What is the difference between a Category and a Class Extension. I believe both are used to add custom methods in existing classes. Can someone throw light on this? Examplification with code will be really appreciated.
A category is a way to add methods to existing classes. They usually reside in files called "Class+CategoryName.h", like "NSView+CustomAdditions.h" (and .m, of course).
A class extension is a category, except for 2 main differences:
The category has no name. It is declared like this:
#interface SomeClass ()
- (void) anAdditionalMethod;
#end
The implementation of the extension must be in the main #implementation block of the file.
It's quite common to see a class extension at the top of a .m file declaring more methods on the class, that are then implemented below in the main #implementation section of the class. This is a way to declare "pseudo-private" methods (pseudo-private in that they're not really private, just not externally exposed).
Category
=> In Objective C, when you want to add some more functionality to a class without inheritance, you simply use category for it.
=> it comes with its own .h and .m file
=> Category uses to add new method not properties.
Class Extension
-> In Objective C, when you want to make behaviour of some property private you use class extension.
-> it comes with **.h** file only.
-> mainly for properties.
Note: when we add a new file and select a option of objective c
category shows category and "category on" not "subclass of" so it
shows like
#interface className (categoryName)
#end
-You will get two file .h and .m with file name as (className+categoryName.h and className+categoryName.m)
and in extension case you will get
#interface className()
#end
-You will get only one file with name as className_extensionName.h
In category you don't own the class but in extension you are.
Category is a way to add methods to a class whether or not source code is available, meaning you can add category to foundation classes like NSString and also to your own custom classes.
Extension can only be added to the classes whose source code is available because compiler compiles the source code and extension at the same time.
We can add extra instance variables and properties in class extension but not in category.
Any variable and method inside the extension is not even accessible to inherited classes.
Category and extension both are basically made to handle large code base, but category is a way to extend class API in multiple source files while extension is a way to add required methods outside the main interface file.
Use category when you have to break your same class code into different source files according to different functionalities, and extension when you just need to add some required methods to existing class outside the main interface file.
Also, when you need to modify a publicly declared instance variable in a class, for example,
readonly to readwrite, you can re-declare it in extension.
Extension: To make methods private and to add properties of our own custom class, not of Apple class.
Category: To add more methods in existing class not the property, it can be used for both custom class and Apple class like NSString.
We can also have properties Using set associated property in category class.
#interface SomeClass (Private)
#property (nonatomic, assign) id newProperty;
#end
NSString * const kNewPropertyKey = #"kNewPropertyKey";
#implementation SomeClass (Private)
#dynamic newProperty;
- (void)setNewProperty:(id)aObject
{
objc_setAssociatedObject(self, kNewPropertyKey, aObject, OBJC_ASSOCIATION_ASSIGN);
}
- (id)newProperty
{
return objc_getAssociatedObject(self, kNewPropertyKey);
}
#end
Refer : http://inchoo.net/dev-talk/ios-development/how-to-add-a-property-via-class-category/
#interface SomeClass ()
- (void) anAdditionalMethod;
#end
I think it is not the way to declare Category.
Category must have a name
#interface SomeClass (XYZ)
- (void) anAdditionalMethod;
#end
for example
#interface NSMutableArray (NSMutableArrayCreation)
+ (id)arrayWithCapacity:(NSUInteger)numItems;
- (id)initWithCapacity:(NSUInteger)numItems;
#end
Declared for NSMutableArray by Apple
ios extension similiar to c#,java abstract class or interface
ios category similiar to c#,java class extension
Categories
Categories are used when you are creating file containing large number of methods.So they provide you with the facility to break a single class into different modules.Also if any changes are made to the categories the compiler does not waste time to compile the entire project.Categories are not able to add new variable or properties and look upto their parent class .You can override a method in a category but it isnt a good idea because the method cannot further be overridden.Also the flow can be effected because all categories have the same hierarchial level and hence two categories belonging to same parent class may exist at run time.Also protected methods can be created using categories
Extensions
Extensions enable you to override the property or add new property to the existing parent class.Syntatically same to categories they do not have name and are represented as #interface class()
No .m file is present and method declared in extension have to be implemented in #implementation of parent file
More help at this link
Here is my understanding :
Extensions are usually used to add extra features to our own "custom class". We can add private methods or properties extending the class interface which can be used within the implementation of the class.
Extensions are to be written within the same file as the class. Hence you cannot write extensions for pre defined types like String, Float, etc.
On the other hand Categories can be used to add extra methods to a pre existing classes. Example we can create our own methods by extending String class. Note that we cannot create extra properties in the categories. Also main advantage of categories is we can write the categories in any other file, outside the file where your class exits.
Also while creating categories you are supposed to give a name for it within the brackets.
But for extension no name is required. Hence some times they are also called anonymous categories.
Categories and Extensions
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. Using categories, you can also distribute the implementation of your own classes among several files. Class extensions are similar, but allow additional required APIs to be declared for a class in locations other than within the primary class #interface block.
Adding Methods to Classes
You can add methods to a class by declaring them in an interface file under a category name and defining them in an implementation file under the same name. The category name indicates that the methods are additions to a class declared elsewhere, not a new class. You cannot, however, use a category to add additional instance variables to a class.
The methods the category adds become part of the class type. For example, methods added to the NSArray class in a category are included as methods the compiler expects an NSArray instance to have in its repertoire. Methods added to the NSArray class in a subclass, however, are not included in the NSArray type. (This matters only for statically typed objects because static typing is the only way the compiler can know an object’s class.)
Category methods can do anything that methods defined in the class proper can do. At runtime, there’s no difference. The methods the category adds to the class are inherited by all the class’s subclasses, just like other methods.
The declaration of a category interface looks very much like a class interface declaration—except the category name is listed within parentheses after the class name and the superclass isn’t mentioned. Unless its methods don’t access any instance variables of the class, the category must import the interface file for the class it extends:
#import "ClassName.h"
#interface ClassName ( CategoryName )
// method declarations
#end
Note that a category can’t declare additional instance variables for the class; it includes only methods. However, all instance variables within the scope of the class are also within the scope of the category. That includes all instance variables declared by the class, even ones declared #private.
There’s no limit to the number of categories that you can add to a class, but each category name must be different, and each should declare and define a different set of methods.
Extensions
Class extensions are like anonymous categories, except that the methods they declare must be implemented in the main #implementation block for the corresponding class. Using the Clang/LLVM 2.0 compiler, you can also declare properties and instance variables in a class extension.
A common use for class extensions is to redeclare property that is publicly declared as read-only privately as readwrite:
#interface MyClass : NSObject
#property (retain, readonly) float value;
#end
// Private extension, typically hidden in the main implementation file.
#interface MyClass ()
#property (retain, readwrite) float value;
#end
// Notice that (in contrast to a category) no name is given in the parentheses in the second #interface block.
It is also generally common for a class to have a publicly declared API and to then have additional methods declared privately for use solely by the class or the framework within which the class resides. Class extensions allow you to declare additional required methods for a class in locations other than within the primary class #interface block, as illustrated in the following example:
#interface MyClass : NSObject
- (float)value;
#end
#interface MyClass () {
float value;
}
- (void)setValue:(float)newValue;
#end
#implementation MyClass
- (float)value {
return value;
}
- (void)setValue:(float)newValue {
value = newValue;
}
#end
The implementation of the setValue: method must appear within the main #implementation block for the class (you cannot implement it in a category). If this is not the case, the compiler emits a warning that it cannot find a method definition for setValue:.
For official documentation follow this link: source
Related
I'm following a tutorial on how to create a popover in Iphone here
what is (overrides) this code:
#interface UIPopoverController (overrides)
+ (BOOL)_popoversDisabled;
#end
#implementation UIPopoverController (overrides)
+ (BOOL)_popoversDisabled
{
return NO;
}
#end
This is an objective-C category. A category is a way of providing extra methods on a class, and they're useful in the following situations:
Extending a library class with domain-specific functionality. ie providing some extra features that will be useful for your application. This works whether or not you have the source-code for that class. For example, you could implement an [NSString asCreditCard] method or [UIColor applicationThemeColor].
Categories are also invaluable for grouping related functionality in a complex class.
Objective-C categories have the restriction that you cannot define additional ivars, and thus ivar-backed properties, on a category, although you can easily work around this using associative references - a run-time feature allowing you to link an object to a given class.
Associative References
To 'fake' a property or ivar on a category class use the following:
Define a 'key' to reference the associated property.
static char const* const carNamekey = "com.myObject.aKey";
The key doesn't necessarily have to have a value, since its the memory address of the object that is actually used.
Now, Implement the properties:
- (void) setCar:(Car*)car
{
objc_setAssociatedObject(self, &carNamekey, car, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (Car*) car
{
return objc_getAssociatedObject(self, &carNamekey);
}
If you wish you can add an #property declaration on the interface definition for the category, as an invitation to users of the class to use property-style (ie class.property = xxx) syntax.
Class Extensions
Another feature that is very similar to Objective-C categories is the class extension. A class extension is defined in the implementation block for a class like so:
#interface MyClass()
Some people refer to these as 'empty categories', but this is not quite correct. Unlike a category, a class extension will allow you to define additional properties and ivars on a class, without using associative references. This is useful for making a readonly property, internally writable, and things like that.
There - now you probably know more about categories than a lot of folks :)
Jasper Blues' answer explains categories nicely, so I'll just add that you should avoid using categories to override methods in the class to which you're adding the category. The reason is that the order in which categories are added to a class is undefined, so you can't know for certain whether the class will get your version of a method or one defined in another category. If you need to override methods in a class, create a subclass and put your overrides there.
However, people sometimes use categories to organize the methods in a class definition. If that's the case here, the method(s) in the overrides category are probably meant to override methods in the superclass, which is fine.
I am wondering why there is twice #interface. One in class.h and other in class.m. For example:
TestTableViewController.h:
#import <UIKit/UIKit.h>
#interface TestTableViewController : UITableViewController
#end
and (automatically generated) class.m i find:
#import "TestTableViewController.h"
#interface TestTableViewController ()
#end
#implementation TestTableViewController
... methods delegated from UITable delegates
#end
So my question is, what the #interface TestTableViewController () in the .m file is about. Why it is there? Do I need it?
Thanks in advance
The second #interface directive is in the implementation file (.m) -- you can infer from it that it's meant for declaring stuff that the creator of the class didn't want to expose to the user of the class. This usually means private and/or internal methods and properties. Also note that there are two types of doing this. The one (which you see here) is called a "class extension" and it's denoted by an empty pair of parentheses:
#interface MyClass ()
This one is particularily important because you can use this to add additional instance variables to your class.
The second one, called a "category", is indicated by a non-empty pair of parentheses, enclosing the name of the category, like this:
#interface MyClass (CategoryName)
and it's also used to extend the class. You can't add instance variables to a class using categories, but you can have multiple categories for the same class, that's the reason why it's mainly used to extend system/framework classes for which you don't have the source code -- so a category, in this sense, is the exact opposite of the class extension.
The second "interface" defines an extension for the "TestTableViewController" class, which is not visible to someone who only imports the h file. This is the de-facto way for creating private methods in objective C.
In there you can declare private methods and properties that you only want to use in your class, but not expose to other classes.
The interface in the TestTableViewController.h file is the declaration of a class extension. There are 2 round brackets that show this. The syntax is the same as for writing a category for a class. But in this case it's used to declare some sort of private methods the author does not want to expose in the header file
A normal category interface looks like this:
#interface TestTableViewController (Your_Category_Name)
- (void)doSomething;
#end
And the corresponding implementation:
#implementation TestTableViewController (Your_Category_Name)
-(void)doSomething {
// Does something...
}
#end
In your example there is no category name specified, so it just extends the class and you can implement the method in the normal implementation.
Normally this technique is used to "hide" methods. They are not declared in the header file and are not visible if you only import the .h file.
in my noob mind and proceedings, when i create a category for a class, I create it in a new file
like NSArray+Shuffle.h and .m
but I have noticed that when I create for example a ViewController,
i have in the implementation:
#interface PingusViewController ()
#end
#implementation PingusViewController
...
#end
So my question is:
what is the
#interface PingusViewController ()
#end
part?, is this for categories? or what use?, and if it is for categories, why use it here and not in some new files?
thanks!
It's called a Class Extension.
Consider it the conventional location for your class' private declarations.
It's normally declared in the implementation file rather than a header file because the declarations are considered private, and are intended to be visible to the class' #implementation only.
Categories OTOH, are interfaces which the author typically uses to extend the class' public interface. The declaration is similar -- after all, the Class Extension is just an unnamed category, but the applications are distinct (by convention).
Class extensions #interface Class () are a lot more powerful and can inject variables into the class. Categories #interface Class (Category) can't.
What other differences are there, and when should one use a category over a class extension?
The main difference is that with an extension, the compiler will expect you to implement the methods within your main #implementation, whereas with a category you have a separate #implementation block. So you should pretty much only use an extension at the top of your main .m file (the only place you should care about ivars, incidentally) -- it's meant to be just that, an extension.
A class extension bears some similarity to a category, but it can only be added to a class for which you have the source code at compile time (the class is compiled at the same time as the class extension). The methods declared by a class extension are implemented in the #implementation block for the original class so you can’t, for example, declare a class extension on a framework class, such as a Cocoa or Cocoa Touch class like NSString.
The syntax to declare a class extension is similar to the syntax for a category, and looks like this:
#interface ClassName ()
#end
Because no name is given in the parentheses, class extensions are often referred to as anonymous categories.
Unlike regular categories, a class extension can add its own properties and instance variables to a class. If you declare a property in a class extension, like this:
#interface XYZAnimal () {
id _someCustomInstanceVariable;
}
...
#end
IMHO, it's best to think of class extensions as private interface to a class. The primary interface (in your .h file) acts as the public interface which defines the class's behavioural contract with other classes.
Use class extensions to Hide Private Information
Class extensions are often used to extend the public interface with additional private methods or properties for use within the implementation of the class itself. It’s common, for example, to define a property as readonly in the interface, but as readwrite in a class extension declared above the implementation, in order that the internal methods of the class can change the property value directly.
As an example, the XYZPerson class might add a property called uniqueIdentifier, designed to keep track of information like a Social Security Number in the US.
It usually requires a large amount of paperwork to have a unique identifier assigned to an individual in the real world, so the XYZPerson class interface might declare this property as readonly, and provide some method that requests an identifier be assigned, like this:
#interface XYZPerson : NSObject
...
#property (readonly) NSString *uniqueIdentifier;
- (void)assignUniqueIdentifier;
#end
In order for the XYZPerson class to be able to change the property internally, it makes sense to redeclare the property in a class extension that’s defined at the top of the implementation file for the class:
#property (readwrite) NSString *uniqueIdentifier;
Note: The readwrite attribute is optional, because it’s the default. You may like to use it when redeclaring a property, for clarity.
Categories are an Objective-C language feature that let you add new methods to an existing class. Extensions are a special case of categories that let you define methods that must be implemented in the main implementation block.
Private declarations can be in class extensions, which mainly are some properties, because we have no need to declare a method before we call it.
ios extension similiar to c#,java abstract class or interface
ios category similiar to c# class extension
Style-wise (and functionally, if there is any difference), for declaring private methods, which of these is better?
#interface MyClass()
#interface MyClass(private)
The two syntaxes serve different purposes.
A named category -- #interface Foo(FooCategory) -- is generally used to:
(1) extend an existing class by adding functionality. Example: NSAttributedString in Foundation is extended by a category in AppKit that adds AppKit specific RTF-like text formatting API.
(2) declare a set of methods that might or might not be implemented by a delegate. Example: Various classes declare -- but don't implement -- #interface NSObject(SetODelegateMethods).
Form (2) has fallen out of favor now that #protocol has been extended to support #optional methods in Objective-C 2.0.
A class extension -- #interface Foo() -- is designed to allow you to declare additional private API -- SPI or System Programming Interface -- that is used to implement the class innards. This typically appears at the top of the .m file. Any methods / properties declared in the class extension must be implemented in the #implementation, just like the methods/properties found in the public #interface.
Class extensions can also be used to redeclare a publicly readonly #property as readwrite prior to #synthesize'ing the accessors.
Example:
Foo.h
#interface Foo:NSObject
#property(readonly, copy) NSString *bar;
-(void) publicSaucing;
#end
Foo.m
#interface Foo()
#property(readwrite, copy) NSString *bar;
- (void) superSecretInternalSaucing;
#end
#implementation Foo
#synthesize bar;
.... must implement the two methods or compiler will warn ....
#end
Yes,
there are the following differences.
1) Using anonymous categories requires implementing its methods in the main #implementation block for the corresponding class; anonymous categories allow you to declare additional required API for a class in locations other than within the primary class #interface block
2) When using MyClass(private), the following must be taken into account: object/category named pairs must be unique. If you declare a private category on your own class, then there are no problems. However, things are different on existing classes. For instance, only one NSString (Private) category can exist in a given Objective-C namespace. This can lead to problems because the Objective-C namespace is shared between the program code and all the libraries,frameworks,and plug-ins.This is especially important for Objective-C programmers writing screensavers,preference panes, and other plug-ins because their code will be injected into application or framework code that they do not control.