As the preceding example demonstrates,a computed instance property function can refer to other instance properties; it can also call instance methods. This is important,because in general the initializer for a stored property can do neither of those things. The reason this is legal for a computed property is that its functions won’t be called until the instance actually exists.

像前面例子示意的那样,一个计算实例属性函数可以引用其它实例属性;它同样可以调用实例方法.

这是非常重要的,因为一般的存储属性的初始化器既不能调用实例方法也不能引用实例变量.而计算属性做这些事合法的原因是它的方法在实例变量实际存在前不会调用.