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，上边是Objective-C代码，下边是等效的Swift代码。必要的地方我会给一些备注来帮助理解。
变量与常量
创建一个变量
//Objective-CNSInteger score = 556;//NSString *name = @"Taylor";//BOOL loggedIn = NO;
//Swiftvar score = 556//var name = "Taylor"//var loggedIn = false创建一个常量
//Objective-Cconst NSInteger score = 556;//NSString * const name = @"Taylor";//const BOOL firstRun = YES;//Objective-C中常量用的很少
//Swiftlet score = 556//let name = "Taylor"//let firstRun = true//Swift中常量很常见
创建一个变量数组
//Objective-CNSMutableArray *items = [NSMutableArray new];//NSMutableArray<NSString *> *results = [NSMutableArray new];//Objective-C中的泛型存在争议，这里列了两种形式
//Swiftvar items = [String]()//var results = Array<String>()//强烈推荐用上面那种
创建一个常量数组
//Objective-CNSArray *grades = @[@90, @85, @97];//NSArray *names = @[@"Taylor", @"Adele", @"Justin"];
//Swiftlet grades = [90, 85, 97]//let names = ["Taylor", "Adele", "Justin"]
向数组中添加一个值类型
//Objective-CNSMutableArray *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
//Swiftvar array = [CGRect]()array.append(CGRect(x: 0, y: 0, width: 32, height: 64))创建一个字典
//Objective-CNSDictionary *houseNumbers = @{ @"Paul": @7, @"Jess": @56, @"Peter": @332 };
//Swiftlet houseNumbers = ["Paul": 7, "Jess": 56, "Peter": 332]
定义一个枚举
//Objective-Ctypedef NS_ENUM(NSInteger, ShapeType) { kCircle, kRectangle, kHexagon};
//Swiftenum ShapeType: Int { case circle case rectangle case hexagon}
附加一串字符
//Objective-CNSString *first = @"Hello, ";NSString *second = [first stringByAppendingString:@" world!"];
//Swiftlet first = "Hello, "let second = first + "world!"
增加数字
//Objective-CNSInteger rating = 4;rating++;rating += 3;
//Swiftvar rating = 4rating += 1rating += 3
插入字符串
//Objective-CNSString *account = @"twostraws";NSString *str = [NSString stringWithFormat:@"Follow me on Twitter: %@", account];
//Swiftlet account = "twostraws"let str = "Follow me on Twitter: \(account)"
打印调试信息
//Objective-CNSString *username = @"twostraws";NSLog(@"Username is %@", username);
//Swiftlet username = "twostraws"print("Username is \(username)")
控制流
检查状态
//Objective-CNSInteger result = 86;if (result >= 85) { NSLog(@"You passed the test!");} else { NSLog(@"Please try again.");}
//Swiftlet result = 86if result >= 85 { print("You passed the test!")} else { print("Please try again.")}
循环一定次数
//Objective-Cfor (NSInteger i = 0; i < 100; ++i) { NSLog(@"This will be printed 100 times.");}
//Swiftfor _ in 0 ..< 100 { print("This will be printed 100 times.")}
在数组中循环
//Objective-CNSArray *companies = @[@"Apple", @"Facebook", @"Twitter"];for (NSString *name in companies) { NSLog(@"%@ is a well-known tech company.", name);}
//Swiftlet companies = ["Apple", "Facebook", "Twitter"]for name in companies { print("\(name) is a well-known tech company.")}
数值切换
//Objective-CNSInteger 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，所以你也许会看到别的语法
//Swiftlet rating = 8switch 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];
//Swiftfunc 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];
//Swiftfunc 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
//Swiftfunc 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];
//Swiftfunc 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<NSString*, NSString*>*)loadAddress { return @{ @"house": @"65, Park Street", @"city": @"Bristol", @"country": @"UK" };}NSDictionary<NSString*, NSString*> *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
//Swiftfunc 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包
//Objective-Cvoid (^printUniversalGreeting)(void) = ^{ NSLog(@"Bah-weep-graaaaagnah wheep nini bong");};printUniversalGreeting();
//Swiftlet universalGreeting = { print("Bah-weep-graaaaagnah wheep nini bong")}universalGreeting()不接收参数返回一个字符串的闭包
//Objective-CNSString* (^getUniversalGreeting)(void) = ^{ return @"Bah-weep-graaaaagnah wheep nini bong";};NSString *greeting = getUniversalGreeting();NSLog(@"%@", greeting);
//Swiftlet 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包
//Objective-CNSString* (^getGreeting)(NSString *) = ^(NSString *name) { return [NSString stringWithFormat:@"Live long and prosper, %@.", name];};NSString *greeting = getGreeting(@"Paul");NSLog(@"%@", greeting);
//Swiftlet 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
//Swiftclass 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
//Swiftclass 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
//Swiftclass 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
//Swiftclass 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-
//Swiftclass 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
//Swiftextension String { func trimmed() -> String { return trimmingCharacters(in: .whitespacesAndNewlines) }}
检查一个对象的类
//Objective-Cif ([object isKindOfClass:[YourClass class]]) { NSLog(@"This is a YourClass.");}
//Swiftif object is YourClass { print("This is a YourClass.")}
类型转换
//Objective-CDog *poodle = (Dog*)animalObject;
//Swiftlet 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-Cdispatch_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"); });});
//SwiftDispatchQueue.global().async { print("Running in the background...") DispatchQueue.main.async { print("Running on the main thread") }}
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