Question

If I add a property to the ViewController

@property (strong, atomic) UIView *smallBox;

and synthesize it in the .m file, the variable can actually be referenced just by smallBox inside of any instance methods.

But then, self.view cannot be replaced by view, even though view is defined as a property of UIViewController too. Why the difference and what is the rule?

Answer 1

self.view and view/_view are not the same thing. Depending on how you create your instance variables, view or _view refer to the actual object instance variable. It is dangerous to access this directly, and you should only do so in init, dealloc or in accessors. Everywhere else, you should use self.view.

self.view is exactly the same as [self view], which passes the message “view” to the object “self” an returns the result. By default, when an object receives a message, it executes the method with that name, and the default implementation of view will return the value of the related instance variable (either view or _view).

In older versions of Xcode, @synthesize view would create an instance variable called view. In the latest versions of Xcode, declaring a property view will will automatically create an instance variable called _view in many cases, even without @synthesize. This change makes it easier to notice when you are accessing the ivar directly.

In short:

• except in init, dealloc and the view accessors (if you custom write them), always use self.view.
• In those methods, you should refer to it as _view.
• If you are writing for the latest Xcode, do not include @synthesize at all. If you are writing for a slightly older Xcode, use @synthesize view=_view;
• self.view does not mean “the value of the instance variable.” It means “the result of passing the message ‘view'” which is generally implemented as returning the instance variable.