Swift编程基础教程

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"Swift_by_Tutorials_1_3 - Swift编程基础教程" 这是一份关于Swift编程语言的基础教程，由raywenderlich.com的专家团队撰写，包括Colin Eberhardt和Matt Galloway等人。本教程的目标是帮助读者掌握Swift 4的核心概念和语法。尽管该教程可能已有些许年代，但Swift 4的基础知识对于理解现代版本的Swift（如Swift 5或6）依然至关重要。教程内容分为多个章节，以逐步深入的方式引导学习者： 1. **介绍**：这部分通常会包含对Swift语言的简要历史、主要特点以及为什么选择学习Swift的讨论。它可能会涵盖Swift在iOS、macOS、watchOS和tvOS开发中的重要性，以及Apple生态系统中的应用。 2. **语言基础I**：这一章将介绍Swift的基本语法，包括变量和常量的声明（let和var）、数据类型（Int, Double, String等）、注释、运算符以及控制流程（if语句，switch语句，for循环，while循环）。此外，还会讲解函数（functions）和闭包（closures）的基础用法。 3. **语言基础II**：在此章节，读者将学习更高级的语法特性，如可选链（optional chaining），类型推断（type inference），模式匹配（pattern matching），以及枚举（enums）和结构体（structs）的使用。此外，还可能涉及类型别名（type aliases）和泛型（generics）。 4. **类（Classes）**：这一章将介绍面向对象编程的关键概念——类。内容可能包括类的定义，初始化（initialization），属性（properties），方法（methods），继承（inheritance），多态（polymorphism）以及访问控制（access control：public, private, internal）。注意，虽然此教程可能没有包含最新版本Swift的特性，如协议扩展（protocol extensions）、异步/await（async/await）等，但它仍能提供对Swift编程语言基本概念的扎实理解，这对于任何想要进入Swift开发领域的初学者来说都是宝贵的资源。在学习过程中，读者应结合最新的官方文档和更新的教程来补充和完善知识体系。

Swift by Tutorials Chapter 1: Language Basics I

raywenderlich.com Page 16

Open Xcode, select File\New Playground…, select the iOS platform and save the

file wherever you like:

A playground is a single Swift file that the template populates as follows:

As you enter your Swift code on the left-hand side of the view, the right-hand side

automatically displays the value of assignments and other useful hints about the

resulting application state. You don’t have to explicitly invoke a Build command

after making a change. In a playground, Xcode continually builds and executes your

code as you type.

Wouldn’t a playground be an excellent place to practice your Swift skills?

Swift by Tutorials Chapter 1: Language Basics I

raywenderlich.com Page 19

println(greeting)

This causes the compiler to report an error, type 'String' does not conform to

protocol 'IntegerLiteralConvertible', at the second line.

Type inference doesn’t weaken your application in any way; it simply saves

keystrokes by removing the need to specify types that are obvious to the compiler

(and hopefully to you). The simple example above shows type inference when

assigning literal values, but it will work everywhere within the language—for

example, when you create a variable and assign it a value from a function.

PRO TIP: We encourage you to use type inference extensively, as it results in

more concise and thus more readable code. However, if there is any instance

where you think adding an explicit type to a variable declaration would

enhance readability, there is certainly nothing wrong with doing so!

Let’s do some simple string manipulation. Update the code as follows:

var greeting = "Hello"

greeting = greeting + " World"

println(greeting)

The console output is now “Hello World” (surely you didn’t need a compiler to tell

you that!).

Swift strings are mutable—that is, you can change their values—where the

mutations have a much simpler syntax than their NSMutableString counterparts.

For example, you could change the above to use the more compact += compound

assignment operator.

There are many times you want to create a value that does not change. Creating

these values as variables is inherently unsafe since accidental assignments can and

do happen.

Swift makes it easy to change a variable into a constant. Update the first line of

code as follows:

let greeting = "Hello"

This results in an error at the second line of code: cannot assign to 'let' value

'greeting'.

You can control the mutability of the built-in Swift types (e.g., String, Int,

Dictionary) by using either the let or var keywords at the point of declaration.

This is in stark contrast to Objective-C and many other languages, where some

type implementations are inherently mutable, such as NSMutableArray, and some

are not, such as NSArray.

Swift by Tutorials Chapter 1: Language Basics I

raywenderlich.com Page 20

PRO-TIP: You should define values as constants wherever possible. This not

only makes your code more robust, it also allows the compiler to further

optimize your code, improving execution times. Thus, you will probably find

yourself using let far more often than var.

The use of let in order to make an instance of a type immutable isn’t restricted to

the built-in Swift types. Any structures (structs) that you define yourself also have

this behavior; however there are some subtle differences between constant

structures and classes. You can read more on this in Chapter 3, “Classes and

Structs”.

Change greeting back to a variable by changing the let to var, then add the

following line just before the println statement:

greeting.append(Character("!"))

This appends an exclamation mark to the end of the string, with the console

outputting the result, “Hello World!”.

The Swift String type has API methods like the above that allow you to manipulate

their values. However, these are quite limited in number. Fortunately, the Swift

String type is bridged to the Objective-C NSString type, providing all the API

methods with which you are no doubt familiar if you’ve used Objective-C.

To introduce these methods, update your playground as follows:

import Foundation

var greeting = "hello world".capitalizedString

println(greeting)

This results in the output “Hello World”.

With the first statement, you import the Foundation framework, which includes

NSString. This adds capitalizedString to Swift’s String.

Why not experiment with some of the other NSString API methods within the

playground? You can even append strings using stringByAppendingString if you

aren’t keen on the more compact Swift style!

import Foundation

var greeting = "hello".stringByAppendingString(" world")

println(greeting)

Finally, the Swift String is a value type. As a result, when you assign a string to a

variable, a constant, or pass it as an argument to a function, its value is copied.

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