MVVM Anwendungen mit Delphi und nicht gegen Delphi

Im folgenden Video meines Youtube-Kanals eine Einführung zum Thema MVVM Anwendungen mit Delphi. Wichtige Motivation ist für mich, dass ich soviel Delphi wie möglich verwende, also eben nicht anfange, alles von Hand im Source-Editor hinzuschreiben.

Hinweis: Das Video versteht sich als erste Einführung in das Thema MVVM mit Delphi. Selbstverständlich gibt es noch eine ganze Reihe von Dingen, die man optimieren und ausbauen kann. Weitere Schritte werde ich hier in noch folgenden Beiträgen skizzieren.

Das Video ist in Deutsch.

TTask / TThreadPool may keep interfaced objects alive

I am using TTask.Run quite a lot in projects and in many of my demos. I won’t go into the details of how to use TTask here, but I only want to highlight that there is still an open issue, which may cause interfaced objects to be destroyed much later, than you would expect. Ultimately this may lead to a much higher memory footprint of your application.

As of Delphi 10.1 Update 2 the issue is still marked open on quality.emabaracdero.com – you might vote for it, if your code is affected by it.

Interfaced objects use automatic reference counting for lifecycle management:

var
  LFoo : IFoo
begin
  LFoo := TFoo.Create;
  //work with LFoo
end;  //LFoo goes out of scope, its reference counter will go to zero,
      // thus it will be destroyed here automatically.

With TTask.Run you can start background threads, that will execute code passed in as anonymous method:

var
  LFoo : IFoo
begin
  LFoo := TFoo.Create;
  TTask.Run(procedure
    begin
      Foobar(LFoo); //Work with LFoo 
    end;
  ).Waitfor; //Wait for task/thread to be completed 
end;  //LFoo goes out of scope, its reference counter SHOULD go to zero,
      // thus it SHOULD be destroyed here automatically.   

Unfortunately the above does not work as expected. The internals of TTask (actually TThreadpool) keep a reference to the (already finished) task and prevent the instance of TFoo, which is referenced by LFoo, to be destroyed automatically. At least when the TThreadPool gets destroyed, all those kept references will be released and the instances will be destroyed, thus they won’t report as memory leak (with ReportMemoryLeaksOnShutDown).

In other words: you will have to set LFoo to nil manually – until this issue gets solved in one of the next updates.

  TTask.Run(procedure
    begin
      Foobar(LFoo); //Work with LFoo 
    end;
  ).Waitfor; //Wait for task/thread to be completed 
  LFoo := nil; //Depending on your logic, this could also be done inside TTask.Run
end;    

Sharing DataModules between VCL and FMX projects

When you start implementing cross-platform applications in Delphi’s FireMonkey framework, then you might consider reusing TDataMdoule instances from your existing VCL applications. If your code is clean enough, i.e. your DataModules are separated from any UI code, that is contain no references to WinApi, Forms, Controls or other UI relevant units, then this is actually a reasonable approach.

You might run into a weird problem though: The IDE will sometimes insist on adding VCL-dependant units to your DataModul, even if you actually compile for FireMonkey. One example is FireDAC.VCL.Wait which will get added automatically under certain conditions (and which won’t compile under FMX).

There is a property that controls this behaviour in shared TDataModule instances:

ClassGroup is basically a „Pseudo Property“, which adds some magic to the project so that the IDE will behave differently depending on the properties value. To make your DataModule really platform independent set it to:   System.Classes.TPersistent

That way, the IDE stops inserting references to either FMX or VCL units in the DataModule’s uses clause. In the case of FireDAC applications, it will just add FireDAC.ConsoleUI.Wait, which will compile just fine on all platforms. You would then add the actual Wait unit in the app’s main form (or in the DPR).