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Sometimes I still got confused when I saw nonatomic , assign, copy, strong, retain
and weak in attributes of property declaration. Therefore write down some notes
and more detail information to memorize it.
| attributes | notes |
|---|---|
| weak | not only don't increase reference count, but which also automatically become nil when the referenced object is destroyed (zeroing weak), to avoid using dangling pointer |
| strong | increase the reference count of this object |
| retain | same as strong actually (called in non-arc decade) |
| assign | used in scalar ivar properties |
| copy | good practice to always set immutable properties to copy, because mutable versions can be passed into immutable properties and also used with block |
| nonatomic | denote that the object being referenced in not thread safe, we often use this attribute with UI component owing to UI operations only executed in main thread |
| atomic | guarantee the atomic operation on this property but might have some performance issue with thead-safety |
| unsafe_unretained | like weak not to increase reference count; however, it don't do zeroing pointer which means we should use this carefully with dangling pointer issue |