Begin reversible block

Command group Flag affected Reversible Execute on client Platform(s)
Constructs NO NO NO All

Syntax

Begin reversible block

Description

This command begins a reversible block of commands. All reversible commands enclosed within the commands Begin reversible block/End reversible block are reversed when the method containing this block finishes. However, a reversible block in the $construct() method of a window class reverses when the window is closed not when the method is terminated as is normally the case. Omnis always steps backwards through a reversible block of commands, thus the first command is reversed last.

Reversible blocks let you create subroutines that restore the values of variables, the current record buffer, and so on, to their previous state when the method terminates. Most commands are reversible: those that are not usually involve an irreversible action such as changing the data in an Omnis data file or running another program. Methods called from within a reversible block are not reversed.

Example

; A method can contain more than one block of reversible commands. In this case,
; commands contained within all the blocks are reversed when the method terminates.
; All the commands in the following example are reversed when the method containing
; the block is finished
Begin reversible block
  Disable menu line mMyMenu/5
  Set current list iMyList
  Build open window list (Clear list)
  Calculate iVar as 0
  Open window instance wMyWindow
End reversible block
; When this block is reversed:
; The window instance wMyWindow is closed
; iVar returns to its former value
; iMyList is restored to its former contents and definition
; The current list is set to the former value
; Menu line 5 is enabled
; The following method hides fields Entry1 and Entry2 and installs the menu mCustomers
Begin reversible block
  Hide fields {Entry1,Entry2}
  Install menu mCustomers
End reversible block
OK message (Icon) {MCUSTOMERS is now visible}
; When this method ends, first MCUSTOMERS is removed, then the fields are shown.
; In the following example, the current list is iMyList
Begin reversible block
  Set current list iMyList2
  Define list {fAccounts.Code,fAccounts.Surname,fAccounts.Balance}
  Set main file {fAccounts}
  Build list from select table
  Enter data
End reversible block
; When this method terminates and the command block is reversed, the Main file is reset,
; the former list definition is restored and the current list is restored to iMyList.