[.NET 5] fix XA1008 warning when $(TargetFrameworkVersion)=v5.0 (#4598)

Context: https://github.com/dotnet/designs/blob/master/accepted/2020/net5/net5.md

Historically, Xamarin.Android has used `$(TargetFrameworkVersion)` to
control the `Mono.Android.dll` binding which is used in apps, and was
treated as the Android version number, e.g.
`$(TargetFrameworkVersion)`=v5.0 caused Xamarin.Android pull in the
`Mono.Android.dll` with bindings for *Android* 5.0 (API-21).

With .NET 5, `$(TargetFrameworkVersion)` *instead* is the version of
.NET, i.e. .NET 5 will have `$(TargetFrameworkVersion)`=v5.0.
The (new?) `$(TargetPlatformVersion)` MSBuild property will now
control which `Mono.Android.dll` binding version should be used.

Unfortunately, not everything within the Xamarin.Android build system
is cognizant of this change.  Consequently, when building within a
.NET 5 environment, *all* builds will emit this XA1008 warning:

	warning XA1008: The TargetFrameworkVersion (Android API level 21) is lower than the targetSdkVersion (29).
	Please increase the `$(TargetFrameworkVersion)` or decrease the `android:targetSdkVersion` in the `AndroidManifest.xml` so that the API levels match.

The XA1008 warning is emitted because Google Play requires a minimum
version of API-29 (Android v10.0), and `$(TargetFrameworkVersion)` was
previously used to infer a minimum API level, and Android v5.0/API-21
is less than the required API-29.

To fix this, I modified the `<CheckGoogleSdkRequirements/>` MSBuild
task so that:

 1. `CheckGoogleSdkRequirements.TargetFrameworkVersion` is optional.
 2. A new `CheckGoogleSdkRequirements.ApiLevel` property is available,
    and allows explicitly providing the API level.

In .NET 5 builds, `CheckGoogleSdkRequirements.TargetFrameworkVersion`
won't be set; `CheckGoogleSdkRequirements.ApiLevel` will instead be
set based on `$(_AndroidApiLevel)`.

This way we will still get some important warnings for .NET 5 such as
`targetSdkVersion` or `minSdkVersion` that need to be changed in the
`AndroidManifest.xml` file.

I also fixed up `LastBuildOutput`, we needed the value for .NET 5
MSBuild tests and I removed unnecessary usage of `yield return`.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.Tooling.targets

@@ -11,9 +11,10 @@ called for "legacy" projects in Xamarin.Android.Legacy.targets.
 
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
-  <UsingTask TaskName="Xamarin.Android.Tasks.ValidateJavaVersion" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
-  <UsingTask TaskName="Xamarin.Android.Tasks.ResolveAndroidTooling" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
-  <UsingTask TaskName="Xamarin.Android.Tasks.RuntimeIdentifierToAbi" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.CheckGoogleSdkRequirements" AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.ResolveAndroidTooling"      AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.RuntimeIdentifierToAbi"     AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.ValidateJavaVersion"        AssemblyFile="$(_XamarinAndroidBuildTasksAssembly)" />
 
   <Target Name="_ResolveAndroidTooling">
     <ValidateJavaVersion
@@ -61,4 +62,12 @@ called for "legacy" projects in Xamarin.Android.Legacy.targets.
     </RuntimeIdentifierToAbi>
   </Target>
 
+  <Target Name="_CheckGoogleSdkRequirements"
+      Condition="Exists('$(IntermediateOutputPath)android\AndroidManifest.xml') And '$(AndroidEnableGooglePlayStoreChecks)' == 'true' ">
+    <CheckGoogleSdkRequirements
+        ApiLevel="$(_AndroidApiLevel)"
+        ManifestFile="$(IntermediateOutputPath)android\AndroidManifest.xml"
+    />
+  </Target>
+
 </Project>

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

@@ -14,17 +14,26 @@ public class CheckGoogleSdkRequirements : AndroidTask
 	{
 		public override string TaskPrefix => "CGS";
 
-		[Required]
+		/// <summary>
+		/// This will be blank for .NET 5 builds
+		/// </summary>
 		public string TargetFrameworkVersion { get; set; }
 
+		/// <summary>
+		/// This is used instead of TargetFrameworkVersion for .NET 5 builds
+		/// </summary>
+		public int ApiLevel { get; set; }
+
 		[Required]
 		public string ManifestFile { get; set; }
 
 		public override bool RunTask ()
 		{
 			ManifestDocument manifest = new ManifestDocument (ManifestFile);
 
-			var compileSdk = MonoAndroidHelper.SupportedVersions.GetApiLevelFromFrameworkVersion (TargetFrameworkVersion);
+			var compileSdk = string.IsNullOrEmpty (TargetFrameworkVersion) ?
+				ApiLevel :
+				MonoAndroidHelper.SupportedVersions.GetApiLevelFromFrameworkVersion (TargetFrameworkVersion);
 
 			if (!int.TryParse (manifest.GetMinimumSdk (), out int minSdk)) {
 				minSdk = 1;

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

@@ -69,6 +69,7 @@ public void DotNetPublish (string runtimeIdentifier, bool isRelease)
 
 			var dotnet = CreateDotNetBuilder (proj);
 			Assert.IsTrue (dotnet.Publish (), "`dotnet publish` should succeed");
+			Assert.IsTrue (StringAssertEx.ContainsText (dotnet.LastBuildOutput, " 0 Warning(s)"), "Should have no MSBuild warnings.");
 
 			var apk = Path.Combine (Root, dotnet.ProjectDirectory, proj.OutputPath,
 				runtimeIdentifier, "UnnamedProject.UnnamedProject.apk");

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/Builder.cs

@@ -30,11 +30,9 @@ public class Builder : IDisposable
 		public IEnumerable<string> LastBuildOutput {
 			get {
 				if (!string.IsNullOrEmpty (buildLogFullPath) && File.Exists (buildLogFullPath)) {
-					foreach (var line in File.ReadLines (buildLogFullPath, Encoding.UTF8)) {
-						yield return line;
-					}
+					return File.ReadLines (buildLogFullPath, Encoding.UTF8);
 				}
-				yield return String.Empty;
+				return Enumerable.Empty<string> ();
 			}
 		}
 		public TimeSpan LastBuildTime { get; protected set; }

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/DotNetCLI.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.IO;
+using System.Linq;
 using System.Text;
 
 namespace Xamarin.ProjectTools
@@ -84,6 +85,15 @@ public bool Publish (string target = null)
 			return Execute (arguments.ToArray ());
 		}
 
+		public IEnumerable<string> LastBuildOutput {
+			get {
+				if (!string.IsNullOrEmpty (BuildLogFile) && File.Exists (BuildLogFile)) {
+					return File.ReadLines (BuildLogFile, Encoding.UTF8);
+				}
+				return Enumerable.Empty<string> ();
+			}
+		}
+
 		List<string> GetDefaultCommandLineArgs (string verb, string target = null)
 		{
 			string testDir = Path.GetDirectoryName (projectOrSolution);

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -40,7 +40,6 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 <UsingTask TaskName="Xamarin.Android.Tasks.CalculateAdditionalResourceCacheDirectories" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CalculateLayoutCodeBehind" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CalculateProjectDependencies" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
-<UsingTask TaskName="Xamarin.Android.Tasks.CheckGoogleSdkRequirements" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CheckForRemovedItems" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CheckForInvalidResourceFileNames" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CompileToDalvik" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
@@ -550,14 +549,6 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
   />
 </Target>
 
-<Target Name="_CheckGoogleSdkRequirements"
-    Condition="Exists('$(IntermediateOutputPath)android\AndroidManifest.xml') And '$(AndroidEnableGooglePlayStoreChecks)' == 'true' ">
-  <CheckGoogleSdkRequirements
-      TargetFrameworkVersion="$(TargetFrameworkVersion)"
-      ManifestFile="$(IntermediateOutputPath)android\AndroidManifest.xml"
-  />
-</Target>
-
 <!--
 *******************************************
           Application Build

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Legacy.targets

@@ -143,7 +143,8 @@ projects. .NET 5 projects will not import this file.
     </CleanDependsOn>
   </PropertyGroup>
 
-  <UsingTask TaskName="Xamarin.Android.Tasks.Legacy.ValidateJavaVersion" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.CheckGoogleSdkRequirements"   AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
+  <UsingTask TaskName="Xamarin.Android.Tasks.Legacy.ValidateJavaVersion"   AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
   <UsingTask TaskName="Xamarin.Android.Tasks.Legacy.ResolveAndroidTooling" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 
   <Target Name="_GetReferenceAssemblyPaths">
@@ -245,4 +246,12 @@ projects. .NET 5 projects will not import this file.
     </ItemGroup>
   </Target>
 
+  <Target Name="_CheckGoogleSdkRequirements"
+      Condition="Exists('$(IntermediateOutputPath)android\AndroidManifest.xml') And '$(AndroidEnableGooglePlayStoreChecks)' == 'true' ">
+    <CheckGoogleSdkRequirements
+        TargetFrameworkVersion="$(TargetFrameworkVersion)"
+        ManifestFile="$(IntermediateOutputPath)android\AndroidManifest.xml"
+    />
+  </Target>
+
 </Project>