Scripting Your World: The Official Guide to Second Life® Scripting

CHAPTER 2

CHAPTER 3

CHAPTER 4

CHAPTER 5

CHAPTER 6

CHAPTER 7

CHAPTER 8

CHAPTER 9

CHAPTER 10

CHAPTER 11

CHAPTER 12

CHAPTER 13

CHAPTER 14

CHAPTER 15

APPENDIC

E

S

25

CHAPTER 1

EVENTS AND EVENT HANDLERS

LSL is an event-driven language. This behavior is a major discriminating feature between LSL and many

other languages. Essentially, the script's flow is determined by events such as receiving messages from other

objects or avatars, or receiving sensor signals. Functionally, events are messages that are sent by a Second

Life server to the scripts active in that sim. Event messages can be received by defining the matching event

handlersJOUIFTUBUFTPGZPVSTDSJQUTNothing happens in a script without event handlers: even the passing

of time is marked by the delivery of timer events. Some events cannot happen without the matching event

IBOEMFSBDUJWFTPNFXIFSFJOUIFPCKFDUJOBUMFBTUPOFPGJUTQSJNT'PSJOTUBODFJGBOPCKFDUEPFTOhUIBWF

a script with a money() event handler, avatars cannot pay that object.

Many library functions have effects that take relatively long periods of time to accomplish. Rather

than blockingTUPQQJOHFYFDVUJPO-4-VTFTasynchronous communications: your script fires off a request

that something happen, and it is notified with an event when the request is satisfied. This pattern is used

BOZUJNFZPVSTDSJQUNBLFTBDBMMUPBMJCSBSZGVODUJPOUIBUDBOOPUCFIBOEMFEMPDBMMZCZUIFIPTUTJNGPS

instance, the function llHTTPRequest() and the http_response() event handler), when physical

TJNVMBUJPOFGGFDUTBSFOhUHPJOHUPCFJOTUBOUBOFPVTllTarget() and at_target()). You could also

UIJOLPGSFQFBUJOHFWFOUTllSensorRepeat() and sensor()PSFWFOMJTUFOFSTllListen()

and listen()) as following the same model: make a request and get responses later. This model of

asynchronous execution allows your script to keep functioning, handling other events and interactions

without stopping to wait for long-term requests to happen.

"TFWFOUTPDDVSBQQMJDBCMFPOFTUIPTFUIBUIBWFIBOEMFSTEFGJOFEPOUIFDVSSFOUTUBUFBSFRVFVFE

for execution by the script in the order they were raised. You should be aware of several important

constraints on the delivery of event messages to handlers:

r #ZEFGBVMUFWFOUTBSFEFMJWFSFEUPBTDSJQUOPNPSFGSFRVFOUMZUIBOFWFSZTFDPOE:PVDBOBEKVTU

this with llMinEventDelay(float seconds) but it can not be less than 0.05 second.

r "NBYJNVNPGFWFOUTDBOCFRVFVFEPOBTDSJQUBOZBEEJUJPOBMFWFOUTXJMMCFTJMFOUMZ

discarded!

r 2VFVFEFWFOUTBSFTJMFOUMZEJTDBSEFEXIFOBTDSJQUUSBOTJUJPOTCFUXFFOTUBUFT

The combination of these factors means that if you have code that expects to get events rapidly and

UBLFTBSFMBUJWFMZMPOHUJNFUPFYFDVUFJODMVEJOHBSUJGJDJBMEFMBZTZPVNBZSVOUIFSJTLPGNJTTJOHFWFOUT"T

BOFYBNQMFTFOEJOHBO*.XIJDIEFMBZTUIFTDSJQUCZTFDPOETJOSFTQPOTFUPDPMMJEFFWFOUTXIJDIDBO

happen as often as every 0.1 second) is probably asking for trouble.

Similarly, the LSL function llSleep(float seconds) pauses script execution for the specified

number of seconds without leaving the current event handler, similar to the way that many LSL functions

introduce artificial delays. In both cases, there is a potential problem if your script is likely to handle lots

of events.

Some events are delivered to a script only if the script has made the appropriate request. For

instance, a listen() event handler is never called unless another handler in the same state has called

llListen(). The SYW website has a list of all the LSL event handlers, the functions required to enable

the event, and the Second Life situation that results in that raised event. And, of course, there are lots of

examples throughout the book.