第一次使用的时候进行计算和初始化，后面的引用不在进行计算。
A lazy stored property is a property whose initial value is not calculated until the first time it is used. You indicate a lazy stored property by writing the lazy modifier before its declaration.
NOTE
You must always declare a lazy property as a variable (with the var keyword), because its initial value might not be retrieved until after instance initialization completes. Constant properties must always have a value before initialization completes, and therefore cannot be declared as lazy.
Lazy properties are useful when the initial value for a property is dependent on outside factors whose values are not known until after an instance’s initialization is complete. Lazy properties are also useful when the initial value for a property requires complex or computationally expensive setup that should not be performed unless or until it is needed.
class Lazytest{
var xx = 1
lazy var ee:String = {
return "\(self.xx)"
}()
lazy var ff = {
return "\(self.xx)"
}
lazy var gg = {para in
return para + "\(self.xx)"
}
}
有无括号的区别。无括号时为匿名函数
ee.storage String? "1" some
ff.storage (() -> String)? nil none
gg.storage ((String) -> String)? nil none
(lldb) po px.ff()
"4"
(lldb) po px.gg("aa")
"aa4"