翻译自：https://docs.swift.org/swift-book/LanguageGuide/OptionalChaining.html

翻译自：https://docs.swift.org/swift-book/LanguageGuide/BasicOperators.html#ID72

翻译自：https://docs.swift.org/swift-book/LanguageGuide/TheBasics.html

翻译自：https://docs.swift.org/swift-book/LanguageGuide/Properties.html

翻译自：https://docs.swift.org/swift-book/LanguageGuide/NestedTypes.html

翻译自：https://docs.swift.org/swift-book/LanguageGuide/Closures.html#ID102

翻译自：https://docs.swift.org/swift-book/LanguageGuide/Concurrency.html

翻译自：https://docs.swift.org/swift-book/LanguageGuide/ControlFlow.html

翻译自：https://docs.swift.org/swift-book/LanguageGuide/Extensions.html

翻译自：https://docs.swift.org/swift-book/LanguageGuide/Methods.html