bool flag = false;
 
         public FavoritesPage()
         {
             Resources.Add("cardView", GetCardViewTemplate());
             InitializeComponent();
+
+            startIndex = -1;
+            nextIndex = 0;
         }
 
         protected override bool IsFavoriteVisible => false;
         protected override ActivityIndicator LoadingIndicator => activityLoading;
-        protected override bool IsDelayLoading => true;
 
         protected override void OnAppearing()
         {
             if (lastUpdated != LastUpdated)
             {
+                startIndex = -1;
+                nextIndex = 0;
                 StartLoad();
             }
             else
@@ -37,6 +45,8 @@ namespace Pixiview.Illust
                 {
                     favorites.Reload();
                     lastUpdated = default;
+                    startIndex = -1;
+                    nextIndex = 0;
                     StartLoad();
                 }
             }
@@ -49,22 +59,51 @@ namespace Pixiview.Illust
 
         protected override IllustItem[] DoLoadIllustData(bool force)
         {
-            var favorites = Stores.GetFavoriteObject(flag);
+            FavoriteList favs;
-            flag = false;
+            if (startIndex < 0)
-            if (favorites == null)
             {
-                return null;
+                var favorites = Stores.GetFavoriteObject(flag);
-            }
+                flag = false;
-            var illusts = favorites.Illusts;
+                if (favorites == null)
-            for (var i = 0; i < illusts.Count; i++)
-            {
-                var item = illusts[i];
-                if (item.RankTitle == null)
                 {
-                    item.RankTitle = item.Title;
+                    return null;
                 }
+                favs = favorites.Illusts;
+                startIndex = 0;
             }
-            return illusts.ToArray();
+            else
+            {
+                favs = Stores.Favorites;
+            }
+            var illusts = favs.Skip(startIndex).Take(STEP).ToArray();
+            nextIndex = startIndex + STEP;
+            if (illusts.Length == 0 || nextIndex >= Stores.Favorites.Count)
+            {
+                // reach the bottom
+                startIndex = nextIndex;
+            }
+            return illusts;
+        }
+
+        private void FlowLayout_MaxHeightChanged(object sender, HeightEventArgs e)
+        {
+            SetOffset(e.ContentHeight - scrollView.Bounds.Height - SCROLL_OFFSET);
+        }
+
+        protected override bool CheckRefresh()
+        {
+            if (nextIndex > startIndex)
+            {
+                startIndex = nextIndex;
+                return true;
+            }
+            return false;
+        }
+
+        private void ScrollView_Scrolled(object sender, ScrolledEventArgs e)
+        {
+            var y = e.ScrollY;
+            OnScrolled(y);
         }
 
         public void Reload(bool force = false)
@@ -74,6 +113,8 @@ namespace Pixiview.Illust
             {
                 flag = true;
             }
+            startIndex = -1;
+            nextIndex = 0;
             StartLoad(force);
         }
 
@@ -105,6 +146,8 @@ namespace Pixiview.Illust
                     {
                         flag = false;
                         lastUpdated = default;
+                        startIndex = -1;
+                        nextIndex = 0;
                         MainThread.BeginInvokeOnMainThread(() => StartLoad(true));
                     }
                 });

Pixiview/Illust/IllustCollectionPage.cs

@@ -12,11 +12,11 @@ using Xamarin.Forms;
 
 namespace Pixiview.Illust
 {
-    public abstract class FavoriteIllustCollectionPage : IllustCollectionPage<IllustItem[]> { }
     public abstract class IllustDataCollectionPage : IllustCollectionPage<IllustData> { }
     public abstract class IllustRankingDataCollectionPage : IllustScrollableCollectionPage<IllustRankingData> { }
     public abstract class IllustRecommendsCollectionPage : IllustScrollableCollectionPage<IllustRecommendsData> { }
     public abstract class IllustUserDataCollectionPage : IllustScrollableCollectionPage<IllustUserData> { }
+    public abstract class FavoriteIllustCollectionPage : IllustScrollableCollectionPage<IllustItem[]> { }
 
     public interface IIllustCollectionPage
     {
@@ -67,7 +67,6 @@ namespace Pixiview.Illust
         protected virtual bool IsFavoriteVisible => true;
         protected virtual bool IsAutoReload => true;
         protected virtual ActivityIndicator LoadingIndicator => null;
-        protected virtual bool IsDelayLoading => false;
         protected virtual double IndicatorMarginTop => 16;
 
         protected readonly Command<IllustItem> commandIllustImageTapped;
@@ -270,18 +269,7 @@ namespace Pixiview.Illust
                     IsLoading = false;
                     IsBottomLoading = false;
 #if __IOS__
-                    if (IsDelayLoading)
+                    Illusts = collection;
-                    {
-                        Device.StartTimer(TimeSpan.FromMilliseconds(250), () =>
-                        {
-                            Illusts = collection;
-                            return false;
-                        });
-                    }
-                    else
-                    {
-                        Illusts = collection;
-                    }
 #else
                     Device.StartTimer(TimeSpan.FromMilliseconds(150), () =>
                     {