Protocol extension is one of the coolest features introduced in swift 2.0. What if we used protocols to take all of the various numerical types within Swift and expose them through a common interface: Number? 63. Swift: Is it possible to add a protocol extension to a protocol? A high school student who loves to make apps during spare time, and really enjoy teaching people how to create apps. If you don’t know well these two features, no worries, I’ll explain them, briefly, in the first two paragraphs—feel free to skip them if you know well the subjects. This is why the m computed property returns self—the expression 1.m is considered to calculate a Double value of 1.0. We’ll start by defining a protocol called Number: Our new protocol has one requirement: floatValue. It adds a fatalError to the function defined in Animal, effectively making Animal an abstract base class, or a class that isn’t instantiated directly. Extensions supports only init (). Our aim is to teach everyone how to build apps with high quality and easy-to-read tutorials. However, there’s one huge feature of protocols that we haven’t talked about yet. The example below adds a new mutating method called square to Swift’s Int type, which squares the original value: Extensions can add new subscripts to an existing type. Because of this, all of the Int.Kind case values can be written in shorthand form inside the switch statement, such as .negative rather than Int.Kind.negative. Protocols don’t have this contraint. to add functionality to them. It’s messy. Save you thousands of dollars. Though you may not have noticed it, this approach also enabled something called polymorphism. They enable new ways to share implementation details between different types, wether they are structs, enums or classes. Question or problem in the Swift programming language: With the addition of protocol extensions in Swift 2.0, it seems like protocols have basically become Java/C# abstract classes. The answer is called multiple inheritance. But, what if we could take a bunch of types that are logically related, but don’t share a common superclass, and expose them through a single interface? Now, how do we apply this protocol to existing types within Swift? This example also adds a new computed instance property to Int, called kind, which returns the appropriate Kind enumeration case for that integer. In Objective-C I could create a category that conformed to a protocol: SomeProtocol.h @protocol SomeProtocol ... @end UIView+CategoryName #import SomeProtocol.h @interface UIView (CategoryName) ... @end I am trying to achieve the same with […] Similarly, there are 3.28084 feet in a meter, and so the ft computed property divides the underlying Double value by 3.28084, to convert it from feet to meters. For example, have you ever divided an Int and a Float, or divided a Double and a UInt? "Initializing an instance of MyClass with, //prints "Initializing an instance of MyClass with hello, world! Let’s see: So this is a situation where subclassing isn’t ideal. Other units require some conversion to be expressed as a value measured in meters. Extensions add new functionality to an existing class, structure, enumeration, or protocol type. Why would we use protocols in a situation like this, as opposed to subclassing? In OOP, polymorphism occurred when classes would inherit functionality from each other, allowing them to behave as instances of their superclasses, along with instances of themselves. Extensions can add new initializers to existing types. Protocol extensions Question 1/12: This extension correctly implements a method from its protocol – true or false? 0. Extensions can add new functionality to a type, but they cannot override existing functionality. So, according to the creators of the Swift language, protocols are a good way to define a set of required functionality that other types can adopt. In this tutorial, we’re going to talk about what protocols are and how you can use them in conjunction with protocol oriented programming. This includes the ability to extend types for which you do not have access to the original source code (known as retroactive modeling ). Swift and the Swift logo are trademarks of Apple Inc. // new functionality to add to SomeType goes here, // implementation of protocol requirements goes here, Adding Protocol Conformance with an Extension, // Prints "Three feet is 0.914399970739201 meters", // Prints "A marathon is 42195.0 meters long", // centerRect's origin is (2.5, 2.5) and its size is (3.0, 3.0), Add computed instance properties and computed type properties, Make an existing type conform to a protocol. It doesn’t make sense if we subclass them from Animal. Since the inception of iOS, we’ve seen object oriented programming, functional reactive programming, protocol oriented programming, and countless other programming paradigms. The protocol name is provided after the type name, separated by a colon, and an implementation of all requirements of the protocol is provided within the extension’s curly braces. One of the most common design patterns in object oriented programming is called subclassing. This looks and feels a lot like subclassing. AppCoda is one of the leading iOS programming communities. Terms of Service | Privacy Policy | RSS Feed | Contact Us. If you use an extension to add an initializer to a structure that was declared in another module, the new initializer can’t access self until it calls an initializer from the defining module. We’ve learned a lot about protocols in this tutorial. Take a look: Thanks to our extensions, every single Double, Float, Int, and UInt within our app is now also a Number. The initializer then calls the structure’s automatic memberwise initializer init(origin:size:), which stores the new origin and size values in the appropriate properties: If you provide a new initializer with an extension, you are still responsible for making sure that each instance is fully initialized once the initializer completes. Protocols and protocol oriented programming are not a replacement for OOP; rather, they are a supplement. However, with protocols, extensions are even more powerful, because they allow you to provide default functionality for your protocols. When I think of protocols, I like to think that protocols provide information about what a type can do, not necessarily what it is. Any type that satisfies the requirements of a protocol is said to conform to that protocol. We’re defining a blueprint for our functionality, providing a default implementation of it, and then selectively overriding it. Similarities. By design Swift will always use static dispatch to call protocol extension’s implementation, no matter if actual implementation of a logger (LogMock in our case) implements verbose(_:String) or … Let’s go back to our animals again! Swift doesn’t allow default values in the protocol declaration, but just in its extension—like in our example–or in the classes which conform to the protocol: 9 1 Also, it is used to specify whether the property is 'gettable' or 'settable'. While many of these paradigms have been simple fads, a select few have stuck around. This post has just gotten into the mechanics of creating and using protocol extensions. Extensions are similar to categories in Objective-C. (Unlike Objective-C categories, Swift extensions do not have names.). This enables you to extend other types to accept your own custom types as initializer parameters, or to provide additional initialization options that were not included as part of the type’s original implementation. We’ll start by explaining what protocols are, highlighting the key differences between protocols and classes / structures. However, in practice you can use the "associated object" technique. Swift 2 add protocol conformance to protocols; ios - Swift access control on protocol conformance; Swift Generics, Constraints, and KeyPaths Have you ever tried combining them during arithmetic operations? We already know what Sound does, but we now know that anything that has the ability to fly() is Flyable. We defined a protocol, Sound, that specifies that something must have a makeSound() function. The following example adds a new instance method called repetitions to the Int type: The repetitions(task:) method takes a single argument of type () -> Void, which indicates a function that has no parameters and does not return a value. We often use them in our projects, and there are moments where we would like having a way to keep the reference of some objects inside these extensions. By combining protocols and protocol extensions, we get to use one of our favorite OOP features (inheritance) while getting all the added benefits of protocols. Pigeon makes sounds and it flies. However, in case of the Sound protocol, it doesn’t matter though. You know as a developer that str is a String. For each integer in the array, the function considers the kind computed property for that integer, and prints an appropriate description. It allows developers to create incredibly detailed and rich relationships between classes. protocol extension 最关键的一点就是能在 protocol extension 方法中获取 protocol 的属性,因为Swift编译器知道任何一个遵守 protocol 的自定义类型,一定会定义这个 protocol 约定的各种属性,既然这样我们就可以在 protocol extension 中添加默认的实现了。 While polymorphism isn’t specific to protocol oriented programming (it’s something we used with OOP all the time), it allows us to go back to the familiar object oriented programming principles we know and love, and use them within our protocol oriented environments. Here’s a list of the similarities I see between C# abstract classes and Swift protocol extensions: Both outline a set of requirements that either a subclass or a protocol adopter must implement. If it looks like a duck, swims like a duck, and quacks like a duck, then it’s a duck. Cool, right? We decided that the only requirement we had for an object to be able to make a sound is to adopt the protocol and implement the required method. The answer is the extension. Protocol Extension: cannot assign to get-only property. Property requirements are declared by 'var' keyword as property variables. Swift: The protocol and extension of the introductory knowledge. We can just extend them and add functionality to every single instance of these types, not just instances of our subclasses. ), it was nice to glance at the .h file and immediately see the external API of the class, while the internals were kept private in the .m file. What this means is that you can declare protocols with requirements that are automatically fulfilled. If Sound and Flyable were classes, Pigeon would only be able to inherit functionality from one of them, not both. Let’s see how this works in practice: Here Sound is a protocol, we can extend any type to conform to Sound, like we did in the above example. In this example, a Double value of 1.0 is considered to represent “one meter”. Simply plug your own RSS feeds and turn the Xcode template into a RSS reader or a Blog reader app. Copyright © 2020 Apple Inc. All rights reserved. MySubclass will automatically inherit all of MyClass‘ functionality, including properties, functions, initializers, etc, meaning that all of MyClass‘ members will automatically be available on MySubclass: Subclasses may also override their superclass’ functionality, meaning they may replace it with their own: As you can see, this is a very powerful design pattern, and rightly so. We know that protocols allow us to define blueprints of functionality that other types can conform to, so let’s see what happens if we creatively use that ability to abstract away type information. We can now treat them as such: Pretty cool, right? I think that the single, most-used debugging tool many developers use is the print statement. An extension can be used to extend an existing generic type, as described in Extending a Generic Type. This website makes no representation or warranty of any kind, either expressed or implied, as to the accuracy, completeness ownership or reliability of the article or any translations thereof. The interesting bit about this example lies in Pigeon. That means that as far as we’re concerned, anything that has a valid implementation of floatValue is a number. This includes the ability to extend types for which you do not have access to the original source code (known as retroactive modeling). Cool! So why is Swift considered a protocol oriented language when Objective C isn’t? but they all make a universal sound. number.kind is already known to be of type Int.Kind. You’ve already seen how protocols let us define contracts that conforming types must adopt. Basically, protocol extensions allow you to retain one of the best features of subclassing (inheritance) while gaining all of the best features of protocols. We have Double, Float, Int and its various widths (Int8, Int16, etc. Suppose this Airplane doesn’t make any sound, so Airplane only conforms to Flyable and inherits the default functionality that comes with it. An object can conform to as many protocols as you want, inheriting default functionality from all of them. Swift Extensions allow us to add new functionality to an existing class, structure, enumeration, or protocol. Inherit from protocols, your first question most likely involves understanding what exactly a protocol called number: our protocol! 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