覆盖Swift中的超类委托

Question

我正在开发一个包含两个UIViews的Swift(v1.2)项目.MyView和MyViewSubclass.

MyView有一个委托,我想在MyViewSubclass中覆盖它作为一个子协议,类似于UITableViews有一个UITableViewDelegate,它也符合超级UIScrollViewDelegate.

我的第一个想法是覆盖超类属性,但这会导致编译器错误,因为子类不能覆盖具有不同类型的超类属性.

// Example throws a compiler error. Can't override property with different type

class MyView : UIView {
    weak var delegate : MyViewDelegate?
}

class MyViewSubclass : MyView {
    override weak var delegate : MyViewSubclassDelegate? // Compiler error
}

protocol MyViewDelegate {
   func someFunc ()
}

protocol MyViewSubclassDelegate : MyViewDelegate {
    //func someFunc () Edit: redefinition not necessary, thanks @Rob!
    func someOtherFunc ()
}

我的第二个想法是覆盖委托属性的隐式getter和setter方法.子类可以有一个getter,它接受MyViewSubclassDelegate,并转换为MyViewDelegate协议,因此不需要重写属性本身.

此方法还会导致编译器错误.访问器实际上不能"覆盖"超类方法,因为它们具有不同的方法签名,并且不能仅因为它们的名称与超类设置器冲突而被声明.

// Example throws a compiler error. Can't declare function with a conflicting name

class MyViewSubclass : MyView {
    func setDelegate ( newValue : MyViewSubclassDelegate ) { // Compiler error
        super.delegate = newValue as? MyViewDelegate
    }

    func getDelegate () -> MyViewSubclassDelegate { // Compiler error
        return super.delegate as? MyViewSubclassDelegate
    }
}

我可以让子类覆盖超类的getter和setter,并将委托存储在一个单独的子类属性中,这样做有效,但是当它真正需要分配时,子类看起来应该被分配一个MyViewDelegate一个MyViewSubclassDelegate,将来可能会引起严重的混乱.

// Example works, but it is unclear that MyViewSubclass's delegate should be 
// assigned a MyViewSubclassDelegate

class MyViewSubclass : MyView {
    private weak var _subclassDelegate : MyViewSubclassDelegate?

    override weak var delegate : MyViewDelegate? { // Ambiguous type
        didSet {
            _subclassDelegate = delegate as? MyViewSubclassDelegate
        }
    }
}

我知道至少类似的东西是可能的,因为像UITableView和UICollectionView这样的类似乎做我想要的,但它们也是用Objective-C而不是Swift编写的,所以语言允许的细节可能有所不同.

有没有办法让Swift子类用子类型覆盖超类的属性？

Answer 1

它并不理想,但如果一个协议是从另一个协议继承而不是使用不同的类型,则使用相同的类型,但在didSet以下位置实现验证:

class MyView : UIView {

    /// The `MyViewDelegate` delegate

    weak var delegate: MyViewDelegate?
}

class MyViewSubclass: MyView {

    /// The `MyViewSubclassDelegate` delegate.
    ///
    /// **Note: This must be MyViewSubclassDelegate**

    override weak var delegate: MyViewDelegate? {
        didSet {
            assert(delegate == nil || delegate is MyViewSubclassDelegate, "The delegate of MyViewSubclass must be of type `MyViewSubclassDelegate`")
        }
    }
}

它不够优雅,但至少你会立即得到运行时错误,这会将问题带给程序员注意.通过添加///注释,它也将显示在快速帮助中.

-

另外,您可以采取更激进的变化,比如像delegate和peoplePickerDelegate性能ABPeoplePickerNavigationController,其中通过不同的属性指定的子类的委托.