如何为每个数组元素生成绑定

Question

如果我有一个@State或一个@ObservedObject具有数组属性的变量，并且我想使用List数组的每个元素的绑定并将其传递到某个子视图（例如Toggle或TextField），是否有标准方法可以做到这一点？

简化示例：

struct Person: Identifiable {
  var id: UUID = .init()
  var name: String
  var isFavorite: Bool = false
}

struct ContentView: View {
  @State var people = [Person(name: "Joey"), Person(name: "Chandler")]

  var body: some View {
     List(people) { person in
        HStack() {
           Text(person.name) 
           Spacer
           Toggle("", isOn: $person.isFavorite) // <- this obviously doesn't work
        }
     }
  }
}

这似乎是一个相当常见的场景，但除了手动构建单独的绑定数组之外，我无法找出明显的解决方案。

我想出的唯一优雅的解决方案（如果没有更好的东西，我会将其添加为答案）是创建Bindinga的扩展，它RandomAccessCollection本身符合 a RandomAccessCollection，它具有绑定作为元素，如下所示：

extension Binding: RandomAccessCollection 
  where Value: RandomAccessCollection & MutableCollection {
  // more code here
}

  // more required extensions to Collection and Sequence here

Answer 1

更新

在iOS13 发行说明（弃用部分）Binding中，SwiftUI 放弃了to的一致性Collection，而是提供了一种解决方法，因此我根据他们的建议更新了这个答案。

这个想法是扩展RandomAccessCollection以添加一个.index()方法，该方法的工作原理类似于.enumerated()创建索引和元素元组的集合，但与符合.enumerated()a RandomAccessCollection、whichList和ForEachrequire 不同。

用法是：

List(people.indexed(), id: \.1.id) { (i, person) in
   HStack() {
      Toggle(person.name, isOn: $people[i].isFavorite)
   }

其实现.indexed()是：

struct IndexedCollection<Base: RandomAccessCollection>: RandomAccessCollection {
    typealias Index = Base.Index
    typealias Element = (index: Index, element: Base.Element)

    let base: Base

    var startIndex: Index { base.startIndex }

    var endIndex: Index { base.startIndex }

    func index(after i: Index) -> Index {
        base.index(after: i)
    }

    func index(before i: Index) -> Index {
        base.index(before: i)
    }

    func index(_ i: Index, offsetBy distance: Int) -> Index {
        base.index(i, offsetBy: distance)
    }

    subscript(position: Index) -> Element {
        (index: position, element: base[position])
    }
}

extension RandomAccessCollection {
    func indexed() -> IndexedCollection<Self> {
        IndexedCollection(base: self)
    }
}

原创 这是我想要实现的目标：

List($people) { personBinding in 
  HStack() {
      Text(personBinding.wrappedValue.name) 
      Spacer()
      Toggle("", isOn: personBinding.isFavorite)
  }
}

换句话说，传递数组的绑定，并在 的List闭包中获取元素的绑定。

为了实现这一点，我创建了一个扩展，Binding使 a Bindingof anyRandomAccessCollection变成 a RandomAccessCollectionof 绑定：

// For all Bindings whose Value is a collection
extension Binding: RandomAccessCollection 
    where Value: RandomAccessCollection & MutableCollection {

  // The Element of this collection is Binding of underlying Value.Element 
  public typealias Element = Binding<Value.Element>
  public typealias Index = Value.Index
  public typealias SubSequence = Self
  public typealias Indices = Value.Indices

  // return a binding to the underlying collection element
  public subscript(position: Index) -> Element {
    get {
      .init(get: { self.wrappedValue[position] },
            set: { self.wrappedValue[position] = $0 })
    }
  }

  // other protocol conformance requirements routed to underlying collection ...

  public func index(before i: Index) -> Index {      
     self.wrappedValue.index(before: i)
  }

  public func index(after i: Index) -> Index {
     self.wrappedValue.index(after: i)
  }

  public var startIndex: Index {
     self.wrappedValue.startIndex
  }

  public var endIndex: Index {
     self.wrappedValue.endIndex
  }
}

这还需要明确遵守继承的协议：

extension Binding: Sequence 
    where Value: RandomAccessCollection & MutableCollection {

  public func makeIterator() -> IndexingIterator<Self> {
    IndexingIterator(_elements: self)
  }
}

extension Binding: Collection 
    where Value: RandomAccessCollection & MutableCollection {

  public var indices: Value.Indices {
    self.wrappedValue.indices
  }
}

extension Binding: BidirectionalCollection 
    where Value: RandomAccessCollection & MutableCollection { 
}

而且，如果基础值是 an Identifiable，那么它也会使 Binding 符合Identifiable，从而无需使用id:：

extension Binding: Identifiable where Value: Identifiable {
  public var id: Value.ID {
    self.wrappedValue.id
  }
}