This is a classic example of the tradeoff between concrete inheritance and delegation to an interface.

这个一个经典的转换例子在类继承和委托到一个接口。

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When it comes to developing a rich and interactive user interface, Cocoa has always excelled. This is partly because of the flexible delegation model that it provides.

需要开发一个交互式富用户界面时，Cocoa 总是技高一筹，部分原因是它提供的灵活的代理模型。

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