Microsoft KB Archive/104054

Knowledge Base

PRB: Calling Screen from Another Without Jumping a READ Level

Article ID: 104054

Article Last Modified on 12/3/2003

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APPLIES TO

• Microsoft Visual FoxPro 3.0 Standard Edition
• Microsoft FoxPro 2.6 Standard Edition
• Microsoft FoxPro 2.6a Standard Edition
• Microsoft FoxPro 2.6 for MS-DOS
• Microsoft FoxPro 2.6a Standard Edition
• Microsoft FoxPro 2.6a Professional Edition for Macintosh

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This article was previously published under Q104054

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
                

Looking at this code, it appears that the READ command will be cleared and that Screen2 will then be executed. Actually, the READ command from Screen1 stays in effect, Screen2 is executed, and the Screen1 READ command is not cleared until the READ command from Screen2 is cleared and control is returned to Screen1.

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.

This code example never leaves the originating program, so all variables declared in each screen set will remain defined until you quit the original calling screen. The program will have to manually release the memory variables in the Cleanup code.

Additional query words: VFoxWin FoxDos FoxWin 2.00 2.50 2.50a 2.50b 2.50c errmsg err msg jump

Keywords: kberrmsg kbcode KB104054

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