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Context:
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Creating objects by keyword, where types of objects to
be created are not known in advance or are extensible during runtime.
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Challenges:
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To separate the capability to actually create specific objects from the
factory.
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To add the capability to create objects to the factory during runtime.
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To fill factory with default population as part of product type definition.
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Skill:
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Participants:
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Factory:
| A Singleton. Registers Creator Plugs. Responsible for creating products by
unique key.
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Creator Plug:
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Plugs into factory to create products of a type by unique key. (Typically
- Factory Method. Also Class Object, Prototype.)
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Concrete Creator Plug:
| Registers itself (or is registered) in the factory. Responsible for creating a
product of a fixed type and/or content known only to itself. Typically
istantiates from a template.
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Product:
| What the factory is declared to create.
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Concrete Product:
| What the factory will create for a certain key.
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Client:
| Obtains products from the factory, supplying key.
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Signature:
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Factory references Creator Plugs by key and
creates products for Client.
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Used patterns and idioms:
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Factory Method. Optionally: Static Registration idiom.
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Used by:
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Serialization mechanisms, visual selection mechanisms
and generally: entry-points for objects from outside program memory.
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Variations:
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Virtual constructor. Creation method is a static method of the abstract
base-class. (Note: the virtual constructor can be implemented by means of any
factory, e.g. Dynamic Pluggable Factory.) Source:
Stroustrup.
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Product Trader . Factory loads Creator Plug if not found. Source:
Lauder.
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Scope:
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General.
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