Intent

Allow new functions to be added to existing class hierarchies without affecting those hierarchies, and without creating the troublesome dependency cycles that are inherent to the GoF Visitor Pattern.

Explanation

Real world example

We have a hierarchy of modem classes. The modems in this hierarchy need to be visited by an external algorithm based on filtering criteria (is it Unix or DOS compatible modem).

Intent

Allows encoding behavior as instructions for a virtual machine.

Explanation

Real world example

A team is working on a new game where wizards battle against each other. The wizard behavior needs to be carefully adjusted and iterated hundreds of times through playtesting. It's not optimal to ask the programmer to make changes each time the game designer wants to vary the behavior, so the wizard behavior is implemented as a data-driven virtual machine.

Intent

The caching pattern avoids expensive re-acquisition of resources by not releasing them immediately after use. The resources retain their identity, are kept in some fast-access storage, and are re-used to avoid having to acquire them again.

Explanation

Real world example

Intent

Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.

Explanation

Real-world example

The Orc King gives loud orders to his army. The closest one to react is the commander, then an officer, and then a soldier. The commander, officer, and soldier form a chain of responsibility.

Intent

Handle costly remote service calls in such a way that the failure of a single service/component cannot bring the whole application down, and we can reconnect to the service as soon as possible.

Explanation

Real world example

Imagine a web application that has both local files/images and remote services that are used for fetching data. These remote services may be either healthy and responsive at times, or may become slow and unresponsive at some point of time due to variety of reasons. So if one of the remote services is slow or not responding successfully, our application will try to fetch response from the remote service using multiple threads/processes, soon all of them will hang (also called thread starvation) causing our entire web application to crash. We should be able to detect this situation and show the user an appropriate message so that he/she can explore other parts of the app unaffected by the remote service failure. Meanwhile, the other services that are working normally, should keep functioning unaffected by this failure.

Also known as

Action, Transaction

Intent

Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.

Explanation

Real-world example

There is a wizard casting spells on a goblin. The spells are executed on the goblin one by one. The first spell shrinks the goblin and the second makes him invisible. Then the wizard reverses the spells one by one. Each spell here is a command object that can be undone.

Intent

Accelerate memory access by arranging data to take advantage of CPU caching.

Modern CPUs have caches to speed up memory access. These can access memory adjacent to recently accessed memory much quicker. Take advantage of that to improve performance by increasing data locality keeping data in contiguous memory in the order that you process it.

Also known as

• IsDirty pattern

Intent

To avoid expensive re-acquisition of resources. The resources retain their identity, are kept in some fast-access storage, and are re-used to avoid having to acquire them again.

Class diagram

Applicability

Intent

Double buffering is a term used to describe a device that has two buffers. The usage of multiple buffers increases the overall throughput of a device and helps prevents bottlenecks. This example shows using double buffer pattern on graphics. It is used to show one image or frame while a separate frame is being buffered to be shown next. This method makes animations and games look more realistic than the same done in a single buffer mode.

Intent

Anticipate that an object’s interface needs to be extended in the future. Additional interfaces are defined by extension objects.

Class diagram

Applicability

Use the Extension Objects pattern when:

• you need to support the addition of new or unforeseen interfaces to existing classes and you don't want to impact clients that don't need this new interface. Extension Objects lets you keep related operations together by defining them in a separate class
• a class representing a key abstraction plays different roles for different clients. The number of roles the class can play should be open-ended. There is a need to preserve the key abstraction itself. For example, a customer object is still a customer object even if different subsystems view it differently.
• a class should be extensible with new behavior without subclassing from it.