Xojo callback function code#

Fortunately that doesn’t seem to be very Lambert, Hare – The standard rules and caveats about threads apply.Īlso, callbacks that accept a varying number of parameters can be difficult if not impossible to implement in Xojo. If you call the declare on a Xojo thread then the callback will also run on that thread. Only module methods and shared class methods may be used. Callback methods can’t be ordinary methods of a class.

There is a limitation on what kinds of methods can be callbacks, however. There really aren’t any limitations to what a callback can do. Only primitive types (Ptrs, Ints, etc.) can be used as parameters and return values, just like with declares. Xojo uses the CDecl convention by default, and can use the StdCall convention if you use a pragma directive. The Xojo compiler and the dylib have to agree about the calling convention. Use the AddressOf operator to get a Ptr to the callback, and then pass the Ptr to the dylib.ĭeclare Sub dylib_foo Lib 'MyLib' (Callback As Ptr)Įxecution then proceeds exactly as in your scenario. I saw some forum discussion from four years ago, where this sort of thing SHOULD work, but it seemed iffy back then. Q3) If such a call can be made, are there any limitations? Can I call XOJO functions that draw to the screen, for example?

Xojo callback function code#

Q2) How does C-compiled code in the dylib make the call into XOJO, provide parameters, and receive return value? Q1) Can I provide an address of a Xojo function to the dylib? I would need to provide dylib_foo() with the address of main_bar(), perhaps once when the program launches? STEP 4) dylib_foo() completes, returns control to the MAIN XOJO APP that originally invoked the external method. STEP 3) main_bar() completes, returns control to dylib_foo(). STEP 2) dylib_foo() executes, but before it completes, it calls main_bar(), a method in the MAIN XOJO APP. STEP 1) From inside the MAIN XOJO APP, a button is pressed, calling an external C-API method dylib_foo() in a dylib. I recognize the hazards of doing so – especially a circular call situation. If I have a dylib (MacOS) or DLL (Windows), could the code in that library call BACK INTO the Xojo app that invoked the call into the dylib in the first place? OK, this may not be possible, but I have to ask: