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Optional Chaining自判断链，可以用来判断调用的目标属性、方法、附属脚本等是否为空。不为空为成功，为空则失败，返回nil。这个链状结构，称为自判断链 - 自判断链接类似于拆包，在属性方法后跟问号。不同之处在于为空时强制拆包会崩溃，但是自判断链接会得到一个很安全的失败返回
numberOfRooms是nonoptional属性，而residence是optional属性，使用自判断链接可以使得到的roomCount变为optional属性，即当residence为空时不会崩溃而是直接返回一个空的numberOfRooms属性if let roomCount = john.residence?.numberOfRooms { print("John's residence has \(roomCount) room(s)") } else { print("Unable to retrieve the number of rooms") } - 自判断链可以用来判断optional属性的nonoptional是否为空，如上residence为optional属性，而numberOfRooms定义时为nonoptional，但是roomCount接收到的非Int而是Int?，这就是自判断链的作用
- 自判断链可以进行多层判断，在等号左边时使用如果该属性为空不会执行为空属性下的属性赋值。
- 当给自判断链为空的属性赋值时，右边函数并不会执行，如下：
没有任何打印。func createAddress() -> Address { print("Function was called.") let someAddress = Address() someAddress.buildingNumber = "29" someAddress.street = "Acacia Road" return someAddress } john.residence?.address = createAddress() - 没有
return的函数实际上是返回一个void类型，也可以理解成一个空的tuple。而在自判断链中，所返回的void类型变为void?类型。也就是说可以使用if来判断是否是一个nil实例调用其方法，如果调用失败也不会崩溃。对于赋值属性也是如此。if john.residence?.printNumberOfRooms() != nil { print("It was possible to print the number of rooms.") } else { print("It was not possible to print the number of rooms.") } // Prints "It was not possible to print the number of rooms." if (john.residence?.address = someAddress) != nil { print("It was possible to set the address.") } else { print("It was not possible to set the address.") } // Prints "It was not possible to set the address." - 自判断链对于附属脚本的使用方式和普通属性和方法类似。需要注意的是
？放在[]前，如下if let firstRoomName = john.residence?[0].name { print("The first room name is \(firstRoomName).") } else { print("Unable to retrieve the first room name.") } // Prints "Unable to retrieve the first room name." - 多重自判断链
if let beginsWithThe = john.residence?.address?.buildingIdentifier()?.hasPrefix("The") { if beginsWithThe { print("John's building identifier begins with \"The\".") } else { print("John's building identifier does not begin with \"The\".") } } // Prints "John's building identifier begins with "The"."
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