Hi, I have questions on using callbacks functions in Delphi. I am using a dll. In this DLL there is a call to a function that returns the response to a callback function. everything works great.

My question is how can I access that information return to the callback function from an existing class in Delphi.


For example:
//Delphi class

Type
TfrmMain = class(TForm)
private
fObjectFieldList: TStringList;
public
Procedure writetoscreen(abuffer:PChar);
end;

//call back function
Type
TGetHttpResponse = procedure(Buffer : Pchar; Id:integer ); stdCall;

Implementation

//call to dll
function HttpRequest(url_ptr:PCHAR;
ip_ptr:PCHAR;
postdata_ptr:PCHAR;
requestheader_ptr:PCHAR;
port:integer;
timeout:integer;
Func:TGetHttpResponse; <---callback
id:integer): integer; stdcall; external 'winhttp_dll.Dll' name '_HttpRequest@32';


//definition to callback
procedure getresponse(aBuffer : Pchar; aId:integer ); stdCall;
begin
// I would like to pass abuffer to writetoscreen procedure that
belongs to TFrmMain
Writetoscreen(abuffer);
end;

PS: i know i can use a global variable but thats out of the question because this is a threaded application where i am trying to used this in.. thanks for your help

: Hi, I have questions on using callbacks functions in Delphi. I am using a dll. In this DLL there is a call to a function that returns the response to a callback function. everything works great.
:
: My question is how can I access that information return to the callback function from an existing class in Delphi.
:
:
: For example:
: //Delphi class
:
: Type
: TfrmMain = class(TForm)
: private
: fObjectFieldList: TStringList;
: public
: Procedure writetoscreen(abuffer:PChar);
: end;
:
: //call back function
: Type
: TGetHttpResponse = procedure(Buffer : Pchar; Id:integer ); stdCall;
:
: Implementation
:
: //call to dll
: function HttpRequest(url_ptr:PCHAR;
: ip_ptr:PCHAR;
: postdata_ptr:PCHAR;
: requestheader_ptr:PCHAR;
: port:integer;
: timeout:integer;
: Func:TGetHttpResponse; <---callback
: id:integer): integer; stdcall; external 'winhttp_dll.Dll' name '_HttpRequest@32';
:
:
: //definition to callback
: procedure getresponse(aBuffer : Pchar; aId:integer ); stdCall;
: begin
: // I would like to pass abuffer to writetoscreen procedure that
: belongs to TFrmMain
: Writetoscreen(abuffer);
: end;
:
: PS: i know i can use a global variable but thats out of the question because this is a threaded application where i am trying to used this in.. thanks for your help
:
:
You should pass the object's address in the ID parameter. Then in the callback you can typecast it to get access to the object's memory space, and its methods:

begin
  HTTPRequest(...., getresponse, integer(frmMain));
   // call to HTTPRequest, left out all the irrelevant parameters
end;

procedure getresponse(aBuffer : Pchar; aId:integer ); stdCall;
begin
  TfrmMain(aID).Writetoscreen(abuffer);
    // aID "points" to frmMain
end;