QuickBasic 4.00b UPDATE.DOC: Adds ON UEVENT to Trap Events (33488)

MORE INFORMATION

New Statement: ON UEVENT

Action: Defines an event trap for a user-defined event

Syntax: ON UEVENT GOSUB {<linenumber>|<linelabel>}

Remarks:

In this syntax, <linenumber> or <linelabel> specifies the number or label of the first line in the event-trapping subroutine. The ON UEVENT statement allows your program to branch to an event-trapping subroutine when a user-defined event occurs. ON UEVENT is unlike other event-trapping statements in that it allows your program, not an external agent, to trigger the event that you expect to trap. For example, assume that your system contains a data acquisition device that generates an interrupt when data arrives. Most of your application is written in QuickBasic. The main module contains an event-trapping subroutine that responds to a user-defined event (in this case, the arrival of data from the acquisition device). The following three assembly-language procedures are also linked into the program:

1. An interrupt handler that responds to the interrupt generated by the acquisition device
2. A routine that installs the interrupt handler in the chain of interrupts
3. A routine that removes the interrupt handler from the chain of interrupts

When the program begins, it calls the assembly-language routine that installs the interrupt handler. Then it executes an ON UEVENT statement to identify the QuickBasic subroutine that will be executed when a user-defined event occurs. The final step in setting up the user-defined event trap is to execute a UEVENT ON statement. Once this is done, the Basic program enters its main execution loop.

When data arrives, the acquisition device generates an interrupt. The assembly-language interrupt handler, in turn, calls the routine SetUEvent. SetUEvent is a Basic run-time-library routine that causes a user-defined event to occur in Basic. This special routine can be called from any Microsoft language. References to SetUEvent are resolved when your program is linked with the run-time library. The user-defined event causes the program to branch to the QuickBasic subroutine identified by the previous ON UEVENT statement. At this point, the event-trapping subroutine performs whatever processing is desired. Just before the application terminates, it calls the assembly-language routine that removes the interrupt handler from the chain of interrupts.

New Statements: UEVENT ON, OFF, STOP

Action: Enable, disable, or suspend event trapping for a user-defined event

Syntax: UEVENT ON
UEVENT OFF
UEVENT STOP

Remarks:

The effect of UEVENT parallels that of other event-trapping statements. For example, UEVENT ON enables a user-defined event trap that you previously set up with an ON UEVENT GOSUB statement.