Design Patterns in the VCL -- Panel as Composite

Revealing the Magician's Secrets

  1. Component Palette ==> Prototype

  2. Panel ==> Composite

Problem

How do I allow the client to construct arbitrarily-complex hierarchies of my classes, while allowing uniform navigation through the hierarchy?

Solution

Composite permits a design in which the container nodes implement the same abstract class as the non-containers.

Consequences

  • The client can treat containers as non-containers, because containers inherit the same interface.
  • The variant of Composite used by the VCL actually pushes the "knowing about children" logic up into the base class (TComponent) to allow even simpler and more homogenous navigation.

    N.B. this simplicity has a cost: it requires all components to share the overhead of parenthood.

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These pages maintained by Tres Seaver of Palladion Software . Copyright 1998, all rights reserved.