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State Pattern

State Pattern. Intent Alter behaviour of an object when its internal state changes Object appears to change its class Alternate names Objects for states. State – Motivation. An object may be in one of many states. It responds differently depending upon its current state Example

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State Pattern

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  1. State Pattern • Intent • Alter behaviour of an object when its internal state changes • Object appears to change its class • Alternate names Objects for states

  2. State – Motivation • An object may be in one of many states. It responds differently depending upon its current state • Example • A Room can be in one of the states • Alright, SomeFaults, ManyFaults • A request to paint the room is made • Alright state – clean and paint room • SomeFaults– repair yourself and paint room • ManyFaults– hire contractor and paint room

  3. + ALRIGHT + SOME_FAULTS + MANY_FAULTS paint + furnish + paint + furnish + paint + furnish + State – Example Structure state + ROOM * ROOM_STATE paint * furnish * paint furnish

  4. * STATE operation * + A_STATE operation + State – Abstract Structure state + CONTEXT operation state.operation

  5. State – Participants • Context Defines client interface • Deferred State Defines interface for common behaviour for different states • Effective State Implements behaviour of that state in context

  6. State – Collaborations • Context delegates state specific behaviour to a concrete state object • Context may pass itself as an argument so that state can access context features • Context is the primary interface with clients • Clients configure context with state objects • Clients do not deal directly with state objects • Context or concrete state can decide which state follows another state

  7. State – Applicability • Object has different behaviour depending on state • Operations have multi-part conditional statement dependent upon state • State is represented by an enumerated constant • Several operations have same conditional structure • Pattern puts each branch of the conditional into a separate class • Object’s state becomes an object that can vary independently of other objects

  8. State – Related Patterns • State objects are often Singletons • Frequently, state classes do not contain any attributes • As a result, state objects of the same type are all the same • To avoid having many different objects that are exactly the same, one can make such classes singletons

  9. Composite Pattern • Intent • Compose objects into tree structures representing part-whole hierarchies • Clients deal uniformly with individual objects and hierarchies of objects

  10. Composite – Motivation • Applications that have recursive groupings of primitives and groups • Drawing programs lines, text, figures and groups • Eiffel static structure classes and clusters • Operations on groups are different than primitives but users treat them in the same way

  11. DIAGRAM DIAGRAM DIAGRAM DIAGRAM TEXT LINE OVAL OVAL TEXT TEXT Composite –  Drawing Example

  12. CLIENT COMPOSITE[T] * GRAPHIC * add * remove * iterate * draw * DIAGRAM + draw + add + remove + iterate + Composite – Example Architecture T TEXT + OVAL + LINE + draw + draw + draw +

  13. COMPONENT * CLIENT op_1 * op_2 * COMPOSITE[T] * add * remove * iterate * Composite – Abstract Architecture T LEAF + COMPOSITE_COMPONENT + op_1 + op_2 + op_1 + op_2 + add + remove + iterate +

  14. Composite – Applicability • Represent part-whole hierarchies of objects • Clients can ignore difference between individual objects and compositions • Clients deal with all objects in a composition in the same way

  15. Composite – Participants • Component Defines properties of an entity • Leaf Defines properties of a primitive entity • Composite Declares properties of a collection of entities • Composite Component Combines properties of a collection of entities and properties of a primitive entity • Client Uses component and composite properties

  16. Composite – Consequences • Whenever client expects a primitive it can accept a composite • Client is simplified by removing tag-case statements to identify parts of the composition • Easy to add new components by subclassing, client does not change • If compositions are to have restricted sets of components have to rely on run-time checking

  17. Composite – Related Patterns • Decorator is a degenerate Composite (only one component) • Visitor localizes operations that would be distributed across composite and leaf classes

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