假设存在以下协议,并且扩展提供了someFuncWithDefaultImplementation()的默认实现。
那么MyClass2是否有可能同时提供someFuncWithDefaultImplementation()的自身实现,该实现也从扩展中调用该方法的默认实现?
protocol MyProtocol : class {
func someFuncWithDefaultImplementation()
func someFunc()
var someInt:Int { get set }
}
extension MyProtocol {
func someFuncWithDefaultImplementation() {
someInt = 5
}
func someFunc() {
someFuncWithDefaultImplementation()
}
}
class MyClass : MyProtocol {
var someInt = 6
}
class MyClass2 : MyProtocol
{
var someInt: Int = 4
func someFuncWithDefaultImplementation()
{
// do some additional stuff
/*** someFuncWithDefaultImplementation() invoke MyProtocol extension implementation here ***/
}
}
let class1 = MyClass()
class1.someFunc()
let class2 = MyClass2()
class2.someFunc()
线程中的以下答案
描述了使用嵌套虚拟类型从已经提供了自己的蓝图方法实现的类型中访问协议的默认实现。我们可以扩展此方法,使其也允许实际使用MyClass2
实例的(蓝图)属性,但是在对MyClass2
已实现其自定义版本的默认实现的调用内(因此优先于默认值)。
我们首先查看示例的轻量级版本,并为该示例提供默认实现 someFuncWithDefaultImplementation()
protocol MyProtocol : class {
func someFuncWithDefaultImplementation()
var someInt: Int { get set }
}
extension MyProtocol {
func someFuncWithDefaultImplementation() {
print("Called default impl. Currently, someInt = \(someInt)")
print("Mutates someInt from within default implementation (0) ...")
someInt = 0
}
}
我们使用非优雅的嵌套式解决方案的定制实现someFuncWithDefaultImplementation()
的MyClass2
,调用后者的默认实现,但存储在参考Dummy
实例回MyClass2
实例,以使someInt
财产MyClass2
在默认实现通话使用(用于读写),即使这是从Dummy
类型中调用的也是如此。
class MyClass2 : MyProtocol
{
var someInt: Int = 42
func someFuncWithDefaultImplementation()
{
// do some additional stuff ...
print("In MyClass2 implementation, currently someInt = \(someInt)")
/* Dummy 'MyClass2'-capturing type used to call the default
implementation of 'MyProtocol', but with read and write
access to 'MyClass2':s self:s 'someInt' instance. */
class Dummy : MyProtocol {
unowned let myClass2: MyClass2
init(_ myClass2: MyClass2) { self.myClass2 = myClass2 }
var someInt: Int {
get { return myClass2.someInt }
set { myClass2.someInt = newValue }
}
}
// call default implementation of 'someFuncWithDefaultImplementation'
// supplying 'self' to read/write access to self.someInt.
Dummy(self).someFuncWithDefaultImplementation()
print("Back in MyClass2:s implementation; now someInt = \(someInt)")
// 0, woah, mutated in default implementation!
}
}
let a = MyClass2()
a.someFuncWithDefaultImplementation()
/* In MyClass2 implementation, currently someInt = 42
Called default impl. Currently, someInt = 42
Mutates someInt from within default implementation (0) ...
Back in MyClass2:s implementation; now someInt = 0 */
您也可以选择Dummy
在函数外部声明嵌套,仅对其private
进行标记以确保不能从外部访问它MyClass2
:
class MyClass2 : MyProtocol
{
var someInt: Int = 42
/* Dummy 'MyClass2'-capturing type used to call the default
implementation of 'MyProtocol', but with read and write
access to 'MyClass2':s self:s 'someInt' instance. */
private class Dummy : MyProtocol {
unowned let myClass2: MyClass2
init(_ myClass2: MyClass2) { self.myClass2 = myClass2 }
var someInt: Int {
get { return myClass2.someInt }
set { myClass2.someInt = newValue }
}
}
func someFuncWithDefaultImplementation()
{
// do some additional stuff ...
print("In MyClass2 implementation, currently someInt = \(someInt)")
// call default implementation of 'someFuncWithDefaultImplementation'
// supplying 'self' to read/write access to self.someInt.
Dummy(self).someFuncWithDefaultImplementation()
print("Back in MyClass2:s implementation; now someInt = \(someInt)")
// 0, woah, mutated in default implementation!
}
}
但是,我将重复与链接答案的作者相同的方法:这种方法不是很好。
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