Asynchronous event through generic interface - delphi

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:
Ask for a value (= send a specific character sequence on
COM port)
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;