Add some task to generate docs (#997)

* Add some task to generate docs
* Use the new native master

build.cake

@@ -1,6 +1,7 @@
 #addin nuget:?package=Cake.Xamarin&version=3.0.2
 #addin nuget:?package=Cake.XCode&version=4.2.0
 #addin nuget:?package=Cake.FileHelpers&version=3.2.1
+#addin nuget:?package=Cake.Json&version=4.0.0&loaddependencies=true
 #addin nuget:?package=SharpCompress&version=0.24.0
 #addin nuget:?package=Mono.ApiTools.NuGetDiff&version=1.1.0-preview.1&prerelease&loaddependencies=true
 #addin nuget:?package=Xamarin.Nuget.Validator&version=1.1.1
@@ -68,7 +69,8 @@ if (!string.IsNullOrEmpty (PythonToolPath) && FileExists (PythonToolPath)) {
     System.Environment.SetEnvironmentVariable ("PATH", dir.FullPath + System.IO.Path.PathSeparator + oldPath);
 }
 
-var AZURE_BUILD_URL = "https://dev.azure.com/xamarin/6fd3d886-57a5-4e31-8db7-52a1b47c07a8/_apis/build/builds/{0}/artifacts?artifactName={1}&%24format=zip&api-version=5.0";
+var AZURE_BUILD_SUCCESS = "https://dev.azure.com/xamarin/6fd3d886-57a5-4e31-8db7-52a1b47c07a8/_apis/build/builds?statusFilter=completed&resultFilter=succeeded&definitions=4&branchName=refs/heads/master&$top=1&api-version=5.1";
+var AZURE_BUILD_URL = "https://dev.azure.com/xamarin/6fd3d886-57a5-4e31-8db7-52a1b47c07a8/_apis/build/builds/{0}/artifacts?artifactName={1}&%24format=zip&api-version=5.1";
 
 var TRACKED_NUGETS = new Dictionary<string, Version> {
     { "SkiaSharp",                          new Version (1, 57, 0) },
@@ -429,6 +431,20 @@ Task ("nuget-validation")
 // DOCS - creating the xml, markdown and other documentation
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
+Task ("download-last-successful-build")
+    .WithCriteria (string.IsNullOrEmpty (AZURE_BUILD_ID))
+    .Does (() =>
+{
+    Warning ("A build ID (--azureBuildId=<ID>) was not specified, using the last successful build.");
+
+    var successUrl = string.Format(AZURE_BUILD_SUCCESS);
+    var json = ParseJson (FileReadText (DownloadFile (successUrl)));
+
+    AZURE_BUILD_ID = (string)json ["value"] [0] ["id"];
+
+    Information ($"Using last successful build ID {AZURE_BUILD_ID}");
+});
+
 Task ("update-docs")
     .IsDependentOn ("docs-api-diff")
     .IsDependentOn ("docs-update-frameworks")

cake/BuildExternals.cake

@@ -745,11 +745,9 @@ Task ("externals-tizen")
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 Task ("externals-download")
+    .IsDependentOn ("download-last-successful-build")
     .Does (() =>
 {
-    if (string.IsNullOrEmpty (AZURE_BUILD_ID))
-        throw new Exception ("Specify a build ID with --azureBuildId=<ID>");
-
     var artifactName = "native-default";
     var artifactFilename = $"{artifactName}.zip";
     var url = string.Format(AZURE_BUILD_URL, AZURE_BUILD_ID, artifactName);

cake/UpdateDocs.cake

@@ -36,11 +36,9 @@ void CopyChangelogs (DirectoryPath diffRoot, string id, string version)
 }
 
 Task ("docs-download-output")
+    .IsDependentOn ("download-last-successful-build")
     .Does (() =>
 {
-    if (string.IsNullOrEmpty (AZURE_BUILD_ID))
-        throw new Exception ("Specify a build ID with --azureBuildId=<ID>");
-
     var url = string.Format(AZURE_BUILD_URL, AZURE_BUILD_ID, "nuget");
 
     EnsureDirectoryExists ("./output");

externals/skia

@@ -1 +1 @@
-Subproject commit 379f64c5526a2872e1a40ba32ffc75910b34b16b
+Subproject commit 6c864a6c858f77d7e20b37cd468103ae8d7e054b