> For the complete documentation index, see [llms.txt](https://wilden-chen.gitbook.io/swift-design-patterns/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://wilden-chen.gitbook.io/swift-design-patterns/singleton.md).

# 單例模式 - Singleton

單例模式確保每個指定的類別只存在一個實例對象，並且可以全局訪問那個實例。一般情況下會使用延時加載的策略，只在第一次需要使用的時候初始化。

注意：在 iOS 中單例模式很常見，`UserDefaults.standard` 、 `UIApplication.shared` 、 `UIScreen.main` 、 `FileManager.default` 這些都是單例模式。

你可能會疑惑了：如果多於一個實例又會怎麼樣呢？程式碼和記憶體有貴重到這個程度嗎？

某些場景下，保持實體物件僅僅只有一份是很有意義的。舉個例子，你的應用程式實體 (`UIApplication`) ，應該只有一個吧，顯然是指你的現在當下執行的應用程式。

還有一個例子：行動裝置的螢幕 (`UIScreen`) 實體也是這樣，所以對於這些類別的情況，有實體存在，但是只想要一個實體物件。

單例模式的應用還有另一種情況：你需要一個全域類別來處理設定資料。我們很容易通過單例模式實現安全的執行緒實體存取，而如果有多個類別可以同時存取設定文件，那可就複雜多了。


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