Delphi Observers Part 4: Active Interception - TCustomEdit

In the previous articles, we established how the observer engine is initialized. Now, let’s look at how a professional-grade component like TEdit uses these hooks to manage UI state. We'll look at its ancestor: TCustomEdit.

The Opt-in Override

For a TCustomEdit to participate in the observer pattern, it must explicitly signal support for specific IDs in CanObserve. Specifically, it looks for EditLinkID (for data binding) and ControlValueID (for simple change monitoring).

function TCustomEdit.CanObserve(const ID: Integer): Boolean;
begin
  Result := False;
  if ID = TObserverMapping.EditLinkID then
    Result := True
  else if ID = TObserverMapping.ControlValueID then
    Result := True;
end;

The State-Saving Handshake

The most sophisticated part of the TCustomEdit implementation is how it handles property overrides. When an observer is added, the component hooks into the OnObserverToggle event. This allows the component to save its current design-time state before the observer potentially modifies it.

procedure TCustomEdit.ObserverAdded(const ID: Integer; const Observer: IObserver);
begin
  if ID = TObserverMapping.EditLinkID then
    Observer.OnObserverToggle := ObserverToggle;
end;

ObserverToggle: The Save/Restore Logic

When an observer becomes active (Value = True), the edit control saves its current ReadOnly, Alignment, and MaxLength properties into "Saved" fields. It then applies the constraints defined by the observer.

procedure TCustomEdit.ObserverToggle(const AObserver: IObserver; const Value: Boolean);
var
  LEditLinkObserver: IEditLinkObserver;
  LEditFormatLink: IEditFormatLink;
begin
  if Supports(AObserver, IEditLinkObserver, LEditLinkObserver) then
  begin
    if Value then // Observer is Activating
    begin
      FSavedReadOnly := ReadOnly;
      FSavedAlignment := Alignment;
      FSavedMaxLength := MaxLength;
      
      if LEditLinkObserver.IsReadOnly then
        ReadOnly := True;
        
      if Supports(LEditLinkObserver.FormatLink, IEditFormatLink, LEditFormatLink) then
      begin
        Alignment := LEditFormatLink.Alignment;
        MaxLength := LEditFormatLink.MaxLength;
      end;
    end
    else // Observer is Deactivating
    begin
      if not (csDestroying in ComponentState) then
      begin
        ReadOnly := FSavedReadOnly;
        Alignment := FSavedAlignment;
        MaxLength := FSavedMaxLength;
      end;
    end;
  end;
end;

This "Save/Restore" pattern is vital. It ensures that if the observer is ever disconnected or disabled, the UI control gracefully reverts to the properties the developer originally set in the Object Inspector.

Why This Matters

This level of integration ensures that the UI remains consistent with the data source. If the underlying data field is ReadOnly, the TEdit hardware property is physically toggled to True, preventing user input at the Windows API level. It isn't just a visual trick; it's a deep structural sync.