Swift中的静态函数和类函数有什么区别？

Question

我可以在 Swift 库中看到这些定义：

extension Bool : BooleanLiteralConvertible {
    static func convertFromBooleanLiteral(value: Bool) -> Bool
}

protocol BooleanLiteralConvertible {
    typealias BooleanLiteralType
    class func convertFromBooleanLiteral(value: BooleanLiteralType) -> Self
}

定义为static func 的成员函数与定义为class func 的另一个成员函数有什么区别？仅仅是static 用于结构和枚举的静态函数，class 用于类和协议吗？是否还有其他需要了解的差异？在语法本身中有这种区别的基本原理是什么？

Answer 1

是不是简单地说，static 用于结构和枚举的静态函数，而 class 用于类和协议？

这是主要区别。其他一些区别是类函数是动态分派的，并且可以被子类覆盖。

协议使用 class 关键字，但它不排除实现协议的结构，它们只是使用 static 代替。为协议选择了类，因此不必使用第三个关键字来表示静态或类。

来自 Chris Lattner 关于这个主题：

我们考虑过统一语法（例如使用“type”作为关键字），但这实际上并不是简单的事情。关键字“类”和“静态”有助于熟悉并且非常具有描述性（一旦您了解 + 方法的工作原理），并为可能向类添加真正的静态方法打开了大门。这个模型的主要奇怪之处在于协议必须选择一个关键字（我们选择了“类”），但总的来说这是正确的权衡。

这里有一个 sn-p，它显示了类函数的一些覆盖行为：

class MyClass {
    class func myFunc() {
        println("myClass")
    }
}

class MyOtherClass: MyClass {
    override class func myFunc() {
        println("myOtherClass")
    }
}

var x: MyClass = MyOtherClass()
x.dynamicType.myFunc() //myOtherClass
x = MyClass()
x.dynamicType.myFunc() //myClass

Answer 2

要声明类型变量属性，请使用 static 声明修饰符标记声明。类可以使用 class 声明修饰符标记类型计算属性，以允许子类覆盖超类的实现。类型属性在类型属性中讨论。

注意
在类声明中，关键字static 与使用class 和final 声明修饰符标记声明的效果相同。

来源：The Swift Programming Language - Type Variable Properties

Answer 3

从 Swift2.0 开始，Apple 说：

“在协议中定义类型属性要求时，始终使用 static 关键字作为前缀。即使类型属性要求在由类实现时可以使用 class 或 static 关键字作为前缀，此规则也适用：”

Answer 4

为了更清楚，我这里举个例子，

class ClassA {
  class func func1() -> String {
    return "func1"
  }

  static func func2() -> String {
    return "func2"
  }

  /* same as above
  final class func func2() -> String {
    return "func2"
  }
  */
}

static func 与final class func 相同

因为它是final，所以我们不能在子类中覆盖它，如下所示：

class ClassB : ClassA {
  override class func func1() -> String {
    return "func1 in ClassB"
  }

  // ERROR: Class method overrides a 'final` class method
  override static func func2() -> String {
    return "func2 in ClassB"
  }
}

Answer 5

我在操场上做了一些实验，得到了一些结论。

TL;DR

如您所见，在class 的情况下，使用class func 或static func 只是习惯问题。

带解释的游乐场示例：

class Dog {
    final func identity() -> String {
        return "Once a woofer, forever a woofer!"
    }

    class func talk() -> String {
        return "Woof woof!"
    }

    static func eat() -> String {
        return "Miam miam"
    }

    func sleep() -> String {
        return "Zzz"
    }
}

class Bulldog: Dog {
    // Can not override a final function
//    override final func identity() -> String {
//        return "I'm once a dog but now I'm a cat"
//    }

    // Can not override a "class func", but redeclare is ok
    func talk() -> String {
        return "I'm a bulldog, and I don't woof."
    }

    // Same as "class func"
    func eat() -> String {
        return "I'm a bulldog, and I don't eat."
    }

    // Normal function can be overridden
    override func sleep() -> String {
        return "I'm a bulldog, and I don't sleep."
    }
}

let dog = Dog()
let bullDog = Bulldog()

// FINAL FUNC
//print(Dog.identity()) // compile error
print(dog.identity()) // print "Once a woofer, forever a woofer!"
//print(Bulldog.identity()) // compile error
print(bullDog.identity()) // print "Once a woofer, forever a woofer!"

// => "final func" is just a "normal" one but prevented to be overridden nor redeclared by subclasses.


// CLASS FUNC
print(Dog.talk()) // print "Woof woof!", called directly from class
//print(dog.talk()) // compile error cause "class func" is meant to be called directly from class, not an instance.
print(Bulldog.talk()) // print "Woof woof!" cause it's called from Bulldog class, not bullDog instance.
print(bullDog.talk()) // print "I'm a bulldog, and I don't woof." cause talk() is redeclared and it's called from bullDig instance

// => "class func" is like a "static" one, must be called directly from class or subclassed, can be redeclared but NOT meant to be overridden.

// STATIC FUNC
print(Dog.eat()) // print "Miam miam"
//print(dog.eat()) // compile error cause "static func" is type method
print(Bulldog.eat()) // print "Miam miam"
print(bullDog.eat()) // print "I'm a bulldog, and I don't eat."

// NORMAL FUNC
//print(Dog.sleep()) // compile error
print(dog.sleep()) // print "Zzz"
//print(Bulldog.sleep()) // compile error
print(bullDog.sleep()) // print "I'm a bulldog, and I don't sleep."

Answer 6

根据苹果发布的Swift 2.2 Book：

“你通过在方法的 func 关键字之前写 static 关键字来指示类型方法。类也可以使用class 关键字允许子类覆盖超类对该方法的实现。”

Answer 7

这称为类型方法，并使用点语法调用，如实例方法。但是，您在类型上调用类型方法，而不是在该类型的实例上。以下是在名为 SomeClass 的类上调用类型方法的方法：

Answer 8

这个例子将清楚每一个方面！

import UIKit

class Parent {
    final func finalFunc() -> String { // Final Function, cannot be redeclared.
        return "Parent Final Function."
    }

    static func staticFunc() -> String { // Static Function, can be redeclared.
        return "Parent Static Function."
    }

    func staticFunc() -> String { // Above function redeclared as Normal function.
        return "Parent Static Function, redeclared with same name but as non-static(normal) function."
    }

    class func classFunc() -> String { // Class Function, can be redeclared.
        return "Parent Class Function."
    }

    func classFunc() -> String { // Above function redeclared as Normal function.
        return "Parent Class Function, redeclared with same name but as non-class(normal) function."
    }

    func normalFunc() -> String { // Normal function, obviously cannot be redeclared.
        return "Parent Normal Function."
    }
}

class Child:Parent {

    // Final functions cannot be overridden.

    override func staticFunc() -> String { // This override form is of the redeclared version i.e: "func staticFunc()" so just like any other function of normal type, it can be overridden.
        return "Child Static Function redeclared and overridden, can simply be called Child Normal Function."
    }

    override class func classFunc() -> String { // Class function, can be overidden.
        return "Child Class Function."
    }

    override func classFunc() -> String { // This override form is of the redeclared version i.e: "func classFunc()" so just like any other function of normal type, it can be overridden.
        return "Child Class Function, redeclared and overridden, can simply be called Child Normal Function."
    }

    override func normalFunc() -> String { // Normal function, can be overridden.
        return "Child Normal Function."
    }
}

let parent = Parent()
let child = Child()

// Final
print("1. " + parent.finalFunc())   // 1. Can be called by object.
print("2. " + child.finalFunc())    // 2. Can be called by object, parent(final) function will be called.
// Parent.finalFunc()               // Cannot be called by class name directly.
// Child.finalFunc()                // Cannot be called by class name directly.

// Static
print("3. " + parent.staticFunc())  // 3. Cannot be called by object, this is redeclared version (i.e: a normal function).
print("4. " + child.staticFunc())   // 4. Cannot be called by object, this is override form redeclared version (normal function).
print("5. " + Parent.staticFunc())  // 5. Can be called by class name directly.
print("6. " + Child.staticFunc())   // 6. Can be called by class name direcly, parent(static) function will be called.

// Class
print("7. " + parent.classFunc())   // 7. Cannot be called by object, this is redeclared version (i.e: a normal function).
print("8. " + child.classFunc())    // 8. Cannot be called by object, this is override form redeclared version (normal function).
print("9. " + Parent.classFunc())   // 9. Can be called by class name directly.
print("10. " + Child.classFunc())   // 10. Can be called by class name direcly, child(class) function will be called.

// Normal
print("11. " + parent.normalFunc())  // 11. Can be called by object.
print("12. " + child.normalFunc())   // 12. Can be called by object, child(normal) function will be called.
// Parent.normalFunc()               // Cannot be called by class name directly.
// Child.normalFunc()                // Cannot be called by class name directly.

/*
 Notes:
 ___________________________________________________________________________
 |Types------Redeclare------Override------Call by object------Call by Class|
 |Final----------0--------------0---------------1------------------0-------|
 |Static---------1--------------0---------------0------------------1-------|
 |Class----------1--------------1---------------0------------------1-------|
 |Normal---------0--------------1---------------1------------------0-------|
 ---------------------------------------------------------------------------

 Final vs Normal function: Both are same but normal methods can be overridden.
 Static vs Class function: Both are same but class methods can be overridden.
 */

输出：

Answer 9

static 和 class 关键字都允许我们将方法附加到类而不是类的实例。例如，您可以创建一个具有名称和年龄等属性的 Student 类，然后创建一个静态方法 numberOfStudents，该方法由 Student 类本身而不是单个实例拥有。

static 和 class 的不同之处在于它们支持继承的方式。当你创建一个静态方法时，它被类拥有并且不能被子类更改，而当你使用类时，它可能会在需要时被覆盖。

这是一个示例代码：

  class Vehicle {
    static func getCurrentSpeed() -> Int {
        return 0
    }

    class func getCurrentNumberOfPassengers() -> Int {
        return 0
    } 

  }

  class Bicycle: Vehicle {
    //This is not allowed
    //Compiler error: "Cannot override static method"
  //  static override func getCurrentSpeed() -> Int {
  //        return 15
  //  }

      class override func getCurrentNumberOfPassengers() -> Int {
        return 1
      }
  }