1329: Translation for LanguageGuide / optional-chaining.md ⭐️⭐️⭐️⭐️⭐️ #1387
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yongfrank
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saitjr
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Oct 14, 2024
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| 为了反映可选链式调用可以在空值（`nil`）上调用的事实，不论这个调用的属性、方法及下标返回的值是不是可选值，它的返回结果都是一个可选值。你可以利用这个返回值来判断你的可选链式调用是否调用成功，如果调用有返回值则说明调用成功，返回 `nil` 则说明调用失败。 | ||
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| 特别地，可选链式调用的返回结果与原本的返回结果具有相同的类型，但是被包装成了一个可选值。例如，使用可选链式调用访问属性，当可选链式调用成功时，如果属性原本的返回结果是 `Int` 类型，则会变为 `Int?` 类型。 |
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具体来说，可选链调用的结果，除了被包装成了一个可选值外，类型与预期的返回值类型是相同的。例如，正常情况下返回 Int 的属性，在通过可选链访问时，返回的是 Int?。
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| 特别地，可选链式调用的返回结果与原本的返回结果具有相同的类型，但是被包装成了一个可选值。例如，使用可选链式调用访问属性，当可选链式调用成功时，如果属性原本的返回结果是 `Int` 类型，则会变为 `Int?` 类型。 | ||
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| 下面几段代码将解释可选链式调用和强制展开的不同。 |
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下面几段代码将演示可选链式调用和强制展开的不同，以及如何判断可选链是否调用成功。
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| ```swift | ||
| class Person { | ||
| var residence: Residence? | ||
| var residence: Residence? |
| In the code below, `john` has a `residence` property value of `nil`: | ||
| `Residence` 有一个 `Int` 类型的属性 `numberOfRooms`，其默认值为 `1`。`Person` 具有一个可选的 `residence` 属性，其类型为 `Residence?`。 | ||
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| 假如你创建了一个新的 `Person` 实例,它的 `residence` 属性由于是是可选型而将初始化为 `nil`,在下面的代码中,`john` 有一个值为 `nil` 的 `residence` 属性： |
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假如你创建了一个新的 Person 实例，由于 residence 属性是可选型，所以初始值是 nil。就像下面的代码中，john 有一个值为 nil 的 residence 属性：
| by placing an exclamation point after `residence` to force the unwrapping of its value, | ||
| you trigger a runtime error, | ||
| because there's no `residence` value to unwrap: | ||
| 如果使用叹号（`!`）强制展开获得这个 `john` 的 `residence` 属性中的 `numberOfRooms` 值，会触发运行时错误，因为这时 `residence` 没有可以展开的值： |
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如果使用叹号（!）对 john 的 residence 属性中的 numberOfRooms 值强制解包，会触发运行时错误，因为这时 residence 没有可以解包的值：
saitjr
requested changes
Oct 20, 2024
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| ```swift | ||
| if let roomCount = john.residence?.numberOfRooms { | ||
| print("John's residence has \(roomCount) room(s).") | ||
| print("John's residence has \(roomCount) room(s).") | ||
| print("John's residence has \(roomCount) room(s).") |
| ``` | ||
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| `john.residence` 现在包含一个实际的 `Residence` 实例，而不再是 `nil`。如果你试图使用先前的可选链式调用访问 `numberOfRooms`，它现在将返回值为 `1` 的 `Int?` 类型的值： | ||
| `john.residence` 现在包含一个实际的 `Residence` 实例，而不再是 `nil`。如果你试图使用先前的可选链式调用访问 `numberOfRooms`，它现在将返回值为 `1` 的 `Int?` 类型的值： |
| 要注意的是，即使 `numberOfRooms` 是非可选的 `Int` 时，这一点也成立。只要使用可选链式调用就意味着 `numberOfRooms` 会返回一个 `Int?` 而不是 `Int`。 | ||
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| 可以将一个 `Residence` 的实例赋给 `john.residence`，这样它就不再是 `nil` 了： | ||
| // 打印 “Unable to retrieve the number of rooms.” | ||
| ``` |
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Translation for LanguageGuide / optional-chaining