Hao Nguyen, I am in no way an expert in this topic but here is what I think the issue is. In Activiti 6 the use of the Web UI (e.g. activiti_app) to create a form and link it to a task is a completely different mechanism and has nothing to do with the way you have done forms in the past. The historical way you've used forms inclusive of "GET form/form-data?<taskId>" continues to be supported in Activiti 6 exactly as you have done it in the past in Activiti 5.22 and is described in the Activiti 6 documentation here - https://www.activiti.org/userguide/#_forms. Think of this new way using the Web UI as an optional new way that provides the ability to visually model forms and link them to a task but this is not a replacement for the old way which should continue to be working fine. I hope this is helpful.
I'm very appreciate for your reply and I understood this mechanism.
By the way, what I'm trying to do is create a form on Activiti and use REST to get the form properties. Then, I render it into a form in my web page for user to type some value into it and submit form to complete task. Everything is work fine. Now I want to make 2 enum fields which are drop down list, when user select item in first enum field, the second enum field will be autofilled. Ex: you select Apple in first enum field and second field will be autofilled with Iphone 4,5,6... Is it possible to do in Activiti?
Sorry for my bad English because English is not my first language. I searched many post in here but no one do this thing before. Hope you can give me some opinion.