[Xamarin.Android.Build.Tasks] Add GetAndroidDependencies Target

Fixes dotnet#1269

This commit adds the `GetAndroidDependencies` target to the
`Xamarin.Android.Common.targets`. Its purpose is to examine
the various settings in the project and report which android
sdk build-tools, platform-tools etc are required.

`GetAndroidDependencies` will output an @(AndroidDependency)
with Version metadata Valid component names are platform,
build-tool, platform-tool and tool.

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

@@ -0,0 +1,57 @@
+using System;
+using System.Collections.Generic;
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+using Xamarin.Android.Tools;
+
+namespace Xamarin.Android.Tasks
+{
+	public class CalculateProjectDependencies : Task
+	{
+		const int DefaultMinSDKVersion = 11;
+
+		[Required]
+		public string TargetFrameworkVersion { get; set; }
+
+		[Required]
+		public ITaskItem ManifestFile { get; set; }
+
+		[Required]
+		public string BuildToolsVersion { get; set; }
+
+		public string PlatformToolsVersion { get; set; }
+
+		public string ToolsVersion { get; set; }
+
+		[Output]
+		public ITaskItem [] Dependencies { get; set; }
+
+		ITaskItem CreateAndroidDependency (string include, string version)
+		{
+			if (string.IsNullOrEmpty (version))
+				return new TaskItem (include);
+
+			return new TaskItem (include, new Dictionary<string, string> {
+				{ "Version", version }
+			});
+		}
+
+		public override bool Execute ()
+		{
+			var dependencies = new List<ITaskItem> ();
+			var targetApiLevel = MonoAndroidHelper.SupportedVersions.GetApiLevelFromFrameworkVersion (TargetFrameworkVersion);
+			var manifest = AndroidAppManifest.Load (ManifestFile.ItemSpec, MonoAndroidHelper.SupportedVersions);
+			var manifestApiLevel = manifest.TargetSdkVersion ?? manifest.MinSdkVersion ?? DefaultMinSDKVersion;
+			dependencies.Add (CreateAndroidDependency ("platform", $"{Math.Max (targetApiLevel.Value, manifestApiLevel)}"));
+			dependencies.Add (CreateAndroidDependency ("build-tool", BuildToolsVersion));
+			if (!string.IsNullOrEmpty (PlatformToolsVersion)) {
+				dependencies.Add (CreateAndroidDependency ("platform-tool", PlatformToolsVersion));
+			}
+			if (!string.IsNullOrEmpty (ToolsVersion)) {
+				dependencies.Add (CreateAndroidDependency ("tool", ToolsVersion));
+			}
+			Dependencies = dependencies.ToArray ();
+			return !Log.HasLoggedErrors;
+		}
+	}
+}

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

@@ -187,6 +187,7 @@
     <Compile Include="Tasks\JavaToolTask.cs" />
     <Compile Include="Tasks\GenerateLayoutCodeBehind.cs" />
     <Compile Include="Tasks\CalculateLayoutCodeBehind.cs" />
+    <Compile Include="Tasks\CalculateProjectDependencies.cs" />
     <Compile Include="$(MonoSourceFullPath)\mcs\tools\pdb2mdb\BitAccess.cs">
       <Link>pdb2mdb\BitAccess.cs</Link>
     </Compile>

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

@@ -32,6 +32,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 <UsingTask TaskName="Xamarin.Android.Tasks.BuildApk" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <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.CheckForRemovedItems" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CheckTargetFrameworks" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
 <UsingTask TaskName="Xamarin.Android.Tasks.CompileToDalvik" AssemblyFile="Xamarin.Android.Build.Tasks.dll" />
@@ -2767,6 +2768,24 @@ because xbuild doesn't support framework reference assemblies.
     DependsOnTargets="$(InstallDependsOnTargets)">
 </Target>
 
+
+<!-- SDK Management Targets -->
+<Target Name="GetAndroidDependencies" DependsOnTargets="$(GetAndroidDependenciesDependsOn)" Returns="@(AndroidDependency)">
+  <PropertyGroup>
+    <_ProjectAndroidManifest>$(ProjectDir)$(AndroidManifest)</_ProjectAndroidManifest>
+  </PropertyGroup>
+  <Error Text="AndroidManifest file does not exist" Condition="'$(_ProjectAndroidManifest)'!='' And !Exists ('$(_ProjectAndroidManifest)')"/>
+  <CalculateProjectDependencies
+    TargetFrameworkVersion="$(TargetFrameworkVersion)"
+    ManifestFile="$(_ProjectAndroidManifest)"
+    BuildToolsVersion="$(AndroidSdkBuildToolsVersion)"
+    PlatformToolsVersion="$(AndroidSdkPlatformToolsVersion)"
+    ToolsVersion="$(AndroidSdkToolsVersion)"
+  >
+    <Output TaskParameter="Dependencies" ItemName="AndroidDependency" />
+  </CalculateProjectDependencies>
+</Target>
+
 <Import Project="$(MSBuildThisFileDirectory)Xamarin.Android.Common.Debugging.targets"
         Condition="Exists('$(MSBuildThisFileDirectory)Xamarin.Android.Common.Debugging.targets')"/>