Are there a differences between these two lines of code?
__weak IBOutlet UITextField *usernameField;
@property (weak) IBOutlet UITextField *usernameField;
What if you declare either of these in interface section of the .h or the .m files?
Yes. The first example declares a weak instance variable called usernameField, but the second declares a weak property called usernameField, and an instance variable called _usernameField that is accessed by the property.
If you declare it in an @interface section of the .m file, then it can only be accessed in that .m file (unless you mess with the Objective-C runtime).
The difference is not in the weak reference but just in the fact that the first is an instance and the second is a @property.
__weak and (weak) is the same thing, but the second is used as attribute for properties.
来源：https://stackoverflow.com/questions/23944361/ios-weak-vs-weak