在objective-c中，类中的全局变量经常使用如下的方式申明。
@property(nonatomic（1）,strong（2）)UIImageView *imageView;
其中的1，2处是对此变量的一些属性申明。有以下几种
*strong *
weak
assign
strong 和 weak 是在arc后引入的关键字，strong类似于retain，引用时候会引用计算+1，weak相反，不会改变引用计数。
weak和assign都是引用计算不变，两个的差别在于，weak用于object type，就是指针类型，而assign用于简单的数据类型，如int BOOL 等。
assign看起来跟weak一样，其实不能混用的，assign的变量在释放后并不设置为nil（和weak不同），当你再去引用时候就会发生错误，崩溃，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 不能修改局部变量，如果需要修改需要加上__block.
block 会对对象强引用，引起retain-cycle，需要使用__weak
__weak __typeof(&*self)weakSelf =self;
在block种使用weakSelf可避免这种强引用。
（两个指针，指向同一块地址（self））；
