Delegate正确声明方式

@property (nonatomic, weak) id delegate;

@property (nonatomic, assign) id delegate;

当delegate所指向的对象(0xxxxxxx)已经销毁的时候
weak： delegate == nil
assign：还会指向那个地址(0xxxxxxx)

参考：
Whats the difference between ‘weak’ and ‘assign’ in delegate property declaration