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BI_PERSISTENT~REFRESH Method 
SAP WebFlow Engine calls the BI_PERSISTENT~REFRESH method when the system has to ensure that all values of an object are valid or that they agree exactly with the persistent display of the object.
The method implementation depends on the internal organization of the class. You can check the object instance data in the database, if necessary.
There is an implementation example in the CL_SWF_FORMABSENCdemo class.

If you do not need the method in your class, carry out a “dummy” implementation (without further coding) to avoid program errors when the system calls the method.