您能否在Swift中的扩展之间覆盖? (编译器似乎很困惑！) [英] Can you override between extensions in Swift or not? (Compiler seems confused!)
问题描述
我一直在使用Swift开发iOS应用程序(其中大部分已从Objective-C移出).我正在使用Core Data，并尝试使用扩展将功能添加到从我的模型自动生成的类中.我在Objective-C中容易做的一件事是在类A的类别中添加一个方法，并在类B的类(从A派生)中覆盖该方法，我希望在Swift中也能做到这一点.
一段时间以来，我的项目中已有以下代码(这只是一个示例)，尽管我尚未使用该功能，但编译器可以很好地编译此代码:
// From CellType.swift -- NOTE: Imports from Foundation and CoreData
@objc(CellType)
class CellType: NSManagedObject {
@NSManaged var maxUses: NSNumber
@NSManaged var useCount: NSNumber
// Other properties removed for brevity
}
// From SwitchCellType.swift -- NOTE: Imports from Foundation and CoreData
@objc(SwitchCellType)
class SwitchCellType: CellType {
@NSManaged var targetCellXIndex: NSNumber
@NSManaged var targetCellYIndex: NSNumber
@NSManaged var targetCellType: CellType
// Other properties removed for brevity
}
// From CellTypeLogic.swift -- NOTE: Imports from Foundation and CoreData
extension CellType
{
var typeLabel : String { get { return "Empty"; } }
func isEqualToType(otherCellType : CellType) -> Bool
{
return (self.typeLabel == otherCellType.typeLabel &&
self.maxUses.isEqualToNumber(otherCellType.maxUses) &&
self.useCount.isEqualToNumber(otherCellType.useCount));
}
// Code removed for brevity
}
// From SwitchCellTypeLogic.swift -- NOTE: Imports from Foundation and CoreData
extension SwitchCellType // YES, this compiles with the overrides!
{
override var typeLabel : String { get { return "Switch"; } }
override func isEqualToType(otherCellType : CellType) -> Bool
{
var answer = false;
if let otherSwitchCellType = otherCellType as? SwitchCellType
{
answer = super.isEqualToType(otherCellType) &&
self.targetCellXIndex.isEqualToNumber(otherSwitchCellType.targetCellXIndex) &&
self.targetCellYIndex.isEqualToNumber(otherSwitchCellType.targetCellYIndex) &&
self.targetCellType.isEqualToType(otherSwitchCellType.targetCellType);
}
return answer;
}
// Code removed for brevity
}
希望那里的某些Swift专家已经看到了我的问题，但是这是我发现的方式:最近，我尝试使用具有参数和/或返回值的方法来添加类似的功能，这些方法和/或返回值不是类型内置的，但是我开始出现此错误:扩展中的声明无法覆盖.
为了探讨这个问题，我将以下内容添加到了我的一个快速文件中，认为它可以正常编译:
class A
{
}
class B : A
{
}
extension A
{
var y : String { get { return "YinA"; } }
}
extension B
{
override var y : String { get { return "YinB"; } } // Compiler error (see below) -- What??
}
令我惊讶的是，我收到了相同的编译器错误(扩展中的声明无法覆盖).什么?但是我已经使用了几次这种模式，而没有编译器错误.
问题: 首先，是否存在关于覆盖扩展的某些规则，以便在某些情况下应该起作用，而在另一些情况下则不能起作用?其次(更令人不安的是)为什么Swift编译器似乎如此不一致?我在这里想念什么?请帮助我恢复对Swift的信仰.
更新:
正如Martin R在正确答案中所指出的那样，只要您(1)仅涉及从NSObject派生的类而(2)不涉及此类，则似乎可以覆盖当前版本的Swift(通过Xcode 6.1通过1.1)中的方法.使用inout修饰符.这是一些示例:
class A : NSObject { }
class B : A { }
class SubNSObject : NSObject {}
class NotSubbed {}
enum SomeEnum { case c1, c2; }
extension A
{
var y : String { get { return "YinA"; } }
func f() -> A { return A(); }
func g(val: SubNSObject, test: Bool = false) { }
func h(val: NotSubbed, test: Bool = false) { }
func j(val: SomeEnum) { }
func k(val: SubNSObject, inout test: Bool) { }
}
extension B
{
// THESE OVERIDES DO COMPILE:
override var y : String { get { return "YinB"; } }
override func f() -> A { return A(); }
override func g(val: SubNSObject, test: Bool) { }
// THESE OVERIDES DO NOT COMPILE:
//override func h(val: NotSubbed, test: Bool = false) { }
//override func j(val: SomeEnum) { }
//override func k(val: SubNSObject, inout test: Bool) { }
}
似乎扩展中的覆盖方法和属性可与 当前的Swift(Swift 1.1/Xcode 6.1)仅用于 Objective-C兼容 方法和属性.
如果一个类是从NSObject派生的，那么它的所有成员都是自动可用的
在Objective-C中(如果可能，请参见下文).因此，
class A : NSObject { }
您的示例代码可以按预期进行编译和运行.您的代码数据扩展名将覆盖
之所以起作用，是因为NSManagedObject是NSObject的子类.
或者，您可以将@objc属性用于方法或属性:
class A { }
class B : A { }
extension A
{
@objc var y : String { get { return "YinA" } }
}
extension B
{
@objc override var y : String { get { return "YinB" } }
}
在Objective-C中无法表示的方法不能用@objc标记
并且不能在子类扩展中覆盖.例如，这适用于
具有inout参数或enum类型参数的方法.
I've been working on an iOS application in Swift (much of it being moved from Objective-C). I'm using Core Data and trying to use extensions to add functionality to classes auto-generated from my model. One thing I readily did in Objective-C was to add a method in a category on class A and override that method in a category on class B (which derived from A), and I was hoping to do the same in Swift.
For a while now I've had the following code in my project (and this is just one example), and though I have not used the functionality yet, the compiler has worked just fine compiling this code:
// From CellType.swift -- NOTE: Imports from Foundation and CoreData
@objc(CellType)
class CellType: NSManagedObject {
@NSManaged var maxUses: NSNumber
@NSManaged var useCount: NSNumber
// Other properties removed for brevity
}
// From SwitchCellType.swift -- NOTE: Imports from Foundation and CoreData
@objc(SwitchCellType)
class SwitchCellType: CellType {
@NSManaged var targetCellXIndex: NSNumber
@NSManaged var targetCellYIndex: NSNumber
@NSManaged var targetCellType: CellType
// Other properties removed for brevity
}
// From CellTypeLogic.swift -- NOTE: Imports from Foundation and CoreData
extension CellType
{
var typeLabel : String { get { return "Empty"; } }
func isEqualToType(otherCellType : CellType) -> Bool
{
return (self.typeLabel == otherCellType.typeLabel &&
self.maxUses.isEqualToNumber(otherCellType.maxUses) &&
self.useCount.isEqualToNumber(otherCellType.useCount));
}
// Code removed for brevity
}
// From SwitchCellTypeLogic.swift -- NOTE: Imports from Foundation and CoreData
extension SwitchCellType // YES, this compiles with the overrides!
{
override var typeLabel : String { get { return "Switch"; } }
override func isEqualToType(otherCellType : CellType) -> Bool
{
var answer = false;
if let otherSwitchCellType = otherCellType as? SwitchCellType
{
answer = super.isEqualToType(otherCellType) &&
self.targetCellXIndex.isEqualToNumber(otherSwitchCellType.targetCellXIndex) &&
self.targetCellYIndex.isEqualToNumber(otherSwitchCellType.targetCellYIndex) &&
self.targetCellType.isEqualToType(otherSwitchCellType.targetCellType);
}
return answer;
}
// Code removed for brevity
}
Hopefully some kind Swift expert out there already sees my issue, but here's how I found out about it: Recently I tried to add similar functionality using methods that have parameters and/or return values that are not built in types, but I started getting this error: Declarations in extensions cannot override yet.
To explore this issue I added the following to one of my swift files, thinking it would compile just fine:
class A
{
}
class B : A
{
}
extension A
{
var y : String { get { return "YinA"; } }
}
extension B
{
override var y : String { get { return "YinB"; } } // Compiler error (see below) -- What??
}
To my surprise, I received the same compiler error (Declarations in extensions cannot override yet). What? But I've used that patter several times already without compiler errors.
Questions: First, are there certain rules about overriding in extensions such that in some cases it is supposed to work but in other cases it is not? Second (and more disconcerting) why does it seem that the Swift compiler is so inconsistent? What am I missing here? Please help me restore my faith in Swift.
UPDATE:
As noted in the correct answer by Martin R, it seems you can override methods in the current version of Swift (1.1 via Xcode 6.1) as long as they (1) involve only classes derived from NSObject and (2) do not use the inout modifier. Here's some examples:
class A : NSObject { }
class B : A { }
class SubNSObject : NSObject {}
class NotSubbed {}
enum SomeEnum { case c1, c2; }
extension A
{
var y : String { get { return "YinA"; } }
func f() -> A { return A(); }
func g(val: SubNSObject, test: Bool = false) { }
func h(val: NotSubbed, test: Bool = false) { }
func j(val: SomeEnum) { }
func k(val: SubNSObject, inout test: Bool) { }
}
extension B
{
// THESE OVERIDES DO COMPILE:
override var y : String { get { return "YinB"; } }
override func f() -> A { return A(); }
override func g(val: SubNSObject, test: Bool) { }
// THESE OVERIDES DO NOT COMPILE:
//override func h(val: NotSubbed, test: Bool = false) { }
//override func j(val: SomeEnum) { }
//override func k(val: SubNSObject, inout test: Bool) { }
}
It seems that overriding methods and properties in an extension works with the current Swift (Swift 1.1/Xcode 6.1) only for Objective-C compatible methods and properties.
If a class is derived from NSObject then all its members are automatically available
in Objective-C (if possible, see below). So with
class A : NSObject { }
your example code compiles and works as expected. Your Code Data extension overrides
work because NSManagedObject is a subclass of NSObject.
Alternatively, you can use the @objc attribute for a method or property:
class A { }
class B : A { }
extension A
{
@objc var y : String { get { return "YinA" } }
}
extension B
{
@objc override var y : String { get { return "YinB" } }
}
Methods which are not representable in Objective-C cannot be marked with @objc
and cannot be overridden in a subclass extension. That applies for example to
methods having inout parameters or parameters of an enum type.
这篇关于您能否在Swift中的扩展之间覆盖? (编译器似乎很困惑！)的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
