error: property's synthesized getter follows Cocoa naming convention for returning 'owned' objects

出现这种情况,主要是属性名中包含  关键字. 

You can solve this by:

Renaming that property:

@property (strong, nonatomic) NSString *theNewTitle;

Keeping the property name and specifying a getter name that doesn’t begin with one of the special method name prefixes:

@property (strong, nonatomic, getter=theNewTitle) NSString *newTitle;

Keeping both the property name and the getter name, and telling the compiler that, even though the getter name starts with 

new

,
it belongs to the 

none

 method
family as opposed to the 

new

method
family:

#ifndef __has_attribute
#define __has_attribute(x) 0  // Compatibility with non-clang compilers
#endif

#if __has_attribute(objc_method_family)
#define BV_OBJC_METHOD_FAMILY_NONE __attribute__((objc_method_family(none)))
#else
#define BV_OBJC_METHOD_FAMILY_NONE
#endif

@interface ViewController : UIViewController
@property (strong, nonatomic) NSString *newTitle;
- (NSString *)newTitle BV_OBJC_METHOD_FAMILY_NONE;
@end

Note that even though this solution allows you to keep 

newTitle

 as
both the property name and the getter name, having a method called 

-newTitle

 that
doesn’t return an object owned by the caller can be confusing for other people reading your code.