Mediator design pattern

Intent

• Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
• Design an intermediary to decouple many peers.
• Promote the many-to-many relationships between interacting peers to "full object status".

Problem

We want to design reusable components, but dependencies between the potentially reusable pieces demonstrates the "spaghetti code" phenomenon (trying to scoop a single serving results in an "all or nothing clump").

Discussion

In Unix, permission to access system resources is managed at three levels of granularity: world, group, and owner. A group is a collection of users intended to model some functional affiliation. Each user on the system can be a member of one or more groups, and each group can have zero or more users assigned to it. Figure 18-1 shows three users that are assigned to all three groups.

If we were to model this in software, we could decide to have User objects coupled to Group objects, and Group objects coupled to User objects. Then when changes occur, both classes and all their instances would be affected.

An alternate approach would be to introduce "an additional level of indirection" - take the mapping of users to groups and groups to users, and make it an abstraction unto itself. This offers several advantages: Users and Groups are decoupled from one another, many mappings can easily be maintained and manipulated simultaneously, and the mapping abstraction can be extended in the future by defining derived classes.

Partitioning a system into many objects generally enhances reusability, but proliferating interconnections between those objects tend to reduce it again. The mediator object: encapsulates all interconnections, acts as the hub of communication, is responsible for controlling and coordinating the interactions of its clients, and promotes loose coupling by keeping objects from referring to each other explicitly.

The Mediator pattern promotes a "many-to-many relationship network" to "full object status". Modelling the inter-relationships with an object enhances encapsulation, and allows the behavior of those inter-relationships to be modified or extended through subclassing.

An example where Mediator is useful is the design of a user and group capability in an operating system. A group can have zero or more users, and, a user can be a member of zero or more groups. The Mediator pattern provides a flexible and non-invasive way to associate and manage users and groups.

Structure

Colleagues (or peers) are not coupled to one another. Each talks to the Mediator, which in turn knows and conducts the orchestration of the others. The "many to many" mapping between colleagues that would otherwise exist, has been "promoted to full object status". This new abstraction provides a locus of indirection where additional leverage can be hosted.

Example

The Mediator defines an object that controls how a set of objects interact. Loose coupling between colleague objects is achieved by having colleagues communicate with the Mediator, rather than with each other. The control tower at a controlled airport demonstrates this pattern very well. The pilots of the planes approaching or departing the terminal area communicate with the tower rather than explicitly communicating with one another. The constraints on who can take off or land are enforced by the tower. It is important to note that the tower does not control the whole flight. It exists only to enforce constraints in the terminal area. [Michael Duell, "Non-software examples of software design patterns", Object Magazine, Jul 97, p54]

Check list

1. Identify a collection of interacting objects that would benefit from mutual decoupling.
2. Encapsulate those interactions in the abstraction of a new class.
3. Create an instance of that new class and rework all "peer" objects to interact with the Mediator only.
4. Balance the principle of decoupling with the principle of distributing responsibility evenly.
5. Be careful not to create a "controller" or "god" object.

Rules of thumb

• Chain of Responsibility, Command, Mediator, and Observer, address how you can decouple senders and receivers, but with different trade-offs. Chain of Responsibility passes a sender request along a chain of potential receivers. Command normally specifies a sender-receiver connection with a subclass. Mediator has senders and receivers reference each other indirectly. Observer defines a very decoupled interface that allows for multiple receivers to be configured at run-time.
• Mediator and Observer are competing patterns. The difference between them is that Observer distributes communication by introducing "observer" and "subject" objects, whereas a Mediator object encapsulates the communication between other objects. We've found it easier to make reusable Observers and Subjects than to make reusable Mediators.
• On the other hand, Mediator can leverage Observer for dynamically registering colleagues and communicating with them.
• Mediator is similar to Facade in that it abstracts functionality of existing classes. Mediator abstracts/centralizes arbitrary communication between colleague objects, it routinely "adds value", and it is known/referenced by the colleague objects (i.e. it defines a multidirectional protocol). In contrast, Facade defines a simpler interface to a subsystem, it doesn't add new functionality, and it is not known by the subsystem classes (i.e. it defines a unidirectional protocol where it makes requests of the subsystem classes but not vice versa).

Code examples

Mediator pattern C++ example

Mediator pattern Java example

Iterator design pattern

Intent

• Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
• The C++ and Java standard library abstraction that makes it possible to decouple collection classes and algorithms.
• Promote to "full object status" the traversal of a collection.
• Polymorphic traversal

Problem

Need to "abstract" the traversal of wildly different data structures so that algorithms can be defined that are capable of interfacing with each transparently.

Discussion

"An aggregate object such as a list should give you a way to access its elements without exposing its internal structure. Moreover, you might want to traverse the list in different ways, depending on what you need to accomplish. But you probably don't want to bloat the List interface with operations for different traversals, even if you could anticipate the ones you'll require. You might also need to have more than one traversal pending on the same list." And, providing a uniform interface for traversing many types of aggregate objects (i.e. polymorphic iteration) might be valuable.

The Iterator pattern lets you do all this. The key idea is to take the responsibility for access and traversal out of the aggregate object and put it into an Iterator object that defines a standard traversal protocol.

The Iterator abstraction is fundamental to an emerging technology called "generic programming". This strategy seeks to explicitly separate the notion of "algorithm" from that of "data structure". The motivation is to: promote component-based development, boost productivity, and reduce configuration management.

As an example, if you wanted to support four data structures (array, binary tree, linked list, and hash table) and three algorithms (sort, find, and merge), a traditional approach would require four times three permutations to develop and maintain. Whereas, a generic programming approach would only require four plus three configuration items.

Structure

The Client uses the Collection class’ public interface directly. But access to the Collection’s elements is encapsulated behind the additional level of abstraction called Iterator. Each Collection derived class knows which Iterator derived class to create and return. After that, the Client relies on the interface defined in the Iterator base class.

Example

The Iterator provides ways to access elements of an aggregate object sequentially without exposing the underlying structure of the object. Files are aggregate objects. In office settings where access to files is made through administrative or secretarial staff, the Iterator pattern is demonstrated with the secretary acting as the Iterator. Several television comedy skits have been developed around the premise of an executive trying to understand the secretary's filing system. To the executive, the filing system is confusing and illogical, but the secretary is able to access files quickly and efficiently.

On early television sets, a dial was used to change channels. When channel surfing, the viewer was required to move the dial through each channel position, regardless of whether or not that channel had reception. On modern television sets, a next and previous button are used. When the viewer selects the "next" button, the next tuned channel will be displayed. Consider watching television in a hotel room in a strange city. When surfing through channels, the channel number is not important, but the programming is. If the programming on one channel is not of interest, the viewer can request the next channel, without knowing its number.

Check list

1. Add a create_iterator() method to the "collection" class, and grant the "iterator" class privileged access.
2. Design an "iterator" class that can encapsulate traversal of the "collection" class.
3. Clients ask the collection object to create an iterator object.
4. Clients use the first(), is_done(), next(), and current_item() protocol to access the elements of the collection class.

Rules of thumb

• The abstract syntax tree of Interpreter is a Composite (therefore Iterator and Visitor are also applicable).
• Iterator can traverse a Composite. Visitor can apply an operation over a Composite.
• Polymorphic Iterators rely on Factory Methods to instantiate the appropriate Iterator subclass.
• Memento is often used in conjunction with Iterator. An Iterator can use a Memento to capture the state of an iteration. The Iterator stores the Memento internally.

Code examples

Iterator pattern C++ example

Iterator pattern Java example

Interpreter design pattern

Intent

• Given a language, define a representation for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
• Map a domain to a language, the language to a grammar, and the grammar to a hierarchical object-oriented design.

Problem

A class of problems occurs repeatedly in a well-defined and well-understood domain. If the domain were characterized with a "language", then problems could be easily solved with an interpretation "engine".

Discussion

The Interpreter pattern discusses: defining a domain language (i.e. problem characterization) as a simple language grammar, representing domain rules as language sentences, and interpreting these sentences to solve the problem. The pattern uses a class to represent each grammar rule. And since grammars are usually hierarchical in structure, an inheritance hierarchy of rule classes maps nicely.

An abstract base class specifies the method interpret(). Each concrete subclass implements interpret() by accepting (as an argument) the current state of the language stream, and adding its contribution to the problem solving process.

Structure

Interpreter suggests modeling the domain with a recursive grammar. Each rule in the grammar is either a 'composite' (a rule that references other rules) or a terminal (a leaf node in a tree structure). Interpreter relies on the recursive traversal of the Composite pattern to interpret the 'sentences' it is asked to process.

Example

The Intepreter pattern defines a grammatical representation for a language and an interpreter to interpret the grammar. Musicians are examples of Interpreters. The pitch of a sound and its duration can be represented in musical notation on a staff. This notation provides the language of music. Musicians playing the music from the score are able to reproduce the original pitch and duration of each sound represented.

Check list

1. Decide if a "little language" offers a justifiable return on investment.
2. Define a grammar for the language.
3. Map each production in the grammar to a class.
4. Organize the suite of classes into the structure of the Composite pattern.
5. Define an interpret(Context) method in the Composite hierarchy.
6. The Context object encapsulates the current state of the input and output as the former is parsed and the latter is accumulated. It is manipulated by each grammar class as the "interpreting" process transforms the input into the output.

Rules of thumb

• Considered in its most general form (i.e. an operation distributed over a class hierarchy based on the Composite pattern), nearly every use of the Composite pattern will also contain the Interpreter pattern. But the Interpreter pattern should be reserved for those cases in which you want to think of this class hierarchy as defining a language.
• Interpreter can use State to define parsing contexts.
• The abstract syntax tree of Interpreter is a Composite (therefore Iterator and Visitor are also applicable).
• Terminal symbols within Interpreter's abstract syntax tree can be shared with Flyweight.
• The pattern doesn't address parsing. When the grammar is very complex, other techniques (such as a parser) are more appropriate.

Code examples

Interpreter pattern C++ example

Interpreter pattern Java example