ARC時代的copy等

• assign implies __unsafe_unretained ownership.
• copy implies __strong ownership, as well as the usual behavior of copy semantics on the setter.
• retain implies __strong ownership.
• strong implies __strong ownership.
• unsafe_unretained implies __unsafe_unretained ownership.
• weak implies __weak ownership.

assign 等同unsafe_unretained

retain 等同 strong

copy的效用和MRC同樣，同時又有strong的效果。

ARC時代copy等不推薦使用的關鍵字能夠用在屬性中，而不能出如今方法調用中。