[LayoutBindings] Fix '[Preserve]' is obsolete warnings (#8529)

Fixes: #7480

Context: e604833

When [Layout Bindings][0] are enabled, a set of `partial` classes
will be generated which "mirrors" the `.axml` structure in C#.

Consider:

	dotnet new android -n com.example.codebehind

You enable Code-Behind by:

 1. Setting the `$(AndroidGenerateLayoutBindings)` MSBuild property
    to `true`.
 2. Using `android:id` on elements within `.axml` files.

Consider this patch to the above `dotnet new`:

	diff --git a/Resources/layout/activity_main.xml b/Resources/layout/activity_main.xml
	index f949852..c74521c 100644
	--- a/Resources/layout/activity_main.xml
	+++ b/Resources/layout/activity_main.xml
	@@ -8,6 +8,7 @@
	         android:layout_width="wrap_content"
	         android:layout_height="wrap_content"
	         android:layout_centerInParent="true"
	+        android:id="@+id/text"
	         android:text="@string/app_text"
	     />
	 </RelativeLayout>
	diff --git a/com.example.codebehind.csproj b/com.example.codebehind.csproj
	index 3fdbcb5..8190f84 100644
	--- a/com.example.codebehind.csproj
	+++ b/com.example.codebehind.csproj
	@@ -8,5 +8,6 @@
	     <ApplicationId>com.companyname.com.example.codebehind</ApplicationId>
	     <ApplicationVersion>1</ApplicationVersion>
	     <ApplicationDisplayVersion>1.0</ApplicationDisplayVersion>
	+    <AndroidGenerateLayoutBindings>true</AndroidGenerateLayoutBindings>
	   </PropertyGroup>
	 </Project>

The resulting output contains
`$(IntermediateOutputPath)codebehind\Binding.activity_main.g.cs`,
which declares a type based on the `.axml` file name, and a mirror of
the structure:

	namespace Binding {
	    // typename based on `activity_main.xml` filename
	    sealed partial class activity_main : global::Xamarin.Android.Design.LayoutBinding {
	        [global::Android.Runtime.PreserveAttribute (Conditional=true)]
	        public activity_main (Activity client, …);
	        // …

	        // `text` is via `android:id="@+id/text"`
	        public TextView text => …;
	    }
	}

The problem is the use of `PreserveAttribute` in the generated code;
it's been `[Obsolete]` since e604833 (over two years ago), which
means all projects using Layout Bindings and Layout Code-Behind always
have [CS0618][1] warnings.

Remove the emission of `PreserveAttribute`, so that CS0618 is no
longer generated.

Additionally, within the generated files disable the [CS1591][2] and
[CS8981][3] warnings:

  * CS1591 is around missing XML documentation comments.  These code
    behind files do not have XML documentation comments, so if/when
    `$(DocumentationFile)` is set, these will be emitted for the
    generated code.  We do not currently plan on emitting XML
    documentation comments, so disable this warning.

  * CS8981 is emitted when a type name consists solely of lowercase
    characters.  As the generated Binding types are based on the
    filename -- which is frequently all lowercase -- this warning
    may be emitted.

Finally, add `#nullable enable` to `LayoutBinding.cs` and fix the
nullable reference type warnings.

[0]: https://github.com/xamarin/xamarin-android/blob/2f4e01ec15102dd9cd922cbd833f6482d69512b5/Documentation/guides/LayoutCodeBehind.md
[1]: https://learn.microsoft.com/dotnet/csharp/language-reference/compiler-messages/cs0618
[2]: https://learn.microsoft.com/dotnet/csharp/language-reference/compiler-messages/cs1591
[3]: https://learn.microsoft.com/dotnet/csharp/language-reference/compiler-messages/warning-waves#cs8981---the-type-name-only-contains-lower-cased-ascii-characters

src/Xamarin.Android.Build.Tasks/Resources/LayoutBinding.cs

@@ -1,27 +1,29 @@
-using System;
+#nullable enable
+
+using System;
 using System.Diagnostics.CodeAnalysis;
 using Android.App;
 using Android.Views;
 
 namespace Xamarin.Android.Design
 {
-	public delegate Java.Lang.Object OnLayoutItemNotFoundHandler (int resourceId, Type expectedViewType);
+	public delegate Java.Lang.Object? OnLayoutItemNotFoundHandler (int resourceId, Type expectedViewType);
 
 	abstract class LayoutBinding
 	{
 		const DynamicallyAccessedMemberTypes Constructors = DynamicallyAccessedMemberTypes.PublicConstructors | DynamicallyAccessedMemberTypes.NonPublicConstructors;
 
-		Activity boundActivity;
-		View boundView;
-		OnLayoutItemNotFoundHandler onLayoutItemNotFound;
+		Activity? boundActivity;
+		View? boundView;
+		OnLayoutItemNotFoundHandler? onLayoutItemNotFound;
 
-		protected LayoutBinding (Activity activity, OnLayoutItemNotFoundHandler onLayoutItemNotFound = null)
+		protected LayoutBinding (Activity activity, OnLayoutItemNotFoundHandler? onLayoutItemNotFound = null)
 		{
 			boundActivity = activity ?? throw new ArgumentNullException (nameof (activity));
 			this.onLayoutItemNotFound = onLayoutItemNotFound;
 		}
 
-		protected LayoutBinding (View view, OnLayoutItemNotFoundHandler onLayoutItemNotFound = null)
+		protected LayoutBinding (View view, OnLayoutItemNotFoundHandler? onLayoutItemNotFound = null)
 		{
 			boundView = view ?? throw new ArgumentNullException (nameof (view));
 			this.onLayoutItemNotFound = onLayoutItemNotFound;
@@ -38,14 +40,14 @@ protected T FindView <
 			if (cachedField != null)
 				return cachedField;
 
-			T ret;
+			T? ret = null;
 			if (boundActivity != null)
 				ret = boundActivity.FindViewById <T> (resourceId);
-			else
+			else if (boundView != null)
 				ret = boundView.FindViewById <T> (resourceId);
 
 			if (ret == null && onLayoutItemNotFound != null)
-				ret = (T)onLayoutItemNotFound (resourceId, typeof (T));
+				ret = (T?)onLayoutItemNotFound (resourceId, typeof (T));
 
 			if (ret == null)
 				throw new global::System.InvalidOperationException ($"View not found (Resource ID: {resourceId})");
@@ -71,16 +73,16 @@ T __FindFragment<
 				T
 		> (
 				int resourceId,
-				Func<Activity, T> finder,
-				ref T cachedField)
+				Func<Activity, T?> finder,
+				ref T? cachedField)
 			where T: Java.Lang.Object
 		{
 			if (cachedField != null)
 				return cachedField;
 
 			var ret = finder (EnsureActivity ());
 			if (ret == null && onLayoutItemNotFound != null)
-				ret = (T)onLayoutItemNotFound (resourceId, typeof (T));
+				ret = (T?)onLayoutItemNotFound (resourceId, typeof (T));
 
 			if (ret == null)
 				throw new InvalidOperationException ($"Fragment not found (ID: {resourceId}; Type: {typeof (T)})");
@@ -89,29 +91,25 @@ T __FindFragment<
 			return ret;
 		}
 #if __ANDROID_11__
+#pragma warning disable CA1422
 		protected T FindFragment<
 				[DynamicallyAccessedMembers (Constructors)]
 				T
 		> (
 				int resourceId,
-				global::Android.App.Fragment __ignoreMe,
-				ref T cachedField
+				global::Android.App.Fragment? __ignoreMe,
+				ref T? cachedField
 		)
 			where T: global::Android.App.Fragment
 		{
-			return __FindFragment<T> (resourceId, (activity) => activity.FragmentManager.FindFragmentById<T> (resourceId), ref cachedField);
+			return __FindFragment<T> (resourceId, (activity) => activity.FragmentManager?.FindFragmentById<T> (resourceId), ref cachedField);
 		}
+#pragma warning restore CA1422
 #endif  // __ANDROID_11__
 
-#if __HAVE_SUPPORT__
-		protected T FindFragment <T> (int resourceId, global::Android.Support.V4.App.Fragment __ignoreMe, ref T cachedField) where T: global::Android.Support.V4.App.Fragment
-		{
-			return __FindFragment<T> (resourceId, (activity) => activity.FragmentManager.FindFragmentById<T> (resourceId), ref cachedField);
-		}
-#endif // __HAVE_SUPPORT__
-
 #if __HAVE_ANDROIDX__
-		protected T FindFragment<T> (int resourceId, global::AndroidX.Fragment.App.Fragment __ignoreMe, ref T cachedField) where T: global::AndroidX.Fragment.App.Fragment
+		protected T FindFragment<T> (int resourceId, global::AndroidX.Fragment.App.Fragment? __ignoreMe, ref T? cachedField)
+			where T : global::AndroidX.Fragment.App.Fragment
 		{
 			return __FindFragment(resourceId, (activity) => {
 				if (activity is AndroidX.Fragment.App.FragmentActivity activity_) {

src/Xamarin.Android.Build.Tasks/Tasks/GenerateLayoutBindings.BindingGenerator.cs

@@ -30,8 +30,6 @@ public sealed class State
 				// generating code for the partial Activity class
 				public string BindingClassName { get; }
 
-				public bool LinkerPreserveConstructors { get; set; }
-
 				public List<string> ExtraImportNamespaces { get; } = new List <string> ();
 
 				public string AndroidFragmentType { get; }
@@ -155,20 +153,18 @@ protected virtual void WriteOnSetContentViewPartials (State state)
 				WritePartialClassOnSetContentViewPartial_Int (state);
 			}
 
-			public State BeginBindingFile (StreamWriter writer, string layoutResourceId, string classNamespace, string className, string androidFragmentType, bool linkerPreserveConstructors = true)
+			public State BeginBindingFile (StreamWriter writer, string layoutResourceId, string classNamespace, string className, string androidFragmentType)
 			{
-				var state = new State (writer, className, !String.IsNullOrWhiteSpace (classNamespace), androidFragmentType) {
-					LinkerPreserveConstructors = linkerPreserveConstructors
-				};
+				var state = new State (writer, className, !String.IsNullOrWhiteSpace (classNamespace), androidFragmentType);
 				BeginBindingFile (state, layoutResourceId, classNamespace, className);
-				WriteBindingConstructors (state, className, state.LinkerPreserveConstructors);
+				WriteBindingConstructors (state, className);
 				return state;
 			}
 
 			protected abstract void BeginBindingFile (State state, string layoutResourceId, string classNamespace, string className);
 			public abstract void EndBindingFile (State state);
 
-			protected abstract void WriteBindingConstructors (State state, string className, bool linkerPreserve);
+			protected abstract void WriteBindingConstructors (State state, string className);
 			protected abstract void WriteBindingViewProperty (State state, LayoutWidget widget, string resourceNamespace);
 			protected abstract void WriteBindingFragmentProperty (State state, LayoutWidget widget, string resourceNamespace);
 			protected abstract void WriteBindingMixedProperty (State state, LayoutWidget widget, string resourceNamespace);
@@ -287,10 +283,11 @@ protected void WriteBindingPropertyBackingField (State state, LayoutWidget widge
 				WriteResetLocation (state);
 			}
 
-			public void WriteComment (State state, string text)
+			public void WriteComment (State state, params string [] text)
 			{
 				EnsureArgument (state, nameof (state));
-				WriteLineIndent (state, $"{LineCommentString}{text}");
+				foreach (string line in text)
+					WriteLineIndent (state, $"{LineCommentString}{line}");
 			}
 
 			public void WriteComment (State state, ICollection<string> lines)

src/Xamarin.Android.Build.Tasks/Tasks/GenerateLayoutBindings.CSharpBindingGenerator.cs

@@ -157,8 +157,33 @@ void WriteClassClose (State state)
 				WriteLineIndent (state, "}");
 			}
 
+			void WriteDisableWarnings (State state)
+			{
+				state.WriteLine ("#pragma warning disable CS8981");
+				state.WriteLine ("#pragma warning disable CS1591");
+			}
+
+			void WriteEnableWarnings (State state)
+			{
+				state.WriteLine ("#pragma warning restore CS1591");
+				state.WriteLine ("#pragma warning restore CS8981");
+			}
+
+			void WriteAutoGeneratedComment (State state)
+			{
+				state.WriteLine (@"//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by a tool.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------");
+			}
+
 			void WriteFilePreamble (State state, string classNamespace)
 			{
+				WriteAutoGeneratedComment (state);
 				WriteUsings (state);
 				state.WriteLine ();
 				WriteNamespaceOpen (state, classNamespace);
@@ -172,13 +197,15 @@ void WriteNamespaceOpen (State state, string classNamespace)
 				state.WriteLine ($"namespace {classNamespace}");
 				state.WriteLine ("{");
 				state.IncreaseIndent ();
+				WriteDisableWarnings (state);
 			}
 
 			void WriteNamespaceClose (State state)
 			{
 				if (!state.IsInNamespace)
 					return;
 
+				WriteEnableWarnings (state);
 				state.DecreaseIndent ();
 				state.WriteLine ("}");
 			}
@@ -200,15 +227,14 @@ void WriteUsing (State state, string ns)
 				state.WriteLine ($"using global::{ns};");
 			}
 
-			protected override void WriteBindingConstructors (State state, string className, bool linkerPreserve)
+			protected override void WriteBindingConstructors (State state, string className)
 			{
-				WriteConstructor (state, className, "Android.App.Activity", linkerPreserve);
-				WriteConstructor (state, className, "Android.Views.View", linkerPreserve);
+				WriteConstructor (state, className, "Android.App.Activity");
+				WriteConstructor (state, className, "Android.Views.View");
 			}
 
-			void WriteConstructor (State state, string className, string clientType, bool linkerPreserve)
+			void WriteConstructor (State state, string className, string clientType)
 			{
-				WritePreserveAtribute (state, linkerPreserve);
 				WriteLineIndent (state, $"public {className} (");
 				state.IncreaseIndent ();
 				WriteLineIndent (state, $"global::{clientType} client,");
@@ -221,14 +247,6 @@ void WriteConstructor (State state, string className, string clientType, bool li
 				state.WriteLine ();
 			}
 
-			void WritePreserveAtribute (State state, bool linkerPreserve)
-			{
-				if (!linkerPreserve)
-					return;
-
-				WriteLineIndent (state, $"[global::Android.Runtime.PreserveAttribute (Conditional=true)]");
-			}
-
 			public override void EndBindingFile (State state)
 			{
 				EnsureArgument (state, nameof (state));

src/Xamarin.Android.Build.Tasks/Tasks/GenerateResourceDesignerIntermediateClass.cs

@@ -24,6 +24,10 @@ public class GenerateResourceDesignerIntermediateClass : AndroidTask
 
 namespace %NAMESPACE% {
 	#pragma warning disable IDE0002
+	/// <summary>
+	/// Android Resource Designer class.
+	/// Exposes the Android Resource designer assembly into the project Namespace.
+	/// </summary>
 	public partial class Resource : %BASECLASS% {
 	}
 	#pragma warning restore IDE0002

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/CodeBehindTests.cs

@@ -339,6 +339,7 @@ void SuccessfulBuild_AndroidX (TestProjectInfo testInfo, bool many, bool dtb, Lo
 
 			CopyLogs (testInfo, true);
 			Assert.That (success, Is.True, "Build should have succeeded");
+			Assert.IsTrue (StringAssertEx.ContainsText (builder.LastBuildOutput, " 0 Warning(s)"), $"{builder.BuildLogFile} should have no MSBuild warnings.");
 
 			CopyGeneratedFiles (testInfo);
 
@@ -508,7 +509,8 @@ string GetBuildTarget (bool isDTB)
 		string[] GetBuildProperties (LocalBuilder builder, bool manyBuild, bool dtbBuild, bool referenceAndroidX, params string[] extraConstants)
 		{
 			var ret = new List <string> {
-				"AndroidGenerateLayoutBindings=true"
+				"AndroidGenerateLayoutBindings=true",
+				"\"NoWarn=CS0414;CA1416;CS1591;XA1005;XA4225\""
 			};
 			if (manyBuild)
 				ret.Add ("ForceParallelBuild=true");

tests/CodeBehind/BuildTests/CodeBehindBuildTests.NET.csproj

@@ -64,6 +64,6 @@
     <ProjectReference Include="..\CommonSampleLibrary\CommonSampleLibrary.NET.csproj" />
   </ItemGroup>
   <ItemGroup Condition=" '$(ReferenceAndroidX)' == 'True' ">
-    <PackageReference Include="Xamarin.AndroidX.AppCompat" Version="1.4.1.1" />
+    <PackageReference Include="Xamarin.AndroidX.AppCompat" Version="1.6.1.5" />
   </ItemGroup>
 </Project>

tests/CodeBehind/BuildTests/MainMergeActivity.cs

@@ -24,6 +24,8 @@ protected override void OnCreate (Bundle savedInstanceState)
 
 #if NOT_CONFLICTING_FRAGMENT
 			CommonSampleLibrary.LogFragment log2 = secondary_log_fragment;
+#elif __HAVE_ANDROIDX__
+			global::AndroidX.Fragment.App.Fragment log2 = secondary_log_fragment;
 #else
 			global::Android.App.Fragment log2 = secondary_log_fragment;
 #endif

tests/CodeBehind/BuildTests/Properties/AndroidManifest.xml

@@ -1,6 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="1" android:versionName="1.0" package="com.xamarin.CodeBehindBuildTests">
-	<uses-sdk android:minSdkVersion="21" />
 	<application android:allowBackup="true" android:icon="@mipmap/icon" android:label="@string/app_name">
 	</application>
 </manifest>

tests/CodeBehind/CommonSampleLibrary/CommonSampleLibrary.NET.csproj

@@ -5,4 +5,10 @@
     <AssemblyName>CommonSampleLibrary</AssemblyName>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
   </PropertyGroup>
+  <PropertyGroup>
+    <DefineConstants Condition=" '$(ExtraConstants)' != '' ">$(DefineConstants);$(ExtraConstants)</DefineConstants>
+  </PropertyGroup>
+  <ItemGroup Condition=" '$(ReferenceAndroidX)' == 'True' ">
+    <PackageReference Include="Xamarin.AndroidX.AppCompat" Version="1.6.1.5" />
+  </ItemGroup>
 </Project>

tests/CodeBehind/CommonSampleLibrary/Logger/LogFragment.cs

@@ -19,14 +19,21 @@
 using Android.Views;
 using Android.Widget;
 using Android.Graphics;
+using Android.Util;
 
 namespace CommonSampleLibrary
 {
+	#pragma warning disable CA1422
 	/**
 	* Simple fraggment which contains a LogView and uses is to output log data it receives
 	* through the LogNode interface.
 	*/
-	public class LogFragment : Fragment
+	public class LogFragment : 
+#if __HAVE_ANDROIDX__
+	AndroidX.Fragment.App.Fragment
+#else
+	Fragment
+#endif
 	{
 		LogView mLogView;
 		ScrollView mScrollView;
@@ -60,7 +67,7 @@ public View InflateViews ()
 
 			mLogView.Gravity = GravityFlags.Bottom;
 			mLogView.SetTextAppearance (Activity, Android.Resource.Style.TextAppearanceMedium);
-
+			
 			mScrollView.AddView (mLogView);
 			return mScrollView;
 		}
@@ -77,5 +84,6 @@ public LogView LogView {
 			get { return mLogView; }
 		}
 	}
+	#pragma warning restore CA1422
 }