Objective-C与Swift的转换速查手册

如果你正要从Objective-C过渡到Swift，或反过来，一个在两种语言间显示等效代码的小手册会很有帮助。本文内容就是这些：苹果开发者的红宝书，包含变量，集合，函数，类等等。
下面例子中，上面是Objective-C代码，下面是等效的Swift代码。必要的地方我会给一些备注来帮助你理解。

变量与常量
创建一个变量

//Objective-C
NSInteger score = 556;
//
NSString *name = @"Taylor";
//
BOOL loggedIn = NO;

//Swift
var score = 556
//
var name = "Taylor"
//
var loggedIn = false

创建一个常量

//Objective-C
const NSInteger score = 556;
//
NSString * const name = @"Taylor";
//
const BOOL firstRun = YES;
//Objective-C中常量用的很少

let score = 556
//
let name = "Taylor"
//
let firstRun = true
//Swift中常量很常见

创建一个变量数组

创建一个常量数组

//Objective-C
NSArray *grades = @[@90, @85, @97];
//
NSArray *names = @[@"Taylor", @"Adele", @"Justin"];

//Swift
let grades = [90, 85, 97]
//
let names = ["Taylor", "Adele", "Justin"]

向数组中添加一个值类型

//Objective-C
NSMutableArray *array = [NSMutableArray new];
//
[array addObject:[NSValue valueWithRect:CGRectMake(0, 0, 32, 64)]];
//在添加到集合前，值类型有对应的引用类型

//Swift
var array = [CGRect]()
//
array.append(CGRect(x: 0, y: 0, width: 32, height: 64))

创建一个字典

//Objective-C
NSDictionary *houseNumbers = @{ @"Paul": @7, @"Jess": @56, @"Peter": @332 };

//Swift
let houseNumbers = ["Paul": 7, "Jess": 56, "Peter": 332]

定义一个枚举

//Objective-C
typedef NS_ENUM(NSInteger, ShapeType) {
    kCircle,
    kRectangle,
    kHexagon
};

//Swift
enum ShapeType: Int {
    case circle
    case rectangle
    case hexagon
}

附加一串字符

//Objective-C
NSString *first = @"Hello, ";
NSString *second = [first stringByAppendingString:@" world!"];

//Swift
let first = "Hello, "
let second = first + "world!"

增加数字

//Objective-C
NSInteger rating = 4;
rating++;
rating += 3;

//Swift
var rating = 4
rating += 1
rating += 3

插入字符串

//Objective-C
NSString *account = @"twostraws";
NSString *str = [NSString stringWithFormat:@"Follow me on Twitter: %@", account];

//Swift
let account = "twostraws"
let str = "Follow me on Twitter: \(account)"

打印调试信息

//Objective-C
NSString *username = @"twostraws";
NSLog(@"Username is %@", username);

//Swift
let username = "twostraws"
print("Username is \(username)")

控制流
检查状态

//Objective-C
NSInteger result = 86;

if (result >= 85) {
    NSLog(@"You passed the test!");
} else {
    NSLog(@"Please try again.");
}

//Swift
let result = 86

if result >= 85 {
    print("You passed the test!")
} else {
    print("Please try again.")
}

循环一定次数

//Objective-C
for (NSInteger i = 0; i < 100; ++i) {
    NSLog(@"This will be printed 100 times.");
}

//Swift
for _ in 0 ..< 100 {
    print("This will be printed 100 times.")
}

在数组中循环

//Objective-C
NSArray *companies = @[@"Apple", @"Facebook", @"Twitter"];

for (NSString *name in companies) {
    NSLog(@"%@ is a well-known tech company.", name);
}

//Swift
let companies = ["Apple", "Facebook", "Twitter"]

for name in companies {
    print("\(name) is a well-known tech company.")
}

数值切换

//Objective-C
NSInteger rating = 8;

switch (rating) {
    case 0 ... 3:
        NSLog(@"Awful");
        break;
    case 4 ... 7:
        NSLog(@"OK");
        break;
    case 8 ... 10:
        NSLog(@"Good");
        break;
    default:
        NSLog(@"Invalid rating.");
}
//很多人不知道Objective-C有范围支持，所以你也许看到二选一的语法

//Swift
let rating = 8

switch rating {
case 0...3:
    print("Awful")
case 4...7:
    print("OK")
case 8...10:
    print("Good")
default:
    print("Invalid rating.")
}
//Swift不会fall through案例，除非你使用fallthrough关键字

函数
不接收参数也没有返回的函数

//Objective-C
- (void)printGreeting {
    NSLog(@"Hello!");
}
[self printGreeting];

//Swift
func printGreeting() {
    print("Hello!")
}
printGreeting()

不接收参数，返回一个字符串的函数

//Objective-C
- (NSString*)printGreeting {
    return @"Hello!";
}
NSString *result = [self printGreeting];

//Swift
func printGreeting() -> String {
    return "Hello!"
}
let result = printGreeting()

接收一个字符串，返回一个字符串的函数

//Objective-C
- (NSString*)printGreetingFor:(NSString*)user {
    return [NSString stringWithFormat:@"Hello, %@!", user];
}

NSString *result = [self printGreetingFor:@"Paul"];
//第一个参数的名称需要为方法名的一部分

//Swift
func printGreeting(for user: String) -> String {
    return "Hello, \(user)!"
}

let result = printGreeting(for: "Paul")

接收一个字符串和一个整数，返回一个字符串的函数

//Objective-C
- (NSString*)printGreetingFor:(NSString*)user withAge:(NSInteger)age {
    if (age >= 18) {
        return [NSString stringWithFormat:@"Hello, %@! You're an adult.", user];
    } else {
        return [NSString stringWithFormat:@"Hello, %@! You're a child.", user];
    }
}

NSString *result = [self printGreetingFor:@"Paul" withAge:38];

//Swift
func printGreeting(for user: String, age: Int) -> String {
    if age >= 18 {
        return "Hello, \(user) You're an adult."
    } else {
        return "Hello, \(user)! You're a child."
    }
}

let result = printGreeting(for: "Paul", age: 38)

从函数返回多个值

//Objective-C
- (NSDictionary*)loadAddress {
    return @{
        @"house": @"65, Park Street",
        @"city": @"Bristol",
        @"country": @"UK"
    };
}

NSDictionary*address = [self loadAddress];
NSString *house = address[@"house"];
NSString *city = address[@"city"];
NSString *country = address[@"country"];
//Objective-C不支持元祖(tuple),所以用字典或数组替代

//Swift
func loadAddress() -> (house: String, city: String, country: String) {
    return ("65, Park Street", "Bristol", "UK")
}

let (city, street, country) = loadAddress()

不接收参数没有返回的闭环

//Objective-C
void (^printUniversalGreeting)(void) = ^{
    NSLog(@"Bah-weep-graaaaagnah wheep nini bong");
};

printUniversalGreeting();

//Swift
let universalGreeting = {
    print("Bah-weep-graaaaagnah wheep nini bong")
}

universalGreeting()

不接收参数返回一个字符串的闭环

//Objective-C
NSString* (^getUniversalGreeting)(void) = ^{
    return @"Bah-weep-graaaaagnah wheep nini bong";
};

NSString *greeting = getUniversalGreeting();
NSLog(@"%@", greeting);

//Swift
let getUniversalGreeting = {
    return "Bah-weep-graaaaagnah wheep nini bong"
}

let greeting = getUniversalGreeting()
print(greeting)

接收一个字符串参数，返回一个字符串的闭环

//Objective-C
NSString* (^getGreeting)(NSString *) = ^(NSString *name) {
    return [NSString stringWithFormat:@"Live long and prosper, %@.", name];
};

NSString *greeting = getGreeting(@"Paul");
NSLog(@"%@", greeting);

//Swift
let getGreeting = { (name: String) in
    return "Live long and prosper, \(name)."
}

let greeting = getGreeting("Paul")
print(greeting)

类
创建空类

//Objective-C
@interface MyClass : NSObject
@end

@implementation MyClass
@end

//Swift
class MyClass: NSObject {
}
//推荐使用结构代替类，这样也许不需要从NSObject继承了

创建有2个属性的类

//Objective-C
@interface User : NSObject
@property (nonatomic, copy) NSString *name;
@property (nonatomic, assign) NSInteger age;
@end

@implementation User

@end

//Swift
class User {
    var name: String
    var age: Int
    
    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}
//Swift要求进行初始化，给这些属性默认值

创建有一个私有属性的类

//Objective-C
//在头文件中
@interface User : NSObject
@property (nonatomic, copy) NSString *name;
@end

//在执行文件中
@interface User()
@property (nonatomic, assign) NSInteger age;
@end

@implementation User

@end
//Objective-C实际上并不支持私有属性，通常都用这种变通方式

//Swift
class User {
    var name: String
    private var age: Int
    
    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}

创建有一个实例方法的类

//Objective-C
@interface Civilization : NSObject
- (NSInteger)getMeaningOfLife;
@end

@implementation Civilization
- (NSInteger)getMeaningOfLife {
    return 42;
}
@end

//Swift
class Civilization {
    func getMeaningOfLife() -> Int {
        return 42
    }
}

创建有一个静态方法的类

//Objective-C
@interface Civilization : NSObject
+ (NSInteger)getMeaningOfLife;
@end

@implementation Civilization
+ (NSInteger)getMeaningOfLife {
    return 42;
}
@end
//差别很小，用+而不是-

//Swift
class Civilization {
    class func getMeaningOfLife() -> Int {
        return 42
    }
}
//Swift也支持静态方法——它不会在子类中被覆盖

用一种新方法扩展一个类型

//Objective-C
@interface NSString (Trimming)
- (NSString*)trimmed;
@end

@implementation NSString (Trimming)

- (NSString*)trimmed {
    return [self stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
}

@end

//Swift
extension String {
    func trimmed() -> String {
        return trimmingCharacters(in: .whitespacesAndNewlines)
    }
}

检查一个对象的类

//Objective-C
if ([object isKindOfClass:[YourClass class]]) {
    NSLog(@"This is a YourClass.");
}

//Swift
if object is YourClass {
    print("This is a YourClass.")
}

类型转换

//Objective-C
Dog *poodle = (Dog*)animalObject;

//Swift
let poodle = animalObject as? Dog
//
let poodle = animalObject as! Dog
//如果不是一个dog，前者会把poodle设为nil，后者则会崩溃

GCD
在不同线程运行代码

//Objective-C
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSLog(@"Running in the background...");
   
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"Running back on the main thread");
    });
});

//Swift
DispatchQueue.global().async {
    print("Running in the background...")
   
    DispatchQueue.main.async {
        print("Running on the main thread")
    }
}

原文参考链接