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

State – Example Structure

*ROOM_STATE

paint *furnish *

+ROOM

paintfurnish

+MANY_FAULTS

paint +

furnish +

+ALRIGHT

paint +

furnish +

+SOME_FAULTS

paint +

furnish +

state

4

State – Abstract Structure

+CONTEXT

operation

*STATE

operation *

operation +

+A_STATE

state.operation

state

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

Composite –  Drawing Example

DIAGRAM

DIAGRAMTEXT LINE

DIAGRAM

OVAL TEXT

DIAGRAM

OVAL TEXT

12

Composite – Example Architecture

LINE +

draw +

DIAGRAM +

draw +

add +

remove +

iterate +

GRAPHIC *draw *

TEXT +

draw +

OVAL +

draw +

COMPOSITE[T] *add *

remove *

iterate *

CLIENTT

13

Composite – Abstract Architecture

COMPOSITE_COMPONENT +

op_1 +

op_2 +

add +

remove +

iterate +

COMPONENT *op_1 *op_2 *

COMPOSITE[T] *add *

remove *

iterate *

CLIENTT

LEAF +

op_1 +

op_2 +

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