自我初始化参数 [英] Self in init params
问题描述
我想像这样使用Self in init参数:
I would like to use Self in init parameters like so:
class A {
public init(finishBlock: ((_ operation: Self) -> Void)? = nil) {...}
}
我知道我可以在这里使用"A",但是我想实现的是,如果某个类从A继承,那么它的初始化程序将知道操作是它的类类型,而不仅仅是A.所以例如,如果我写道:
I know I could use "A" in this place, but I would like to achieve that if some class inherits from A, then it's initializer would know operation as it's class type and not as just A. So for example if I wrote:
class B: A {
public init(finishBlock: ((_ operation: Self) -> Void)? = nil) {...}
public func fooOnlyInB() {}
}
然后我可以使用:
let b = B { (operation) in
operation.fooOnlyInB()
}
这有可能吗?
推荐答案
代替在每个初始化程序中使用Self
或A
,您可以简单地覆盖每个子类的初始化程序以使用其自己的类型作为operation
Instead of using Self
or A
in each of the initialisers, you can simply override each subclass' initialiser to use its own type as operation
.
之所以有效,是因为A
的初始化程序指出operation
应该是符合A
的类型,并且当您覆盖它时,您可以自由地使用A
的子类作为operation
.但是,如果将operation
更改为不相关的类型,例如String
或Int
,则编译器将不会覆盖现有的初始化程序.
This works because A
's initialiser states that operation
should be a type that conforms to A
, and when you override it you have the liberty to use a subclass of A
as operation
instead. However, if you change operation
to an unrelated type such as String
or Int
, the compiler will not override the existing initialiser.
首先,用init
定义A
:
class A {
init(finishBlock: ((_ operation: A) -> Void)?) {...}
}
现在要创建子类,您必须使用子类的类型代替operation
覆盖init
.在对super.init
的调用中,强制向上转换operation
($0
)为子类的类型,并使用强制转换的operation
调用finishBlock
.
Now to create a subclass, you must override init
using the subclass' type as operation
instead. In your call to super.init
, force upcast operation
($0
) to your subclass' type, and call finishBlock
with this casted operation
.
class B: A {
override init(finishBlock: ((_ operation: B) -> Void)?) {
// Perform custom initialisation...
super.init { finishBlock?($0 as! B) }
}
func fooOnlyInB() {
print("foo")
}
}
B
的初始化程序现在将B
传递为operation
,这意味着您不再需要自己转换它!这要归功于您可以使用更具体的类型(在本例中为B
)覆盖init
.
B
's initialiser now passes B
as operation
, which means that you don't need to cast it yourself anymore! This is thanks to the fact that you can override an init
with a more specific type, in this case B
.
let b = B { operation in
operation.fooOnlyInB() // prints "foo"
}
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