测试实例是否是Swift中的typealias协议？

Question

这与this question相关。使用typealias，我可以创建一个具有“children”var但具有特定类型的类，这正是我正在寻找的。但是当我这样做时，我不能再测试以查看实例是否遵循该协议。我也试过'如果让'，那不行。有没有办法做到这一点？

protocol Parent { 
    typealias Child 
    var children: [Child] { get set } 
} 

protocol Child { 
    typealias Parent 
    var parent: Parent { get set } 

} 

class Foo: Parent { 
    var children = [Bar]() 

    init(){} 
} 

class Bar: Child { 
    var parent = Foo() 
} 

let testVar = Foo() 
let cVar = Bar() 

testVar.children.append(cVar) 
cVar.parent = testVar 

//I get the error here saying protocol is limited to generic constraints 
if testVar is Parent { 

} 

Answer 1

不幸的是，没有办法解决这个问题（至少现在是这样）。所以更具动态性的方法是建立一个没有自身相关类型要求的“超级”协议。这允许您将这些“超级”的协议之间进行转换：

protocol _Parent { 
    var anyChildren: [Any] { get set } 
} 
protocol Parent: _Parent { 
    typealias Child 
    var children: [Child] { get set } 
} 

extension Parent { 
    // requirement of _Parent 
    var anyChildren: [Any] { 
     get { 
      return children.map{ $0 as Any } 
     } 
     set { 
      // use `as!` and `map` if you are sure that all elements are of type Child 
      // or if you want a crash if not 
      children = newValue.flatMap{ $0 as? Child } 
     } 
    } 
} 

protocol _Child { 
    var anyParent: Any { get set } 
} 

protocol Child: _Child { 
    typealias Parent 
    var parent: Parent { get set } 
} 

extension Child { 
    // requirement of _Child 
    var anyParent: Any { 
     get { 
      return parent 
     } 
     set { 
      parent = newValue as! Parent 
     } 
    } 
} 

现在你可以使用它们的属性来操作数据：

class Foo: Parent { 
    var children = [Foo]() 
} 

let anything: Any = Foo() 

// use `if let` and `as?` instead of `is` in order to have a more static type 
if let aDynamicParent = anything as? _Parent { 
    aDynamicParent.anyChildren.map{ $0 as! Foo } // cast the array back to its original type 

    aDynamicParent.children = [Foo(), Foo(), Foo()] 
} 

Answer 2

你不需要Child和Parent类型别名如果Parent包含符合Child协议和Child包含Parent协议属性的对象数组。

protocol Parent { 
    var children: [Child] { get set } 
} 

protocol Child { 
    var parent: Parent { get nonmutating set } 
} 

class Foo: Parent { 
    var children: [Child] 
    init(children: [Child]) { 
     self.children = children 
     self.children.forEach { $0.parent = self } 
    } 
} 

class Bar: Child { 
    var parent: Parent 
    init(parent: Parent) { 
     self.parent = parent 
     self.parent.children.append(self) 
    } 
} 

let testVar = Foo(children: [Bar]()) 
let cVar = Bar(parent: testVar) 

let parent = testVar as Parent 
let children = parent.children