Preface

If you are moving from Objective-C to Swift, or vice versa, a brochure showing equivalent code between two languages will be helpful. That's what this article is about: Apple developer's Red Treasure Book, which includes variables, collections, functions, classes, and so on.

In the following example, above is the Objective-C code, and below is the equivalent Swift code. Where necessary, I'll give you some notes to help you understand.

Variables and Constants

Create a variable

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

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

Create a constant

//Objective-C
const NSInteger score = 556;
//
NSString * const name = @"Taylor";
//
const BOOL firstRun = YES;
//Object-C is rarely used

//Swift
let score = 556
//
let name = "Taylor"
//
let firstRun = true
//Swift constants are common

Create an array of variables

Create a constant array

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

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

Add a value type to the array

//Objective-C
NSMutableArray *array = [NSMutableArray new];
//
[array addObject:[NSValue valueWithRect:CGRectMake(0, 0, 32, 64)]];
//Before adding to a collection, the value type has a corresponding reference type

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

Create a dictionary

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

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

Define an enumeration

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

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

Attach a string of characters

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

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

add number

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

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

Insert string

//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)"

Print debugging information

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

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

control flow

Check state

//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.")
}

Cycle a certain number of times

//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.")
}

Loop in an array

//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.")
}

Numerical switching

//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.");
}
//Many people don't know that Objective-C has scope support, so you may see a grammar of two choices.

//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 won't fall through the case unless you use the fall through keyword

function

Functions that do not receive parameters and do not return

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

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

Returns a function of a string without receiving parameters

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

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

Receive a string and return a function of a string

//Objective-C
- (NSString*)printGreetingFor:(NSString*)user {
 return [NSString stringWithFormat:@"Hello, %@!", user];
}
 
NSString *result = [self printGreetingFor:@"Paul"];
//The name of the first parameter needs to be part of the method name

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

let result = printGreeting(for: "Paul")

Receive a string and an integer and return a function of a string

//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)

Returns multiple values from a function

//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"];
//Object-C does not support tuple, so use dictionaries or arrays instead.

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

Closed Loop without Receiving Parameters and Not Returning

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

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

Closed loop that returns a string without receiving parameters

/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)

Receive a string parameter and return a closed loop of a string

//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)

class

Create empty class

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

//Swift
class MyClass: NSObject {
}
//Structures are recommended instead of classes, so you may not need to inherit from NSObject

Create a class with two attributes

//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 requires initialization and default values for these properties

Create a class with a private property

//Objective-C
//In the header file
@interface User : NSObject
@property (nonatomic, copy) NSString *name;
@end
 
//Execution Documents
@interface User()
@property (nonatomic, assign) NSInteger age;
@end
 
@implementation User
 
@end
//Objective-C does not actually support private attributes, usually in this flexible way

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

Create a class with an instance method

//Objective-C
@interface Civilization : NSObject
- (NSInteger)getMeaningOfLife;
@end
 
@implementation Civilization
- (NSInteger)getMeaningOfLife {
 return 42;
}
@end

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

Create a class with a static method

//Objective-C
@interface Civilization : NSObject
+ (NSInteger)getMeaningOfLife;
@end
 
@implementation Civilization
+ (NSInteger)getMeaningOfLife {
 return 42;
}
@end
//The difference is very small. Use + instead of -

//Swift
class Civilization {
 class func getMeaningOfLife() -> Int {
  return 42
 }
}
//Swift also supports static methods -- it won't be overridden in subclasses

Extending a type in a new way

//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)
 }
}

Check the class of an object

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

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

Type conversion

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

//Swift
let poodle = animalObject as? Dog
//
let poodle = animalObject as! Dog
//If it's not a dog, the former sets poodle to nil, and the latter crashes.

GCD
Running code in different threads

//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")
 }
}

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