Saturday, May 24, 2014

MV* with Lazarus: between Presenter and ViewModel

The MVC conundrum


Sooner or later a programmer will get in touch with the acronym MVC (Model-View-Controller). Despite its ubiquitous presence in discussions or articles about code design, there's few comprehensive examples of using this pattern with Delphi / Lazarus. Most of the examples are just "one form application" that does not show how to organize a large scale application. There's not even a common pattern between them, some have reference to the controller in the view while others do the opposite.

This is not an object pascal exclusive issue. Other languages have the same problem and the reason is simple: the MVC as was designed to Smalltalk decades ago does not fits naturally in the modern, event driven, GUI architecture. The MVP (Model-View-Presenter) and its variations Passive View and Supervising Controller updates the pattern to match the requirements of today user interfaces. There's also Presentation Model and its most famous deviation MVVM (Model-View-ViewModel).

Meeting the presentation layer


All in all, the objective of all these patterns are to separate the presentation from the business layer thus facilitating the code maintenance. The difference lies in the responsibility of each presentation layer component and how they interact with the model (business object). In order to improve the architecture of my Lazarus projects, i found that the MVP is the most doable to be used with object pascal. There are good examples of MVP/PassiveView with Delphi that could be easily adapted but, in my opinion, is overkill and counterproductive to define read and write properties for each GUI element.

I have forms as simple as seem below

procedure TAppConfigViewForm.FormShow(Sender: TObject);
begin
  BaseURLEdit.Text := Config.BaseURL;
end;

procedure TAppConfigViewForm.SaveButtonClick(Sender: TObject);
begin
  Config.BaseURL := BaseURLEdit.Text;
  Config.Save;
end;

Having to define a view and a presenter interfaces and implement a presenter to such a simple view is a no-no to me. On the other hand, in complexes views, handling the GUI logic in a separated component is worth the work.

With these in mind, i defined an interface (IPresentation) to abstract how a view (TForm) is configured and show. To use just reference one by a string id, call SetProperties to set published properties and ShowModal to show it.

  
var
  Presentation: IPresentation;

Presentation := PresentationManager['myview'];
Presentation.SetProperties(['ConfigProp', FConfig]).ShowModal;

The presentations are registered to a specialized IoC container through two overloaded methods:

  
IPresentationManager = interface
  procedure Register(const PresentationName: String; ViewClass: TFormClass);
  procedure Register(const PresentationName: String; PresenterClass: TPresenterClass);
end;

Both has a PresentationName argument that will identify the presentation. The first overload accepts a TFormClass, the view is instantiated directly and there's no presenter. The second, accepts a PresenterClass that will be responsible to show the view.

This is how the presenter and view classes looks:

//presenter 
interface

  TNutritionEvaluationPresenter = class(TBasePresenter)
  public
    function ShowModal: TModalResult; override;
    function CanImportPreviousEvaluation: Boolean;
    procedure ImportPreviousEvaluation;
    procedure SaveEvaluation;
    property EvaluationData: TJSONObject read GetEvaluationData;
  end;

implementation

uses
  NutritionEvaluationView;


function TNutritionEvaluationPresenter.ShowModal: TModalResult;
var
  View: TNutritionEvaluationViewForm;
begin
  View := TNutritionEvaluationViewForm.Create(nil);
  try
    View.Presenter := Self;
    Result := View.ShowModal;
  finally
    View.Destroy;
  end;
end;

//view
interface

uses
  NutritionEvaluationPresenter;


  TNutritionEvaluationViewForm = class(TForm)
  [..]
  published
    property Presenter: TNutritionEvaluationPresenter read FPresenter write SetPresenter;
  end;

procedure TNutritionEvaluationViewForm.ImportPreviousLabelClick(Sender: TObject);
begin
  FPresenter.ImportPreviousEvaluation;
end;

procedure TNutritionEvaluationViewForm.SaveButtonClick(Sender: TObject);
begin
  FPresenter.SaveEvaluation;
end;

procedure TNutritionEvaluationViewForm.FormShow(Sender: TObject);
begin
  ImportPreviousLabel.Visible := FPresenter.CanImportPreviousEvaluation;
  //update GUI with evaluation data
end;


The Presenter here is acting more like a ViewModel (expose data, state, operations to view) than a true presenter. It works fine but with serious caveats:
  • The view and the presenter know each other which defeats the purpose of independent implementations. Also is not possible to hold a view reference in presenter interface (circular unit reference)
  • The TForm presenter property must be set manually (subject to forget)
  • Registering a TForm class that expects a presenter directly will crash since there'll be no presenter

Interfaces and conventions to the rescue


I was not not really satisfied with the above approach, so reworked the code and got the following design:

  • The presentation register method now has three arguments: name, view class and presenter class (optional). When the presenter class is not defined, the view is instantiated directly
  • The view (TForm) is show by the internal code. No need to the presenter do it.
  • If a presenter class is specified, the view class must define a published property named Presenter. An error is throw if the property does not exists or if is of an incompatible type
  • The presenter property can be declared as a interface also, allowing to completely decouple the presenter from the view implementations
  • There's the possibility to bind a view instance to a presenter property. Not implemented since, until now I did not need.
So much talk. The current code can be found here  and a example how I use it here.

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