swift - 将自定义属性包装器与 @Published 结合使用

Question

我希望将自定义属性包装器应用于已包装在 @Published 中的变量。 ，嵌套它们像
(一) @Custom @Published var myVar或者
(B) @Published @Custom var myVar(注意包装器的应用顺序)。
在 (A) 的情况下，我收到错误

'wrappedValue' is unavailable: @Published is only available on properties of classes

对于(B)

error: key path value type 'Int' cannot be converted to contextual type 'Updating<Int>'

两者都不是特别有用。任何想法如何使它工作？
最小代码示例

import Combine

class A {
    @Updating @Published var b: Int
    
    init(b: Int) {
        self.b = b
    }
}

@propertyWrapper struct Updating<T> {
    var wrappedValue: T {
        didSet {
            print("Update: \(wrappedValue)")
        }
    }
}

let a = A(b: 1)
let cancellable = a.$b.sink {
    print("Published: \($0)")
}
a.b = 2
// Expected output:
// ==> Published: 1
// ==> Published: 2
// ==> Update: 2

Answer 1

我找到的唯一解决方案是一种解决方法:制作自定义 @propertyWrapper有一个 @Published里面的属性(property)。
例子:

/// Workaround @Published not playing nice with other property wrappers. 
/// Use this to replace @Published to temporarily help debug a property being accessed off the main thread.

@propertyWrapper
public class MainThreadPublished<Value> {
    @Published
    private var value: Value
    
    
    public var projectedValue: Published<Value>.Publisher {
        get {
            assert(Thread.isMainThread, "Accessing @MainThread property on wrong thread: \(Thread.current)")
            return $value
        }
        @available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
        set {
            assert(Thread.isMainThread, "Accessing @MainThread property on wrong thread: \(Thread.current)")
            $value = newValue
        }
    }
    
    public var wrappedValue: Value {
        get {
            assert(Thread.isMainThread, "Accessing @MainThread property on wrong thread: \(Thread.current)")
            return value
        }
        set {
            assert(Thread.isMainThread, "Accessing @MainThread property on wrong thread: \(Thread.current)")
            value = newValue
        }
    }
    
    public init(wrappedValue value: Value) {
        self.value = value
    }

    public init(initialValue value: Value) {
        self.value = value
    }
}

进一步阅读:

属性包装器提案:https://github.com/apple/swift-evolution/blob/main/proposals/0258-property-wrappers.md#referencing-the-enclosing-self-in-a-wrapper-type

不是 Apple 的 @Published 来源，而是一个有用的例子:https://github.com/OpenCombine/OpenCombine/blob/master/Sources/OpenCombine/Published.swift

编辑:
我也刚刚发现这篇文章可能提供了另一种方法，但我没有时间调查:

嵌套属性包装器:https://noahgilmore.com/blog/nesting-property-wrappers/