[WIP] Generate empty code when remapping is absent

Author: grendello

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

@@ -26,7 +26,6 @@ public MamNativeCodeInfo (int replacementTypeCount, int replacementMethodIndexEn
 
 		public override string TaskPrefix => "GMAMNC";
 
-		[Required]
 		public ITaskItem RemappingXmlFilePath { get; set; }
 
 		[Required]
@@ -35,7 +34,29 @@ public MamNativeCodeInfo (int replacementTypeCount, int replacementMethodIndexEn
 		[Required]
 		public string [] SupportedAbis { get; set; }
 
+		public bool GenerateEmptyCode { get; set; }
+
 		public override bool RunTask ()
+		{
+			if (!GenerateEmptyCode) {
+				if (RemappingXmlFilePath == null) {
+					throw new InvalidOperationException ("RemappingXmlFilePath parameter is required");
+				}
+
+				Generate ();
+			} else {
+				GenerateEmpty ();
+			}
+
+			return !Log.HasLoggedErrors;
+		}
+
+		void GenerateEmpty ()
+		{
+			Generate (new MamRemappingAssemblyGenerator (), typeReplacementsCount: 0);
+		}
+
+		void Generate ()
 		{
 			var typeReplacements = new List<MamTypeReplacement> ();
 			var methodReplacements = new List<MamMethodReplacement> ();
@@ -52,15 +73,17 @@ public override bool RunTask ()
 				}
 			}
 
-			Log.LogMessage ($"Creating MAM generator. Type replacements: {typeReplacements.Count}; method replacements: {methodReplacements.Count}");
-			var mamGenerator = new MamRemappingAssemblyGenerator (typeReplacements, methodReplacements);
+			Generate (new MamRemappingAssemblyGenerator (typeReplacements, methodReplacements), typeReplacements.Count);
+		}
+
+		void Generate (MamRemappingAssemblyGenerator mamGenerator, int typeReplacementsCount)
+		{
 			mamGenerator.Init ();
 
 			foreach (string abi in SupportedAbis) {
 				string baseAsmFilePath = Path.Combine (OutputDirectory, $"mam_remap.{abi.ToLowerInvariant ()}");
 				string llFilePath  = $"{baseAsmFilePath}.ll";
 
-				Log.LogMessage ($"Generating file: {llFilePath}");
 				using (var sw = MemoryStreamPool.Shared.CreateStreamWriter ()) {
 					mamGenerator.Write (GeneratePackageManagerJava.GetAndroidTargetArchForAbi (abi), sw, llFilePath);
 					sw.Flush ();
@@ -70,11 +93,9 @@ public override bool RunTask ()
 
 			BuildEngine4.RegisterTaskObjectAssemblyLocal (
 				MamNativeCodeInfoKey,
-				new MamNativeCodeInfo (typeReplacements.Count, mamGenerator.ReplacementMethodIndexEntryCount),
+				new MamNativeCodeInfo (typeReplacementsCount, mamGenerator.ReplacementMethodIndexEntryCount),
 				RegisteredTaskObjectLifetime.Build
 			);
-
-			return !Log.HasLoggedErrors;
 		}
 
 		void ReadXml (XmlReader reader, List<MamTypeReplacement> typeReplacements, List<MamMethodReplacement> methodReplacements)

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

@@ -196,14 +196,21 @@ sealed class MAMTypeReplacementEntry
 
 		public int ReplacementMethodIndexEntryCount { get; private set; } = 0;
 
+		public MamRemappingAssemblyGenerator ()
+		{}
+
 		public MamRemappingAssemblyGenerator (List<MamTypeReplacement> typeReplacements, List<MamMethodReplacement> methodReplacements)
 		{
-			this.typeReplacementsInput = typeReplacements;
-			this.methodReplacementsInput = methodReplacements;
+			this.typeReplacementsInput = typeReplacements ?? throw new ArgumentNullException (nameof (typeReplacements));
+			this.methodReplacementsInput = methodReplacements ?? throw new ArgumentNullException (nameof (methodReplacements));
 		}
 
 		public override void Init ()
 		{
+			if (typeReplacementsInput == null) {
+				return;
+			}
+
 			typeReplacements = new List<StructureInstance<MAMTypeReplacementEntry>> ();
 			foreach (MamTypeReplacement mtr in typeReplacementsInput) {
 				var entry = new MAMTypeReplacementEntry {
@@ -330,6 +337,24 @@ protected override void Write (LlvmIrGenerator generator)
 			generator.WriteEOL ();
 			generator.WriteEOL ("MAM remapping data");
 
+			if (typeReplacements == null) {
+				generator.WriteStructureArray (
+					mamTypeReplacementEntryStructureInfo,
+					0,
+					LlvmIrVariableOptions.GlobalConstant,
+					"mam_type_replacements"
+				);
+
+				generator.WriteStructureArray (
+					mamIndexTypeEntryStructureInfo,
+					0,
+					LlvmIrVariableOptions.GlobalConstant,
+					"mam_method_replacement_index"
+				);
+
+				return;
+			}
+
 			generator.WriteStructureArray (
 				mamTypeReplacementEntryStructureInfo,
 				typeReplacements,

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

@@ -1626,6 +1626,17 @@ because xbuild doesn't support framework reference assemblies.
   </PrepareAbiItems>
 </Target>
 
+<Target Name="_GenerateEmptyAndroidRemapNativeCode"
+        DependsOnTargets="_PrepareAndroidRemapNativeAssemblySources"
+        Condition=" '@(_AndroidRemapMembers->Count())' == '0' "
+        Outputs="@(_AndroidRemapAssemblySource)">
+  <GenerateMamNativeCode
+      OutputDirectory="$(_NativeAssemblySourceDir)"
+      GenerateEmptyCode="True"
+      SupportedAbis="@(_BuildTargetAbis)"
+   />
+</Target>
+
 <Target Name="_GenerateAndroidRemapNativeCode"
         DependsOnTargets="_PrepareAndroidRemapNativeAssemblySources"
         Condition=" '@(_AndroidRemapMembers->Count())' != '0' "
@@ -1651,6 +1662,7 @@ because xbuild doesn't support framework reference assemblies.
     _PrepareEnvironmentAssemblySources;
     _GenerateEnvironmentFiles;
     _GenerateAndroidRemapNativeCode;
+    _GenerateEmptyAndroidRemapNativeCode;
     _IncludeNativeSystemLibraries;
   </_GeneratePackageManagerJavaDependsOn>
 </PropertyGroup>