BBC BASIC Programmers' Reference

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=====Adding an event to the event queue=====

//by Richard Russell, August 2014//\\ \\  Normally, **event interrupts** (e.g. ON MOUSE, ON MOVE, ON SYS and ON TIME) take place as a result of user actions such as mouse clicks, menu selections, moving/re-sizing a window or (in the case of ON TIME) periodically. The events are initially added to a 32-event FIFO queue, and then automatically dispatched to the specified BASIC handler routine (if any).\\ \\  For test purposes, or other special uses, it is possible to insert a 'pseudo event' into the queue (if there is enough space). You can do that using the following code:

<code bb4w> 
        SYS @fn%(18), handler%, msg%, wparam%, lparam%
        @flags% OR= &20000000
</code>

Here **handler%** is the address of the event handler (see below), **msg%** is the value which will ultimately be received by the handler in the **@msg%** system variable, **wparam%** is the value which will be received in **@wparam%** and **lparam%** is the value which will be received in **@lparam%**.\\ \\  The **handler%** parameter can be the address of a custom handler or can specify one of the standard ON... event handlers as follows:

<code bb4w>
        handler%=!388 : REM ON TIME
        handler%=!396 : REM ON MOVE
        handler%=!400 : REM ON SYS
        handler%=!404 : REM ON MOVE
</code>

(if no relevant **ON event** statement has yet been executed, or an **ON event OFF** has been executed, handler% will be set to zero).\\ \\  If you want to determine whether the event was successfully added to the queue you can test the return value:

<code bb4w>
        SYS @fn%(18), handler%, msg%, wparam%, lparam% TO ok%
        IF ok% @flags% OR= &20000000
</code>

If **ok%** is non-zero the event was added, if it is zero the queue was already full.\\ \\  Here is a simple example of the use of this technique:

<code bb4w>
        ON SYS PRINT @msg%,@wparam%,@lparam% : RETURN

        SYS @fn%(18), !400, 123, 456, 789 TO ok%
        IF ok% @flags% OR= &20000000

        REPEAT
          WAIT 0
        UNTIL FALSE
</code>