Taking action only the first time an event executes

Question

I want a certain action to occur only the first time an event handler for a button is called. Is there a simple way to do this?
Bill F

Answer 1

There are two pretty easy ways to do this.
If you want to continue to trap other events (other event cases in your event structure), use a shift register:
1. Add a shift register to the loop containing your event structure. (right-click on the left-hand border of your loop and select Add Shift Register).
2. Wire a numeric constant = 0 from outside the loop to the input (left-hand) terminal of your shift register.
3. Inside your event structure, in the event case you want to handle only once, put a case structure around the code you want to execute only once. That code should be within the True case.
4. Add an Equal To 0? function (from the comparison palette) to the loop. Wire the input shift register to the input of Equal To 0?. Wire the output of Equal To 0? t
o the selctor for your new case.
5. Within the True case of your new case structure, add a numeric constant = 1. Wire that constant out to the output (right-hand) side of your shift register.
6. Within the False case of your new case structure, wire the input side of your shift register to the output tunnel created by the wiring in step 5.
This way the event structure will keep running and handling other events, but the case you just modified will only run the first time: the first time you get the event, the shift register will = 0 so you'll execute the True case of your new case structure. Within that case, you set the shift register to 1. The next time you get that event, the shift register = 1, not 0, so you'll execute the False case and not the code in your True case.
If you want to stop all event handling after the first event and your event handler is the only thing with its loop, from the event case (for the event you want to stop on), wire a True boolean constant out to
the Stop terminal of the loop containing your event structure.