If I have a method to attach an animation to an UI Element, instead of copy/pasting the code for this method into many viewcontrollers, can I just create one class to hold the method and then import it?
I assume it doesn't make sense to use a category in this instance because I need this method in multiple viewcontrollers, and a category is an extension of a single viewcontroller.
A category is an extension of a class, not of a specific instance of a class. And, any modification that a category makes to a class is available to all subclasses (as with other methods on classes in OOP).
So, if you add a category on NSObject, basically all classes in your entire app have access to the methods in that category - hence you need to be very careful what methods you add there.
Whether you add a category or a helper class is personal preference in a lot of cases, both will work.
Edited 9 April 2015
You have two basic choices:
Create a category, or create a custom subclass.
Categories allow you to add methods to an entire class, even classes for which you don't have the source code. Categories are especially useful for adding new methods to system classes where the system creates the object inside frameworks, where you can't change things to create a custom subclass instead.
You can add a category to a base class like UIView, and the methods you add become available to that class and all it's subclasses. So you could add an animation category to UIView and then all UIView objects including image views, buttons, sliders, etc, would gain the methods defined in that category.
Categories help you get around the fact that there is no multiple inheritance in Objective-C. (In the example above of adding animation behavior to UIViews, you can't create a subclass of UIView AnimationView, and then create a UITextView that inherits from both UITextView and AnimationView and also create a UIImageView that inherits from AnimationView.)
There are a couple of significant limitations to categories:
They can't really override the code of already-existing methods. (They can, but you can't call the super implementation, and if there are multiple categories with implementations of the same method, the results are undefined, so you should not do this.)
Categories can't add new instance variables to the classes they extend. (There are ways to simulate this using a technique called associative storage, but that's beyond the scope of this post
A custom subclass can override existing methods. It can also add new methods, properties, and instance variables. However, it can't add methods to an existing subclass like a category can.
Why use categories by example :
Lets say you have a base Class Engine that is inherited by Classes like Car, Bike and Scooter. Company was start-up and willing to add a functionality of maintenance that wasn't provided earlier.
Engine of Car, Bike and Scooter will be maintenaned under same policies. So it is better to add more functionalities on into Engine class instead of each Vehicle. In doing so we are not supposed to alter the Engine class for it may not be available to alter. Just create a category
Engine+Maintenance
and all the functionalities you put in maintenance will be available in all the subclasses. A better approach.
Categories are a way to split a single class definition into multiple files. Their goal is to ease the burden of maintaining large code bases by modularizing a class. This prevents your source code from becoming monolithic 10000+ line files that are impossible to navigate and makes it easy to assign specific, well-defined portions of a class to individual developers
Categories are an alternate of sub-classing. When we add a method to an existing class, it becomes a part of that class methods. So, any instance of that class can access that method. However, it doesn't mean that when you add a method to some existing class like NSString or NSArray, the class is going to add your method to its definition universally. It only means that inside that project, your NSString or NSArray class will have another method defined by you, and any class(inside that particular project) that is sub-classing it, or creating an instance of that class will have that method as a part of that class' definition.
So, you can add your animation to your desired UI Element by adding a method through category to that UI Element's UIView class. Let's say your UI Element is a button. Now, if you create an category for the UIButton then, whenever you create an instance of UIButton, you can automatically have a method to animate it.
Subclassing is one way to add functionality to an object, but avoiding unnecessary subclassing by using a category will help reduce the amount of code and keep your projects more organized.
categories are a subclass of specific class.for example if you want to change text color, background and font of label without programming code,then you have to make a catogories of UILabel class.
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