19.鸭子类型

鸭打字

在编程中，鸭子类型是动态编程语言中使用的类型系统。对象的类型或类不如它定义的方法重要。
使用鸭子类型，您可以检查一个类是否具有给定的方法或属性。

为什么叫 Duck Typing？

它来自那句话：
如果它走路像鸭子，嘎嘎也像鸭子，那它一定是鸭子

基本上在程序的上下文中，只要一个类具有函数的确切名称；我们既不关心函数具体做什么，也不关心具体函数来自哪个类。

Is关键字_

对于鸭子类型和检查空值，我们需要使用is关键字。
is关键字用于检查给定对象的数据类型并返回一个布尔值：

5 is int # true
Frog is Animal # Does the Frog Class inherit from the Animal Class and is not empty?

鸭子打字的例子

# Animal Class
extends Node
class_name Animal
func fly():
   print('Animal flies')

# Duck Class
extends Animal
class_name Duck
func fly():
   print('this duck flies')

# Circle Class
extends Node
class_name Circle
func fly():
   print('Circles are flying???')

# Node Class
extends Node2D
var animal = Animal.new()
var duck = Duck.new()
var circle = Circle.new()
# No Type Safety
"""
In this case we can pass it in everything and the function call will work
as long as the class object has that specific function name
letItFly(animal)
letItFly(duck)
letItFly(circle)
"""
func letItFly(flyingObject):
   flyingObject.fly()
# With Type Safety
"""
But what if we want type safety?
Say we only want Classes and Sub-Classes of the 'Animal' class???
"""
func animalFlies(animalObject: Animal):
   animalObject.fly() # comes with auto complete
"""
The problem is that an error will be thrown if the class is a null value
For Example:
   var nullObject: Animal # Null Instantiation w/ type safety
In this case nullObject will make it through the animalFlies() method without warning.
The game will crash when a null object tries to call the 'fly()' because it does not exist
on a null object
In this case we have to check that null values are not sneaking through
"""
# Check for the objects casted as Nulls
# var nullObject: Animal # casted as null but will make it through this function regardless
func animalFliesSafely(animalObject: Animal):
   # Option 1
   if animalObject == null:
      print('object/value does not fly')
      return
   # Option 2
   if (animalObject is Animal) == false:
      print('Animal Class not part of Inheritance Chain')
      return
   # Do whatever you want; you're an animal!
   print('continue on ;)')
   animalObject.fly() # without a null check, throws an error if null