Understanding how the VCL uses observers internally is fascinating, but to use this pattern in our own applications, we need a clean way to implement these interfaces. Without a base framework, you would find yourself writing the same boilerplate code for GetActive, SetActive, and interface management over and over.

Step 1: The Core Boilerplate (TBaseObserver)

First, we need a class that implements the basic IObserver interface. This class handles the Active state and the OnObserverToggle event, which we've seen is crucial for UI synchronization.

TBaseObserver = class(TInterfacedObject, IObserver)
  protected
    FActive: LongBool;
    FOnObserverToggle: TObserverToggleEvent;
  public
    constructor Create;
    procedure Removed; virtual;
    function GetActive: Boolean; virtual;
    procedure SetActive(Value: Boolean); virtual;
    property Active: Boolean read GetActive write SetActive;
    // ... Getters/Setters for OnObserverToggle
end;

Step 2: Linking to the VCL (TBaseVclObserver)

Since we are primarily working with UI controls, our framework needs to know which TWinControl we are observing. By creating a TBaseVclObserver, we can automate the process of adding ourselves to the component's observer list.

TBaseVclObserver = class(TBaseObserver)
  protected
    FControl: TWinControl;
    procedure AddControlValueObserver(const AControl: TWinControl); virtual;
    procedure AddEditLinkObserver(const AControl: TWinControl); virtual;
    procedure AddAllObservers(const AControl: TWinControl); virtual;
  public
    constructor Create(const AControl: TWinControl); virtual;
end;

Automating Registration

The beauty of this framework is the AddAllObservers method. Instead of manually calling AddObserver for every ID, the base class can iterate through the standard IDs and register the observer where supported. This utilizes the Observers property we explored in Part 2.

constructor TBaseVclObserver.Create(const AControl: TWinControl);
begin
  inherited Create;
  FControl := AControl;
  if FControl <> nil then
    AddAllObservers(FControl);
end;

procedure TBaseVclObserver.AddControlValueObserver(const AControl: TWinControl);
begin
  AControl.Observers.AddObserver(TObserverMapping.ControlValueID, Self);
end;

By moving this logic into a base framework, we've turned a complex interface implementation into a simple inheritance task. Now, creating an observer for a TMemo or a custom control is as easy as overriding a few specific data-handling methods.