The buffered and unbuffered modes

The buffered and unbuffered mode control the synchronization of program variables and form fields.

Data model / view / controller paradigm

When bound to an interactive instruction (such as dialog), program variables act as a data model to display data or to get user input. To change the values of form fields by program code, the corresponding variables must be set and displayed.

Synchronization of program variables with the form fields depends on the buffer mode used by the dialog. Use the unbuffered mode to get automatic data model / form field synchronization.

Configuring the buffer mode

By default, singular dialogs (INPUT, DISPLAY ARRAY) and procedural DIALOG blocks use the buffered mode.

The unbuffered mode can be set per (modal) dialog instruction, with the UNBUFFERED dialog attribute:
INPUT BY NAME p_site.* ATTRIBUTES(UNBUFFERED) 
   ...
END INPUT
When using a procedural DIALOG block, all sub dialogs defined locally or included with the SUBDIALOG clause inherit the buffer mode of the parent procedural dialog block:
DIALOG ATTRIBUTES(UNBUFFERED)
  INPUT BY NAME p_site.* -- unbuffered
     ...
  END INPUT
  DISPLAY ARRAY a_events TO sr_events.* -- unbuffered
     ...
  END DISPLAY
  SUBDIALOG d_comments -- unbuffered
END DIALOG
The unbuffered mode can also be set globally with the ui.Dialog.setDefaultUnbuffered() method, for singular and procedural dialogs:
CALL ui.Dialog.setDefaultUnbuffered(TRUE)
...
INPUT BY NAME rec_cust.* WITHOUT DEFAULTS -- uses unbuffered mode 
   ...
END INPUT

The buffered mode

When you use the default "buffered" mode, program variable changes are not automatically displayed to form fields; you need to execute DISPLAY TO or DISPLAY BY NAME. Additionally, if an action is triggered, the value of the current field is not validated and is not copied into the corresponding program variable. The only way to get the text of a field is to use GET_FLDBUF() or DIALOG.getFieldBuffer(). These functions return the current text, which might not be a valid representation of a value of the field data type:
INPUT BY NAME p_item.*
   ON ACTION zoom 
      CALL select_item()
           RETURNING p_item.code, p_item.desc
         DISPLAY BY NAME p_item.code, p_item.desc
      END IF
   ...
END INPUT

The unbuffered mode

With the unbuffered mode, program variables and form fields are automatically synchronized, and the dialog instruction is sensitive to program variable changes: You don't need to display values explicitly with DISPLAY TO or DISPLAY BY NAME. When an action is triggered, the value of the current field is validated and is copied into the corresponding program variable. If you need to display new data during the dialog execution, just assign the values to the program variables; the runtime system will automatically display the values to the screen after user code of the current control or interaction block has been executed:
INPUT BY NAME p_site.* ATTRIBUTES(UNBUFFERED)
   ON ACTION zoom 
      CALL select_item()
           RETURNING p_item.code, p_item.desc
         -- no need to display desc.
      END IF
   ...
END INPUT

Actions configuration for field validation

During data input, values entered by the user in form fields are automatically validated and copied into the program variables. Actually the value entered in form fields is first available in the form field buffer. This buffer can be queried with built-in functions or dialog class methods. With the unbuffered mode, the field buffer is used to synchronize program variables each time control returns to the runtime system - for example, when the user clicks on a button to execute an action.

With the unbuffered mode, data validation must be prevented for some actions such as cancel or close. To avoid field validation for a given action, set the validate action default attribute to "no", in the .4ad file or in the ACTION DEFAULTS section of the form file:
ACTION DEFAULTS
   ACTION undo (TEXT = "Undo", VALIDATE = NO)
   ...
END
Note:

Actions such as dialogtouched, cancel, delete, close, help are by default defined with the validate=no attribute in the $FGLDIR/lib/default.4ad file.

If field validation is disabled for an action, the code executed in the ON ACTION block acts as if the dialog was in buffered mode. The program variable is not set. However, the input buffer of the current field is updated. When returning from the user code, the dialog will not synchronize the form fields with program variables, and the current field will display the input buffer content. Therefore, if you change the value of the program variable during an ON ACTION block where validation is disabled, you must explicitly display the values to the fields with DISPLAY TO / BY NAME.

To illustrate this case, imagine that you want to implement an undo action to allow the modifications done by the user to be reverted (before these have been saved to the database of course). You typically copy the current record into a clone variable when the dialog starts, and copy these old values back to the input record when the undo action is invoked. An undo action is a good candidate to avoid field validation, since you want to ignore current values. If you don't re-display the values, the input buffer of the current field will remain when returning from the ON ACTION block:
DIALOG ATTRIBUTES(UNBUFFERED)
 INPUT BY NAME p_cust.*
   BEFORE INPUT
     LET p_cust_copy = p_cust
   ON ACTION undo -- Defined with VALIDATE=NO
     LET p_cust = p_cust_copy
     DISPLAY BY NAME p_cust.*
 END INPUT
END DIALOG

For more details, see Data validation at action invocation.