Objective-C Equivalent of Public Get/Protected Set Property in C#

c++objective-cproperties

Is there any way to create a property like this C# property in Objective-C?

public int prop { get; protected set;}

Essentially, I want to make it possible to get the value from outside the class, but to only be able to set the value from within the class.

Best Answer

The easiest way to accomplish this is to declare the property as readonly in the public interface for MyClass (i.e. the .h file):

@property (readonly) NSInteger prop;

Then, in .m file for that class, declare a class extension (a category with an empty name). In class extensions, you may redeclare a @property to change its writeability to readwrite:

@interface MyClass ()
@property (readwrite) NSInteger prop;
@end

@implementation MyClass
@synthesize prop;
...
@end

Of course, Objective-C does not enforce access restrictions, so there's nothing stopping some rogue code from calling -[MyClass setProp:]. The compiler will flag this as a warning, however, which is as good as you can get in Objective-C. Unfortunately, there is no standard way to document this "protected" writeable property to subclasses; you'll have to settle on a convention for your team and or put it in the public documentation for the class if it's in a framework that you're going to release.

Related Solutions

Objective-C – Property Access Guide

Nothing! Dot-notation is "syntactic sugar" introduced in Objective-C 2.0. In fact, the compiler converts foo.property to [foo property] during compile-time, so they compile to exactly the same thing.

It's simply a matter of which you prefer.

Objective-C Properties – How to Make a Private Property

You want to use a "class extension" rather than a category:

@interface MyClass ()
@property (nonatomic, retain) NSMutableArray *stuff;
@end

@implementation MyClass
@synthesize stuff; // ok

Class extensions were created in Objective-C 2.0 in part specifically for this purpose. The advantage of class extensions is that the compiler treats them as part of the original class definition and can thus warn about incomplete implementations.

Besides purely private properties, you can also create read-only public properties that are read-write internally. A property may be re-declared in a class extensions solely to change the access (readonly vs. readwrite) but must be identical in declaration otherwise. Thus you can do:

//MyClass.h
@interface MyClass : NSObject
{ }
@property (nonatomic,retain,redonly) NSArray *myProperty;
@end

//MyClass.m
@interface MyClass ()
@property (nonatomic, retain, readwrite) NSArray *myProperty;
@end

@implementation MyClass
@synthesize myProperty;
//...
@end

Best Answer

Related Solutions

Objective-C – Property Access Guide

Objective-C Properties – How to Make a Private Property

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