From c09c373e97abd746515aa7f556ffff01982e9965 Mon Sep 17 00:00:00 2001
From: adrianknight89 <adrianknight89@outlook.com>
Date: Thu, 26 Sep 2019 19:16:03 -0500
Subject: [PATCH] [iOS] Fix MeasureFirstItem for CollectionView (#7622)

* fix MeasureFirstItem

* added test

* changed file name

* fix path

* added text

* remove 5455

* drop private
fixes #7621
---
 .../Issue7621.xaml                            |  33 ++++++
 .../Issue7621.xaml.cs                         | 104 ++++++++++++++++++
 ...rin.Forms.Controls.Issues.Shared.projitems |  12 ++
 .../HeightConstrainedTemplatedCell.cs         |   3 +-
 .../CollectionView/TemplatedCell.cs           |  15 ++-
 .../WidthConstrainedTemplatedCell.cs          |   3 +-
 6 files changed, 166 insertions(+), 4 deletions(-)
 create mode 100644 Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7621.xaml
 create mode 100644 Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7621.xaml.cs

diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7621.xaml b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7621.xaml
new file mode 100644
index 000000000..477cbcb3a
--- /dev/null
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7621.xaml
@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<controls:TestContentPage
+    xmlns="http://xamarin.com/schemas/2014/forms"
+    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+    xmlns:controls="clr-namespace:Xamarin.Forms.Controls"
+    x:Class="Xamarin.Forms.Controls.Issues.Issue7621">
+    <Grid>
+        <Grid.RowDefinitions>
+            <RowDefinition Height="Auto"/>
+            <RowDefinition/>
+        </Grid.RowDefinitions>
+
+        <StackLayout Grid.Row="0" Margin="10" Spacing="10" Orientation="Vertical" BackgroundColor="Beige">
+            <Label LineBreakMode="WordWrap" Text="Verify that all Frames have the same width and height. Scroll to the end of CollectionView and back. Verify that item sizes do not change. If so, MeasureFirstItem is working." HorizontalTextAlignment="Center" VerticalTextAlignment="Center"/>
+            <Label LineBreakMode="WordWrap" Text="MeasureAllItems is not part of this test, but you could switch to it for manual testing." HorizontalTextAlignment="Center" VerticalTextAlignment="Center"/>
+            <Button Text="Switch to MeasureAllItems" HorizontalOptions="Center" VerticalOptions="Center" Clicked="ButtonClicked"/>
+        </StackLayout>
+
+        <CollectionView Grid.Row="1" ItemSizingStrategy="MeasureFirstItem" ItemsSource="{Binding Items}">
+            <CollectionView.ItemsLayout>
+                <GridItemsLayout Span="2" Orientation="Horizontal" HorizontalItemSpacing="5" VerticalItemSpacing="5"/>
+            </CollectionView.ItemsLayout>
+
+            <CollectionView.ItemTemplate>
+                <DataTemplate>
+                    <Frame CornerRadius="5" HasShadow="False" Padding="10" BackgroundColor="{Binding BackgroundColor}">
+                        <Image Source="{Binding Source}" Aspect="AspectFit"/>
+                    </Frame>
+                </DataTemplate>
+            </CollectionView.ItemTemplate>
+        </CollectionView>
+    </Grid>
+</controls:TestContentPage>
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7621.xaml.cs b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7621.xaml.cs
new file mode 100644
index 000000000..f85d4ec4a
--- /dev/null
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Issue7621.xaml.cs
@@ -0,0 +1,104 @@
+using System.Collections.ObjectModel;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+using System;
+using Xamarin.Forms.Xaml;
+using System.Threading;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+#if UITEST
+using Xamarin.UITest;
+using Xamarin.UITest.Queries;
+using NUnit.Framework;
+using Xamarin.Forms.Core.UITests;
+using System.Linq;
+#endif
+
+namespace Xamarin.Forms.Controls.Issues
+{
+#if UITEST
+	[Category(UITestCategories.CollectionView)]
+#endif
+#if APP
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+#endif
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Github, 7621, "[iOS] MeasureFirstItem is broken for CollectionView", PlatformAffected.iOS)]
+	public partial class Issue7621 : TestContentPage
+	{
+		bool isMeasuringAllItems = false;
+
+#if APP
+		public Issue7621()
+		{
+			Device.SetFlags(new List<string> { CollectionView.CollectionViewExperimental });
+
+			InitializeComponent();
+
+			BindingContext = new ViewModel7621();
+		}
+#endif
+
+		protected override void Init()
+		{
+
+		}
+
+		void ButtonClicked(object sender, EventArgs e)
+		{
+			var button = sender as Button;
+			var grid = button.Parent.Parent as Grid;
+			var collectionView = grid.Children[1] as CollectionView;
+
+			isMeasuringAllItems = !isMeasuringAllItems;
+
+			collectionView.ItemSizingStrategy = isMeasuringAllItems ? ItemSizingStrategy.MeasureAllItems : ItemSizingStrategy.MeasureFirstItem;
+			button.Text = isMeasuringAllItems ? "Switch to MeasureFirstItem" : "Switch to MeasureAllItems";
+		}
+	}
+
+	[Preserve(AllMembers = true)]
+	public class ViewModel7621
+	{
+		public ObservableCollection<Model7621> Items { get; set; }
+
+		public ViewModel7621()
+		{
+			var collection = new ObservableCollection<Model7621>();
+			Color[] _colors =
+			{
+				Color.Red,
+				Color.Blue,
+				Color.Green,
+				Color.Yellow
+			};
+			string[] _images =
+			{
+				"cover1.jpg",
+				"oasis.jpg",
+				"photo.jpg",
+				"Vegetables.jpg"
+			};
+
+			for (var i = 0; i < 30; i++)
+			{
+				collection.Add(new Model7621
+				{
+					BackgroundColor = _colors[i % 4],
+					Source = _images[i % 4]
+				});
+			}
+
+			Items = collection;
+		}
+	}
+
+	[Preserve(AllMembers = true)]
+	public class Model7621
+	{
+		public Color BackgroundColor { get; set; }
+
+		public string Source { get; set; }
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
index db64afd36..c4eb204fe 100644
--- a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
@@ -19,6 +19,9 @@
     <Compile Include="$(MSBuildThisFileDirectory)CollectionViewHeaderFooterView.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)CollectionViewItemsUpdatingScrollMode.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue3475.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Issue7621.xaml.cs">
+      <SubType>Code</SubType>
+    </Compile>
     <Compile Include="$(MSBuildThisFileDirectory)Issue6889.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue6945.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Issue7329.cs" />
@@ -1381,6 +1384,9 @@
     <Compile Update="C:\Users\hartez\Documents\Xamarin\Xamarin.Forms\Xamarin.Forms.Controls.Issues\Xamarin.Forms.Controls.Issues.Shared\Issue7519Xaml.xaml.cs">
       <DependentUpon>Issue7519Xaml.xaml</DependentUpon>
     </Compile>
+    <Compile Update="$(MSBuildThisFileDirectory)Issue7621.xaml.cs">
+      <DependentUpon>Issue7621.xaml</DependentUpon>
+    </Compile>
   </ItemGroup>
   <ItemGroup>
     <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue1455.xaml">
@@ -1418,4 +1424,10 @@
       <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
     </EmbeddedResource>
   </ItemGroup>
+  <ItemGroup>
+    <EmbeddedResource Include="$(MSBuildThisFileDirectory)Issue7621.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </EmbeddedResource>
+  </ItemGroup>
 </Project>
diff --git a/Xamarin.Forms.Platform.iOS/CollectionView/HeightConstrainedTemplatedCell.cs b/Xamarin.Forms.Platform.iOS/CollectionView/HeightConstrainedTemplatedCell.cs
index 3d782eecd..efdc14e44 100644
--- a/Xamarin.Forms.Platform.iOS/CollectionView/HeightConstrainedTemplatedCell.cs
+++ b/Xamarin.Forms.Platform.iOS/CollectionView/HeightConstrainedTemplatedCell.cs
@@ -12,8 +12,9 @@ namespace Xamarin.Forms.Platform.iOS
 
 		public override void ConstrainTo(CGSize constraint)
 		{
+			base.ConstrainTo(constraint);
+
 			ConstrainedDimension = constraint.Height;
-			Layout(constraint);
 		}
 
 		protected override (bool, Size) NeedsContentSizeUpdate(Size currentSize)
diff --git a/Xamarin.Forms.Platform.iOS/CollectionView/TemplatedCell.cs b/Xamarin.Forms.Platform.iOS/CollectionView/TemplatedCell.cs
index e6eb5e69c..cbf1f734a 100644
--- a/Xamarin.Forms.Platform.iOS/CollectionView/TemplatedCell.cs
+++ b/Xamarin.Forms.Platform.iOS/CollectionView/TemplatedCell.cs
@@ -10,6 +10,8 @@ namespace Xamarin.Forms.Platform.iOS
 	{
 		public event EventHandler<EventArgs> ContentSizeChanged;
 
+		protected CGSize ConstrainedSize;
+
 		protected nfloat ConstrainedDimension;
 
 		DataTemplate _currentTemplate;
@@ -25,9 +27,18 @@ namespace Xamarin.Forms.Platform.iOS
 
 		internal IVisualElementRenderer VisualElementRenderer { get; private set; }
 
+		public override void ConstrainTo(CGSize constraint)
+		{
+			ConstrainedSize = constraint;
+		}
+
 		public override void ConstrainTo(nfloat constant)
 		{
 			ConstrainedDimension = constant;
+
+			// Reset constrained size in case ItemSizingStrategy changes
+			// and we want to measure each item
+			ConstrainedSize = default(CGSize);
 		}
 
 		public override UICollectionViewLayoutAttributes PreferredLayoutAttributesFittingAttributes(
@@ -35,8 +46,8 @@ namespace Xamarin.Forms.Platform.iOS
 		{
 			var preferredAttributes = base.PreferredLayoutAttributesFittingAttributes(layoutAttributes);
 
-			// Measure this cell (including the Forms element)
-			var size = Measure();
+			// Measure this cell (including the Forms element) if there is no constrained size
+			var	size = ConstrainedSize == default(CGSize) ? Measure() : ConstrainedSize;
 
 			// Update the size of the root view to accommodate the Forms element
 			var nativeView = VisualElementRenderer.NativeView;
diff --git a/Xamarin.Forms.Platform.iOS/CollectionView/WidthConstrainedTemplatedCell.cs b/Xamarin.Forms.Platform.iOS/CollectionView/WidthConstrainedTemplatedCell.cs
index b1fc3a579..c56c6ce4a 100644
--- a/Xamarin.Forms.Platform.iOS/CollectionView/WidthConstrainedTemplatedCell.cs
+++ b/Xamarin.Forms.Platform.iOS/CollectionView/WidthConstrainedTemplatedCell.cs
@@ -12,8 +12,9 @@ namespace Xamarin.Forms.Platform.iOS
 
 		public override void ConstrainTo(CGSize constraint)
 		{
+			base.ConstrainTo(constraint);
+
 			ConstrainedDimension = constraint.Width;
-			Layout(constraint);
 		}
 
 		protected override (bool, Size) NeedsContentSizeUpdate(Size currentSize)