LastWillTestament - Disconnect - Destroy

DEV TMS MQTT
1

Hello,

in the OnDestroy-Event of my Form I check if the mqttclient is connected and if so, I disconnect and destroy it:

procedure TForm1.FormDestroy(Sender: TObject);
begin
  if FMqttClient.IsConnected then begin
    FMqttClient.Disconnect();
  end;

  FreeAndNil(FMqttClient);
end;

However, the broker sends the last will testament even though the connection should be closed gracefully.

I assume, that the disconnecting process is not yet finished when I call the FreeAndNil(FMqttClient) and therefore the disconnect packet does not arrive at the broker. Without FreeAndNil(FMqttClient) the LWT is not send.

How can I wait with the FreeAndNil until the connection is closed gracefully?

3

The disconnection is asynchronous and the application should wait until the TMSMQTTClient.OnConnectedStatusChanged event has AConnected set to false before destroying the TMSMQTTClient component.

All this is implemented in these demos :

  • \Demo\HighPerformance
  • \Demo\LazarusSimple

They use the TForm.OnCloseQuery and TMSMQTTClient.OnConnectedStatusChanged events to close the connection gracefully following these steps :

  1. TMainForm.FormCloseQuery set CanClose to false, sets FClosing to true and calls TMSMQTTClient.Disconnect.
  2. TMSMQTTClient.OnConnectedStatusChanged is triggered with AConnected set to false and calls close.
4

Thank you, the HighPerformance example works!
I tried the lazarus example first (in Delphi), but here the Application got stuck at closing (mqtt was disconnected but the application hangs and is not responsive)

5

I'm not sure I understand. The Lazarus example is not supposed to open in Delphi?

6

Sorry, I was not clear about that. I copied the relavant code snippets from the Lazarus example into my project and tried it.

The TThread.ForceQueue from the HighPerformance example did the trick.
TThread.ForceQueue works also in Lazarus, so I wonder why this is not also present in the Lazarus example?

7

The Lazarus demo always sets TMSMQTTClient.SyncedEvents to True but the FMX demo has a switch to set a custom TMSMQTTClient.SyncedEvents value.

The TMSMQTTClient.OnConnectedStatusChanged event might run in the context of a background thread or the main application thread in the FMX demo.

The form must always be destroyed in the main application thread and that's why it has a TThread.ForceQueue call.

Besides that, LCL, VCL and FMX have a completely different implementation underneath. Even different Delphi versions have significant implementation changes that may require slightly different application code.

The Lazarus demo works correctly without a TThread.ForceQueue call but using the same code in Delphi may require a few changes

8

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