Proxy, Observer, Symbolic Links

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Proxy, Observer, Symbolic Links

• Rebecca Chernoff

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Proxy Pattern

• Intent
• Provide a surrogate or placeholder for another
  object to control access to it
• Other Names
• Surrogate

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Proxy Motivations Applicability

• Remote Proxy
• Virtual Proxy
• Protection Proxy
• Smart Reference

• Local representative of object in a different
  address space.
• Defer cost of creation and initialization.
• Protection via access control.
• Reference counting
• Load a persistent object into memory when first
  referenced
• Object locking

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Proxy Participants

• RealSubject
• Defines the real object that the proxy
  represents.
• Subject
• Defines the common interface for RealSubject and
  Proxy.
• Allows for a Proxy to be used wherever a
  RealSubject is expected.

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Proxy Participants

• Proxy
• Maintains a reference to the real subject. May
  refer to a Subject if the RealSubject and Subject
  interfaces are the same.
• Identical interface to Subject so the Proxy can
  be substituted.
• Controls access to the real subject.
• May be responsible for creating and deleting the
  real subject.

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Proxy Participants

• Additional Responsibilities of Proxy
• Remote Proxy
• Encode and send the request to the real subject
  in a different address space.
• Virtual Proxy
• Cache additional information in order to postpone
  accessing the real subject.
• Protection Proxy
• Check access permissions.

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Proxy Structure
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Proxy Consequences

• Introduces a level of indirection
• Remote Proxy
• Hides fact that object resides in a different
  address space.
• Virtual Proxy
• Perform optimizations such as creation on demand.
• Protection Proxy and Smart References
• Allow additional housekeeping tasks when object
  is accessed.
• Copy-On-Write

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Proxy Related Patterns

• Adapter
• Provides a different interface to an object, but
  since Proxy can refuse to perform a request, the
  interface is effectively a subset.
• Decorator
• Similar implementation to proxy, but different
  purpose. Decorator adds responsibilities whereas
  proxy controls access.

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Observer Pattern

• Intent
• Define a one-to-many dependency between objects
  so that when one object changes state, all its
  dependents are notified and updated automatically
• Also Known As
• Dependents
• Publish-Subscribe

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Observer Motivation

• Maintain consistency between related objects.
• Application Data vs. Presentation
• Avoid tightly coupled classes that reduce
  reusability.

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Observer Applicability

• An abstraction has two aspects, one dependent on
  the other.
• A change to one object requires changing others.
• An object should be able to notify other objects
  without making assumptions about the objects.

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Observer Participants

• Observer
• Defines an interface for objects to be notified
  when a subject changes.
• Subject
• Defines an interface for attaching and detaching
  Observer objects.
• Unknown number of Observer objects.

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Observer Participants

• ConcreteObserver
• Implements the Observer interface to keep its
  state consistent with the subjects.
• Maintains a reference to the ConcreteSubject
• Stores state that should stay consistent with the
  subjects.
• ConcreteSubject
• Implements the Subject interface to update
  Observers.
• Stores state of interest to ConcreteObserver
  objects.
• Sends a notification to its observers when its
  state changes.

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Observer Structure
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Observer Collaborations
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Observer Consequences

• Vary subjects and observers independently.
• Abstract coupling between Subject and Observer.
• Support for broadcast communication.
• Simple update to Server may cause a cascade of
  updates to Observer and its dependent objects.

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Observer Implementation

• Mapping subjects to their observers.
• Tradeoff between time and space.
• Observing more than one subject.
• Observer needs to know which Subject is
  notifying.
• Who triggers the update?
• Subject client doesnt have to remember to
  update, but consecutive operations cause multiple
  updates.
• Observer client can wait to trigger the update,
  but client must remember to call update.
• Dangling references to deleted subjects.
• Notify observers right before deletion.
  Observers of multiple Subjects cant be deleted
  when just one Subject is deleted.

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Observer Implementation

• Making sure the Subject state is self-consistent
  before notification.
• Use TemplateMethod pattern with Notify as last
  operation.
• Avoiding observer-specific update protocols the
  push and pull models.
• Push Model Detailed information sent
  regardless.
• Pull Model Observers ask for details
  explicitly.
• Specifying modifications of interest explicitly.
• Observers register for a specific aspect of
  interest.

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Observer Implementation

• Encapsulating complex update semantics.
• Change-Manager
• Takes responsibility of maintaining references to
  observers away from the Subject.
• Defines a particular update strategy.
• Updates all dependent observers at the request of
  a subject.
• Combining the Subject and Observer classes.
• When multiple inheritance not supported by
  language, both interfaces may reside in one class.

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Observer

• Related Patterns
• Mediator
• The ChangeManager encapsulates complex update
  semantics, thus acting as a mediator between the
  Subject and its Observers.
• Singleton
• ChangeManager may be unique and globally
  accessible.

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But Where Do Surrogates Fit into This?

• File System Example
• Symbolic Links / Aliases / Shortcuts
• Reference to another node in the file system.
• Surrogate for a node, not the node itself.
• Own access rights.
• Edit and save a file.
• Add and remove nodes to a directory.

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But Where Do Surrogates Fit into This?

• How do I find the right design pattern for the
  task?
• Consider how design patterns solve design
  problems.
• Scan the Intent sections for something that
  sounds right.
• Study how patterns interrelate.
• Look at patterns whose purpose corresponds to
  what youre trying to do.
• Examine a relevant cause of redesign and apply
  the patterns that help you avoid it.
• Consider what should be variable in your design.

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But Where Do Surrogates Fit into This?

• Proxy Pattern
• Subject gt Node
• Proxy gt Link
• Real Subject gt ?

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But Where Do Surrogates Fit Into This?