Calling DYLIB callbacks to Xojo app

FINALLY - I am moving to the latest Xojo, working in macOS 10.14 Mojave, after using RealStudio 2011r3 for the past 8 years (which has been very successful for me). I find that I have to do very little to get it to work, mostly fixing my threading cheating (altering UI elements in a thread).

In that light, there is one major violation (I think) which doesn’t throw any exceptions, and you’ll see why. I’d like to know what would be the best fundamental way to refactor this. It’s a pretty simple question; I’m just making it sound complicated.

I use REAL/Xojo mostly to serve as a front end, although I do plenty within it. But most of the work of my app(s) is in a DYLIB/DLL, which does long operations and calls a callback function in my REAL/XOjo UI app to display progress.

This is the process I use, and it works great on Mac and Windows, using 2011r3, but I very much suspect it does not - causes hard crashes - in Xojo.

[code]Function FakeWorkFunc() As Integer

// not called within a thread

Soft Declare Function DoConvert Lib “conversionengine.dylib” (Inst1 As Integer, Inst2 As Integer) As Integer

Dim Param1, Param2 As Integer
Dim ErrCode As Integer

// function takes say 10 minutes. It calls the Xojo-contained callback function (it has the address)
ErrCode = DoConvert(Param1, Param2)

return ErrCode

End Function

Function ProgressCallback(Buff As Ptr) As Integer

// decodes Buff into a MemoryBlock and essentially gets a string and float out of it
// never mind how

Dim ProgressText As String
Dim ProgressAmt As Single

frmProgress.txtProgress.Text = ProgressText
frmProgress.ProgresBar1.Value = ProgressAmt * 1000

return 1

End Function[/code]

I think obviously the DYLIB/DLL banging on the UI thread while it’s still held up with the called function, is NOT GOOD.

And, additionally, I have some Xojo-based functions which the DYLIB/DLL calls - not UI related - where it gets some work done.

So, smart people everywhere, how would you refactor this so it wold play ball with the more thread-safe nature of the latest Xojo?

I would move the UI-related code from the callback into a Timer, and have the callback trigger the timer. The Timer’s action event is guaranteed to run on the main/UI thread.

[code]Function ProgressCallback(Buff As Ptr) As Integer

// decodes Buff into a MemoryBlock and essentially gets a string and float out of it
// never mind how

// convert these variables to properties accessible to both the callback and the timer
Dim ProgressText As String
Dim ProgressAmt As Single

myTimer.Mode = Timer.ModeSingle
App.YieldToNextThread() // so the main thread can run

return 1

End Function[/code]

Hi @Garth Hjelte

Would you use the new Thread.UserInterfaceUpdate available since 2019r2?


If the callback is triggered from a background thread (which is not officially supported), you have to take some precautions:
Use pragmas to disable backgroundtasks and Stackoverflow checks.
Do not access instances or their properties and don‘t create them.
And of course don‘t access the GUI.

(First a note: where did I say anywhere that I was using Threads?)

OK, thank you Andrew Lambert, what you said made sense, but… it doesn’t work. And I think why is because (I don’t think) I’m not violating anything in the first place. Maybe everything I’m doing occurs in the same, main thread. So this was kind of a trick question?

This is all a serial operation, if you consider that the dylib is simply an extension of the Xojo app. And it is, right? It all shares the same process space. Every one of these operations are synchronous, they don’t continue until they are individually finished.

App pulls up a dialog
User inputs some parameters
Clicks OK
The button handler calls a Xojo function
The function calls a dylib function
The dylib does some stuff, at times calls back to to Xojo app
The Xojo app modifies the UI, and returns control back to the dylib
The dylib finishes up what it was doing and the function that was call returns control to Xojo
Xojo shuts down the progress window and throws up a MsgBox finished prompt

So perhaps I’m not doing anything wrong in the first place?

that would really depend on how the dylib code is doing what it does
if its not multi threaded in and of itself then it shouldn’t be an issue
but if it is then IF you could get a callback on a non-main thread then andrew advice to set a property and start a timer may prevent issues (there are precious few thigns you can do from an async callback - setting a property that is a simple type is one)
however you may have needed to start the timer before starting the call to the dylib as one thing you should not do from an async callback is call any Xojo code beyond setting a simply property

that doesnt seem to be the issue here but without more detail about how the dylib operates its hard to give specific advice

is it possible that the dylib links against something that does spawn a thread ?

All purely functional, no spawning nothing, no threads, no timers. I’m just surprised that I can click on the button on my dialog. But it all works, just like it did on 2011r3 in OSX 10.6.

you were expecting it to NOT work ?
there’s no reason it should fail I can think of if its as straight forward as you say
if the call to the dylib is entirely synchronous then xojo basically waits until that call returns like always
no reason that would just randomly change