Event and signal handling functions

to onevent :arg1 :arg2 ...
    instructions
end

to onsignalname :turtle :data
    instructions
end

to onsignal :name :turtle :data
    instructions
end

Events are actions which occur during the program execution in order difficult to predict a priori when the code is written. They include actions on the user interface like mouse moves, cicks, keyboard hits and actions of turtles like collisions or edge crossing. Events can launch default or user-written handling functions.
Signals are a special case of events. They are raised programmatically by turtles and timers. Signals are used to synchronize parallel work of multiple turtles.

In order to implement a handling function for an event you should create the function with the name and arguments corresponding to the event. Name of the function is made of prefix "on" and the event name.
Signal handling function should be named as "onsignal" followed by the signal name; arguments for the signal handling function are: :turtle (a turtle which has sent the signal) and :data (a variable which contains additional information added to the signal when it was raised). Function named "onsignal" without any following name handles all signals which were not handled by dedicated "onsignalnazwa" functions; :name argument with the signal name is available in this case.

Handling functions are executed as separate tasks of turtles. Tasks are queued in turtle's task queues in the order of raised events.

Note 1: Definition of a handling function is not required to use all arguments available for an event, however it is required to preserve the order of arguments as documented in opisie zdarzenia.
Note 2: Event (signal) handling function is active for the turtle which contains definition. It is also visible bot not active for child turtles. Instructions eventenable, eventdisable can be used to activate and disactivate the handling function after it was defined.

Example 1:

to onmouseclick :mousepos
print :mousepos
end

Output:
Position of the mouse is written to the text output after each click.

{-295 130}
{119 18}
{-14 -100}
{87 145}

Example 2:

to onsignaltick :turtle :data
(print :turtle :data)
end

"t := (timer "tick 300 50 4)

Output:

first {1 50.0078125 50.0078125}
first {2 301.0234375 351.03125}
first {3 302.015625 653.046875}
first {4 302.015625 955.0625}

Example 3:

to onsignal :name :turtle
(print "signal :name "from :turtle)
end

to onsignaltwo
print "dedicated_handler_two
end

signal "one
signal "two
signal "three

Output:

signal one from first
dedicated_handler_two
signal three from first

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