Swift基础

Type Aliases

typealias AudioSample = UInt16

Tuples(元组）

let http404Error = (404, "Not Found")

使用方法

对多个对象进行赋值：

let (statusCode, statusMessage) = http404Error
print("The status code is \(statusCode)")
// Prints "The status code is 404"
print("The status message is \(statusMessage)")
// Prints "The status message is Not Found"

通过index获取Tuples成员

print("The status code is \(http404Error.0)")
// Prints "The status code is 404"
print("The status message is \(http404Error.1)")
// Prints "The status message is Not Found"

命名Tuples成员

let http200Status = (statusCode: 200, description: "OK")
// 获取成员方法
print("The status code is \(http200Status.statusCode)")
// Prints "The status code is 200"
print("The status message is \(http200Status.description)")
// Prints "The status message is OK"

Optional Binding

用在if和while判断条件里面，避免对optionals对象的展开(!)：

if let constantName = someOptional {
    statements
}

if let actualNumber = Int(possibleNumber) {
    print("\"\(possibleNumber)\" has an integer value of \(actualNumber)")
} else {
    print("\"\(possibleNumber)\" could not be converted to an integer")
}
// Prints ""123" has an integer value of 123"

Implicitly Unwrapped Optionals

在声明的类型后面加!

let assumedString: String! = "An implicitly unwrapped optional string."
let implicitString: String = assumedString // no need for an exclamation mark

Error Handling

catch语法

func makeASandwich() throws {
    // ...
}

do {
    try makeASandwich()
    eatASandwich()
} catch SandwichError.outOfCleanDishes {
    washDishes()
} catch SandwichError.missingIngredients(let ingredients) {
    buyGroceries(ingredients)
}

Assertions(断言) and Preconditions(前置条件)

区别：断言只在debug模式下起作用，前置条件在debug和release下都起作用

// 断言
let age = -3
assert(age >= 0, "A person's age can't be less than zero.")
// This assertion fails because -3 is not >= 0.

// 前置条件
// In the implementation of a subscript...
precondition(index > 0, "Index must be greater than zero.")

Lazy Stored Properties

在第一次引用到的时候才初始化，注意：在多线程中，同时进行第一次引用的时候，不保证属性只初始化一次。

class DataImporter {
    /*
    DataImporter is a class to import data from an external file.
    The class is assumed to take a nontrivial amount of time to initialize.
    */
    var filename = "data.txt"
    // the DataImporter class would provide data importing functionality here
}

class DataManager {
    lazy var importer = DataImporter()
    var data = [String]()
    // the DataManager class would provide data management functionality here
}

let manager = DataManager()
manager.data.append("Some data")
manager.data.append("Some more data")
// the DataImporter instance for the importer property has not yet been created

print(manager.importer.filename)
// the DataImporter instance for the importer property has now been created
// Prints "data.txt"