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Cache cell measurements
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@hartez
hartez committed Dec 18, 2020
1 parent 8fa8157 commit a9fd28a
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Showing 10 changed files with 197 additions and 77 deletions.
18 changes: 14 additions & 4 deletions ...in.Forms.Controls/GalleryPages/CollectionViewGalleries/ItemSizeGalleries/ChatExample.xaml
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Original file line number Diff line number Diff line change
Expand Up @@ -13,19 +13,23 @@
<ColumnDefinition Width="2*"></ColumnDefinition>
</Grid.ColumnDefinitions>
<Frame BorderColor="Green" BackgroundColor="LightGreen" CornerRadius="5" Grid.Column="1">
<StackLayout Margin="2">
<Label Text="{Binding Text}" LineBreakMode="WordWrap" FontSize="10" HorizontalTextAlignment="End" />
</StackLayout>
</Frame>
</Grid>
</DataTemplate>

<DataTemplate x:Key="Remote">
<Grid>
<Grid Padding="5,0,5,0">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="2*"></ColumnDefinition>
<ColumnDefinition Width="*"></ColumnDefinition>
</Grid.ColumnDefinitions>
<Frame BorderColor="Blue" BackgroundColor="LightBlue" CornerRadius="5">
<Label Text="{Binding Text}" LineBreakMode="WordWrap" FontSize="10" HorizontalTextAlignment="Start" />
<StackLayout Margin="2">
<Label Text="{Binding Text}" LineBreakMode="WordWrap" FontSize="10" HorizontalTextAlignment="Start" />
</StackLayout>
</Frame>
</Grid>
</DataTemplate>
Expand All @@ -45,13 +49,19 @@
</Grid.RowDefinitions>

<StackLayout Orientation="Horizontal">
<Button x:Name="AppendRandomSizedItem" Text="Append Random Message" />
<Button x:Name="AppendRandomSizedItem" Text="Append Random Message" Margin="2" />
<Button x:Name="Clear" Text="Clear" Margin="2" />
<Button x:Name="Lots" Text="Add 1000 Messages" Margin="2" />
</StackLayout>

<CollectionView ItemsSource="{Binding ChatMessages}"
ItemTemplate="{StaticResource ChatTemplateSelector}"
ItemSizingStrategy="MeasureAllItems"
Grid.Row="1"/>
Grid.Row="1">
<CollectionView.ItemsLayout>
<LinearItemsLayout Orientation="Vertical" ItemSpacing="5"/>
</CollectionView.ItemsLayout>
</CollectionView>

</Grid>
</ContentPage.Content>
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69 changes: 50 additions & 19 deletions ...Forms.Controls/GalleryPages/CollectionViewGalleries/ItemSizeGalleries/ChatExample.xaml.cs
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Original file line number Diff line number Diff line change
@@ -1,18 +1,14 @@
using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace Xamarin.Forms.Controls.GalleryPages.CollectionViewGalleries.ItemSizeGalleries
{
[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class ChatExample : ContentPage
{
Random _random = new Random(DateTime.Now.Millisecond);
ChatExampleViewModel _vm = new ChatExampleViewModel();

public ChatExample()
Expand All @@ -21,38 +17,73 @@ public ChatExample()
BindingContext = _vm;

AppendRandomSizedItem.Clicked += AppendRandomChatMessage;
Clear.Clicked += ClearMessages;
Lots.Clicked += LotsOfMessages;
}

private void AppendRandomChatMessage(object sender, EventArgs e)
void AppendRandomChatMessage(object sender, EventArgs e)
{
const string lorem = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut " +
"labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip " +
"ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat ";
//+ "nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.";
_vm.ChatMessages.Add(GenerateRandomMessage());
}

var random = new Random(DateTime.Now.Millisecond);
void LotsOfMessages(object sender, EventArgs e)
{
var newVm = new ChatExampleViewModel(GenerateMessages(1000));
_vm = newVm;
BindingContext = _vm;
}

var local = random.Next(0, 2) == 1;
void ClearMessages(object sender, EventArgs e)
{
_vm.ChatMessages.Clear();
}

ChatMessage GenerateRandomMessage()
{
const string lorem = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut "
+ "labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip "
+ "ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat "
+ "nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.";

var textLength = random.Next(0, lorem.Length - 1);
var local = _random.Next(0, 2) == 1;

var textLength = _random.Next(0, lorem.Length - 1);

var text = lorem.Substring(0, textLength);

_vm.ChatMessages.Add(new ChatMessage(text, local));
return new ChatMessage(text, local);
}

IEnumerable<ChatMessage> GenerateMessages(int count)
{
for (int n = 0; n < count; n++)
{
yield return GenerateRandomMessage();
}
}
}

public class ChatExampleViewModel
public class ChatExampleViewModel
{
public ObservableCollection<ChatMessage> ChatMessages { get; } = new ObservableCollection<ChatMessage>();
public ObservableCollection<ChatMessage> ChatMessages { get; }

public ChatExampleViewModel()
{
ChatMessages = new ObservableCollection<ChatMessage>();
}

public ChatExampleViewModel(IEnumerable<ChatMessage> chatMessages)
{
ChatMessages = new ObservableCollection<ChatMessage>(chatMessages);
}
}

public class ChatMessage
{
public class ChatMessage
{
public bool IsLocal { get; set; }
public string Text { get; set; }

public ChatMessage(string text) : this(text, true){ }
public ChatMessage(string text) : this(text, true) { }

public ChatMessage(string text, bool isLocal)
{
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26 changes: 25 additions & 1 deletion Xamarin.Forms.Platform.iOS.UnitTests/ObservableItemsSourceTests.cs
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Original file line number Diff line number Diff line change
@@ -1,4 +1,7 @@
using NUnit.Framework;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using Foundation;
using NUnit.Framework;

namespace Xamarin.Forms.Platform.iOS.UnitTests
{
Expand Down Expand Up @@ -58,5 +61,26 @@ public void GenerateIndexPathRangeForLoop()
Assert.That((int)result[7].Item, Is.EqualTo(13));
Assert.That((int)result[8].Item, Is.EqualTo(14));
}

[Test]
public void IndexPathValidTest()
{
var list = new List<string>
{
"one",
"two",
"three"
};

var source = new ListSource(list);

var valid = NSIndexPath.FromItemSection(2, 0);
var invalidItem = NSIndexPath.FromItemSection(7, 0);
var invalidSection = NSIndexPath.FromItemSection(1, 9);

Assert.IsTrue(source.IsIndexPathValid(valid));
Assert.IsFalse(source.IsIndexPathValid(invalidItem));
Assert.IsFalse(source.IsIndexPathValid(invalidSection));
}
}
}
15 changes: 15 additions & 0 deletions Xamarin.Forms.Platform.iOS/CollectionView/IndexPathHelpers.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -32,5 +32,20 @@ public static NSIndexPath[] GenerateLoopedIndexPathRange(int section, int sectio

return result;
}

public static bool IsIndexPathValid(this IItemsViewSource source, NSIndexPath indexPath)
{
if (indexPath.Section >= source.GroupCount)
{
return false;
}

if (indexPath.Item >= source.ItemCountInGroup(indexPath.Section))
{
return false;
}

return true;
}
}
}
43 changes: 41 additions & 2 deletions Xamarin.Forms.Platform.iOS/CollectionView/ItemsViewController.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,7 @@ public abstract class ItemsViewController<TItemsView> : UICollectionViewControll
UIView _emptyUIView;
VisualElement _emptyViewFormsElement;
Dictionary<NSIndexPath, TemplatedCell> _measurementCells = new Dictionary<NSIndexPath, TemplatedCell>();
Dictionary<object, CGSize> _cellSizeCache = new Dictionary<object, CGSize>();

protected UICollectionViewDelegateFlowLayout Delegator { get; set; }

Expand Down Expand Up @@ -111,11 +112,16 @@ void CheckForEmptySource()

_isEmpty = ItemsSource.ItemCount == 0;

if (_isEmpty)
{
_measurementCells.Clear();
}

if (wasEmpty != _isEmpty)
{
UpdateEmptyViewVisibility(_isEmpty);
}

if (wasEmpty && !_isEmpty)
{
// If we're going from empty to having stuff, it's possible that we've never actually measured
Expand Down Expand Up @@ -243,12 +249,15 @@ protected virtual void UpdateDefaultCell(DefaultCell cell, NSIndexPath indexPath
protected virtual void UpdateTemplatedCell(TemplatedCell cell, NSIndexPath indexPath)
{
cell.ContentSizeChanged -= CellContentSizeChanged;
cell.LayoutAttributesChanged -= CellLayoutAttributesChanged;

// If we've already created a cell for this index path (for measurement), re-use the content
if (_measurementCells.TryGetValue(indexPath, out TemplatedCell measurementCell))
{
System.Diagnostics.Debug.WriteLine($">>>>>> Reusing a measurement cell ");
_measurementCells.Remove(indexPath);
measurementCell.ContentSizeChanged -= CellContentSizeChanged;
measurementCell.LayoutAttributesChanged -= CellLayoutAttributesChanged;
cell.UseContent(measurementCell);
}
else
Expand All @@ -257,10 +266,11 @@ protected virtual void UpdateTemplatedCell(TemplatedCell cell, NSIndexPath index
}

cell.ContentSizeChanged += CellContentSizeChanged;
cell.LayoutAttributesChanged += CellLayoutAttributesChanged;

ItemsViewLayout.PrepareCellForLayout(cell);
}

public virtual NSIndexPath GetIndexForItem(object item)
{
return ItemsSource.GetIndexForItem(item);
Expand All @@ -279,6 +289,15 @@ void CellContentSizeChanged(object sender, EventArgs e)
Layout?.InvalidateLayout();
}

void CellLayoutAttributesChanged(object sender, LayoutAttributesChangedEventArgs args)
{
var item = ItemsSource[args.NewAttributes.IndexPath];
if (item != null)
{
_cellSizeCache[item] = args.NewAttributes.Size;
}
}

protected virtual string DetermineCellReuseId()
{
if (ItemsView.ItemTemplate != null)
Expand Down Expand Up @@ -486,5 +505,25 @@ public TemplatedCell CreateMeasurementCell(NSIndexPath indexPath)

return templatedCell;
}

internal CGSize GetSizeForItem(NSIndexPath indexPath)
{
if (ItemsViewLayout.EstimatedItemSize.IsEmpty)
{
return ItemsViewLayout.ItemSize;
}

if (ItemsSource.IsIndexPathValid(indexPath))
{
var item = ItemsSource[indexPath];

if (item != null && _cellSizeCache.TryGetValue(item, out CGSize size))
{
return size;
}
}

return ItemsViewLayout.EstimatedItemSize;
}
}
}
48 changes: 1 addition & 47 deletions Xamarin.Forms.Platform.iOS/CollectionView/ItemsViewDelegator.cs
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Original file line number Diff line number Diff line change
@@ -1,10 +1,8 @@
using System;
using System.Collections.Generic;
using System.Linq;
using CoreGraphics;
using Foundation;
using UIKit;
using Xamarin.Forms.Internals;

namespace Xamarin.Forms.Platform.iOS
{
Expand All @@ -16,7 +14,6 @@ public class ItemsViewDelegator<TItemsView, TViewController> : UICollectionViewD
public TViewController ViewController { get; }

protected float PreviousHorizontalOffset, PreviousVerticalOffset;
readonly Dictionary<DataTemplate, CGSize> _templateSizeEstimates = new Dictionary<DataTemplate, CGSize>();

public ItemsViewDelegator(ItemsViewLayout itemsViewLayout, TViewController itemsViewController)
{
Expand Down Expand Up @@ -165,52 +162,9 @@ static NSIndexPath GetCenteredIndexPath(UICollectionView collectionView)
return centerItemIndex;
}

// Note: we are deliberately avoiding calculating the exact size of every item that the UICollectionViewFlowLayout
// requests from us; instead, we use the exact ItemSize (when possible) or the EstimatedItemSize.
// UICollectionViewFlowLayout will request the size for _every single item_ in our datasource, even if it's not going
// to be on screen yet. For small datasources, realizing the Forms content and measuring it is no problem.
// But for large datasets (hundreds or thousands of items), we'd be realizing a Forms datatemplate and binding it for
// every single item, which defeats virtualization almost entirely.
// So we only create a measurement cell and measure it in this method if we don't already have a cached estimate for
// that item's data template.

public override CGSize GetSizeForItem(UICollectionView collectionView, UICollectionViewLayout layout, NSIndexPath indexPath)
{
//if (ItemsViewLayout.EstimatedItemSize.IsEmpty)
//{
// return ItemsViewLayout.ItemSize;
//}

//var itemTemplate = ViewController.ItemsView.ItemTemplate;

//if (!(itemTemplate is DataTemplateSelector dataTemplateSelector))
//{
// // If the DataTemplate only maps to a single template, then our original size estimate will be fine
// return ItemsViewLayout.EstimatedItemSize;
//}

//// Determine the template type for the current item
//var targetTemplate = dataTemplateSelector.SelectDataTemplate(ViewController.ItemsSource[indexPath], ViewController.ItemsView);

//if (_templateSizeEstimates.TryGetValue(targetTemplate, out CGSize templateSizeEstimate))
//{
// // We've seen this template before; use the cached estimate
// return templateSizeEstimate;
//}

var measurementCell = ViewController.CreateMeasurementCell(indexPath);

//if (measurementCell == null)
//{
// // If we couldn't get a measurement cell for some reason, fall back to the old estimate
// return ItemsViewLayout.EstimatedItemSize;
//}

// Measure the cell and cache the result as our estimate for this template
var size = measurementCell.Measure();
_templateSizeEstimates[measurementCell.CurrentTemplate] = size;

return size;
return ViewController.GetSizeForItem(indexPath);
}
}
}
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