extending swift values to the server pdf pdf
Extending Swift Value(s) to the Server David Ungar and Robert Dickerson
Extending Swift Value(s) to the Server
by David Ungar and Robert Dickerson Copyright © 2017 IBM Corporation. All rights reserved. Printed in the United States of America. Published by O’Reilly Media, Inc., 1005 Gravenstein Highway North, Sebastopol, CA 95472. O’Reilly books may be purchased for educational, business, or sales promotional use. Online editions are also available for most titles ( ). For more information, contact our corporate/institutional sales department: 800-998-9938 or corporate@oreilly.com.
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January 2017: First Edition
Revision History for the First Edition
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While the publisher and the authors have used good faith efforts to ensure that the information and instructions contained in this work are accurate, the publisher and the authors disclaim all responsibility for errors or omissions, including without limitation responsibility for damages resulting from the use of or reliance on this work. Use of the information and instructions contained in this work is at your own risk. If any code samples or other technology this work contains or describes is subject to open source licenses or the intellectual property rights of others, it is your responsibility to ensure that your use thereof complies with such licenses and/or rights. 978-1-491-97196-3 [LSI]
Preface: Swift for the Rest of Your Application
Q: Why did the human put on his boxing gloves? A: He had to punch some cards.
Today’s applications do not run on a single platform. Rather, some parts run on resource-limited devices, and other parts run on a vast and mysterious cloud of servers. This separation has led to a schism in how we build these applications because different platforms have different requirements: the mobile portions must conserve battery power, while the server portions must handle a large number of requests simultaneously. Consequently, programmers use different languages for different parts of applications—for instance, JavaScript for the browser, and Java for the server.
However, constructing an application out of multiple languages is fraught with drawbacks: different teams in the same organization speak different languages—literally—and must master different developer ecosystems. Precious time must be spent translating concepts across language barriers and a few developers must know all of the languages in order to be effective. Test cases and data models must be replicated in different languages, introducing bugs and incurring future maintenance efforts. Because third-party libraries cannot be shared across groups, each team must learn different APIs to obtain merely the same functionality.
​Swift was introduced by Apple in 2014 and replaced Objective-C as the recommended language for all new applications running on Apple devices. Later, when Swift became open source in 2015, it spread to new platforms. Currently, Swift is available on x86, ARM (including Raspberry Pi), and zOS architectures, as well as Linux, macOS, tvOS, watchOS, and iOS operating systems. So, it is now possible to write a whole end-to-end mobile application—front-end, middle, back, and even toaster—all in Swift. That’s why we wrote this book; we wanted to help you, the developer, who is most likely writing in Java or JavaScript, to consider a switch to Swift. Why adopt Swift? The Swift language may well be better than what you are currently using.
You can develop and debug in a consistent environment. Integrated development environments (IDEs) offer a tremendous amount of functionality such as text editing, static analysis, code completion, debugging, profiling, and even source-control integration. Switching back and forth between say, Eclipse and Xcode is a bit like switching between French horn and electric guitar: neither easy nor productive.
You can reuse code. When each bit of functionality is expressed exactly once, there is less work, more understanding, and fewer bugs.
You can leverage Swift’s features—such as optional types, value types, and functional programming facilities—to detect many bugs at compile time that would otherwise be hard to find.
Since Swift uses the for producing native-code binaries, your applications have the potential for competitive performance in terms of speed, startup time, and memory usage. However, examination of performance is outside the scope of this book. You will find an active and approachable community of Swift developers who are creating web posts, books, and videos. In 2016, Swift was cited as the second “Most Loved” language in a
, and the third most upward trending technology.
This book will introduce you to the Swift language, illustrate some of its most interesting features, and show you how to create and deploy a simple web service. Let’s get started!
CODING STYLE & IMPLEMENTATIONS
In the examples, the space constraints of this medium have led us to indent, break lines, and place brackets differently than
we would in actual code. In addition, space has precluded the inclusion of full implementations and blocks of code in this edition contain inconsistencies in color and font. If the inconsistencies confuse you, please consult the repositories inTable P-1. Where to find code examples Repository name Referenced in Created in
Acknowledgments This book would not have been possible without the support, encouragement, and guidance of the IBM Cloud and Swift@IBM leadership team, including Pat Bohrer, Eli Cleary, Jason Gartner, Sandeep Gopisetty, Heiko Ludwig, Giovanni Pacifici, John Ponzo, and Karl Weinmeister. In
addition, we want to extend our thanks to the many IBM Swift engineers and Swift community members working to bring Swift to the server—including Chris Bailey, Hubertus Franke, David Grove, David Jones, and Shmuel Kallner—for sharing their collective technical insights and creating the tools and libraries described herein. The Swift community’s embrace of Swift on the server reassured us that our contribution would be valued. The growing number of their instructive blog posts, videos, conference talks, and books have been of great help. We would like to thank our team had the daunting task of editing our lengthy technical drafts and producing this book. We owe a huge debt of gratitude to the Apple Core Swift Team for their courage, intelligence, talent, wisdom, and generosity for bringing a new language and ecosystem into existence and moving it to open source. Language design involves many difficult and complex tradeoffs, and bringing a new language to the world requires a tremendous amount of work. The rapid acceptance of Swift by developers is powerful testimony to the quality of the language.
Words fall short in plumbing the depths of our gratitude for the support and love of our sweethearts, Barbara Hurd and Valerie Magolan.
Chapter 1. A Swift Introduction
Swift supports several different programming paradigms. This chapter provides a brief overview of the parts of the Swift language that will be familiar to a Java or JavaScript programmer. Swift is not a small language, and this chapter omits many of its conveniences, including argument labels, shorthand syntax for closures, string interpolation, array and dictionary literals, ranges, and scoping attributes. Swift’s breadth lets you try Swift without changing your programming style while you master its basics. Later, when ready, you can exploit the additional paradigms it offers.
A beginning Swift developer may initially be overwhelmed by the cornucopia of features in the Swift language, since it gives you many ways to solve the same problem. But taking the time to choose the right approach can often catch bugs, shorten, and clarify your code. For instance, value types help prevent unintended mutation of values. Paradigms borrowed from functional programming such as generics, closures, and protocols provide ways to factor out not only common code, but also variations on common themes. As a result, the underlying themes can be written once, used in varying contexts, and still be statically checked. Your programs will be much easier to maintain and debug, especially as they grow larger.
Types and Type Inference
Swift combines strong and static typing with powerful type inference to keep code relatively concise. Swift’s type system and compile-time guarantees help improve the reliability of nontrivial code.
DECIPHERING TYPE ERRORS IN LONG STATEMENTS
If your program won’t compile, you can often clarify a type error by breaking up an assignment statement into smaller ones
with explicit type declarations for each intermediate result.Syntax
Swift’s syntax borrows enough from other languages to be easily readable. Here’s a trivial example:
let aHost = "someMachine.com" aHost = "anotherMachine.com" // ILLEGAL: can't change a constant that changes a constant after it has been initialized. (Throughout this book, ILLEGAL means “will not compile.”) This constant is initialized at its declaration, but Swift requires only that a constant be initialized before being used.
var aPath = "something" aPath = "myDatabase" // OK
aPath is also a String, but is a mutable variable. Swift functions use keywords to prevent mixing up arguments at a call site. For example, here is a function:
func combine ( host : String , withPath path : String ) -> String {
- return host "/" path
}
and here is a call to it:
// returns "someMachine.com/myDatabase" combine ( host : aHost , withPath : aPath ) Swift’s syntax combines ease of learning, convenience of use, and prevention of mistakes.
Simple Enumerations
In Swift, as in other languages, an enumeration represents some fixed, closed set of alternatives that might be assigned to some variable. Unlike enumerations in other languages, Swift’s come in three flavors, each suited for a particular use. The flavor of an enumeration depends on how much information its values are specified to include.
An enumeration may be specified by 1) only a set of cases, 2) a set of cases, each with a fixed value, or 3) a set of cases, each with a set of assignable values. (The last flavor is covered in
Chapter 2 .) The simplest flavor merely associates a unique identifier with each case. For example:
enum Validity case { valid , invalid }
The second flavor of enumeration provides for each case to be associated with a value that is always the same for that case. Such a value must be expressed as a literal value, such as 17 or "abc". For example:
enum StatusCode : Int { case ok = 200 case created = 201
// more cases go here … case badRequest = 400 case unauthorized = 401
} The value of this enumeration can be accessed via the rawValue attribute:
func printRealValue ( of e : StatusCode ) { print ( "real value is" , e . rawValue )
} Tuples
As in some other languages, a Swift tuple simply groups multiple values together. For example, here’s a function that returns both a name and serial number:
func lookup ( user : String ) -> ( String , Int ) { // compute n and sn return
( n , sn ) }
Tuple members can be accessed by index:
let userInfo = lookup ( user : "Washington" ) print ( "name:" , userInfo . , "serialNumber:" , userInfo . 1 )
or can be unpacked simultaneously:
let ( name , serialNumber ) = lookup ( user : "Adams" ) print ( "name:" , name , "serialNumber:" , serialNumber )
Members can be named in the tuple type declaration:
func lookup ( user : String )
- > ( name : String , serialNumber : Int ) {
// compute n and sn return ( n , sn )
}
and then accessed by name:
let userInfo = lookup ( user : "Washington" ) print ( "name:" , userInfo . name ,
"serialNumber:" , userInfo . serialNumber )
When an identifier is declared with let, every element of the tuple behaves as if it were declared with a let:
let second = lookup ( user : "Adams" ) second . name = "Gomez Adams" // ILLEGAL: u is a let var anotherSecond = lookup ( user : "Adams" )
Chapter 2 .) The opposite of a value type is a reference type. The only reference types are instances-of-classes and closures. Consequently, these are the only types that allow shared access to mutable state. Tuples combine nicely with other language features: the standard built-in method for iterating through
switch ( peg , hole ) {
As in some other statically-typed languages, Swift allows you to define your own operators based on static types.
Custom Operators
Tuples are a convenient way to aggregate a few constant fields that have no meaningful type apart from the types of the fields.
} }
"fits four ways" default : return "does not fit"
// switches on a tuple case (. roundPeg , . roundHole ): return "fits any orientation" case (. squarePeg , . squareHole ): return
enum PegShape { case roundPeg , squarePeg } enum HoleShape { case roundHole , squareHole , triangularHole } func howDoes ( _ peg : PegShape , fitInto hole : HoleShape )
anotherSecond . name = "Gomez Adams" // Legal: x is a var print ( anotherSecond . name ) // prints Gomez Adams
a dictionary uses key-value tuples. Also, Swift’s switch statements become very concise and descriptive by switching on a tuple:
first and anotherFirst are decoupled; changes to one do not affect the other. This isolation enables you to reason about your program one small chunk at a time. Swift has other constructs with this tidy property; they are all lumped into the category of value types. (See
// returns "George Washington" because // anotherFirst is an unchanged copy
// returns "George Washington" anotherFirst . name // returns "George Washington" as expected first . name = "George Jefferson" first . name // was changed, so returns "George Jefferson" anotherFirst . name
var first = lookup ( user : "Washington" ) var anotherFirst = first first . name
When you assign a tuple to a new variable, it gets a fresh copy:
- > String {
// returns 3 9.0 |> sqrt
This lets us read code from left to right, in the order of execution. For example:
send ( compress ( getImage ()))
can be rewritten as:
getImage () |> compress |> send
To define this custom operator, you first tell Swift about the syntax:
LeftFunctionalApply precedencegroup { associativity : left higherThan : AssignmentPrecedence lowerThan : TernaryPrecedence
} infix operator |> : LeftFunctionalApply
then provide a (generic) implementation:
func throws |> < In , Out > ( lhs : In , rhs : ( In ) -> Out ) rethrows -> Out { return try rhs ( lhs )
} Judiciously used, custom operators can help make code clear and concise.
Closures
Swift includes closures: anonymous functions defined inline, which can read and write to their
enclosing lexical scope closure is a first-class entity, a reference type which can be assigned to constants and variables, passed as arguments, and returned as results. Functions and methods are just special cases of closures with a slightly different syntax for their definition. Closures support functional programming, which can be particularly helpful in dealing with asynchrony ( .
DECIPHERING TYPES ERRORS IN CLOSURES
Most of the time, the Swift compiler can infer the types of closure arguments and results. When it cannot, or when the code
includes a type error, the error message from the compiler can be obscure. You can often clarify type errors by addingtypes to a closure that are not strictly necessary. In the example below, if the compiler were to complain about a type, you
could add the (previously implicit) types:func makeChannel ()
- > ( send : ( String ) -> Void , receive : () -> String ) {
var message : String = "" return ( send : { ( s : String ) -> Void in message = s }, receive : { (
_ : Void ) -> String in return message }
) } Object Orientation
Swift includes full support for class-based object-oriented programming, including classes, instances, constructors (called “initializers”), instance variables, static variables, computed virtual getters and setters, inheritance, and final attributes. When one method overrides another, it must be annotated in the code. This requirement prevents some errors. Experienced Swift programmers tend to reserve objects for things requiring shared access to a mutable state. (See
Chapter 2 .) Protocols Define Interfaces As with other typed languages, Swift includes a notion that the expected behavior of an entity is
separate from any particular embodiment of that entity. The former is called a protocol and the latter a concrete type—think interface versus class if you’re a Java programmer. A Swift protocol lets you define what is expected of a type without specifying any implementation. For example, the operations expected of any HTTP_Request might be that it can supply a URL and a requestString:
protocol HTTP_Request_Protocol { var url : URL { get } var requestString : String { get }
}
In other languages, Abstract_HTTP_Request might need an abstract requestString. But in Swift, the protocol serves that purpose:
class Abstract_HTTP_Request { let url : URL // A constant instance variable init ( url : URL ) { self . url = url }
} class Get_HTTP_Request :
Abstract_HTTP_Request , HTTP_Request_Protocol { var requestString : String { return "GET" }
} class Post_HTTP_Request :
Abstract_HTTP_Request , HTTP_Request_Protocol
var requestString : String { return
} struct FavoriteFoodResponse { let city : String let answer : String
: ResponseProtocol { let city : String let answer : Int
} struct TemperatureResponse
} var answer : Answer { get }
protocol ResponseProtocol { associatedtype Answer var city : String { get
Even though each answer is a different type, they can share the same description by adopting a common protocol:
}
struct TemperatureResponse { let city : String let answer : Int
"POST" } var data : String init ( url : URL , data : String ) { self . data = data super
Generic entities allow the same basic code to apply to different types. In addition to concrete types, Swift also allows protocols to be generalized to different types, although the mechanism differs. For example, suppose you have two responses, a TemperatureResponse and a FavoriteFoodResponse:
Generic Protocols
In the following, you’ll see an example of a generic protocol, which could not be used as a type. As in other languages, protocols help to prevent errors as well as remove dependencies on the representation.
let aRequest : HTTP_Request_Protocol = Get_HTTP_Request ( url : … /* some URL */ ) aRequest . requestString // returns "GET"
When declaring a variable to hold a request, instead of using the type Abstract_HTTP_Request, use the type HTTP_Request_Protocol:
( url : url ) } }
. init
} struct FavoriteFoodResponse : ResponseProtocol {
let answer : String }
The associatedtype in the protocol works a bit like a type parameter, except that instead of being passed in, it is inferred from context. Any protocol with an associated type is considered to be generic. Self, which denotes the concrete type conforming to the protocol, is also considered to be an associated type.
Unfortunately, generic protocols such as this one are more difficult to use than nongeneric ones. Specifically, they cannot be used in place of types, but only as generic constraints. So, you cannot write a declaration to hold a value that conforms to ResponseProtocol:
var someResponse : ResponseProtocol // ILLEGAL
But you can write a function that will work on any type that conforms to ResponseProtocol:
func handleResponse < SomeResponseType : ResponseProtocol > ( response : SomeResponseType ) { … }
Because a generic protocol cannot be used as a type, it is often helpful to split up a generic protocol into generic and nongeneric protocols. A full discussion of these generic protocols is beyond the scope of this book. Generic protocols support generic functions, structures, and objects by providing a way to express requirements and implementations that apply to entities that themselves generalize over the types of their arguments, results, or constituents.
Extending Classes, Structures, and Enumerations
Like many other languages, Swift allows you to add new behavior to a preexisting construct. In Swift, this capability is called an extension, and can be used with classes, structures, enumerations, and protocols. The last case is a bit different because protocol extensions supply default behavior, just as method bodies in Java interfaces do.
As you might expect, Swift’s extension facility is especially useful for large programs because the entity you want to extend is likely to have been defined in a separate library. You might not even have source code for it! Less obviously, Swift’s extensions help in two other situations: 1. An extension can add a bit of specialized functionality that is only visible within a single file.
Suppose that in some computation you find yourself squaring a number often, such as (a/b) * (a/b)
- (c/d) * (c/d). You could add the following to the file containing that code:
private extension Int {
- var squared : Int { return self self }
}
Now you can rewrite the above as (a/b).squared + (c/d).squared. The extension adds a new member to Int without cluttering up its namespace everywhere.
2. You might have a set of classes where each performs the same set of functions. Extensions let you group the code by function as opposed to class. An extension need not be in the same file or even the same module as the original definition. For example, you might have classes for city and state that each perform a country lookup:
class City { let name : String init ( name : String ) { self . name = name } func lookupCountry () -> String { … }
} class State { let name : String init ( name : String ) { self . name = name } func lookupCountry () -> String { … }
}
Extensions let you group the lookup functions together:
extension City { func lookupCountry () -> String { … } } extension State { func lookupCountry () -> String { … } }
This lets you put functionality where it makes the most sense, whether in a type defined by a library, limited to the scope of a single file, or together with similar functionality for different types.
Unlike Smalltalk, Swift closures cannot return from the home method’s scope (a.k.a., nonlocal return), so they cannot be used to extend the built-in control structures.
Chapter 2. Optional Types, Structures, &
Enumerations
Programming is hard and debugging is harder, but maintaining and debugging large programs that run asynchronously and concurrently is hardest. It makes sense to place the burden of checking certain runtime properties of your program on the compiler rather than the developer. Swift’s optional types and structures let you tell the compiler more about your program now so that you spend less time debugging later. These features rely on a combination of compile-time checks and runtime techniques that, in most cases, do not reduce performance.
Optional Types Exterminate Nil-Value Bugs
Programs represent uninitialized or absent values with nil (a.k.a., null). If your code fails to handle a nil value anywhere one can occur, bad things can happen. So Swift incorporated the might-be-nil versus can-never-be-nil distinction into its static type system. These are called “just” and “maybe” in Haskell. For example, suppose you are trying to extract the “Content-Type” entry from the header fields of an HTTP request. You have the header fields represented as a dictionary with String keys and values:
let headerFields : [ String : String ] = …
Swift uses subscript notation to look up the value of a given key in a dictionary:
let contentType = headerFields [ "Content-Type" ]
and Swift will infer a type for contentType. But that type is not “String”! It is “String?” with a
question mark, because String represents a value that can never be nil, whereas String? represents a
value that be either a String or nil. The latter is called an optional type. The dictionary lookup returns an optional type because the dictionary might not contain a key for “Content-Type.” The type String? is not the same type as String and Swift won’t let you use the value of an optional type without an explicit check:
if contentType . hasPrefix ( "text" ) // ILLEGAL
There are many convenient ways to perform this check. For example, the if-let form checks if the value is nonnil. If so, it assigns the value to a new variable that is local to the then-part. If the value is nil, it executes the else-part, if any.
if let ct = headerFields [ "Content-Type" ] { contentType = ct
} else { contentType = "no contentType"
}
The ?? nil-coalescing operator allows you to substitute a value for nil. It only evaluates the expression on the right if the one on the left is nil. For example, you might use ?? to supply a default value for a missing dictionary entry:
let contentType = headerFields [ "Content-Type" ] ?? "none"
Swift’s treatment of nil values will significantly improve the quality of your programs over many other languages. Swift’s optionals add security without inconvenience.
SURPRISINGLY HELPFUL, A PERSONAL NOTE FROM DAVID
For decades, I had programmed in languages that treat null as a universal value of any type. When I adopted Swift, I was quite surprised by how much its treatment of nil improved the quality of my code. The unexpected benefit arose where a variable was not optional because it was guaranteed to never be nil. As a result, that variable was eliminated as a potential source of crashes.
Structures Isolate Mutation
A Swift structure (struct) is like an instance of a class: it groups values together, the values can be fixed or variable, and it includes methods that operate on those values. However, unlike an instance of a class, a structure is a value type. Assigning a structure to a variable creates a fresh copy, decoupling the assignee from the assigned value. Whenever a struct is assigned to a new variable, as for instance when passed as an argument to a function, the struct is passed by value; in other words, the new variable receives a copy of every value in the structure.
STRUCTURE MUTATION GUARANTEES
A structure provides two guarantees about its mutability:
1. When passed to another routine or placed in another variable, the original structure is insulated from any changes to the
2. When a structure is associated with an identifier via a let statement, the value of the identifier may not change. current temperature in a given city, and you also need to check and see how long the database took. You could use a class with an instance variable, startTime:
class TemperatureRequestClass { let city : String var startTime : Date ? = nil // an optional Date init ( city : String ) { self
. city = city } }
After creating a request object:
let request = TemperatureRequestClass ( city : "Paris" )
you might hand it off to be processed:
request . startTime = Date . now sendToDB ( request : request , callback : receivedResponse )
and later print the time difference:
func receivedResponse ( temperature : Int ) { let dbTime = Date . now . timeIntervalSince ( request . startTime !) print ( "It took" , dbTime ,
"seconds to discover that the temperature in" , request . city , "is" , temperature )
}
But if there’s a bug in that sendToDB routine, it could corrupt the startTime!
func sendToDB ( request : TemperatureRequestClass , callback : ( Int ) -> Void
) { …
// Do lots of slow work to prepare to connection request . startTime = Date . now //The BUG! …
// Send the request on the prepared connection }
Now your requestTime calculation would be wrong. (See .)
Figure 2-1. Because instances of classes are reference types, when request is passed to sendDB that function gets a reference
to the same object. Then when it incorrectly mutates startTime, the original object is corrupted.
Swift provides a better way—using a structure instead of a class:
struct TemperatureRequestStruct { let city : String var startTime : Date ? = nil init ( city : String ) { self . city = city
} }
Because your calling code alters the startTime, which is contained in a structure, it must put that structure in a var, not a let:
var request = TemperatureRequestStruct ( city : "Paris" )
Now, the Swift compiler catches the error before the program can even run!
func sendToDB ( request : TemperatureRequestStruct , callback : ( Int ) -> Void
) { … // Do lots of slow work to prepare to connection request . startTime = Date . now
// ILLEGAL: will not compile!! … // Send the request on the prepared connection
}
Why is this assignment an error? Function parameters are lets by default, and the let-ness of the request parameter “flows down” into the startTime field. But what if you see the compile error and try a quick fix to get your code through the compiler?
var mutableRequest = request mutableRequest . startTime = Date . now
Swift will still protect your code from the bug because even though sendToDB has changed the startTime, it would not change for the caller. The caller would not see the mutation because when the
Figure 2-2. Because structures are value types, the sendToDB function receives a whole new copy of the request. The first
attempt at mutation won’t even compile because parameters are lets. If the request is assigned to a mutableRequest var, the
mutation will compile but won’t corrupt the original.
Mutating Methods
Unlike any other value type, a structure can include methods that mutate the contents of var fields in the structure. As a helpful signal to the programmer, such methods must be annotated with mutating. This requirement highlights the places where side effects could occur:
struct TemperatureRequestStruct { … var startTime : Date ? = nil
mutating func clearStartTime () { startTime = nil
} }
Since a let prohibits mutation, a mutating function may be invoked only upon a var. Unlike mutating an instance variable of a class, a mutation to a structure only changes the copy referenced by one variable:
let request1 = TemperatureRequestStruct ( city : "Paris" ) var request2 = request1 // makes a copy because is a struct request1 . clearStartTime () // ILLEGAL: cannot mutate a let request2 . clearStartTime () // OK, but does not change request1
STRUCTURES CONTAINING REFERENCES TO OBJECTS
A structure may contain a reference to an instance of a class, and vice versa. If a structure contains such a reference,
some of the desirable properties of a value type may be lost. In particular, if a structure includes an attribute that computes
its result by querying a class instance, the result may be subject to unexpected changes. This problem is transitive: it can occur even if your value type contains a value type from a library and that type contains a reference to a class instance.Despite its potential for havoc, it is the above feature that enables the Swift standard library to provide one of the most valuable features of Swift, collection value-types: Swift’s Arrays, Dictionaries, and Sets are structures, and therefore
provide strong support for mutation isolation and debugging. Under the covers, they use class instances to optimize mutation
with the goal of providing both performance and robust semantics. For instance, if you write a function that passes an array
into a library, you can rest assured that the library cannot change your array. It behaves as if copied, but can often avoid the
actual expense of copying.Default Implementations with Protocol Extensions
Swift classes can inherit from each other, but if you want to factor out behavior that two structures share, instead of creating a super-class, you must use a protocol extension. This technique is sometimes called protocol-oriented programming. For example, suppose you write two similar structures, a temperature request and an ozone-level request:
struct TemperatureRequest { let city : String let state : String var startTime : Date ? = nil init
( city : String , state : String ) { self . city = city self . state = state
} var cityAndState : String { return city + ", " + state }
} struct OzoneRequest
{ let city : String
let state : String var startTime : Date ? = nil init
( city : String , state : String ) { self . city = city self . state = state
} var cityAndState + + : String { return city ", " state }
} Both structures have identical cityAndState properties that return a string combining the city and state.
You can factor out the common code with a protocol and a protocol extension. First, the Request protocol ensures that the properties used by the common code, in this case city and state, are implemented by the adopter of the protocol. Then the protocol extension adds the new cityAndState property to all objects that conform to the Request protocol.
protocol Request { var city : String { get } var state : String { get }
} extension Request { var cityAndState : String { return + city + ", " state } } struct TemperatureRequest : Request { let city : String let state : String var startTime : Date ? = nil init
( city : String , state : String ) { self . city = city self . state = state
} } struct OzoneRequest : Request { let city : String let state : String var startTime : Date ? = nil init ( city : String , state : String ) { self . city = city self . state = state
} }
A protocol extension cannot add new requirements to a protocol, nor can it add additional data fields to an object; all it can do is add implementations of new behavior to the types that adopt the protocol.
What if you need to express a multilevel hierarchy? Protocols can inherit from other protocols, but it’s not quite the same as subclassing—for example, there is no super.
Structures with protocols and protocol extensions provide one way to replace a class hierarchy and limit mutation. Swift includes yet another: . These can do an even better job in the right situations.
Structures are like instances of classes that are always copied when being passed around. Use structures wherever you don’t need separate references to the same mutable state. With structures, you don’t have to code the copy operation, you don’t have to clutter up the code with copying, and in the case of larger structures, the compiler can optimize away much of the expense of copying. Substituting a structure for an instance of a class changes the easiest thing to write from passing a reference to copying a value. Bye-bye bugs.
Enumerations with Associated Values
In Swift, there is usually more than one way to express a given concept. For instance, you can choose between a class and a structure. Moreover, Swift includes many features from the functional programming school. Judiciously exploited, these constructs can clarify, shorten, and generalize your code. Like color TV, air conditioning, or the smartphone, you may never have missed this next construct, yet may soon find you would not think of living without it. One of these is the enumeration with associated values.
As described in Swift enumeration represents a value taken from a fixed set—for example, one of a fixed set of HTTP requests:
enum HTTP_Request_Kind { case get , post // other request types omitted for brevity
}
But Swift enumerations can do so much more because any case can also include one or more
associated values. Such an enumeration is more like a discriminated union than a Java or C
enumeration. It can replace a small class hierarchy or a group of structures that implement a common protocol. For example, suppose you need to implement HTTP requests with an equality (==) operation. You could create a class hierarchy, with an abstract superclass containing common code and concrete subclasses for each kind of request. But since you don’t want any asynchronous code to mutate requests, you would do better to use structures and a protocol. The protocol would have to be generic (because a requirement of == references the associated type, Self). However, recall that Swift disallows the use of a generic protocol as a type:
protocol HTTP_Request { static func == ( a : Self , b : Self ) -> Bool } struct Get : HTTP_Request {...} struct Post : HTTP_Request {...} let // ILLEGAL someRequest : HTTP_Request = ...
let requests : [ HTTP_Request ] = ...
// also ILLEGAL
So you could neither declare a variable that could contain any kind of request, nor an array that could contain a mixture of requests. However, since the set of requests is fixed, there is no need to allow others to create new kinds of requests. Thus, an enumeration with associated values is the best fit.
enum HTTP_Request { case get ( destination : URL , headerFields : [ String : String ] ) case post ( destination : URL , headerFields : [ String : String ], data : Data )
// also delete, put, & patch func send ( completionHandler :
@ escaping ( Data ?, URLResponse ?, Error ?) -> Void ) { let s = URLSession () let r = URLRequest ( url : someURL ) switch self { case
. get : // also delete s . dataTask ( with : r , completionHandler : completionHandler )
. resume () case . post ( _ , _ , let data ): // also put, patch s . uploadTask ( with : r , from : data , completionHandler : completionHandler )
. resume () } } static func
== ( lhs : HTTP_Request , rhs : HTTP_Request )
- > Bool {
switch ( lhs , rhs ) { case let (. get ( u1 , h1 ), . get ( u2 , h2 )) where u1 == u2 && h1 == h2 : return true case let (. post ( u1 , h1 , d1 ), . post ( u2 , h2 , d2 )) where u1 == u2 && h1 == h2 && d1 == d2 : return true
// also delete, put & patch default : return false
} } } HTTP_Request, that subsumes every kind of request. As a consequence, the same array can hold any kind of HTTP_Request:
let requests : [ HTTP_Request ] = [ . get ( destination : url1 , headerFields : [:]), . post ( destination : url2 , headerFields : [:], data : someData ) ]
and you can test different kinds of requests for equality with each other:
if aRequest == anotherRequest Enumerations as Inside-Out Class Hierarchies
When you replace a class hierarchy with an enumeration, it’s like turning the hierarchy inside out: the hierarchy first divides the code into various types, such as the request types above; then, within each class the methods divide up the code by function. But the methods of an enumeration first divide up the code by function, and the switches within each function then divide up the code by case.
The enumeration isn’t always better than the hierarchy. There are drawbacks to using an enumeration: More code
Instead of implicit dynamic dispatching within a class hierarchy, you must write explicit switch statements. Less extensibility
You can add a subclass to a class that was defined in a different file, but you cannot add a case to an enumeration that was defined in a different file. (This restriction can be a benefit where appropriate.) An enumeration is favored when the set of cases is less likely to expand later; a class hierarchy is favored when the set of cases is more likely to expand, or be expanded by importers of your library. Awkward for common data
A superclass or protocol offers better support than an enumeration for data attributes that are common to all the alternatives. It may be better to use a structure holding the common attributes and an enumeration for the specifics. As you get used to Swift, you will find more and more uses for generic enumerations with associated values.
An enumeration with associated values provides a single, concrete type for disparate cases. Where the set of alternatives is fixed and has few data attributes in common, an enumeration with associated values can significantly improve the quality of your code.
Choosing an Aggregate
Tuples, classes, structures, protocols, enumerations: Swift offers many different aggregates. Which to use in any given situation? and the following subsections provide a simplified guide. It neglects two aspects: some constructs may run faster than others, and classes offer more implicitly dynamic dispatch of methods than do other aggregates.
Figure 2-3. Issues to ponder when choosing an aggregate
Instantiate a Class for a Thing with an Identity