在objective-c中，類中的全局變量經(jīng)常使用如下的方式申明。

@property(nonatomic（1）,strong（2）)UIImageView *imageView;

其中的1，2處是對此變量的一些屬性申明。有以下幾種
*strong *
weak
assign
strong 和 weak 是在arc后引入的關鍵字，strong類似于retain，引用時候會引用計算+1，weak相反，不會改變引用計數(shù)。
weak和assign都是引用計算不變，兩個的差別在于，weak用于object type，就是指針類型，而assign用于簡單的數(shù)據(jù)類型，如int BOOL 等。
assign看起來跟weak一樣，其實不能混用的，assign的變量在釋放后并不設置為nil（和weak不同），當你再去引用時候就會發(fā)生錯誤，崩潰，EXC_BAD_ACCESS.

Has an assign attribute, AND Has been deallocated. Thus when you attempt to call the respondsToSelector method on delegate, you get a EXC_BAD_ACCESS. This is because objects that use the assign property will not be set to nil when they are deallocated. (Hence why doing a !self.delegate before the respondsToSelector does not prevent the responseToSelector from being called on a deallocated object, and still crashes your code) The solution is to use the weak attribute, because when the object deallocates, the pointer WILL be set the nil . So when your code calls respondsToSelector on a nil, Objective C will ignore the call, and not crash.

block 不能修改局部變量，如果需要修改需要加上__block.
block 會對對象強引用，引起retain-cycle，需要使用__weak

__weak __typeof(&*self)weakSelf =self;

在block種使用weakSelf可避免這種強引用。
（兩個指針，指向同一塊地址（self））；