这个是针对arc来说的
__weak是弱引用
__strong是强引用
官方的说明如下
- __strong is the default. An object remains “alive” as long as there is a strong pointer to it.
- 强引用是默认的,对象像强指针一样持久的存在
- __weak specifies a reference that does not keep the referenced object alive. A weak reference is set to nil when there are no strong references to the object.
- 弱引用指定一个引用不保证存在，当没有强引用对象时，弱引用被设置为nil