Asynchronous event through generic interface

Question

I have to connect several measurement devices to my app (ie. caliper, weight scale, ...), not being tied to a specific brand nor model, so on client side I use interfaces with generic methods (QueryValue). Devices are connected on COM port and accessed on an asynchronous way:

1. Ask for a value (= send a specific character sequence on COM port)
2. Wait for a response

On 'business' side my components use TComPort internally, which data reception event is TComPort.OnRxChar. I wonder how I could fire this event through an interface? Here is what I've done so far:

IDevice = interface
  procedure QueryValue;
  function GetValue: Double;
end;

TDevice = class(TInterfacedObject, IDevice)
private
  FComPort: TComPort;
  FValue: Double;
protected
  procedure ComPortRxChar;
public
  constructor Create;
  procedure QueryValue;
  function GetValue: Double;
end;

constructor TDevice.Create;
begin
  FComPort := TComPort.Create;
  FComPort.OnRxChar := ComPortRxChar;
end;

// COM port receiving data
procedure TDevice.ComPortRxChar;
begin
  FValue := ...
end;

procedure TDevice.GetValue;
begin
  Result := FValue;
end;

But I need an event to know when to call GetValue on client side. What's the usual way to perform that kind of data flow?

Answer 1

You can add event property to interface

IDevice = interface
  function GetValue: Double;
  procedure SetMyEvent(const Value: TNotifyEvent);
  function GetMyEvent: TNotifyEvent;
  property MyEvent: TNotifyEvent read GetMyEvent write SetMyEvent;
end;

and realize it in TDevice class

TDevice = class(TInterfacedObject, IDevice)
private
  FMyEvent: TNotifyEvent;
  procedure SetMyEvent(const Value: TNotifyEvent);
  function GetMyEvent: TNotifyEvent;
public
  function GetValue: Double;
  procedure EmulChar;
end;

Then as usually call FMyEvent handler (if assigned) in the end of ComPortRxChar.

 Tform1...
  procedure EventHandler(Sender: TObject);

procedure TForm1.EventHandler(Sender: TObject);
var
  d: Integer;
  i: IDevice;
begin
  i := TDevice(Sender) as IDevice;
  d := Round(i.GetValue);
  ShowMessage(Format('The answer is %d...', [d]));
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  id: IDevice;
begin
  id:= TDevice.Create;
  id.MyEvent := EventHandler;
  (id as TDevice).EmulChar; //emulate rxchar arrival
end;

procedure TDevice.EmulChar;
begin
  if Assigned(FMyEvent) then
    FMyEvent(Self);
end;

function TDevice.GetMyEvent: TNotifyEvent;
begin
  Result := FMyEvent;
end;

function TDevice.GetValue: Double;
begin
  Result := 42;
end;

procedure TDevice.SetMyEvent(const Value: TNotifyEvent);
begin
  FMyEvent := Value;
end;