Anyone working on this atm? I would be interested in doing it. But I have difficulty getting started. Is there anywhere a tutorial of how to make a such a plugin for workflow? Any other helpful tips to get started with this?

See the guide for general tips, but this plugin is probably more of a special case. (Turning GroovyPostbuildRecorder into a SimpleBuildStep would not work well.)

It is not obvious to me what aspects of the plugin are useful to support from Workflow. You can already run Groovy code of your choice, and update the build description:

currentBuild.description = "This was building ${someComputedValue}!"

So we need to clearly determine which features are unique to this plugin—previously mentioned were badges and summary actions—and decide on a reasonably intuitive way of exposing those to Workflow scripts.

The simplest approach would be to update BadgeManager to support Run rather than only AbstractBuild, then expose it using the GlobalVariable extension point as manager. Then from Workflow you could use essentially the same syntax as in a post-build script from a freestyle project:

manager.addWarningBadge 'Lots of tests skipped, watch out!'

There is a catch: to find the build associated with the CpsScript would seem to require an API change, as seen by this use of package access. I actually introduced such an API in an aborted attempt to solve JENKINS-30222:

diff --git a/cps/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsScript.java b/cps/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsScript.java
index 69a24c9..23501bf 100644
--- a/cps/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsScript.java
+++ b/cps/src/main/java/org/jenkinsci/plugins/workflow/cps/CpsScript.java
@@ -136,6 +140,10 @@ public abstract class CpsScript extends SerializableScript {
         }
     }
 
+    public CpsFlowExecution $execution() {
+        return execution;
+    }
+
     @Override
     public Object evaluate(String script) throws CompilationFailedException {
         // this might throw the magic CpsCallableInvocation to execute the script asynchronously

A more integrated approach would be to create an extension point so that RunWrapper (used for currentBuild in core Workflow) could be given additional methods and properties by plugins, so that you could write

currentBuild.addWarningBadge 'Lots of tests skipped, watch out!'

but this would be more work and require more knowledge of Groovy intricacies.

The "native" ability to add badges and actions of some kind to the workflow build itself would be a valuable improvement to workflow (job plugin) itself IMO.

to find the build associated with the CpsScript would seem to require an API change

I am making another pass at JENKINS-30222 which would involve making $build public in WF 1.11, removing that blocker.

Code changed in jenkins
User: Jesse Glick
Path:
pom.xml
src/main/java/org/jvnet/hudson/plugins/groovypostbuild/GroovyPostbuildRecorder.java
src/main/java/org/jvnet/hudson/plugins/groovypostbuild/WorkflowManager.java
src/main/resources/org/jvnet/hudson/plugins/groovypostbuild/WorkflowManager/help.jelly
src/test/java/org/jvnet/hudson/plugins/groovypostbuild/WorkflowTest.java
http://jenkins-ci.org/commit/groovy-postbuild-plugin/0fa6ec9c1f547892e2a15be131c600949d875399
Log:
[FIXED JENKINS-26918] Binding BadgeManager to a global variable for Workflows.

Code changed in jenkins
User: ikedam
Path:
pom.xml
src/main/java/org/jvnet/hudson/plugins/groovypostbuild/GroovyPostbuildRecorder.java
src/main/java/org/jvnet/hudson/plugins/groovypostbuild/WorkflowManager.java
src/main/resources/org/jvnet/hudson/plugins/groovypostbuild/WorkflowManager/help.jelly
src/test/java/org/jvnet/hudson/plugins/groovypostbuild/WorkflowTest.java
http://jenkins-ci.org/commit/groovy-postbuild-plugin/fd4558976706395cd6752720277a90eef92a7be9
Log:
Merge pull request #24 from jglick/workflow-JENKINS-26918

JENKINS-26918 Workflow support

Compare: https://github.com/jenkinsci/groovy-postbuild-plugin/compare/a12047227760...fd4558976706

Released groovy-postbuild 2.3.
You can use manager in workflow scripts.

Code changed in jenkins
User: Jesse Glick
Path:
COMPATIBILITY.md
http://jenkins-ci.org/commit/workflow-plugin/c4519c3f20066219c2a67c085a4a22eea27d4420
Log:
JENKINS-26918 Noting.

Hi,

I understand that Groovy Postbuild is now compatible with pipeline, however I can't find how to add it to my pipeline code. I expected to find a postbuild command in pipeline snippet generator, but it is not there (or at least I cannot find it).

We have Jenkins Enterprise 2.7.21, Groovy Postbuild 2.3.1.

Will appreciate any help with this.

Look under Global Variables IIRC.