PRB: Calling Screen from Another Without Jumping a READ Level

• Microsoft Visual FoxPro for Windows, version 3.0
• Microsoft FoxPro for Windows, versions 2.5x, 2.6, 2.6a
• Microsoft FoxPro for MS-DOS, versions 2.5x, 2.6, 2.6a
• Microsoft FoxPro for Macintosh, versions 2.5x, 2.6a

SYMPTOMS

Many FoxPro programmers want to be able to execute one screen from another. This may present problems if the READ command from the calling screen is not cleared before the next screen is executed and more than five screens are executed. The most common error message encountered is "Too many READs in effect," which is the result of having more than five READS in effect, which is a FoxPro limit.

CAUSE

The most common, though incorrect, way to try to solve this problem is to enter something similar to the following code in the VALID clause of a push button on Screen1:

   CLEAR READ
   DO screen2.spr

This behavior occurs because in order for the CLEAR READ command to be recognized by the current READ command, program control has to be returned to that READ command. In this example, the CLEAR READ command is issued and then program control is immediately transferred to SCREEN2.SPR; the READ command for Screen1 never has a chance to act on the CLEAR READ command and therefore Screen1's READ command is not cleared before the DO command for SCREEN2.SPR is issued.

RESOLUTION

*************************************************************************** IMPORTANT: This technique is not intended to replace a foundation READ, but is a way to get around the limitation of not being able to have more than five READs in effect at one time. Because a DO command is being executed from another DO command, there is a form of recursion occurring and the error "DO nesting too deep" will appear if the limit of 32 DO levels is reached. ***************************************************************************

To achieve the desired behavior, issue the CLEAR READ command as done above, but don't issue the DO screen2.spr command until after the READ command has been cleared. For example:

1. In the Screen Builder, create a new screen.

   NOTE: In FoxPro for MS-DOS, choose Screen Layout from the Screen menu, and change the screen type from Desktop to Window.

2. Create a push button with two prompts: <Screen2> and <Quit>. Make the variable "m.choice". Open the Valid code snippet and add this code:

         IF m.choice=1
            tobedone="screen2.spr"
         ENDIF
         CLEAR READ
   
      If the Screen2 button is chosen, TOBEDONE is set to the name of the
      proper program file. The CLEAR READ command is issued if either button
      is chosen. Control is returned to the READ command and it is CLEARed.
   
   

3. Open the Setup code snippet and add this line of code:

         tobedone=space(12)
   

   This variable stores the next screen to be executed.

4. Open the Cleanup code snippet and add these lines of code:

         IF EMPTY(PROGRAM(30))     && Check if we are about to exceed the
            IF NOT EMPTY(tobedone) && 32-level DO limit. If we are, the next
               DO (tobedone)       && program isn't executed and an error
         *                            message appears.
            ENDIF
         ELSE
            WAIT WINDOW "32 DO Level Limit exceeded"
         ENDIF
   
      This code tests whether TOBEDONE has been assigned a value that means
      the Screen2 button was chosen. If Screen2 was chosen, the value in
      TOBEDONE is executed using a name expression. If the screen is exited in
      some other fashion, that is, the ESC key or the Quit button was chosen,
      nothing is executed.
   
      NOTE: Make sure this is the last line in the Cleanup code.