[Xamarin.Android.Build.Tasks] remove TFI == MonoAndroid checks

In preparation for .NET 5, we think the `$(TargetFrameworkMoniker)`
*might* be:

    .NETCoreApp,Version=5.0,Profile=Xamarin.Android

It might also be `Profile=Android`, we don't know for sure yet.

The TFM currently is:

    MonoAndroid,Version=v10.0

I think we can (and should) remove any code during the build that
looks at `$(TargetFrameworkIdentifier)`.

The places we are currently doing this are for performance:

* The `_ResolveLibraryProjectImports` or `_GenerateJavaStubs` targets
  do not need to run against .NET standard assemblies.

We can rely on a reference to `Mono.Android.dll` instead. We are
already using System.Reflection.Metadata to see if the assembly
actually has this reference.

So an example of the condition:

    Condition="'%(ResolvedUserAssemblies.TargetFrameworkIdentifier)' == 'MonoAndroid' Or '%(ResolvedUserAssemblies.HasMonoAndroidReference)' == 'True'"

Can just be:

    Condition="'%(ResolvedUserAssemblies.HasMonoAndroidReference)' == 'True'"

I also took the opportunity for two small perf improvements:

* `<FilterAssemblies/>` only needs to look for obsolete attributes in
  assemblies that reference `Mono.Android.dll`.

* `<ResolveAssemblies/>` was collection a dictionary of API levels and
  then iterating over it later to emit warnings. This now just emits
  warnings as assemblies are processed.

~~ Results ~~

Testing a build with no changes for the SmartHotel360 app:

    Before:
     60 ms  FilterAssemblies                           2 calls
    318 ms  ResolveAssemblies                          1 calls
    After:
     49 ms  FilterAssemblies                           2 calls
    317 ms  ResolveAssemblies                          1 calls

I would think there is a small ~12ms performance improvement to all
builds of this app.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Designer.targets

@@ -57,7 +57,7 @@ Copyright (C) 2016 Xamarin. All rights reserved.
     />
     <_ResolvedUserMonoAndroidAssembliesForDesigner
         Include="@(ResolvedUserAssemblies)"
-        Condition="'%(ResolvedUserAssemblies.TargetFrameworkIdentifier)' == 'MonoAndroid' Or '%(ResolvedUserAssemblies.HasMonoAndroidReference)' == 'True'"
+        Condition="'%(ResolvedUserAssemblies.HasMonoAndroidReference)' == 'True'"
     />
   </ItemGroup>
 </Target>

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

@@ -19,7 +19,6 @@ public class FilterAssemblies : AndroidTask
 	{
 		public override string TaskPrefix => "FLT";
 
-		const string TargetFrameworkIdentifier = "MonoAndroid";
 		const string MonoAndroidReference = "Mono.Android";
 
 		public bool DesignTimeBuild { get; set; }
@@ -42,19 +41,9 @@ public override bool RunTask ()
 				}
 				using (var pe = new PEReader (File.OpenRead (assemblyItem.ItemSpec))) {
 					var reader = pe.GetMetadataReader ();
-					var assemblyDefinition = reader.GetAssemblyDefinition ();
-					var targetFrameworkIdentifier = GetTargetFrameworkIdentifier (assemblyDefinition, reader);
-					if (string.Compare (targetFrameworkIdentifier, TargetFrameworkIdentifier, StringComparison.OrdinalIgnoreCase) == 0) {
-						output.Add (assemblyItem);
-						continue;
-					}
-
-					// In the rare case, [assembly: TargetFramework("MonoAndroid,Version=v9.0")] may not match
-					Log.LogDebugMessage ($"{nameof (TargetFrameworkIdentifier)} did not match: {assemblyItem.ItemSpec}");
-
-					// Fallback to looking for a Mono.Android reference
+					// By default look for a reference to Mono.Android.dll
 					if (HasReference (reader)) {
-						Log.LogDebugMessage ($"{MonoAndroidReference} reference found: {assemblyItem.ItemSpec}");
+						CheckAssemblyAttributes (reader);
 						output.Add (assemblyItem);
 						continue;
 					}
@@ -71,27 +60,13 @@ public override bool RunTask ()
 			return !Log.HasLoggedErrors;
 		}
 
-		string GetTargetFrameworkIdentifier (AssemblyDefinition assembly, MetadataReader reader)
+		void CheckAssemblyAttributes (MetadataReader reader)
 		{
-			string targetFrameworkIdentifier = null;
+			var assembly = reader.GetAssemblyDefinition ();
 			foreach (var handle in assembly.GetCustomAttributes ()) {
 				var attribute = reader.GetCustomAttribute (handle);
 				var name = reader.GetCustomAttributeFullName (attribute);
 				switch (name) {
-					case "System.Runtime.Versioning.TargetFrameworkAttribute":
-						var arguments = attribute.GetCustomAttributeArguments ();
-						foreach (var p in arguments.FixedArguments) {
-							// Of the form "MonoAndroid,Version=v8.1"
-							var value = p.Value?.ToString ();
-							if (!string.IsNullOrEmpty (value)) {
-								int commaIndex = value.IndexOf (",", StringComparison.Ordinal);
-								if (commaIndex != -1) {
-									targetFrameworkIdentifier = value.Substring (0, commaIndex);
-									break;
-								}
-							}
-						}
-						break;
 					case "Android.IncludeAndroidResourcesFromAttribute":
 					case "Android.NativeLibraryReferenceAttribute":
 					case "Java.Interop.JavaLibraryReferenceAttribute":
@@ -102,7 +77,6 @@ string GetTargetFrameworkIdentifier (AssemblyDefinition assembly, MetadataReader
 						break;
 				}
 			}
-			return targetFrameworkIdentifier;
 		}
 
 		bool HasReference (MetadataReader reader)

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

@@ -81,6 +81,8 @@ void Execute (MetadataResolver resolver)
 				lockFile = LockFileUtilities.GetLockFile (ProjectAssetFile, logger);
 			}
 
+			mainApiLevel = MonoAndroidHelper.SupportedVersions.GetApiLevelFromFrameworkVersion (TargetFrameworkVersion);
+
 			try {
 				foreach (var assembly in Assemblies) {
 					// Add each user assembly and all referenced assemblies (recursive)
@@ -123,14 +125,6 @@ void Execute (MetadataResolver resolver)
 			// Add I18N assemblies if needed
 			AddI18nAssemblies (resolver, assemblies);
 
-			var mainapiLevel = MonoAndroidHelper.SupportedVersions.GetApiLevelFromFrameworkVersion (TargetFrameworkVersion);
-			foreach (var item in api_levels.Where (x => mainapiLevel < x.Value)) {
-				var itemOSVersion = MonoAndroidHelper.SupportedVersions.GetFrameworkVersionFromApiLevel (item.Value);
-				LogCodedWarning ("XA0105", ProjectFile, 0,
-					"The $(TargetFrameworkVersion) for {0} ({1}) is greater than the $(TargetFrameworkVersion) for your project ({2}). " +
-					"You need to increase the $(TargetFrameworkVersion) for your project.", Path.GetFileName (item.Key), itemOSVersion, TargetFrameworkVersion);
-			}
-
 			var resolvedAssemblies          = new List<ITaskItem> (assemblies.Count);
 			var resolvedSymbols             = new List<ITaskItem> (assemblies.Count);
 			var resolvedFrameworkAssemblies = new List<ITaskItem> (assemblies.Count);
@@ -157,7 +151,7 @@ void Execute (MetadataResolver resolver)
 		}
 
 		readonly List<string> do_not_package_atts = new List<string> ();
-		readonly Dictionary<string, int> api_levels = new Dictionary<string, int> ();
+		int? mainApiLevel;
 		int indent = 2;
 
 		string ResolveRuntimeAssemblyForReferenceAssembly (LockFile lockFile, string assemblyPath)
@@ -214,23 +208,16 @@ void AddAssemblyReferences (MetadataResolver resolver, Dictionary<string, ITaskI
 			if (resolutionPath == null)
 				resolutionPath = new List<string>();
 
-			CheckAssemblyAttributes (assembly, reader, out string targetFrameworkIdentifier);
+			CheckAssemblyAttributes (assembly, reader);
 
 			LogMessage ("{0}Adding assembly reference for {1}, recursively...", new string (' ', indent), assemblyName);
 			resolutionPath.Add (assemblyName);
 			indent += 2;
 
 			// Add this assembly
-			ITaskItem assemblyItem = null;
-			if (topLevel) {
-				if (assemblies.TryGetValue (assemblyName, out assemblyItem)) {
-					if (!string.IsNullOrEmpty (targetFrameworkIdentifier) && string.IsNullOrEmpty (assemblyItem.GetMetadata ("TargetFrameworkIdentifier"))) {
-						assemblyItem.SetMetadata ("TargetFrameworkIdentifier", targetFrameworkIdentifier);
-					}
-				}
-			} else {
+			if (!assemblies.TryGetValue (assemblyName, out ITaskItem assemblyItem) && !topLevel) {
 				assemblies [assemblyName] = 
-					assemblyItem = CreateAssemblyTaskItem (assemblyPath, targetFrameworkIdentifier);
+					assemblyItem = CreateAssemblyTaskItem (assemblyPath);
 			}
 
 			// Recurse into each referenced assembly
@@ -272,10 +259,8 @@ void AddAssemblyReferences (MetadataResolver resolver, Dictionary<string, ITaskI
 			resolutionPath.RemoveAt (resolutionPath.Count - 1);
 		}
 
-		void CheckAssemblyAttributes (AssemblyDefinition assembly, MetadataReader reader, out string targetFrameworkIdentifier)
+		void CheckAssemblyAttributes (AssemblyDefinition assembly, MetadataReader reader)
 		{
-			targetFrameworkIdentifier = null;
-
 			foreach (var handle in assembly.GetCustomAttributes ()) {
 				var attribute = reader.GetCustomAttribute (handle);
 				switch (reader.GetCustomAttributeFullName (attribute)) {
@@ -293,22 +278,24 @@ void CheckAssemblyAttributes (AssemblyDefinition assembly, MetadataReader reader
 							var arguments = attribute.GetCustomAttributeArguments ();
 							foreach (var p in arguments.FixedArguments) {
 								// Of the form "MonoAndroid,Version=v8.1"
+								// If this is a .NET 5 TFM, we won't emit these warnings
 								var value = p.Value?.ToString ();
 								if (!string.IsNullOrEmpty (value)) {
 									int commaIndex = value.IndexOf (",", StringComparison.Ordinal);
 									if (commaIndex != -1) {
-										targetFrameworkIdentifier = value.Substring (0, commaIndex);
+										string targetFrameworkIdentifier = value.Substring (0, commaIndex);
 										if (targetFrameworkIdentifier == "MonoAndroid") {
 											const string match = "Version=";
 											var versionIndex = value.IndexOf (match, commaIndex, StringComparison.Ordinal);
 											if (versionIndex != -1) {
 												versionIndex += match.Length;
 												string version = value.Substring (versionIndex, value.Length - versionIndex);
 												var apiLevel = MonoAndroidHelper.SupportedVersions.GetApiLevelFromFrameworkVersion (version);
-												if (apiLevel != null) {
+												if (apiLevel > mainApiLevel) {
 													var assemblyName = reader.GetString (assembly.Name);
-													LogDebugMessage ("{0}={1}", assemblyName, apiLevel);
-													api_levels [assemblyName] = apiLevel.Value;
+													LogCodedWarning ("XA0105", ProjectFile, 0,
+														"The $(TargetFrameworkVersion) for {0} ({1}) is greater than the $(TargetFrameworkVersion) for your project ({2}). " +
+														"You need to increase the $(TargetFrameworkVersion) for your project.", assemblyName, version, TargetFrameworkVersion);
 												}
 											}
 										}
@@ -368,14 +355,12 @@ void ResolveI18nAssembly (MetadataResolver resolver, string name, Dictionary<str
 			assemblies [name] = CreateAssemblyTaskItem (assembly);
 		}
 
-		static ITaskItem CreateAssemblyTaskItem (string assembly, string targetFrameworkIdentifier = null)
+		static ITaskItem CreateAssemblyTaskItem (string assembly)
 		{
 			var assemblyFullPath = Path.GetFullPath (assembly);
-			var dictionary = new Dictionary<string, string> (2) {
+			var dictionary = new Dictionary<string, string> (1) {
 				{ "ReferenceAssembly", assemblyFullPath },
 			};
-			if (!string.IsNullOrEmpty (targetFrameworkIdentifier))
-				dictionary.Add ("TargetFrameworkIdentifier", targetFrameworkIdentifier);
 			return new TaskItem (assemblyFullPath, dictionary);
 		}
 	}

src/Xamarin.Android.Build.Tasks/Utilities/MonoAndroidHelper.cs

@@ -290,10 +290,6 @@ public static string GetNativeLibraryAbi (ITaskItem lib)
 
 		public static bool IsMonoAndroidAssembly (ITaskItem assembly)
 		{
-			var tfi = assembly.GetMetadata ("TargetFrameworkIdentifier");
-			if (string.Compare (tfi, "MonoAndroid", StringComparison.OrdinalIgnoreCase) == 0)
-				return true;
-
 			var hasReference = assembly.GetMetadata ("HasMonoAndroidReference");
 			return bool.TryParse (hasReference, out bool value) && value;
 		}

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

@@ -2032,7 +2032,7 @@ because xbuild doesn't support framework reference assemblies.
   <ItemGroup>
     <_ResolvedUserMonoAndroidAssemblies
         Include="@(_ResolvedUserAssemblies)"
-        Condition=" '%(_ResolvedUserAssemblies.TargetFrameworkIdentifier)' == 'MonoAndroid' Or '%(_ResolvedUserAssemblies.HasMonoAndroidReference)' == 'True' "
+        Condition=" '%(_ResolvedUserAssemblies.HasMonoAndroidReference)' == 'True' "
     />
   </ItemGroup>
 </Target>