unowned & weak

https://docs.swift.org/swift-book/documentation/the-swift-programming-language/automaticreferencecounting/#Unowned-References

结论：unowned 修饰的对象，需要保证其访问时总是有值的，也就是引用 unowned 变量的对象，生命周期要相等或更长。否则 unowned 对象释放后再访问就会 crash