AsyncCall with Delphi 2007

Question

What I basically want is to start AsyncCall and proceed with my code loading. I have Interface section that consumes lots of time (600+ms) and I want to load this code in independent thread.

I've tried to use AsyncCall to make something like this:

procedure Load;
begin
...
end;

initialization
  AsyncCall(@Load, []); // or LocalAsyncCall(@Load)

However, this Load procedure actually starts in Main thread and not in the new created thread. How can I force the Load procedure to be loaded in any thread other than MainThread?

I can create TThread and Execute this but I want to force AsyncCall or LocalAsyncCall or anything from AsyncCall library to make to work.

Thanks for your help.

Answer 1

The problem is that your code is not retaining the IAsyncCall interface that is returned by the AsyncCall function.

AsyncCall(@Load, []);
//AsyncCall returns an IAsyncCall interface,
//but this code does not take a reference to it

Because of this, the interface that is returned has its reference count decremented to zero as soon as the initialization section completes. This therefore frees the object that implements the interface which does this:

destructor TAsyncCall.Destroy;
begin
  if FCall <> nil then
  begin
    try
-->   FCall.Sync; // throw raised exceptions here
    finally
      FCall.Free;
    end;
  end;
  inherited Destroy;
end;

The key line is the call to Sync which forces the asynchronous call to be executed to completion. All this happens in the main thread which explains the behaviour that you report.

---

The solution is that you simply need to keep the IAsyncCall interface alive by storing it in a variable.

var
  a: IAsyncCall;

initialization
  a := AsyncCall(@Load, []);

In the real code you need to ensure that Load had completed before running any code that is reliant on Load. When your program reached a point where it required Load to have been called it has to call Sync on the IAsyncCall interface.

So you might write it something like this.

unit MyUnit;

interface

procedure EnsureLoaded;

implementation

uses
  AsyncCalls;

....

procedure Load;
begin
  ....
end;

var
  LoadAsyncCall: IAsyncCall;

procedure EnsureLoaded;
begin
  LoadAsyncCall := nil;//this will effect a call to Sync
end;

initialization
  LoadAsyncCall := AsyncCall(@Load, []);

end.

The call EnsureLoaded from other units that required Load to have run. Or, alternatively, call EnsureLoaded from any methods exported by MyUnit that depended on Load having run. The latter option has much better encapsulation.

Answer 2

Have you tried something like this?:

procedure Load;
begin
  if GetCurrentThreadId <> MainThreadID then
    Beep;
end;

var a: IAsyncCall;

initialization
  a := AsyncCall(@Load, []);
  a.ForceDifferentThread;

ForceDifferentThread() tells AsyncCalls that the assigned function must not be executed in the current thread.