Thoughts about application architeture with Lazarus

The Delphi books of my days (or why i'm not guilt of my application's poor design)

As most of Lazarus developers, i started to code in Delphi (in fact i learned computer programming with turbo pascal) and to get most of the tool i read some books, i bought three or four and read part of others in bookstores. This is supposed to be a good practice when learning a new technology.

The problem, noticed by me only years later, is the lack of teaching of good application design like separation of concerns (view, business, persistence layers) and how to achieve them with Delphi. Most of the books focused in the visual aspect (how to create a good looking form, reports etc) and how to setup datasets and the db aware controls. The closer to a good practice advice was putting datasets and datasources in data modules instead of forms.

We can't even blame the book authors. The Delphi's greatest selling point was (is?) the Rapid Application Development (RAD) features.

Recipes for a bulky spaghetti

In early days, when developing my applications, i was a diligent student: i put database logic in data modules and designed the forms as specified in the books. But, as all developers that created applications with more than three forms knows, things started to get hard to evolve and maintain.

Keeping the database components in data modules did not help much. You end with shared dataset states, and all problems that comes with it, across different parts of applications.

Below is a data module's snapshot of my first big application (still in production, by the way).

It could be even worse if i had not started to use a TDataset factory in the middle of development 

In the end, the project has code like:

  // a form to select a profile
  DataCenter.PrescriptionProfilesDataset.Open;
  with TLoadPrescriptionProfileForm.Create(AOwner) do
  try
    Result := ShowModal;
    if Result = mrYes then
      DataCenter.LoadPrescriptionProfile;
  finally
    DataCenter.PrescriptionProfilesDataset.Close;
    Destroy;
  end;
  
  //snippet of DataCenter.LoadPrescriptionProfile (copy the selected profile to PrescriptionItemsDataset)
  with PrescriptionItemsDataset do
  begin
    DisableControls;
    try
      FilterPrescriptionProfileItems(PrescriptionProfilesDataset.FieldByName('Id').AsInteger);
      while not PrescriptionProfileItemsDataset.Eof do
      begin
        NewMedication := PrescriptionProfileItemsDataset.FieldByName('Medication').AsString;
        if Lookup('Medication', NewMedication, 'Id') = Null then
        begin
          Append;
          FieldByName('PatientId').AsInteger := PatientsDatasetId.AsInteger;
          FieldByName('Medication').AsString := NewMedication;
          FieldByName('Dosage').AsString := PrescriptionProfileItemsDataset.FieldByName('Dosage').AsString;
          [..]
          Post;
        end;
        PrescriptionProfileItemsDataset.Next;
      end;
      ApplyUpdates;
      PrescriptionProfileItemsDataset.Close;
    finally
      EnableControls;
    end;
  end;

Its not necessary to be a software architect guru to know that this is unmanageable

Eating the pasta with business objects and inversion of control

In the projects that succeeded the first one, most of the data related code is encapsulated in business objects. The data module does not contain TDataset instances anymore, it's responsible only to act as a TDataset factory and to implement some specific data action. To work with dataset it's necessary just reference one from a key which leads to code like the below:

  FWeightHistoryDataset := DataModule.GetQuery(Self, 'weighthistory');
  FWeightHistoryDataset.ParamByName('prontuaryid').AsInteger := FId;
  FWeightHistoryDataset.Open;

This fixes the shared state issue since each dataset has a clear, limited scope. But does not solve the  business objects dependency of a global instance (DataModule), which  makes testing harder.

In the project that i'm starting, i solved the dependency to the global instance by using the service locator pattern through the IoC Container i cited in a previous post. I defined a resource factory service that is resolved as soon as the business object is created, opening the doors to setup testing environments in a clear manner.

All done?

Not yet. The business logic is contained in specific classes, there's no shared state across application and no hardcoded global dependency but the view layer (forms) is still (dis)organized  in the classic way with each TForm calling and being called by other ones directly. This problem, and the solutions i'm working, will be the subject to a future post.

Number of units, optimization and executable size

I tend to split my code in small units instead of writing a big unit with lot of classes or functions. The drawback is the large number of files but, in my opinion, the benefits outweight it.


One thing that always bothered me was if this practice has any effect in file size.


Seems not. I wrote two versions of the same program. In the first, all classes (one descendant of another) are defined in the same unit while in the second each class lives in a separated unit. When compiled with the debugging info the separated program is a little bigger. This difference in size does not exist when compiled without debugging info.


As a side note, i noticed that compiling with -O2 flag leads to smaller executables compared with -O1, the Lazarus default. It's just a few kilobytes but worth the note.  

Using TComponent with automatic reference count

For some time, i know the concepts of Inversion of Control (IoC) and Dependency Injection (DI) as well the benefits they bring but never used them in my code.  Now that i'm a starting a new project from scratch, and the deadline is not so tight, i decided to raise the bar for my code design.

I'll implement an IoC container in the line of VSoft's one. While adding the possibility of doing DI through constructor injection would be great, i won't implement it. It's not a hard requirement of mine and fpc currently does not support the features (basically Delphi's new RTTI) needed to implement it without hacks.

Automatic reference counting

Most of Delphi IoC implementations use COM interfaces and rely on the automatic reference count to manage object instance life cycle. So do i. This approach's drawback is that the class to be instantiated must handle the reference count. When designing new classes or when class hierarchy can be modified, is sufficient to inherit from TInterfaced* classes. The problem rises when is necessary to use a class that has a defined hierarchy and does not handle reference counting, like LCL ones.

Since i plan to decouple TForm descendants, i need a way to use them with the IoC container. Below is the (rough) design, in pseudo code:

//Define interface
  IPersonView = interface
  ['{9B5BBA42-E82B-4CA0-A43D-66A22DCC10DE}']
    procedure DoIt;
  end;

  //Implement an IPersonView
  TPersonViewForm = class(TForm, IPersonView)   
    procedure DoIt;
  end;

  //Register implementation   
  Container.Register(IPersonView, TPersonViewForm); 

  //Instantiate the view
  Container.Resolve(IPersonView)

At first look, it should work seamlessly. And in fact does: a TPersonViewForm is instantiated and returned as IPersonView. The only issue is that the object instance will never be freed even when the interface reference goes out of scope. This occurs because _AddRef and _Release methods of TComponent does not handle reference count by default.

VCLComObject to the rescue

Examining the code, we observe that TComponent _AddRef and _Release forwards to VCLComObject property. There's not good documentation or examples of using this property. So i wrote an example to see if it would solve my problem.

Basically i wrote TComponentReference, a descendant of TInterfacedObject with a dummy implementation of IVCLComObject that gets a TComponent reference in the constructor and free it in BeforeDestruction.

constructor TComponentReference.Create(Component: TComponent);
begin
  FComponent := Component;
end;
procedure TComponentReference.BeforeDestruction;
begin
  inherited BeforeDestruction;
  FComponent.Free;
end;

And this is how i tested:

function GetMyIntf: IMyIntf;
var
  C: TMyComponent;
  R: IVCLComObject;
begin
  C := TMyComponent.Create(nil);
  R := TComponentReference.Create(C);
  C.VCLComObject := R;
  Result := C as IMyIntf;
end;
var
  MyIntf: IMyIntf;
begin
  MyIntf := GetMyIntf;
  MyIntf.DoIt;
end.   

It worked! I get a IMyIntf reference and no memory leaks. Easier than i initially think.

The code can be downloaded here.