swift 协议“弱”不能应用于非类类型

Question

I'm a bit confused. What's the difference between protocol A : class { ... } and protocol A{ ... } , and which one we should use in swift?

PS： we got an error when we wrote like this

protocol A{ ... }

weak var delegate: A

error : 'weak' cannot be applied to non-class type

Answer 1

Swift >= 4:

protocol A : AnyObject { ... {

Swift < 4:

protocol A : class { ... }

defines a "class-only protocol" : Only class types (and not structures or enumerations) can adopt this protocol.

Weak references are only defined for reference types . Classes are reference types, structures and enumerations are value types. (Closures are reference types as well, but closures cannot adopt a protocol, so they are irrelevant in this context.)

Therefore, if the object conforming to the protocol needs to be stored in a weak property then the protocol must be a class-only protocol.

Here is another example which requires a class-only protocol:

protocol A { 
    var name : String { get set }
}

func foo(a : A) {
    a.name = "bar" // error: cannot assign to property: 'a' is a 'let' constant
}

This does not compile because for instances of structures and enumerations, a.name = "bar" is a mutation of a . If you define the protocol as

protocol A : class { 
    var name : String { get set }
}

then the compiler knows that a is an instance of a class type to that a is a reference to the object storage, and a.name = "bar" modifies the referenced object, but not a .

So generally, you would define a class-only protocol if you need the types adopting the protocol to be reference types and not value types.

Answer 2

如果您使用的是Swift 4 及更高版本

protocol A : AnyObject { ... }

Answer 3

You can make the protocol derive from any class type like NSObject or AnyObject:

protocol TopNewsTableDelegate : AnyObject {
    func topNewsTableDidLoadedStories()
}

Answer 4

Or you can type like this

@objc protocol A { ... }

then you can make a weak delegate reference

Answer 5

protocol CustomProtocolName : NSObjectProtocol {
  // ...
}