# 擴展

擴展是一種十分強大的機制，可以讓你在不用繼承的情況下，給已存在的類別、結構或者列舉添加一些新的功能。最重要的一點是，你可以在你沒有訪問權限的情況下擴展已有類別。這意味著你甚至可以擴展 Cocoa 的類，比如 `UIView` 或者 `UIImage` 。

舉個例子，在編譯時新加的方法可以像擴展類別的正常方法一樣執行。這和裝飾器模式有點不同，因為擴展不會有擴展類別的實體。


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