Implementing Nscopying in Swift with Subclasses

Implementing NSCopying in Swift with subclasses

The Short Answer

You cannot use self.dynamicType() without marking init() as required because there's no guarantee subclasses of Vehicle will also implement init().

Exploring The Problem

Taking a look at The Swift Programming Language: Initialization, it's mentioned how

subclasses do not inherit their superclass initializers by default

The situations in which a subclass will inherit its superclass' initialisers are:

Assuming that you provide default values for any new properties you
introduce in a subclass, the following two rules apply:

Rule 1

If your subclass doesn’t define any designated initializers, it
automatically inherits all of its superclass designated initializers.

Rule 2

If your subclass provides an implementation of all of its superclass
designated initializers—either by inheriting them as per rule 1, or by
providing a custom implementation as part of its definition—then it
automatically inherits all of the superclass convenience initialisers.

Take a look at the example:

class MySuperclass {
    let num = 0

    // MySuperclass is given `init()` as its default initialiser
    // because I gave `num` a default value.
}

class MySubclass : MySuperclass {
    let otherNum: Int

    init(otherNum: Int) {
        self.otherNum = otherNum
    }
}  

According to the information above, since MySubclass defined the property otherNum without an initial value, it doesn't automatically inherit init() from MySuperclass.

Now suppose I want to add the following method to MySuperclass:

func myMethod() {
    println(self.dynamicType().num)
}

You'll get the error you described because there is no guarantee subclasses of MySuperclass will implement init() (and in this example they don't).

To solve this problem you therefore need to mark init() as required, to ensure all subclasses of MySuperclass implement init(), and so calling self.dynamicType() is a valid thing to do. It's the same problem as in your question: Swift knows Vehicle implements init(), however it doesn't know any subclasses will implement init() and so you need to make it required.

An alternative solution, which isn't suitable in your example, is to mark Vehicle as final, meaning Vehicle can't be subclassed. Then you'll be able to use self.dynamicType(); but you might as well just use Vehicle() in that case.

Implementing NSCopying in Subclass of Subclass

One of those things you realize right after asking...

The implementation of copyWithZone: in the superclass (Shape) shouldn't be assuming it's a Shape. So instead of the wrong way, as I mentioned above:

- (id)copyWithZone:(NSZone *)zone {
    Shape *s = [[Shape allocWithZone:zone] init];
    s.sides = self.sides;
    return s;
}

You should instead use:

- (id)copyWithZone:(NSZone *)zone {
    Shape *s = [[[self class] allocWithZone:zone] init]; // <-- NOTE CHANGE
    s.sides = self.sides;
    return s;
}

NSCopying copy(with:) - Does it really need to return Any?

This is the Objective-C API for the NSObject method copy:

- (id)copy;

That translates into Swift as:

func copy() -> Any

Thus, no type information is attached to the new object; that's why you have to cast.

Now, if you don't like that, there's an easy solution in your situation: don't adopt NSCopying and don't use the built-in NSObject copy method! Write your own one-off method that clones a Person — e.g. an instance method called makeClone(). NSObject copy is a convenience, but no law requires you to use it (unless there is some other reason why Person needs to conform to NSCopying, e.g. it is to be used as the key to an NSDictionary, but I'm betting you are never encountering any such reason).

Is this ok to do in a swift subclass copy()?

Your Method

override func copy() -> AnyObject {
    let clone = super.copy() as SubClassType
    return clone
}

My Answer

I'm not sure exactly what you want the method to do.

let clone = super.copy() as SubClassType

statically types the constant clone to be of type SubClassType. It doesn't make any changes to the object. The very next line of code

return clone

statically types the return value to be AnyObject. Again, it doesn't make any changes to the object.

The code is identical to

override func copy() -> AnyObject {
    return super.copy()
}

Which is the default behavior when you don't override a method.

In the end, you have 4 lines of code that are identical to 0 lines of code.

How to implement copy constructor in Swift subclass?

init(copyFrom: Square) is an overload, not an override, of init(copyFrom: Shape). What I mean is that they are unrelated methods because they accept different types. In Swift that's acceptable. In ObjC, that's illegal. There are no overloads in ObjC.

Swift initializers don't automatically inherit. So in Swift, you couldn't try to copy a random Shape as a Square. The initializer isn't available. But in ObjC, initializers do automatically inherit (and you can't stop them from doing so). So if you have a method initWithCopyFrom:(*Shape), it is required that every subclass be willing to accept it. That means you could (in ObjC) try to create a copy of a Circle as a Square. That's of course nonsense.

If this is an NSObject subclass, you should use NSCopying. Here's how you would go about that:

import Foundation

class Shape : NSObject, NSCopying { // <== Note NSCopying
  var color : String

  required override init() { // <== Need "required" because we need to call dynamicType() below
    color = "Red"
  }

  func copyWithZone(zone: NSZone) -> AnyObject { // <== NSCopying
    // *** Construct "one of my current class". This is why init() is a required initializer
    let theCopy = self.dynamicType()
    theCopy.color = self.color
    return theCopy
  }
}

class Square : Shape {
  var length : Double

  required init() {
    length = 10.0
    super.init()
  }

  override func copyWithZone(zone: NSZone) -> AnyObject { // <== NSCopying
    let theCopy = super.copyWithZone(zone) as Square // <== Need casting since it returns AnyObject
    theCopy.length = self.length
    return theCopy
  }

}

let s = Square()      // {{color "Red"} length 10.0}

let copy = s.copy() as Square // {{color "Red"} length 10.0} // <== copy() requires a cast

s.color = "Blue"               // {{color "Blue"} length 10.0}
s                              // {{color "Blue"} length 10.0}
copy                           // {{color "Red"}

Swift 3

class Shape: NSObject, NSCopying {

    required override init() {
        super.init()
    }    

    func copy(with zone: NSZone? = nil) -> Any {
        let copy = type(of: self).init()
        return copy
    }

}

class Square: Shape {

    required override init() {
        super.init()
    }    

    func copy(with zone: NSZone? = nil) -> Any {
        let copy = super.copy(with: zone) as! Square
        copy.foo = self.foo
        ......
        return copy
    }

}

Implementing NSCopying

To implement NSCopying, your object must respond to the -copyWithZone: selector. Here’s how you declare that you conform to it:

@interface MyObject : NSObject <NSCopying> {

Then, in your object’s implementation (your .m file):

- (id)copyWithZone:(NSZone *)zone
{
    // Copying code here.
}

What should your code do? First, create a new instance of the object—you can call [[[self class] alloc] init] to get an initialized obejct of the current class, which works well for subclassing. Then, for any instance variables that are a subclass of NSObject that supports copying, you can call [thatObject copyWithZone:zone] for the new object. For primitive types (int, char, BOOL and friends) just set the variables to be equal. So, for your object Vendor, it’d look like this:

- (id)copyWithZone:(NSZone *)zone
{
    id copy = [[[self class] alloc] init];

    if (copy) {
        // Copy NSObject subclasses
        [copy setVendorID:[[self.vendorID copyWithZone:zone] autorelease]];
        [copy setAvailableCars:[[self.availableCars copyWithZone:zone] autorelease]];

        // Set primitives
        [copy setAtAirport:self.atAirport];
    }

    return copy;
}

Right way to implement deep copy for object which contains nested objects in Swift?

Assuming your filter choice also conforms:

let copy = ZLFilters(filterChoices: filterChoices!.copyWithZone(zone))

Best practice when implementing copyWithZone:

1. You should always use [[self class] allocWithZone:zone] to make sure you are creating a copy using the appropriate class. The example you give for 002 shows exactly why: Subclasses will call [super copyWithZone:zone] and expect to get back an instance of the appropriate class, not an instance of the super class.

2. I access the ivars directly, so I don't need to worry about any side effects I might add to the property setter (e.g., generating notifications) later on. Keep in mind, subclasses are free to override any method. In your example, you are sending two extra messages per ivar. I would implement it as follows:

Code:

- (id)copyWithZone:(NSZone *)zone {
    Crime *newCrime = [super copyWithZone:zone];
    newCrime->_month = [_month copyWithZone:zone];
    newCrime->_category = [_category copyWithZone:zone];
    // etc...
    return newCrime;
}

Of course, whether you copy the ivars, retain them, or just assign them should mirror what the setters do.

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