Swift：苹果创新编程语言

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"《Swift编程语言》是苹果公司推出的一种新型编程语言，专为iOS和OS X应用程序设计，它在C和Objective-C的基础上发展，但不受C语言兼容性的限制。Swift强调安全的编程模式，并引入现代特性，使编程更易用、更灵活且更有趣。Swift与成熟且广受欢迎的Cocoa和Cocoa Touch框架相结合，为重新构想软件开发工作提供了机会。 Swift的诞生历经多年。苹果通过改进现有的编译器、调试器和框架基础设施为其打下了基础。自动引用计数（ARC）简化了内存管理。基于Foundation和Cocoa构建的框架堆栈已经现代化和标准化。Objective-C本身也进化支持了blocks、集合字面量和模块，使得现代语言技术的采用可以在不中断的情况下进行。得益于这些准备工作，现在我们可以为苹果软件开发引入一种面向未来的新语言。对于Objective-C开发者来说，Swift感觉十分亲切。它借鉴了Objective-C命名参数的可读性，同时摒弃了旧语言的一些复杂性。Swift的语法更加简洁，类型推断使得代码编写更加高效。它的.playground特性允许开发者实时查看代码效果，提高了学习和调试的效率。 Swift引入了一些关键特性，如可选类型（Optionals），用于避免空指针异常，确保程序安全性。协议（Protocols）允许定义行为规范，类和其他类型可以遵循，实现多态。泛型（Generics）则让代码更具复用性，可以在多种数据类型上使用。在函数式编程方面，Swift支持闭包（Closures），可以捕获和存储上下文中的数据，增强了代码的灵活性。元组（Tuples）允许一次返回多个值，简化了函数的返回和参数传递。同时，Swift还引入了扩展（Extensions），可以为已有类型添加新功能，而无需修改原始代码。 Swift的错误处理（Error Handling）机制通过do-catch结构，让异常处理更加清晰，降低了程序的出错风险。另外，Swift的类型系统是强类型的，它不允许隐式转换，有助于防止潜在的类型错误。《Swift编程语言》是苹果对编程工具的一次革新，它结合了现代编程语言的最佳实践，提供了更高效、更安全的开发环境，是iOS和macOS应用开发者的首选语言。随着Swift的不断发展和完善，其生态系统也在不断扩大，包括开源社区的支持，使得学习和使用Swift的资源越来越丰富。"

Comments in Swift are very similar to comments in C. Single-line comments begin with two forward-slashes (//):

// this is a comment

You can also write multiline comments, which start with a forward-slash followed by an asterisk (/*) and end with an asterisk followed by a forward-slash (*/):

1 /* this is also a comment,

2 but written over multiple lines */

Unlike multiline comments in C, multiline comments in Swift can be nested inside other multiline comments. You write nested comments by starting a multiline comment

block and then starting a second multiline comment within the first block. The second block is then closed, followed by the first block:

1 /* this is the start of the first multiline comment

2 /* this is the second, nested multiline comment */

3 this is the end of the first multiline comment */

Nested multiline comments enable you to comment out large blocks of code quickly and easily, even if the code already contains multiline comments.

SemicolonsSemicolons

Unlike many other languages, Swift does not require you to write a semicolon (;) after each statement in your code, although you can do so if you wish. Semicolons

are

required, however, if you want to write multiple separate statements on a single line:

1 let cat = ""; println(cat)

2 // prints ""

IntegersIntegers

Integers

are whole numbers with no fractional component, such as 42 and -23. Integers are either

signed

(positive, zero, or negative) or

unsigned

(positive or zero).

Swift provides signed and unsigned integers in 8, 16, 32, and 64 bit forms. These integers follow a naming convention similar to C, in that an 8-bit unsigned integer

is of type UInt8, and a 32-bit signed integer is of type Int32. Like all types in Swift, these integer types have capitalized names.

Integer BoundsInteger Bounds

You can access the minimum and maximum values of each integer type with its min and max properties:

1 let minValue = UInt8.min // minValue is equal to 0, and is of type UInt8

2 let maxValue = UInt8.max // maxValue is equal to 255, and is of type UInt8

The values of these properties are of the appropriate-sized number type (such as UInt8 in the example above) and can therefore be used in expressions alongside other

values of the same type.

IntInt

In most cases, you don’t need to pick a specific size of integer to use in your code. Swift provides an additional integer type, Int, which has the same size as the

current platform’s native word size:

Unless you need to work with a specific size of integer, always use Int for integer values in your code. This aids code consistency and interoperability. Even on 32-

bit platforms, Int can store any value between -2,147,483,648 and 2,147,483,647, and is large enough for many integer ranges.

UIntUInt

Swift also provides an unsigned integer type, UInt, which has the same size as the current platform’s native word size:

N O T E

Use UInt only when you specifically need an unsigned integer type with the same size as the platform’s native word size. If this is not the case, Int is preferred, even when the

values to be stored are known to be non-negative. A consistent use of Int for integer values aids code interoperability, avoids the need to convert between different number types,

and matches integer type inference, as described in Type Safety and Type Inference.

On a 32-bit platform, Int is the same size as Int32.

•

On a 64-bit platform, Int is the same size as Int64.

•

On a 32-bit platform, UInt is the same size as UInt32.

•

On a 64-bit platform, UInt is the same size as UInt64.

•

Floating-Point NumbersFloating-Point Numbers

Floating-point numbers

are numbers with a fractional component, such as 3.14159, 0.1, and -273.15.

Floating-point types can represent a much wider range of values than integer types, and can store numbers that are much larger or smaller than can be stored in an

Int. Swift provides two signed floating-point number types:

N O T E

Double has a precision of at least 15 decimal digits, whereas the precision of Float can be as little as 6 decimal digits. The appropriate floating-point type to use depends on

the nature and range of values you need to work with in your code.

Type Safety and Type InferenceType Safety and Type Inference

Swift is a

type safe

language. A type safe language encourages you to be clear about the types of values your code can work with. If part of your code expects a

String, you can’t pass it an Int by mistake.

Because Swift is type safe, it performs

type checks

when compiling your code and flags any mismatched types as errors. This enables you to catch and fix errors as

early as possible in the development process.

Type-checking helps you avoid errors when you’re working with different types of values. However, this doesn’t mean that you have to specify the type of every

constant and variable that you declare. If you don’t specify the type of value you need, Swift uses

type inference

to work out the appropriate type. Type inference

enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.

Because of type inference, Swift requires far fewer type declarations than languages such as C or Objective-C. Constants and variables are still explicitly typed, but

much of the work of specifying their type is done for you.

Type inference is particularly useful when you declare a constant or variable with an initial value. This is often done by assigning a

literal value

(or

literal

) to

the constant or variable at the point that you declare it. (A literal value is a value that appears directly in your source code, such as 42 and 3.14159 in the

examples below.)

For example, if you assign a literal value of 42 to a new constant without saying what type it is, Swift infers that you want the constant to be an Int, because you

have initialized it with a number that looks like an integer:

1 let meaningOfLife = 42

2 // meaningOfLife is inferred to be of type Int

Likewise, if you don’t specify a type for a floating-point literal, Swift infers that you want to create a Double:

1 let pi = 3.14159

2 // pi is inferred to be of type Double

Swift always chooses Double (rather than Float) when inferring the type of floating-point numbers.

If you combine integer and floating-point literals in an expression, a type of Double will be inferred from the context:

1 let anotherPi = 3 + 0.14159

2 // anotherPi is also inferred to be of type Double

The literal value of 3 has no explicit type in and of itself, and so an appropriate output type of Double is inferred from the presence of a floating-point literal as

part of the addition.

Numeric LiteralsNumeric Literals

Integer literals can be written as:

All of these integer literals have a decimal value of 17:

1 let decimalInteger = 17

2 let binaryInteger = 0b10001 // 17 in binary notation

3 let octalInteger = 0o21 // 17 in octal notation

4 let hexadecimalInteger = 0x11 // 17 in hexadecimal notation

Floating-point literals can be decimal (with no prefix), or hexadecimal (with a 0x prefix). They must always have a number (or hexadecimal number) on both sides of

the decimal point. They can also have an optional

exponent

, indicated by an uppercase or lowercase e for decimal floats, or an uppercase or lowercase p for

hexadecimal floats.

Double represents a 64-bit floating-point number. Use it when floating-point values must be very large or particularly precise.

•

Float represents a 32-bit floating-point number. Use it when floating-point values do not require 64-bit precision.

•

decimal

number, with no prefix•

binary

number, with a 0b prefix

•

octal

number, with a 0o prefix

•

hexadecimal

number, with a 0x prefix

•

For decimal numbers with an exponent of exp, the base number is multiplied by 10

exp

For hexadecimal numbers with an exponent of exp, the base number is multiplied by 2

exp

All of these floating-point literals have a decimal value of 12.1875:

1 let decimalDouble = 12.1875

2 let exponentDouble = 1.21875e1

3 let hexadecimalDouble = 0xC.3p0

Numeric literals can contain extra formatting to make them easier to read. Both integers and floats can be padded with extra zeroes and can contain underscores to

help with readability. Neither type of formatting affects the underlying value of the literal:

1 let paddedDouble = 000123.456

2 let oneMillion = 1_000_000

3 let justOverOneMillion = 1_000_000.000_000_1

Numeric Type ConversionNumeric Type Conversion

Use the Int type for all general-purpose integer constants and variables in your code, even if they are known to be non-negative. Using the default integer type in

everyday situations means that integer constants and variables are immediately interoperable in your code and will match the inferred type for integer literal values.

Use other integer types only when they are are specifically needed for the task at hand, because of explicitly-sized data from an external source, or for performance,

memory usage, or other necessary optimization. Using explicitly-sized types in these situations helps to catch any accidental value overflows and implicitly documents

the nature of the data being used.

Integer ConversionInteger Conversion

The range of numbers that can be stored in an integer constant or variable is different for each numeric type. An Int8 constant or variable can store numbers between -

128 and 127, whereas a UInt8 constant or variable can store numbers between 0 and 255. A number that will not fit into a constant or variable of a sized integer type is

reported as an error when your code is compiled:

1 let cannotBeNegative: UInt8 = -1

2 // UInt8 cannot store negative numbers, and so this will report an error

3 let tooBig: Int8 = Int8.max + 1

4 // Int8 cannot store a number larger than its maximum value,

5 // and so this will also report an error

Because each numeric type can store a different range of values, you must opt in to numeric type conversion on a case-by-case basis. This opt-in approach prevents

hidden conversion errors and helps make type conversion intentions explicit in your code.

To convert one specific number type to another, you initialize a new number of the desired type with the existing value. In the example below, the constant twoThousand

is of type UInt16, whereas the constant one is of type UInt8. They cannot be added together directly, because they are not of the same type. Instead, this example calls

UInt16(one) to create a new UInt16 initialized with the value of one, and uses this value in place of the original:

1 let twoThousand: UInt16 = 2_000

2 let one: UInt8 = 1

3 let twoThousandAndOne = twoThousand + UInt16(one)

Because both sides of the addition are now of type UInt16, the addition is allowed. The output constant (twoThousandAndOne) is inferred to be of type UInt16, because it is

the sum of two UInt16 values.

SomeType(ofInitialValue) is the default way to call the initializer of a Swift type and pass in an initial value. Behind the scenes, UInt16 has an initializer that accepts

a UInt8 value, and so this initializer is used to make a new UInt16 from an existing UInt8. You can’t pass in

any

type here, however—it has to be a type for which

UInt16 provides an initializer. Extending existing types to provide initializers that accept new types (including your own type definitions) is covered in Extensions.

Integer and Floating-Point ConversionInteger and Floating-Point Conversion

Conversions between integer and floating-point numeric types must be made explicit:

1 let three = 3

2 let pointOneFourOneFiveNine = 0.14159

3 let pi = Double(three) + pointOneFourOneFiveNine

4 // pi equals 3.14159, and is inferred to be of type Double

Here, the value of the constant three is used to create a new value of type Double, so that both sides of the addition are of the same type. Without this conversion in

place, the addition would not be allowed.

1.25e2 means 1.25 × 10

, or 125.0.

•

1.25e-2 means 1.25 × 10

-2

, or 0.0125.

•

0xFp2 means 15 × 2

, or 60.0.

•

0xFp-2 means 15 × 2

-2

, or 3.75.

•

The reverse is also true for floating-point to integer conversion, in that an integer type can be initialized with a Double or Float value:

1 let integerPi = Int(pi)

2 // integerPi equals 3, and is inferred to be of type Int

Floating-point values are always truncated when used to initialize a new integer value in this way. This means that 4.75 becomes 4, and -3.9 becomes -3.

N O T E

The rules for combining numeric constants and variables are different from the rules for numeric literals. The literal value 3 can be added directly to the literal value 0.14159,

because number literals do not have an explicit type in and of themselves. Their type is inferred only at the point that they are evaluated by the compiler.

Type AliasesType Aliases

Type aliases

define an alternative name for an existing type. You define type aliases with the typealias keyword.

Type aliases are useful when you want to refer to an existing type by a name that is contextually more appropriate, such as when working with data of a specific size

from an external source:

typealias AudioSample = UInt16

Once you define a type alias, you can use the alias anywhere you might use the original name:

1 var maxAmplitudeFound = AudioSample.min

2 // maxAmplitudeFound is now 0

Here, AudioSample is defined as an alias for UInt16. Because it is an alias, the call to AudioSample.min actually calls UInt16.min, which provides an initial value of 0 for

the maxAmplitudeFound variable.

BooleansBooleans

Swift has a basic

Boolean

type, called Bool. Boolean values are referred to as

logical

, because they can only ever be true or false. Swift provides two Boolean

constant values, true and false:

1 let orangesAreOrange = true

2 let turnipsAreDelicious = false

The types of orangesAreOrange and turnipsAreDelicious have been inferred as Bool from the fact that they were initialized with Boolean literal values. As with Int and Double

above, you don’t need to declare constants or variables as Bool if you set them to true or false as soon as you create them. Type inference helps make Swift code more

concise and readable when it initializes constants or variables with other values whose type is already known.

Boolean values are particularly useful when you work with conditional statements such as the if statement:

1 if turnipsAreDelicious {

2 println("Mmm, tasty turnips!")

3 } else {

4 println("Eww, turnips are horrible.")

5 }

6 // prints "Eww, turnips are horrible."

Conditional statements such as the if statement are covered in more detail in Control Flow.

Swift’s type safety prevents non-Boolean values from being be substituted for Bool. The following example reports a compile-time error:

1 let i = 1

2 if i {

3 // this example will not compile, and will report an error

4 }

However, the alternative example below is valid:

1 let i = 1

2 if i == 1 {

3 // this example will compile successfully

4 }

The result of the i == 1 comparison is of type Bool, and so this second example passes the type-check. Comparisons like i == 1 are discussed in Basic Operators.

As with other examples of type safety in Swift, this approach avoids accidental errors and ensures that the intention of a particular section of code is always clear.

TuplesTuples

Tuples

group multiple values into a single compound value. The values within a tuple can be of any type and do not have to be of the same type as each other.

In this example, (404, "Not Found") is a tuple that describes an

HTTP status code

. An HTTP status code is a special value returned by a web server whenever you request a

web page. A status code of 404 Not Found is returned if you request a webpage that doesn’t exist.

1 let http404Error = (404, "Not Found")

2 // http404Error is of type (Int, String), and equals (404, "Not Found")

The (404, "Not Found") tuple groups together an Int and a String to give the HTTP status code two separate values: a number and a human-readable description. It can be

described as “a tuple of type (Int, String)”.

You can create tuples from any permutation of types, and they can contain as many different types as you like. There’s nothing stopping you from having a tuple of

type (Int, Int, Int), or (String, Bool), or indeed any other permutation you require.

You can

decompose

a tuple’s contents into separate constants or variables, which you then access as usual:

1 let (statusCode, statusMessage) = http404Error

2 println("The status code is \(statusCode)")

3 // prints "The status code is 404"

4 println("The status message is \(statusMessage)")

5 // prints "The status message is Not Found"

If you only need some of the tuple’s values, ignore parts of the tuple with an underscore (_) when you decompose the tuple:

1 let (justTheStatusCode, _) = http404Error

2 println("The status code is \(justTheStatusCode)")

3 // prints "The status code is 404"

Alternatively, access the individual element values in a tuple using index numbers starting at zero:

1 println("The status code is \(http404Error.0)")

2 // prints "The status code is 404"

3 println("The status message is \(http404Error.1)")

4 // prints "The status message is Not Found"

You can name the individual elements in a tuple when the tuple is defined:

let http200Status = (statusCode: 200, description: "OK")

If you name the elements in a tuple, you can use the element names to access the values of those elements:

1 println("The status code is \(http200Status.statusCode)")

2 // prints "The status code is 200"

3 println("The status message is \(http200Status.description)")

4 // prints "The status message is OK"

Tuples are particularly useful as the return values of functions. A function that tries to retrieve a web page might return the (Int, String) tuple type to describe the

success or failure of the page retrieval. By returning a tuple with two distinct values, each of a different type, the function provides more useful information about

its outcome than if it could only return a single value of a single type. For more information, see Functions with Multiple Return Values.

N O T E

Tuples are useful for temporary groups of related values. They are not suited to the creation of complex data structures. If your data structure is likely to persist beyond a

temporary scope, model it as a class or structure, rather than as a tuple. For more information, see Classes and Structures.

OptionalsOptionals

You use

optionals

in situations where a value may be absent. An optional says:

N O T E

The concept of optionals doesn’t exist in C or Objective-C. The nearest thing in Objective-C is the ability to return nil from a method that would otherwise return an object,

with nil meaning “the absence of a valid object.” However, this only works for objects—it doesn’t work for structs, basic C types, or enumeration values. For these types,

Objective-C methods typically return a special value (such as NSNotFound) to indicate the absence of a value. This approach assumes that the method’s caller knows there is a

special value to test against and remembers to check for it. Swift’s optionals let you indicate the absence of a value for

any type at all

, without the need for special

constants.

Here’s an example. Swift’s String type has a method called toInt, which tries to convert a String value into an Int value. However, not every string can be converted

There

a value, and it equals

•

There

isn’t

a value at all•

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Swift：苹果创新编程语言

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