ServiceState is a member of the APLTree library. The library is a collection of classes etc. that aim to support the Dyalog APL programmer. Search GitHub for "apltree" and you will find solutions to many every-day problems Dyalog APL programmers might have to solve.
ServiceState is a namespace script that offers an interface between the Windows SCM (Service Control Manager) and a Dyalog APL application running as a service.
In you application workspace you need to have a function on ⎕LX similar to this:
Start;ps
[1] ps←#.ServiceState.CreateParmSpace
[2] ps.ride←1
[3] #.ServiceState.Init ps
[4] :If #.ServiceState.IsRunningAsService
[5] {#.TestService.Run ⍵}&⍬
[6] ⎕DQ'.'
[7] #.TestService.Off
[8] :Else
[9] #.TestService.Run ⍬
[10] :EndIf
Note that this example focuses on getting the application running as a service. For the purpose of demonstrating how to overwrite defaults (no RIDE) it shows how to create a parameter space with default settings on line [1] and then overwrite a default on line [2].
If you are happy with the defaults anyway (which you can check by executing #.ServiceState.CreateParmSpace.∆List) then you can specify ⍬ as the right argument of #.ServiceState.Init in line [3].
Remarks:
[4]: Checks whether the application is actually running as a Windows Service. This allows to execute the application in its own thread in case its running as a service but not otherwise - that makes debugging the application significantly easier.
[5]: Here we start the real application. Note that this must run in its own thread because otherwise the application might well become unresponsive.
[6]: We need to execute ⎕DQ '.' in order to make sure that the interpreter processes all events.
[7]: At this stage we need to execute ⎕OFF; that's what #.TestService.Off is doing.
The application must make sure in its main loop that it checks for state changes, and act accordingly. This is all done by the Operator #.ServiceState.CheckServiceMessages which takes the name of a logging function as operand:
MainLoop dummmy;buffer;data;event;obj;rc;r;msg;cs
[1] :Repeat
[2] ⎕DL 1 ⍝ What is reasonable here depends on the application
[3] :If (Log #.ServiceState.CheckServiceMessages)#.ServiceState.IsRunningAsService
[4] Log'The service is in the process of shutting down...'
[5] :Leave
[6] :EndIf
[7] :Until 0
Remarks:
Line [3] checks for any changes signalled from the Service Control Manager (SCM). If a "Pause" is detected, then the function loops until either a "Continue" or a "Stop" is detected. The explicit result is 1 in case a "Stop" is detected and 0 otherwise.