Very recently all the new classes I create in Xcode are prefaced by this extra interface in the .m file:
#interface NewViewController ()
#end
What does Apple expect me to put here and how is it different from the #interface declaration in my .h file?
That is a class extension. In a nutshell, you declare your class's public interface in the header and use a class extension to declare any private interface you might want to provide (e.g. properties for which you want accessors but which you don't want to make public).
Related
I'm still fairly new to Objective-C but I'd love to learn more about how it should be done.
I'm building a simple cheat sheet that I'd like to print and put on my office wall as a reminder.
Here's what I have so far:
// Headers (.h)
// Shows what's available to other classes
#interface ExampleViewController : UIViewController
// Declare public methods, ivars &
// properties that are synthesized.
#end
// Implementation (.m)
// Defines the content of the class
#interface ExampleViewController ()
// Class extension allowing to declare
// private methods, ivars & properties that are synthesized.
#end
#implementation ExampleViewController
// Private Properties
// Method definitions
#end
One thing I don't understand is why have both #interface and #implementation inside the implementation .m file?
I get that we can declare private stuff but why not simply throw them in #implementation like:
#implementation ExampleViewController
UIView *view; // private property
- (void)...more code
#end
#1 - Why should I ever use #interface from within my implementation .m file?
#2 - For header .h, why should I ever use #class more than #import?
#import actually gets the whole definition and #class tells the compiler that the symbol is a class. So I just don't see why I should ever use #class?
#3 - Otherwise, is there anything I should be adding somewhere in my .h or .m cheat sheet?
That's not a problem-related question but a more wiki-esque question so we everybody can look it up and completely and quickly understand those concepts as they are very hard to grasp for any newcomer.
Why should I ever use #interface from within my implementation .m file?
Because it's better to clearly separate public and private parts of the class.
For header .h, why should I ever use #class more than #import?
When forward-declaring classes for use in protocols. Like this:
#class Foo;
#protocol FooDelegate
// this wouldn't compile without a forward declaration of `Foo'
- (void)fooDidFinishAction:(Foo *)f;
#end
Otherwise, is there anything I should be adding somewhere in my .h or .m cheat sheet?
That's way too general to be answered in one post.
1 - Why should I ever use #interface from within my implementation .m file?
When you do not intend to expose that interface to any other component. That's certainly the case for private class extensions but may also apply for something like a test which doesn't need a .h file at all because while it does define a class it does not need to expose an interface for any other component to use.
2 - For header .h, why should I ever use #class more than #import?
Invert your question; why should I ever use #import rather than #class?
#class informs the compiler that a class definition of that name will exist to be linked but says nothing about it's interface.
#import makes the class' interface available to you.
A forward declaration requires less work and can allow for faster builds. It is also not always possible to #import a class at all times (as in circular references like #H2CO3's protocol example). If all you need to know is that a class exists then just use the forward declaration. When you actually need to interact with its specific interface (usually in your class' implementation) then you need to #import.
3 - Otherwise, is there anything I should be adding somewhere in my .h or .m cheat sheet?
Unless you intend to actually expose ivars as a public interface (almost certainly not the case) leave them out of your .h and expose only properties instead.
Keep your public interface as simple as possible. Try not to reveal implementation details. However keep it informative enough that users of the class can verify its behavior using that public interface. (I find test driving the design of the class using only it's public interface a good tool for striking this balance.)
Imports and forward declarations expose dependencies. Keep them to the minimum you actually need so that you can understand what the class in question actually depends on.
Delegate protocols and block types are a common part of a class' interface but not part of the #interface. Include them in the .h if they are needed by other classes (e.g. to register callbacks).
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
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