[英]Escaping closure captures mutating 'self' parameter: struct
I have the following code I'm using with SwiftUI:我有以下与 SwiftUI 一起使用的代码:
import Foundation
public struct Trigger {
public var value = false
public mutating func toggle() {
value = true
let responseDate = Date().advanced(by: 3)
OperationQueue.main.schedule(after: .init(responseDate)) {
moveBack()
}
}
private mutating func moveBack() {
value = false
}
}
However, I'm getting an error:但是,我收到一个错误:
Escaping closure captures mutating 'self' parameter
Escaping 闭包捕获变异的“自我”参数
I understand that changing the struct to a class would solve this issue, but is there any way to actually capture a mutating self in an escaping closure in a struct?我知道将结构更改为 class 可以解决这个问题,但是有没有办法在结构中的 escaping 闭包中实际捕获变异自我?
As you have found, the quick solution is to use a reference type , a class .如您所见,快速解决方案是使用参考类型class 。 But why is this the case?
但为什么会这样呢?
Swift structs are value types, so they are immutable. Swift 结构是值类型,因此它们是不可变的。 You can mark a function as
mutating
to indicate to the compiler that a function mutates the struct, but what does that actually mean?您可以将 function 标记为正在
mutating
,以向编译器指示 function 会变异该结构,但这实际上意味着什么?
Consider a simple struct:考虑一个简单的结构:
struct Counter {
var count
init(_ count: Int = 0)
{
self.count = count
}
mutating func increment() {
self.count+=1
}
}
Now, try and assign an instance of this to a let
constant:现在,尝试将 this 的一个实例分配给一个
let
常量:
let someCounter = Counter()
someCounter.increment()
print(someCounter.count)
You will get an error;你会得到一个错误; you need to use a
var
.你需要使用
var
。
var someCounter = Counter()
someCounter.increment()
print(someCounter.count)
What actually happens when you call a mutating
func is that a new Counter
is created, with the new count
and it is assigned to someCounter
.当你调用一个
mutating
func 时,实际发生的是一个新的Counter
被创建,新的count
被分配给someCounter
。 It is effectively saying someCounter = Counter(someCounter.count+1)
它实际上是在说
someCounter = Counter(someCounter.count+1)
Now, think what would happen if you could mutate self
in an escaping closure - That new Counter
is going to be created at some unspecified time in the future, but execution has already moved on.现在,想想如果你可以在 escaping 闭包中改变
self
会发生什么 - 新的Counter
将在未来某个未指定的时间创建,但执行已经继续。 It is too late to update someCounter
.更新
someCounter
为时已晚。
The other advantage of using a class
, as you have found, is that you can use ObservableObject
, which makes updating your SwiftUI views much easier.正如您所发现的,使用
class
的另一个优点是您可以使用ObservableObject
,这使得更新 SwiftUI 视图更加容易。
Solution I finished with:我完成的解决方案:
import Foundation
import Combine
public final class IntervalTrigger: ObservableObject {
private let timeInterval: TimeInterval
@Published var value = false
public init(_ timeInterval: TimeInterval) {
self.timeInterval = timeInterval
}
public func toggle() {
value = true
let responseDate = Date().advanced(by: timeInterval)
OperationQueue.main.schedule(after: .init(responseDate)) { [weak self] in
self?.value = false
}
}
}
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