Callback functionality between Delphi DLL and Delphi EXE

Question

I am writting a Delphi DLL. In the Delphi DLL I would like to implement a callback function, this callback function should call back to the caller Delphi program. Main purpose of callback function is that certain long operation will happen in the DLL, but progress reporting (through a progressbar) and operation cancellation should happen in the caller Delphi program.

I would like to get some support how to implement correctly the callback function within the DLL. I can proceed until assigning the callback function passed from the EXE but i do nit know how to initiate a call from the DLL itself.

This is the definition part (used by EXE and DLL):

uses Windows;

Type
  PTCallBackStruct = ^TCallBackStruct;
  TCallBackStruct = packed record
  Handle: THandle;
  Caller: Pointer;           
  FileSize: LongInt;
end;

type

 TFunctionPointerType = function(ZCallbackRec: PTCallBackStruct): Longbool;
  stdcall;

type
  PTDLLParamaters = ^TDLLParamaters;
  TDLLParamaters = packed record
   Handle: THandle;
   Caller: Pointer; 
   CallbackFunction: TFunctionPointerType;
 end;

 var
   DLLCallback: TFunctionPointerType;

EXE file:

 uses ....    

 type

  function DLL_Callback(ZCallBackRec: PTCallBackStruct): LongBool; stdcall;
    forward;

  implementation

   function DLL_Callback(ZCallBackRec: PTCallBackStruct): LongBool; stdcall;
   begin
      // progress reporting this function should be called back from 
      //the DLL. The Handle and Self parameter should help with identifying 
      // which object initiated the callback
   end; 

via PTDLLParameters passed to the DLL as follows from the Delphi exe:

// init callback
 DLLParameters := AllocMem(SizeOf(TDLLParamaters));
 with DLLParameters^ do
   begin
     Handle := Application.Handle;
     Caller := Self;
     CallbackFunction:= DLL_Callback;
  end;

loading the DLL

   .....
   .....

calling the DLL

   CompressionCreateLibrary(DLLParameters);
   ....

certain operations

     Free DLL
     ....
     .....
     FreeMem(DLLParameters);

DLL file:

This function should be called from certain part of the DLL making the progress back to the EXE regarding actual operation:

   function CallCallBackFromDLL(Size: integer): Integer;
    begin
       //
       .... 
       set up callbackstruct
       .... 
       // calling back
        DLLCallback(CallbackStruct);
      end;

I think this part should be ok:

 // main init call assigning the callback function to the DLL
 function CompressionCreateLibrary(DLLParametersID: PTDLLParamaters): Integer;

 begin

     DLLParametersID.CallbackFunction:= @DLLCallback;

 end;

Please help me out how to implement the callback part properly in the DLL. Some example code would be appreciated. While debuging initialization works fine, but making a callback fails. Thank you in advance.

Answer 1

Your assignment statement is backward. In the DLL function, DLLParametersID holds the information about the callback, but then you overwrite it with your global variable:

DLLParametersID.CallbackFunction:= @DLLCallback;

Swap those to assign DLLCallback.