[msbuild] Change the logic about how the prebuilt app bundle is decompressed.

1. Compute the path of the zip in the .props file, and use it as Inputs for
   the PrepareAppBundle target. This way we only run PrepareAppBundle if the
   zip file has changed (and we actually run it if it has changed).
2. Delete any previous unzipped contents if we're unzipping (to make sure we
   don't have files leftover from an earlier version of the prebuilt zip).
3. Compute the 'HotRestartAppBundlePath' in a new and earlier task. This means
   we can also depend on this targe in the _CleanHotRestartBundle target to
   avoid having to compute the 'HotRestartAppBundlePath' property there as
   well.
4. Move the touching of the stamp file (the 'Extracted' file) to MSBuild. This
   way it shows up in logs.
5. Enable nullability in the PrepareAppBundle target and fix any issues.

msbuild/Xamarin.iOS.Tasks.Windows/Tasks/PrepareAppBundle.cs

@@ -5,25 +5,29 @@ using System.IO.Compression;
 using Xamarin.iOS.Windows;
 using Xamarin.MacDev;
 
+#nullable enable
+
 namespace Xamarin.iOS.HotRestart.Tasks {
 	public class PrepareAppBundle : Task {
 		#region Inputs
 
 		[Required]
-		public string AppBundleName { get; set; }
+		public string AppBundleName { get; set; } = string.Empty;
 
 		[Required]
-		public string SessionId { get; set; }
+		public string SessionId { get; set; } = string.Empty;
 
 		[Required]
 		public bool ShouldExtract { get; set; }
 
+		public string PreBuiltAppBundlePath { get; set; } = string.Empty;
+
 		#endregion
 
 		#region Outputs
 
 		[Output]
-		public string AppBundlePath { get; set; }
+		public string AppBundlePath { get; set; } = string.Empty;
 
 		#endregion
 
@@ -32,17 +36,14 @@ namespace Xamarin.iOS.HotRestart.Tasks {
 			if (string.IsNullOrEmpty (AppBundlePath))
 				AppBundlePath = HotRestartContext.Default.GetAppBundlePath (AppBundleName, SessionId.Substring (0, 8));
 
-			if (!Directory.Exists (AppBundlePath) && ShouldExtract) {
-				var preBuiltAppBundlePath = Path.Combine (
-					Path.GetDirectoryName (typeof (PrepareAppBundle).Assembly.Location),
-					"Xamarin.PreBuilt.iOS.app.zip");
+			if (ShouldExtract) {
+				Directory.Delete (AppBundlePath, true);
 
-				ZipFile.ExtractToDirectory (preBuiltAppBundlePath, AppBundlePath);
+				Log.LogMessage (MessageImportance.Low, $"Extracting '{PreBuiltAppBundlePath}' into {AppBundlePath}.");
+				ZipFile.ExtractToDirectory (PreBuiltAppBundlePath, AppBundlePath);
 
 				// Ensure the archived-expanded-entitlements.xcent is empty. If there are any entitlements in that file, the new signature will be invalid
 				new PDictionary ().Save (Path.Combine (AppBundlePath, "archived-expanded-entitlements.xcent"));
-
-				File.WriteAllText (Path.Combine (AppBundlePath, "Extracted"), string.Empty);
 			}
 
 			return true;

msbuild/Xamarin.iOS.Tasks.Windows/Xamarin.iOS.HotRestart.props

@@ -13,5 +13,7 @@
 		<HotRestartIPAPath>$(HotRestartSignedAppOutputDir)$(_AppBundleName).ipa</HotRestartIPAPath>
 
 		<UnpackHotRestartFrameworks Condition="'$(UnpackHotRestartFrameworks)' == ''">true</UnpackHotRestartFrameworks>
+
+		<HotRestartPreBuiltZipPath Condition="'$(HotRestartPreBuiltZipPath)' == ''">$(MSBuildThisFileDirectory)Xamarin.PreBuilt.iOS.app.zip</HotRestartPreBuiltZipPath>
 	</PropertyGroup>
 </Project>

msbuild/Xamarin.iOS.Tasks.Windows/Xamarin.iOS.HotRestart.targets

@@ -75,19 +75,39 @@
 
 	<Target Name="_CreateHotRestartCachedBundle" DependsOnTargets="$(_CreateHotRestartCachedBundleDependsOn)" />
 	
+	<Target Name="_ComputeHotRestartAppBundlePath"
+		DependsOnTargets="_GenerateHotRestartBuildSessionId"
+		Condition="'$(IsHotRestartBuild)' == 'true' And '$(HotRestartAppBundlePath)' == ''"
+		>
+
+		<PrepareAppBundle
+			Condition="'$(IsHotRestartBuild)' == 'true'"
+			AppBundleName="$(_AppBundleName)"
+			SessionId="$(HotRestartBuildSessionId)"
+			ShouldExtract="false"
+			>
+
+			<Output TaskParameter="AppBundlePath" PropertyName="HotRestartAppBundlePath" />
+		</PrepareAppBundle>
+	</Target>
+
 	<!-- Creates HotRestart app bundle -->
-	<Target Name="_PrepareHotRestartAppBundle" DependsOnTargets="_GenerateHotRestartBuildSessionId" 
-					Condition="'$(_CanOutputAppBundle)' == 'true' And '$(IsAppExtension)' == 'false' And '$(IsHotRestartBuild)' == 'true'">
+	<Target Name="_PrepareHotRestartAppBundle"
+			DependsOnTargets="_GenerateHotRestartBuildSessionId;_ComputeHotRestartAppBundlePath"
+			Condition="'$(_CanOutputAppBundle)' == 'true' And '$(IsAppExtension)' == 'false' And '$(IsHotRestartBuild)' == 'true'"
+			Inputs="$(HotRestartPreBuiltZipPath)"
+			Outputs="$(HotRestartAppBundlePath)\Extracted"
+			>
 
 		<!--Create app bundle dir and get its path-->
 		<PrepareAppBundle 
 			AppBundleName="$(_AppBundleName)"
 			AppBundlePath="$(HotRestartAppBundlePath)"
+			PreBuiltAppBundlePath="$(HotRestartPreBuiltZipPath)"
 			SessionId="$(HotRestartBuildSessionId)" 
-			ShouldExtract="true">
+			ShouldExtract="true" />
 
-			<Output TaskParameter="AppBundlePath" PropertyName="HotRestartAppBundlePath" />
-		</PrepareAppBundle>
+		<Touch AlwaysCreate="true" Files="$(HotRestartAppBundlePath)\Extracted" />
 
 		<!-- Delete the build signature to force XMA do a full build next time -->
 		<Delete Files="$(BuildSignatureFile)" Condition="Exists('$(BuildSignatureFile)')"	/>
@@ -243,17 +263,7 @@
 		</CleanDependsOn>
 	</PropertyGroup>
 
-	<Target Name="_CleanHotRestartBundle" DependsOnTargets="_GenerateHotRestartBuildSessionId" >
-		<!-- Gets the bundle path -->
-		<PrepareAppBundle 
-			Condition="'$(IsHotRestartBuild)' == 'true'"
-			AppBundleName="$(_AppBundleName)"
-			SessionId="$(HotRestartBuildSessionId)" 
-			ShouldExtract="false">
-
-			<Output TaskParameter="AppBundlePath" PropertyName="HotRestartAppBundlePath" />
-		</PrepareAppBundle>
-		
+	<Target Name="_CleanHotRestartBundle" DependsOnTargets="_GenerateHotRestartBuildSessionId;_ComputeHotRestartAppBundlePath" >
 		<RemoveDir Directories="$(HotRestartAppBundlePath)" />
 		<RemoveDir Directories="$(HotRestartSignedAppOutputDir)" />
 		<RemoveDir Directories="$(DeviceSpecificIntermediateOutputPath)UnpackedFrameworks" />