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- What does an exclamation mark mean in the Swift language? 22 answers
- 在Swift语言中，惊叹号是什么意思?22日答案
- Swift variable decorations with “?” (question mark) and “!” (exclamation mark) 1 answer
- Swift可变装饰用“?”(问号)和“!”(感叹号)1答案。
There are three way to declare a property in Swift:
在Swift中申报财产有三种方式:
var optStr: String?
var normStr: String = "normStr"
var exactStr: String!
The first one is property with an optional type, a type that can contain either nil or the String in our case. The second one is a property that always contain the String. It should be initialized in init or in the declaration.
第一个是具有可选类型的属性，该类型可以包含nil或字符串。第二个属性总是包含字符串。它应该在init或声明中初始化。
But what about the third way?
但是第三条呢?
var exactStr: String!
I made some experiments in the playground, and it turned out that a function that takes type? can take both type, type? and type! variables as an argument:
我在操场上做了一些实验，结果发现一个需要打字的函数?两种都可以吗?和类型!变量作为参数:
var optStr: String?
var normStr: String
var forcedStr: String!
func printStr(str: String?) {
println("str: \(str)")
}
printStr(optStr) //prints: "str: nil"
//printStr(normStr) //doesn't compile as not initialized
printStr(forcedStr) //prints: "str: nil"
optStr = "optStr"; normStr = "normStr"; forcedStr = "forcedStr"
printStr(optStr) //prints "str: optStr"
printStr(normStr) //prints "str: normStr"
printStr(forcedStr) //prints "str: forcedStr"
So why and when should I use type!?
那么为什么我应该在什么时候使用type呢?
Update: this is not a duplicate of What does an exclamation mark mean in the Swift language?. I'm not asking about unwrapping a variable: I'm asking about declaring a property with an exclamation point (Type!).
更新:这不是一个重复的什么惊叹号在斯威夫特的语言?我不是在询问如何展开一个变量:我是在询问如何使用感叹号(类型!)声明一个属性。
1 个解决方案
#1
49
It's a variable of type "implicitly-unwrapped optional String". Essentially, every access of implicitStr is treated as if it were written implicitStr! (thus unwrapping the value).
它是“隐含未包装的可选字符串”类型的变量。本质上，所有对implicitStr的访问都被视为“implicitStr”字。(因此展开的值)。
This, of course, will cause a crash if the value is nil. You can still test the implicit optional via if implicitStr != nil, or use it in optional chaining as var foo = implicitStr?.uppercaseString. So you can still use it just as safely as a normal optional; it's just biased toward the case where the value is not nil.
当然，如果值为nil，这将导致崩溃。您仍然可以通过if implicitStr != nil测试隐式可选项，或者在可选链接中使用它作为var foo = implicitStr?.upper casestring。所以你仍然可以安全地使用它就像一个普通的可选的;它只是偏向于值不是nil的情况。
Implicitly-unwrapped optionals are quite useful in cases where the value may not be present at initialization, but are set early and unlikely to become nil again. (For example, a variable you set in -awakeFromNib might reasonably be an implicitly-unwrapped optional.)
当值在初始化时可能不存在，但在早期就被设置，并且不太可能再次变为nil时，隐含的未包装选项是非常有用的。(例如，您在- awfromakenib中设置的变量可能是一个隐含的未包装的可选变量。)
Further, since Objective-C methods can return both nil and object types, their return values cannot be modeled as non-optional. To avoid requiring liberal use of forced unwrapping whenever dealing with Cocoa APIs, though, the parameters and return types of Cocoa APIs are usually represented as implicitly-unwrapped optionals.
此外，由于Objective-C方法可以同时返回nil和对象类型，因此不能将它们的返回值建模为非可选的。但是，为了避免在处理Cocoa api时使用强制展开，Cocoa api的参数和返回类型通常被表示为隐含的未封装的选项。
#1
49
It's a variable of type "implicitly-unwrapped optional String". Essentially, every access of implicitStr is treated as if it were written implicitStr! (thus unwrapping the value).
它是“隐含未包装的可选字符串”类型的变量。本质上，所有对implicitStr的访问都被视为“implicitStr”字。(因此展开的值)。
This, of course, will cause a crash if the value is nil. You can still test the implicit optional via if implicitStr != nil, or use it in optional chaining as var foo = implicitStr?.uppercaseString. So you can still use it just as safely as a normal optional; it's just biased toward the case where the value is not nil.
当然，如果值为nil，这将导致崩溃。您仍然可以通过if implicitStr != nil测试隐式可选项，或者在可选链接中使用它作为var foo = implicitStr?.upper casestring。所以你仍然可以安全地使用它就像一个普通的可选的;它只是偏向于值不是nil的情况。
Implicitly-unwrapped optionals are quite useful in cases where the value may not be present at initialization, but are set early and unlikely to become nil again. (For example, a variable you set in -awakeFromNib might reasonably be an implicitly-unwrapped optional.)
当值在初始化时可能不存在，但在早期就被设置，并且不太可能再次变为nil时，隐含的未包装选项是非常有用的。(例如，您在- awfromakenib中设置的变量可能是一个隐含的未包装的可选变量。)
Further, since Objective-C methods can return both nil and object types, their return values cannot be modeled as non-optional. To avoid requiring liberal use of forced unwrapping whenever dealing with Cocoa APIs, though, the parameters and return types of Cocoa APIs are usually represented as implicitly-unwrapped optionals.
此外，由于Objective-C方法可以同时返回nil和对象类型，因此不能将它们的返回值建模为非可选的。但是，为了避免在处理Cocoa api时使用强制展开，Cocoa api的参数和返回类型通常被表示为隐含的未封装的选项。