I am getting crazy. Output field become gray after invalidate();

Question

Before post, i have carefully read SAP docs, including the course JA310, but still don't find the solution.
My problem is that when i called a RFC, and execute output.invalidate(), the output field becomes gray.
(There won't be any authorization issues, i use the rfc_user for sap backend system, it has sap_new and sap_all authorization).
I will make the case most simple.
BAPI backed:
- IMPORT: orgin type num2, EXPORT result type num2.
- the function of BAPI is simple : result = orgin + 1.
In the NWDS.
- I have already tested the input bind, it works fine. (with an other FM has IMPORT data only)
- but when I try to bind the output data, the display field is gray and empty. (I have check the binding between model and controller, controller and view, all is fine)
Here is the 'logic flow'
Part I: wdDoInit()
  public void wdDoInit()
    //@@begin wdDoInit()
// out put field is now gray
     Ztestbapi_Output outbapi = new Ztestbapi_Output();
     outbapi.setResult("05");
     wdContext.nodeZtestbapi_Output().bind(outbapi);
// output field is now availble
Pari II: Button Action
  public void Execute1( )
    //@@begin Execute1()
     try {
//         wdContext.nodeZtestbapi_Output().invalidate();
// out putfield is now gray again
          wdContext.currentZtestbapi_InputElement().modelObject().execute();
// out putfield is now gray again
          wdContext.nodeZtestbapi_Output().invalidate();
          Ztestbapi_Output outbapi = new Ztestbapi_Output();
          outbapi.setResult("07");
          wdContext.nodeZtestbapi_Output().bind(outbapi);
// output field is now availble
          wdContext.nodeZtestbapi_Output().invalidate();
// out putfield is now gray again
     } catch (WDDynamicRFCExecuteException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
Thank u very much in advance!

Answer 1

My recent research is :
when i change my execute() function like:
     Ztestbapi_Input inbapi = new Ztestbapi_Input();
     inbapi.setOrgin("02");
     wdContext.nodeZtestbapi_Input().bind(inbapi);
     try {
                     // here is the key change
                     wdContext.nodeZtestbapi_Output().bind(inbapi.getOutput());
                     // End of key change.
          wdContext.currentZtestbapi_InputElement().modelObject().execute();
          wdContext.nodeZtestbapi_Output().bind(inbapi.getOutput());
          wdContext.nodeZtestbapi_Output().invalidate();
          if (wdContext.nodeZtestbapi_Output().isEmpty()){
               inbapi.setOrgin("07");
               wdContext.nodeZtestbapi_Input().bind(inbapi);
     } catch (WDDynamicRFCExecuteException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
The output and the input should be binded automatically, shouldn't them?
Always confused.