Common Design Patterns for Symbian OS: The Foundations of Smartphone Software

9.4. Handle–Body

Intent

Decouple an interface from its implementation so that the two can vary independently.

AKA

Cheshire Cat

9.4.1. Problem

9.4.1.1. Context

One of your components needs to expose an interface for use by external clients.

9.4.1.2. Summary

• You wish your component's interface to be as encapsulated as possible.

• You wish to maintain the compatibility of the exposed interface across multiple versions of your component whilst retaining the freedom to change how it is implemented.

• You would like to rapidly develop a component but also allow for optimization or the addition of new features later.

9.4.1.3. Description

As the software in mobile devices becomes ever more complicated, it also becomes more difficult to both maintain and extend it.

You will often find yourself asking questions such as:

• Does my software implement a specification that is subject to changes and updates?

• Will I need to allow a different implementation to be provided?

• Do I need to cope with different versions of software?

• Do I need to develop an early version of a component as soon as possible which is'good enough' now but will need to be re-factored later on?

The problems of developing software that is easy to expand, maintain and re-factor has and always will be a key issue that you will face. A standard way to achieve this is to ensure your interfaces are well encapsulated and don't 'leak' any unnecessary details or dependencies especially when external clients are relying on the interface ...