Declare properties in .h interface or in an extension in .m file?

问题

In Objective-C, is it best practice to:

1. Declare objects such as buttons in the .h and then synthesize in the .m

   .h
   @interface SomeViewController : UIViewController  
     @property (strong, nonatomic) UIButton *someButton;  
   @end
   
   .m
   @implementation SomeViewController  
     @synthesize someButton = _someButton;  
   @end
   

2. or declare them as ivars in the .m

   @interface SomeViewController ()  
     @property (strong, nonatomic) UIButton *someButton;  
   @end  
   

I notice that in a lot of Apple code, specifically their Breadcrumbs sample code, many of their properties are declared in the interface. Is there a difference between the two? I also noticed that when properties are declared in the @interface, they are automatically synthesized with an underscore prefix, making the someButton = _someButton synthesis useless.

回答1:

First, as of Xcode 4.4 there is no longer a need to @synthesize(unless you change both the setter and getter method), either when the @property is declared in the @interface or @implementation.

If the @property is only accessed from within the class then declare the @property in a class extension in the .m file. This provides encapsulation and make it easy to see that the @property is not used from another class.

If the @property is used by other classes, by design, then define it in the @interface in the .h file.

来源：https://stackoverflow.com/questions/14132129/declare-properties-in-h-interface-or-in-an-extension-in-m-file