设计模式之Application Programs and Toolkits

Application Programs 应用程序

If you're building an application programsuch as a document editor or spreadsheet, then internal reuse, maintainability, andextension are high priorities. Internal reuse ensures that you don't design andimplement any more than you have to. Design patterns that reduce dependencies canincrease internal reuse. Looser coupling boosts the likelihood that one class ofobject can cooperate with several others.For example, when you eliminatedependencies on specific operations by isolatingand encapsulating each operation, you makeit easier to reuse an operation in different contexts. The same thing canhappen when you remove algorithmic and

representational dependencies too.

如果你建立了一个应用程序比如说文本编辑或电子数据表，然后内部重用，可维护性和 扩展是高优先级。内部重用保证你不需要再一次设计和实现已经有的内容，设计模式就是减少依赖来增加内部重用。松耦合性提高增强了几个人合作来做一类对象的可能性。例如，当你通过使用孤立或者封装的运算，淘汰了使用依赖性很强的运算，在不同的文件中就可以很容易的重用运算，当你移除算法和有代表性的依存关系的时候，你都可以做和上面类似的事情。

Design patterns also make an application more maintainable when they're used to limit platform dependencies and to layer a system. They enhance extensibility by showing you how to extend class hierarchies and how to exploit object composition. Reduced coupling also enhances extensibility. Extending a class in isolation is easier if the class doesn't depend on lots of other classes.

当你用有限制的平台并且分层放置一个系统，设计模式就会使应用的可维护性增强。扩展类的继承和利用类的组合会提高其扩展性。减少链接也会提高类的可扩展性。

如果一个类不依靠大量其他的类，扩展一个孤立的类是很容易的。

Toolkits 工具包

Often an application will incorporateclasses from one or more libraries ofpredefined classes called toolkits. Atoolkit is a set of related and reusable classes designed to provide useful,general-purpose functionality. An example of a toolkit is a set of collection classesfor lists, associative tables, stacks,and the like. The C++ I/O stream library isanother example. Toolkits don't impose a particular design on your application;they just provide functionality that can help your application do its job. Theylet you as an implementer avoid recoding

common functionality. Toolkits emphasize code reuse. They are the object-oriented equivalent of subroutine libraries.

经常性的，应用会调用工具包中一个或更多库中预定义的类。工具包是和重用、通用功能的相关并且重用类设计会提供这些重用、通用功能。举个简单的例子工具包是一套包含链表类、关联表和栈等等像这样的类的集合。C++ I/O流库是工具包的另一个例子。在应用中，工具包不能强加特别的设计；它们只是提供了帮助你应用工作的功能。它们只是帮助你避免重新编写普遍的实现。工具包强调的是代码的重用，它们是面向对象的子程序库。

Toolkit design is arguably harder than application design, because toolkits have to work in many applications to be useful.Moreover, the toolkit writer isn't in a position to know what those applications will be or their special needs. That makes it all the more important toavoid assumptions and dependencies that can limit the toolkit's flexibility and consequently its applicability andeffectiveness.

工具设计可以认为难于应用设计，因为在应用中工具包会起效。更多的是，工具不是在应用需要或特殊的位置编写。这些使避免承担和依赖变得很重要，可以限制工具包的灵活性，应用性和有效性。