In 2017, with the release of Delphi 10.2 Tokyo, Embarcadero introduced a specialized implementation of the Observer pattern into the System.Classes unit. While it has been in the wild for 9 years, it remains a "hidden" architecture for many, primarily because it serves as the invisible engine behind LiveBindings. Other than live bindings, you can also use the Observer pattern as a way to update component settings to the Windows registry, an .ini file, or persist it elsewhere.

However, this architecture isn't just for the internal framework. It is a powerful tool for any developer looking to decouple UI controls from business logic or create self-synchronizing components.

The Core Contract: IObserver

Unlike a simple notification event, IObserver is designed to be a stateful manager. It doesn't just watch for a change; it participates in the lifecycle of the object it observes.

IObserver = interface
  ['{B03253D8-7720-4B68-B10A-E3E79B91ECD3}']
  procedure Removed;
  function GetActive: Boolean;
  procedure SetActive(Value: Boolean);
  property Active: Boolean read GetActive write SetActive;
  // ... Toggle events and properties
end;

Crucially, the interface includes an Active property. This allows a subject to "silence" an observer without having to unregister it—a vital feature when you need to programmatically update a control without triggering a feedback loop of change notifications.

Single-Cast vs. Multi-Cast

Delphi makes a rigid distinction between how observers are distributed. This is reflected in two descendant interfaces:

  • ISingleCastObserver: Designed for "one-to-one" relationships. For example, a TEdit usually has one primary data source (an EditLink).
  • IMultiCastObserver: Designed for "one-to-many" relationships. Think of a status bar that needs to update alongside several different UI elements simultaneously.

Unfortunately, in the Delphi RTL, there's a bug in TObservers.GetSingleCastObserver. Here's what's in the loop in GetSingleCastObserver: 

    for I := 0 to LList.Count - 1 do
    begin
      if Supports(LList.Items[0], ISingleCastObserver, LObserver) then
        ...
    end;

The RTL loops through the entire list, intending to look for an implementation of a ISingleCastObserver, but it always looks at the first item, instead of every item. It's a single letter change, and the bug, RSS-1873: Indexing problem in TObservers.GetSingleCastObserver was filed back in 23 Sep 2024, and it's now 2026, and it's still not fixed.

The Registry: TObservers

Managing these interfaces manually would be a nightmare. Instead, Delphi provides the TObservers class. This class acts as a central registry within a component, mapping specific Integer IDs to lists of observers.

TObservers = class
public
  procedure AddObserver(const ID: Integer; const AIntf: IInterface);
  procedure RemoveObserver(const ID: Integer; const AIntf: IInterface);
  function IsObserving(const ID: Integer): Boolean;
end;

This ID-based system (using constants from TObserverMapping) allows a single component to support multiple types of observation—such as tracking a value, a position, or an edit state—all through the same management object.

In the next installment, we’ll look at how TComponent brings this engine to life through lazy initialization and the component lifecycle.

In Part 2: We dive into the lazy-loading of the Observers property and how Delphi ensures zero memory overhead for components that don't use them.