Cora Data. Category provides helper methods to managed object. Custom objects are stored as transformable attributes. Objective-C types are stored through NSValue.

Cocoa Programming Guidelines.tex

@@ -949,4 +949,133 @@ The following getter implementation traverses view controller hierarchy, and ret
 \end{codelisting}
 
 
+\section{Core Data}
+
+
+\begin{importantlisting}
+NSManagedObject's properties must not be prefixed with "new". Due to @dynamic compiler directive, the following compiler error is suppressed: "property's synthesized getter follows Cocoa naming convention for returning 'owned' objects".
+\end{importantlisting}
+
+
+\subsection{Category provides helper methods to managed object}
+
+NSManagedObject subclasses can be generated by Xcode schema editor automatically.
+
+\begin{codelisting}
+@interface Person : NSManagedObject
+
+@property (nonatomic, retain) NSString * name;
+@property (nonatomic, retain) NSString * surname;
+
+@end
+\end{codelisting}
+
+Therefore, all helper methods are kept in a category in a separate file, otherwise they would be overridden by the editor.
+
+\begin{codelisting}
+@interface Person (Additions)
+
+- (NSString *)fullName;
+
+@end
+\end{codelisting}
+
+\subsection{Custom objects are stored as transformable attributes}
+
+A custom object can be stored by Core Data as a transformable attribute, if it conforms to NSCoding protocol. Xcode schema editor uses id for this attributes, which deprives developer of good type checking.
+
+\begin{codelisting}
+@interface Person : NSManagedObject
+
+@property (nonatomic, retain) id eyeColorTransformableType;
+
+@end
+\end{codelisting}
+
+To ameliorate the situation, a property with the proper class type is provided by a category.
+
+\begin{codelisting}
+@interface Person (Additions)
+
+@property (strong, nonatomic) UIColor * eyeColor;
+
+@end
+\end{codelisting}
+
+Consequently the original property is never used except by the shadowing property's accessors.
+
+\begin{codelisting}
+@implementation Person (Additions)
+
+- (void)setEyeColor:(UIColor *)eyeColor
+{
+    self.eyeColorTransformableType = eyeColor;
+}
+
+- (UIColor *)eyeColor
+{
+    return self.eyeColorTransformableType;
+}
+
+@end
+\end{codelisting}
+
+
+\subsection{Objective-C types are stored through NSValue}
+
+NSValue is a simple container for Objective-C data types. It conforms to NSCoding protocol, thus can be stored by Core Data as transformable attribute. 
+
+\begin{codelisting}
+@interface Figure : NSManagedObject
+
+@property (nonatomic, retain) id imaginaryPositionTransformableType;	          // stores NSValue object
+
+@end
+\end{codelisting}
+
+A category provides a shadowing property for the convenience.
+
+\begin{codelisting}
+struct ComplexNumber {
+    float real;
+    float imaginary;
+};
+typedef struct ComplexNumber ComplexNumber;
+
+extern ComplexNumber const ComplexNumberZero;
+
+
+@interface Figure (Additions)
+
+@property (assign, nonatomic) ComplexNumber imaginaryPosition;
+
+@end
+\end{codelisting}
+
+The getter initializes the local variable so it does not return uninitialized value, if the shadowed property is nil.
+
+\begin{codelisting}
+@implementation Figure (Additions)
+
+- (ComplexNumber)imaginaryPosition
+{
+    ComplexNumber imaginaryPosition = ComplexNumberZero;
+    [self.imaginaryPositionTransformableType getValue:&imaginaryPosition];
+    return imaginaryPosition;
+}
+\end{codelisting}
+
+The setter creates NSValue object with the passed value, and stores it.
+
+\begin{codelisting}
+- (void)setImaginaryPosition:(ComplexNumber)imaginaryPosition
+{
+    NSValue * value = [NSValue valueWithBytes:&imaginaryPosition objCType:@encode(ComplexNumber)];
+    self.imaginaryPositionTransformableType = value;
+}
+
+@end
+\end{codelisting}
+
+
 \end{document}