[Xamarin.Android.Build.Tasks] combine CheckTargetFrameworks/ResolveAssemblies

Context: https://gist.github.com/jonathanpeppers/a207a8828317460239a12a8b0cc918d7

When reviewing how many times we are using Mono.Cecil to open every
assembly, it was happening 9 times!

Unfortunately it is not as easy as just "caching" something, the
simplest approach for now is to start combining functionality that are
in multiple tasks right now.

A simple first example is the `CheckTargetFrameworks` MSBuild task,
which runs after the `Compile` and `ResolveAssemblies` MSBuild tasks.
`CheckTargetFrameworks` has the job of emitting warnings about
`[assembly: System.Runtime.Versioning.TargetFramework]`. We can easily
do this work in `ResolveAssemblies` as it is already looping over all
the assemblies' custom attributes for other reasons.

Changes to make this happen:
- Removed `CheckTargetFrameworks` and the `_CheckTargetFrameworks`
  target, adding the required `TargetFrameworkVersion` and
  `ProjectFile` properties to `ResolveAssemblies`.
- Added a new `CheckAssemblyAttributes` where we can add to a `switch`
  statement for new attribute types in the future as needed.
- Added a dictionary of `api_levels`, and copied over the logic from
  `CheckTargetFrameworks`

Before:

    105 ms  CheckTargetFrameworks                      1 calls
    188 ms  ResolveAssemblies                          1 calls

After:

    216 ms  ResolveAssemblies                          1 calls

I timed the `tests/Xamarin.Forms-Performance-Integration` project in
this repository: a build with no changes.

Since `CheckTargetFrameworks` is removed completely, we can avoid the
time it was taking to open every assembly again.

This saves around 77ms.

Author: jonathanpeppers

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

@@ -28,6 +28,12 @@ public class ResolveAssemblies : AsyncTask
 		[Required]
 		public string ReferenceAssembliesDirectory { get; set; }
 
+		[Required]
+		public string TargetFrameworkVersion { get; set; }
+
+		[Required]
+		public string ProjectFile { get; set; }
+
 		public string ProjectAssetFile { get; set; }
 
 		public string TargetMoniker { get; set; }
@@ -122,6 +128,14 @@ void Execute (DirectoryAssemblyResolver 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);
+				Log.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);
@@ -149,6 +163,7 @@ void Execute (DirectoryAssemblyResolver resolver)
 		}
 
 		readonly List<string> do_not_package_atts = new List<string> ();
+		readonly Dictionary<string, int> api_levels = new Dictionary<string, int> ();
 		int indent = 2;
 
 		AssemblyDefinition ResolveRuntimeAssemblyForReferenceAssembly (LockFile lockFile, DirectoryAssemblyResolver resolver, string assemblyPath)
@@ -198,13 +213,8 @@ void AddAssemblyReferences (DirectoryAssemblyResolver resolver, Dictionary<strin
 
 			if (resolutionPath == null)
 				resolutionPath = new List<string>();
-			
-			foreach (var att in assembly.CustomAttributes.Where (a => a.AttributeType.FullName == "Java.Interop.DoNotPackageAttribute")) {
-				string file = (string) att.ConstructorArguments.First ().Value;
-				if (string.IsNullOrWhiteSpace (file))
-					LogError ("In referenced assembly {0}, Java.Interop.DoNotPackageAttribute requires non-null file name.", assembly.FullName);
-				do_not_package_atts.Add (Path.GetFileName (file));
-			}
+
+			CheckAssemblyAttributes (assembly);
 
 			LogMessage ("{0}Adding assembly reference for {1}, recursively...", new string (' ', indent), assembly.Name);
 			resolutionPath.Add (assembly.Name.Name);
@@ -245,6 +255,38 @@ void AddAssemblyReferences (DirectoryAssemblyResolver resolver, Dictionary<strin
 			resolutionPath.RemoveAt (resolutionPath.Count - 1);
 		}
 
+		void CheckAssemblyAttributes (AssemblyDefinition assembly)
+		{
+			foreach (var att in assembly.CustomAttributes) {
+				switch (att.AttributeType.FullName) {
+					case "Java.Interop.DoNotPackageAttribute": {
+							string file = (string)att.ConstructorArguments.First ().Value;
+							if (string.IsNullOrWhiteSpace (file))
+								LogError ("In referenced assembly {0}, Java.Interop.DoNotPackageAttribute requires non-null file name.", assembly.FullName);
+							do_not_package_atts.Add (Path.GetFileName (file));
+						}
+						break;
+					case "System.Runtime.Versioning.TargetFrameworkAttribute": {
+							foreach (var p in att.ConstructorArguments) {
+								var value = p.Value.ToString ();
+								if (value.StartsWith ("MonoAndroid")) {
+									var values = value.Split ('=');
+									var apiLevel = MonoAndroidHelper.SupportedVersions.GetApiLevelFromFrameworkVersion (values [1]);
+									if (apiLevel != null) {
+										var assemblyName = assembly.Name.Name;
+										Log.LogDebugMessage ("{0}={1}", assemblyName, apiLevel);
+										api_levels [assemblyName] = apiLevel.Value;
+									}
+								}
+							}
+						}
+						break;
+					default:
+						break;
+				}
+			}
+		}
+
 		static LinkModes ParseLinkMode (string linkmode)
 		{
 			if (string.IsNullOrWhiteSpace (linkmode))

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

@@ -444,7 +444,6 @@
     <Compile Include="Tasks\JavaCompileToolTask.cs" />
     <Compile Include="Tasks\AsyncTask.cs" />
     <Compile Include="Utilities\ResourceMerger.cs" />
-    <Compile Include="Tasks\CheckTargetFrameworks.cs" />
     <Compile Include="Tasks\Dx.cs" />
     <Compile Include="Tasks\CreateMsymManifest.cs" />
     <Compile Include="Utilities\AndroidResource.cs">

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Build.Tasks.csproj

@@ -37,7 +37,6 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 <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.CheckForRemovedItems" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
-<UsingTask TaskName="Xamarin.Android.Tasks.CheckTargetFrameworks" 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" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CollectPdbFiles" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
@@ -487,14 +486,6 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
  </CreateItem>
 </Target>
 
-<Target Name="_CheckTargetFrameworks"
-		DependsOnTargets="_ResolveAssemblies">
-	<CheckTargetFrameworks
-		ResolvedAssemblies="@(ResolvedAssemblies)"
-		ProjectFile="$(MSBuildProjectFullPath)"
-		TargetFrameworkVersion="$(TargetFrameworkVersion)" />
-</Target>
-
 <Target Name="_StripEmbeddedLibraries"
   Inputs="@(ResolvedAssemblies)" 
   Outputs="$(_AndroidStripFlag)"
@@ -1891,6 +1882,8 @@ because xbuild doesn't support framework reference assemblies.
 		Assemblies="@(FilteredAssemblies)"
 		I18nAssemblies="$(MandroidI18n)"
 		LinkMode="$(AndroidLinkMode)"
+		ProjectFile="$(MSBuildProjectFullPath)"
+		TargetFrameworkVersion="$(TargetFrameworkVersion)"
 		ProjectAssetFile="$(ProjectLockFile)"
 		TargetMoniker="$(NuGetTargetMoniker)"
 		ReferenceAssembliesDirectory="$(TargetFrameworkDirectory)">
@@ -2143,7 +2136,6 @@ because xbuild doesn't support framework reference assemblies.
 	<_PrepareAssembliesDependsOnTargets>
 		_ResolveAssemblies
 		;_ResolveSatellitePaths
-		;_CheckTargetFrameworks
 		;_CreatePackageWorkspace
 		;_StripEmbeddedLibraries
 		;_LinkAssemblies