Inheritance in swift

Inherit （Inheritance）

1、 Value type （ enumeration 、 Structure ） Inheritance is not supported , Only classes support inheritance

2、 A class without a parent , be called ： Base class

        Swift Did not like OC、Java That rule ： Any class will eventually inherit from a base class .

3、 Subclasses can override the subscript of the parent class 、 Method 、 attribute , Rewrite must add override keyword .

Memory structure

class Animal {
    var age = 0
}

class Dog : Animal {
    var weight = 0
}

class ErHa : Dog {
    var iq = 0
}

let a = Animal()
a.age = 10

to glance at a How much memory is needed ,a It's heap space , So it must be 16 Multiple , At the front 8 Bytes are used to put type information , the second 8 Bytes put reference count related things , Further back 8 Bytes is put age, A total of 24 Bytes , But the guarantee is 16 Multiple , So it is 32 Bytes .

Dog Because there is inheritance , So it's equivalent to

class Animal {
    var age = 0
}

class Dog : Animal {
    var weight = 0
}

class Dog {
    var age = 0
    var weight = 0
}

class ErHa : Dog {
    var iq = 0
}

let d = Dog()
d.age = 10
d.weight = 20

d There are two properties in an object ,age and weight, Each account 8 Bytes , And generally speaking, the attribute memory of the parent class is higher ,d Objects also occupy 32 Bytes , The first is type related 8 Bytes , The second block is related to reference counting 8 Bytes , The third is storage age Of 8 Bytes , The fourth is storage weight Of 8 Bytes .

The same one ErHa Object must have 24 Byte store age、weight、iq, In addition, there is the front 16 Bytes , So it is 40 Bytes , But make sure it is 16 Multiple , So is 48.

Override instance methods 、 Subscript

class Animal {
    func speak() {
        print("Animal speak")
    }
    
    subscript(index: Int) -> Int {
        return index
    }
}

class Cat : Animal {
    override func speak() {
        super.speak()
        print("Cat speak")
    }
    
    override subscript(index: Int) -> Int {
        return super[index] + 1
    }
}

var anim: Animal
anim = Animal()
//Animal speak
anim.speak()
//6
print(anim[6])

anim = Cat()
//Animal speak
//Cat speak
anim.speak()
// 7
print(anim[6])

Override type methods 、 Subscript

1、 By class Decorated type method 、 Subscript , Allow to be overridden by subclasses

2、 By static Decorated type method 、 Subscript , It is not allowed to be overridden by subclasses

class Animal {
    class func speak() {
        print("Animal speak")
    }
    
    class subscript(index: Int) -> Int {
        return index
    }
}

class Cat : Animal {
    override class func speak() {
        super.speak()
        print("Cat speak")
    }
    
    override class subscript(index: Int) -> Int {
        return super[index] + 1
    }
}

static Decorated type method 、 An error is reported when the subscript is rewritten

Subclass overrides can be used static modification , Just can't continue to be rewritten

Overridden properties  

1、 Subclasses can attribute the parent class （ Storage 、 Calculation ） Rewrite to compute property

2、 Subclasses cannot override parent class attributes to storage attributes

3、 Can only override var attribute , Can not rewrite let attribute

4、 When rewriting , Property name 、 The type should be consistent

5、 Subclass overridden attribute permissions , Cannot be less than the permission of the parent attribute

        If the parent property is read-only , Then the overridden attributes of subclasses can be read-only , It can also be readable and writable .

        If the parent property is readable and writable , Then the properties of subclasses after rewriting must also be readable and writable .

Override instance properties

class Circle {
    var radius: Int = 0
    var diameter: Int {
        set {
            print("Circle setDiameter")
            radius = newValue / 2
        }
        get {
            print("Circle getDiameter")
            return radius * 2
        }
    }
}

class SubCircle: Circle {
    override var radius: Int {
        set {
            print("SubCircle setRadius")
            super.radius = newValue > 0 ? newValue : 0
        }
        get {
            print("SubCircle getRadius")
            return super.radius
        }
    }
    override var diameter: Int {
        set {
            print("SubCircle setDiameter")
            super.diameter = newValue > 0 ? newValue : 0
        }
        get {
            print("SubCircle getDiameter")
            return super.diameter
        }
    }
}

var circle = SubCircle()
circle.radius = 6
//SubCircle setRadius

print(circle.diameter)
//SubCircle getDiameter
//Circle getDiameter
//SubCircle getRadius
//12

circle.diameter = 20
//SubCircle setDiameter
//Circle setDiameter
//SubCircle setRadius

print(circle.radius)
//SubCircle getRadius
//10

Override type properties

1、 By class Decorated calculation type attribute , Can be overridden by subclass

        The storage type attribute can only be used static To modify .

2、 By static Decorated type attribute （ Storage 、 Calculation ）, Can't be overridden by subclass

Attribute viewer

1、 It can be a parent class attribute in a subclass （ Except for read-only calculation properties 、let attribute ） Add attribute viewer

class Circle {
    var radius: Int = 1
}

class SubCircle: Circle {
    override var radius: Int {
        willSet {
            print("SubCircle willSetRadius", newValue)
        }
        didSet {
            print("SubCircle didSetRadius", oldValue, radius)
        }
    }
}

var circle = SubCircle()
circle.radius = 10
//SubCircle willSetRadius 10
//SubCircle didSetRadius 1 10

2、 There are attribute observers in both parent and child classes

class Circle {
    var radius: Int = 1 {
        willSet {
            print("Circle willSetRadius", newValue)
        }
        didSet {
            print("Circle didSetRadius", oldValue, radius)
        }
    }
}

class SubCircle: Circle {
    override var radius: Int {
        willSet {
            print("SubCircle willSetRadius", newValue)
        }
        didSet {
            print("SubCircle didSetRadius", oldValue, radius)
        }
    }
}

var circle = SubCircle()
circle.radius = 10
//SubCircle willSetRadius 10
//Circle willSetRadius 10
//Circle didSetRadius 1 10
//SubCircle didSetRadius 1 10

3、 Subclasses can add attribute observers to the calculated attributes in the parent class .

class Circle {
    class var radius: Int {
        set {
            print("Circle setRadius", newValue)
        }
        get {
            print("Circle getRadius")
            return 20
        }
    }
}

class SubCircle: Circle {
    override static var radius: Int {
        willSet {
            print("SubCircle willSetRadius", newValue)
        }
        didSet {
            print("SubCircle didSetRadius", oldValue, radius)
        }
    }
}

SubCircle.radius = 10
//        Circle getRadius (oldValue)
//        SubCircle willSetRadius 10
//        Circle setRadius 10
//        Circle getRadius (radius)
//        SubCircle didSetRadius 20 20

final

1、 By final The method of decoration 、 Subscript 、 attribute , Cannot be rewritten

2、 By final Modified class , Forbidden to be inherited