initial value：必须初始化、不影响观察者
Classes and structures must set all of their stored properties to an appropriate initial value by the time an instance of that class or structure is created. Stored properties cannot be left in an indeterminate state.
You can set an initial value for a stored property within an initializer, or by assigning a default property value as part of the property’s definition. These actions are described in the following sections.
When you assign a default value to a stored property, or set its initial value within an initializer, the value of that property is set directly, without calling any property observers.
Default Property Values：另一种初始化方式；
Parameter Names and Argument Labels
As with function and method parameters, initialization parameters can have both a parameter name for use within the initializer’s body and an argument label for use when calling the initializer.
Optional Property Types
Properties of optional type are automatically initialized with a value of nil,
Assigning Constant Properties During Initialization
常量可以在初始化时赋值；
Default Initializers
条件：1、所有参数都有缺省参量；2、没有初始化函数；
Memberwise Initializers for Structure Types
结构体可以有以属性为参量的缺省构造器，类没有；
Initializer Delegation for Value Types：初始化代理
区分于引用类型：值类型、引用类型。
？没有明白？
self.init只能在初始器内部使用。
For value types, you use self.init to refer to other initializers from the same value type when writing your own custom initializers. You can call self.init only from within an initializer.