Design pattern學習 – 3 (Singleton單例模式)

保證一個類別只會產生一個物件，而且要提供存取該物件的統一方法

	// lazy Singleton: create the singleton object when needed
	// Note: Global constants and variables are always computed lazily
	// ref: https://docs.swift.org/swift-book/LanguageGuide/Properties.html#ID263
	class SingletonGreed {
	static let shared = SingletonGreed()
	private init() {}
	}

	class SingletonThreadSafe {
	private static var shared: SingletonThreadSafe?
	private init() {}
	static func getInstance() -> SingletonThreadSafe {
	if shared == nil {
	synced(self) {
	if shared == nil {
	shared = SingletonThreadSafe()
	}
	}
	}
	return shared!
	}

	static func synced(_ lock: Any, closure: () -> ()) {
	objc_sync_enter(lock)
	closure()
	objc_sync_exit(lock)
	}
	}

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Java sample:

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