[Lazarus] WinINet STDCALL callback crash

[Sven Barth]

On 25.02.2013 17:42, matthew at accordancebible.com wrote:
> Ludo, Hans-Peter,
>
> Many thanks for your insights.
>
> Ludo, I am reviewing the msdn code example you posted; I have read quite a few but don't remember seeing that one.
>
> Hans-Peter, our project has so far been able to avoid the additional complexity posed by threads.  It would be with reluctance that we would turn to threading; however, if experimenting with Ludo's post content (see below) does not work, then would will use threads.  Might you know of an example implementation or code for asynchronous HTTP and/or FTP downloads?

In principle you could do it as follows:

=== example begin ===

type
   TDownloader = class(TThread)
   public type
     TCallback = procedure(aSender: TDownloader; aData: TStream) of object;
   private
     fCallback: TCallback;
     fUrl: String;
   protected
     procedure Execute; override;
   public
     constructor Create(const aUrl: String; aCallback: TCallback);
   end;

constructor TDownloader.Create(const aUrl: String; aCallback: TCallback);
begin
   // check that aCallback and aUrl are given and set up local variables
   // ...

   FreeOnTerminate := True;
   inherited Create(False);
end;

procedure TDownloader.Execute;
var
   ms: TMemoryStream;
begin
   ms := TMemoryStream.Create;
   try
     // here you use what ever synchronous API you want and store it's 
result in ms

     // when all data is received
     fCallback(Self, ms);
   finally
     ms.Free;
   end;
end;

// somewhere else

TDownloader.Create('http://example.org/whatever', @MyCallback);
// fire and forget

=== example end ===

Note: if you want to be able to cancel the download (which could be done 
by calling the thread's "Terminate" method which sets "Terminated" to 
"True") you should not set "FreeOnTerminate" to "True", check for 
"Terminated" in the thread's "Execute" method and capture the instance 
returned by "TDownloader.Create".

Regards,
Sven