I am learning objective-C and I know we can use extension to add some private members to an interface.
But the book said there should be nothing in the ()
I read the following code snippet
#interface Tree(Private)
- (void)blah:(int)num;
#end
I am wondering why there is Private inside ()
You can put any name in the class category declaration, usually indicating the purpose of that category.
So in your case author of the code wanted to tell that those methods are for internal use of the class itself and are not intented to be called from other classes
The declaration in your example is precisely called a category (not an extension).
You can add as many categories as you like to any given Class (even if you don't have access to the source code). Categories allow you to add new methods to a class, but not new ivars nor properties. [1]
Each category has a name, which is the bit between parenthesis. There should not be two different categories for the same Class with the same name.
When the name is empty, this is called an extension. Extensions have some slight differences with categories: you can add ivars and properties to extensions and you can only use them for Classes for which you have access to the source code. [1]
Usually, extensions (like the example in your book) are declared at the top of the .m file, and are used for declaring methods, ivars and/or properties that are to be used only within that file (usually comprised of a single Class).
P.D.: If you really want to add new properties through categories as opposed to through extensions you can actually do so using associated objects [2][3].
http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html
Defining a property in iOS class extension
http://oleb.net/blog/2011/05/faking-ivars-in-objc-categories-with-associative-references/
In the case that is considered as a category, but since it's on the .m file, it will have the same effect. You can also see the tips from Xcode:
Being the mid one considered something like Tree(description) and the last one Tree ()
There are two closely related concepts here: class categories and class extensions. Class categories include a name inside the parenthesis and are most commonly used to add methods to existing classes or to organize methods into logical groups. Class extensions extend the internal implementation of the class (i.e. are used to define private properties, methods, etc).
More details can be found on Apple's dev site:
http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html
Related
I'am using a third-party framework that provides a class whose instances only have properties. In my app, I'd like to add an extra property to this instances. What the appropriate way to do this would it be for this scenario?
a) Extending the framework's class in my app
b) Creating a subclass of the framework's class and define the new property I need
Thanks in advance
It's
b)
because adding (stored) properties in a class extension is not supported.
There are two important rules for using extensions:
Extensions can add new functionality to a type, but they cannot override existing functionality
Extensions can add new computed properties, but they cannot add stored properties, or add property observers to existing properties
It depends on what is the behaviour you are expecting to achieve.
Extending: You can only add new methods and computed vars, but you will achieve seamless effort in your code. the new functionality is available anywhere without adding new classes in your code
Subclassing: You can add new vars and override function but there is a bigger footprint in your code. You will need to use that specific subclass throughout your project.
I guess it is more of a design question.
My suggestion: if the entire project needs this new behaviour use extensions, otherwise subclass.
I have a class that I want to split across multiple files. I tried using categories, but can't figure out how to make it work.
My class is named UserManager and I want to create UserManager+Amazon and UserManager+Facebook.
The problem is that I do need to access private properties and/or methods in UserManager+Amazon that are implemented in UserManager+Facebook, and vice-versa.
How can I extract methods outside the main UserManager.m file, while maintaining the access to private stuff?
EDIT: #Avi has an excellent idea below, although I have not tested it.
I have also just discovered a solution: Properties for Class and Its Subclasses Only
It works with categories and subclasses. I have tested it with my code and it works. It uses a class extension on the BaseClass.h, complete with an implemented example immediately below the accepted answer.
OLD:
I've been struggling with this recently as well.
My current idea is to create a third category UserManager+Private that implements all of the private methods and handles the properties via associated objects (http://nshipster.com/associated-objects/).
It feels very unwieldy, but it might work for you. I would still be interested in a better solution if one exists.
I have just starting developing application in ios and i have read so many tutorial for that but still i have not satisfy from that tutorial.
i want to know theoretical why we need to use category and what are the benefits.
Referenced from
http://www.g8production.com/post/37787310116/categories-in-objective-c-how-to-extend-methods
and
Difference between Category and Class Extension?
Categories and extensions allow you to extend the functionality of existing classes without subclassing (inherit nothing) adding functionality to an existing class, even to one for which you do not have the source.
A category allow you to add (only) methods to a class by declaring them in an interface file (.h) and defining them in an implementation file (.m), like in a basic Objective-C class. Sadly a category can’t declare additional instance variable for a class.
Now this declared methods become part of the categorized-class!!!
There’s no limit to the number of categories that you can add to a categorized-class, but each category name must be different should declare and define a different set of methods.
Edit
Categories
-> In objective c, when you want to add some more functionality to a class without inheritance, you simply use category for it.
-> Category use to add new method not properties
.
Class Extension
-> In objective c, when you want to make behavior of some property private you use class extension.
->mainly for properties.
A category allows you to add methods to an existing class—even to one for which you do not have the source.
Categories are a powerful feature that allows you to extend the functionality of existing classes without subclassing
How to use category in obj video
Benefit of using categories.
if you use categories then no need to keep in mind which custom class you have created for that specific feature. simply by using categories you can add new capability to existing class & by creating object of same class u can access it.
In one of my apps I wanted to change the default font. So instead of constantly changing the font where ever there was text, I set up a category were it changed the default font to the one I wanted.
I'm trying to add a new file to my Xcode project using Xcode 6.1.1 and Xcode now has a "File type" option where you select between "Empty File, Category, Protocol, Extension"
Can someone explain the differences between these and what the default to select would be? My file is a subclass of NSObject.
Thanks
Category
Categories are used to help modularize and organize class definitions. They allow you to take a (complex) class definition and spread it over several organized classes. It is not the same as subclassing. Although categories do allow you to override methods, Objective-C has no way of determining which method definition should be used, so you should never use a category to override methods. Instead, create a subclass that overrides the method as per usual.
Categories can contain protected methods, which "allow arbitrary files to 'opt-in' to a portion of an API by simply importing the category." (Check out the articles linked below.)
Extension
Extensions provide similar functionality to categories, except that you must implement the extension's API in the main implementation file.
Extensions can also be used to create a formal private API. Ordinarily, if you wanted to create private methods, you would write them in the implementation block, but would exclude them from the interface block. However, if you have an extensive group of methods that you would like to remain private, this becomes cumbersome and difficult to read/maintain. Using extensions, you can define the private methods in both the interface and implementation blocks of the .m file. As long as you do not include it in the respective .h file, these methods will be treated as private methods.
Extensions can also be used to make previously declared properties that are read-only outside the class read-write within the class (using the "self." syntax).
Protocol
Protocols allow for abstracted horizontal relationships across various (sometimes unrelated) classes and class hierarchies. A protocol consists of an API that can be used by a variety of classes, regardless of whether or not they are related. This allows you to modify/add some class functionality through a potentially wide range of classes without having to subclass them and alter their own class hierarchies.
In order to use a protocol, a class only needs to:
1. Include the protocol's name inside angled brackets <> after the class/superclass name declaration
2. Implement the protocol's methods
Protocols can also be useful for type checking.
Empty File
An empty file is just that - an empty file. You give it a name, but it contains no class information whatsoever (no generated methods, blocks, comments, etc.).
Sources: RyPress article on Categories and Extensions and RyPress article on Protocols. Both articles have helpful examples of each tool.
I want to extend my generated NSManagedObject classes (data mapping, init, utility functions). I was doing this with categories, but doing two imports got old real fast. I saw that MagicalRecord subclassed the generated files (https://github.com/magicalpanda/MagicalRecord/tree/develop/Samples/iOS/Application/Models). I tried this approach, but ran into the following problem.
[foo.bar myExtensionMethod];
Where foo is of class _foo and bar is of class _bar. The method myExtensionMethod is in the subclass of _bar named bar. I get a syntax error along the lines of _bar does not have function myExtensionMethod.
question
Is there a way to extend generated NSManagedObject classes that doesn't require two imports or something like modgenerator? As I regenerate the files, manually editing the generated files isn't an option.
The classes with underscore (_Foo, _Bar) are overwritten by mogenerator with the
current properties of your Core Data entities, and you should not modify these.
Your extension methods should go into the classes without underscore (Foo, Bar),
and you need only include "Foo.h" or "Bar.h".