Objective-C style guide.

Language

US English should be used.

Preferred:

UIColor *myColor = [UIColor whiteColor];

Not Preferred:

UIColor *myColour = [UIColor whiteColor];

Naming

Apple naming conventions should be adhered to wherever possible, especially those related to memory management rules (NARC).

Long, descriptive method and variable names are good.

Preferred:

UIButton *settingsButton;
@property (strong, nonatomic) NSString *descriptiveVariableName;

Not Preferred:

UIButton *setBut;
id varnm;

Golden Path

When coding with conditionals, the left hand margin of the code should be the "golden" or "happy" path. That is, don't nest if statements. Multiple return statements are OK.

Preferred:

- (void)someMethod 
{
  if (![someOther boolValue]) {
    return;
  }

  //Do something important
}

Not Preferred:

- (void)someMethod 
{
  if ([someOther boolValue]) {
    //Do something important
  }
}

Spacing

• Indent using 4 spaces (this conserves space in print and makes line wrapping less likely). Never indent with tabs. Be sure to set this preference in Xcode.
• Method braces open on next line.

Preferred:

- (void)methodName
{
    //Do something
}

Not Preferred:

- (void)methodName {
    //Do something
}

• Other braces (if/else/switch/while etc.) always open on the same line as the statement but close on a new line.

Preferred:

if (user.isHappy) {
    //Do something
} else {
    //Do something else
}

Not Preferred:

if (user.isHappy)
{
    //Do something
}
else {
    //Do something else
}

• There should be exactly one blank line between methods to aid in visual clarity and organization. Whitespace within methods should separate functionality.
• Prefer using auto-synthesis. But if necessary, @synthesize and @dynamic should each be declared on new lines in the implementation.
• Colon-aligning method invocation should often be avoided. There are cases where a method signature may have >= 3 colons and colon-aligning makes the code more readable. Please do NOT however colon align methods containing blocks because Xcode's indenting makes it illegible.

Preferred:

// blocks are easily readable
[UIView animateWithDuration:1.0 animations:^{
  // something
} completion:^(BOOL finished) {
  // something
}];

Not Preferred:

// colon-aligning makes the block indentation hard to read
[UIView animateWithDuration:1.0
                 animations:^{
                     // something
                 }
                 completion:^(BOOL finished) {
                     // something
                 }];

Comments

When they are needed, comments should be used to explain why a particular piece of code does something. Any comments that are used must be kept up-to-date or deleted.

Block comments should generally be avoided, as code should be as self-documenting as possible, with only the need for intermittent, few-line explanations. Exception: This does not apply to those comments used to generate documentation.

Preferred:

// UIButton *setBut;

Not Preferred:

/* UIButton *settingsButton; */

Do not use #if 0 as Xcode's syntax heighliter won't undrestand.

Methods

In method signatures, there should be a space after the method type (-/+ symbol). There should be a space between the method segments (matching Apple's style). Always include a keyword and be descriptive with the word before the argument which describes the argument.

The usage of the word "and" is reserved. It should not be used for multiple parameters as illustrated in the initWithWidth:height: example below.

Preferred:

- (void)setExampleText:(NSString *)text image:(UIImage *)image;
- (instancetype)initWithWidth:(CGFloat)width height:(CGFloat)height;

Not Preferred:

-(void)setT:(NSString *)text i:(UIImage *)image;
- (instancetype)initWith:(int)width and:(int)height;  // Never do this. "and" is reserved.

Variables

Variables should be named as descriptively as possible. Single letter variable names should be avoided except in for() loops.

Asterisks indicating pointers belong with the variable, e.g., NSString *text not NSString* text or NSString * text, except in the case of constants.

Private properties should be used in place of instance variables whenever possible. Although using instance variables is a valid way of doing things, by agreeing to prefer properties our code will be more consistent.

Preferred:

@interface RWTTutorial : NSObject

@property (strong, nonatomic) NSString *tutorialName;

@end

Not Preferred:

@interface RWTTutorial : NSObject {
  NSString *tutorialName;
}

Property Attributes

Property attributes should be explicitly listed, and will help new programmers when reading the code. The order of properties should be atomicity then storage.

Preferred:

@property (nonatomic, weak) IBOutlet UIView *containerView;
@property (nonatomic, strong) NSString *tutorialName;

Not Preferred:

@property (weak, nonatomic) IBOutlet UIView *containerView;
@property (nonatomic) NSString *tutorialName;

Properties with mutable counterparts (e.g. NSString) should prefer copy instead of strong. Why? Even if you declared a property as NSString somebody might pass in an instance of an NSMutableString and then change it without you noticing that.

Preferred:

@property (copy, nonatomic) NSString *tutorialName;

Not Preferred:

@property (strong, nonatomic) NSString *tutorialName;

Dot-Notation Syntax

Dot syntax is purely a convenient wrapper around accessor method calls. When you use dot syntax, the property is still accessed or changed using getter and setter methods. Read more here

Dot-notation should always be used for accessing and mutating properties, as it makes code more concise. Bracket notation is preferred in all other instances.

Preferred:

NSInteger arrayCount = [self.array count];
view.backgroundColor = [UIColor orangeColor];
[UIApplication sharedApplication].delegate;

Not Preferred:

NSInteger arrayCount = self.array.count;
[view setBackgroundColor:[UIColor orangeColor]];
UIApplication.sharedApplication.delegate;

Literals

NSString, NSDictionary, NSArray, and NSNumber literals should be used whenever creating immutable instances of those objects. Pay special care that nil values can not be passed into NSArray and NSDictionary literals, as this will cause a crash.

Preferred:

NSArray *names = @[@"Brian", @"Matt", @"Chris", @"Alex", @"Steve", @"Paul"];
NSDictionary *productManagers = @{@"iPhone": @"Kate", @"iPad": @"Kamal", @"Mobile Web": @"Bill"};
NSNumber *shouldUseLiterals = @YES;
NSNumber *buildingStreetNumber = @10018;

Not Preferred:

NSArray *names = [NSArray arrayWithObjects:@"Brian", @"Matt", @"Chris", @"Alex", @"Steve", @"Paul", nil];
NSDictionary *productManagers = [NSDictionary dictionaryWithObjectsAndKeys: @"Kate", @"iPhone", @"Kamal", @"iPad", @"Bill", @"Mobile Web", nil];
NSNumber *shouldUseLiterals = [NSNumber numberWithBool:YES];
NSNumber *buildingStreetNumber = [NSNumber numberWithInteger:10018];

Constants

Constants are preferred over in-line string literals or numbers, as they allow for easy reproduction of commonly used variables and can be quickly changed without the need for find and replace. Constants should be declared as static constants and not #defines unless explicitly being used as a macro.

Preferred:

static NSString * const RWTAboutViewControllerCompanyName = @"RayWenderlich.com";

static CGFloat const RWTImageThumbnailHeight = 50.0;

Not Preferred:

#define CompanyName @"RayWenderlich.com"

#define thumbnailHeight 2

Enumerated Types

When using enums, it is recommended to use the new fixed underlying type specification because it has stronger type checking and code completion. The SDK now includes a macro to facilitate and encourage use of fixed underlying types: NS_ENUM()

For Example:

typedef NS_ENUM(NSInteger, RWTLeftMenuTopItemType) {
  RWTLeftMenuTopItemMain = 0,
  RWTLeftMenuTopItemShows,
  RWTLeftMenuTopItemSchedule
};

You can also make explicit value assignments (showing older k-style constant definition):

typedef NS_ENUM(NSInteger, RWTGlobalConstants) {
  RWTPinSizeMin   = 1,
  RWTPinSizeMax   = 5,
  RWTPinCountMin  = 100,
  RWTPinCountMax  = 500,
};

Older k-style constant definitions should be avoided unless writing CoreFoundation C code (unlikely).

Not Preferred:

enum GlobalConstants {
  kMaxPinSize = 5,
  kMaxPinCount = 500,
};

Booleans

Preferred:

if (someObject) {}
if (![anotherObject boolValue]) {}

Not Preferred:

if (someObject == nil) {}
if ([anotherObject boolValue] == NO) {}

If the name of a BOOL property is expressed as an adjective, the property can omit the “is” prefix but specifies the conventional name for the get accessor, for example:

@property (assign, getter=isEditable) BOOL editable;

Text and example taken from the Cocoa Naming Guidelines.

Conditionals

Conditional bodies should always use braces even when a conditional body could be written without braces (e.g., it is one line only) to prevent errors. These errors include adding a second line and expecting it to be part of the if-statement. Another, even more dangerous defect may happen where the line "inside" the if-statement is commented out, and the next line unwittingly becomes part of the if-statement. In addition, this style is more consistent with all other conditionals, and therefore more easily scannable.

Preferred:

if (!error) {
  return success;
}

Not Preferred:

if (!error)
  return success;

or

if (!error) return success;

Ternary Operator

The Ternary operator, ?: , should only be used when it increases clarity or code neatness. A single condition is usually all that should be evaluated. Evaluating multiple conditions is usually more understandable as an if statement, or refactored into instance variables. In general, the best use of the ternary operator is during assignment of a variable and deciding which value to use.

Non-boolean variables should be compared against something, and parentheses are added for improved readability. If the variable being compared is a boolean type, then no parentheses are needed.

Preferred:

NSInteger value = 5;
result = (value != 0) ? x : y;

BOOL isHorizontal = YES;
result = isHorizontal ? x : y;

Not Preferred:

result = a > b ? x = c > d ? c : d : y;

Init Methods

Init methods should follow the convention provided by Apple's generated code template. A return type of 'instancetype' should also be used instead of 'id'.

- (instancetype)init 
{
  self = [super init];
  if (self) {
    // ...
  }
  return self;
}

See Class Constructor Methods for link to article on instancetype.

Class Constructor Methods

Where class constructor methods are used, these should always return type of 'instancetype' and never 'id'. This ensures the compiler correctly infers the result type.

@interface Airplane
+ (instancetype)airplaneWithType:(RWTAirplaneType)type;
@end

More information on instancetype can be found on NSHipster.com.

CGRect Functions

When accessing the x, y, width, or height of a CGRect, always use the CGGeometry functions instead of direct struct member access. From Apple's CGGeometry reference:

All functions described in this reference that take CGRect data structures as inputs implicitly standardize those rectangles before calculating their results. For this reason, your applications should avoid directly reading and writing the data stored in the CGRect data structure. Instead, use the functions described here to manipulate rectangles and to retrieve their characteristics.

Preferred:

CGRect frame = self.view.frame;

CGFloat x = CGRectGetMinX(frame);
CGFloat y = CGRectGetMinY(frame);
CGFloat width = CGRectGetWidth(frame);
CGFloat height = CGRectGetHeight(frame);
CGRect frame = CGRectMake(0.0, 0.0, width, height);

Not Preferred:

CGRect frame = self.view.frame;

CGFloat x = frame.origin.x;
CGFloat y = frame.origin.y;
CGFloat width = frame.size.width;
CGFloat height = frame.size.height;
CGRect frame = (CGRect){ .origin = CGPointZero, .size = frame.size };

Error handling

When methods return an error parameter by reference, switch on the returned value, not the error variable.

Preferred:

NSError *error = nil;
if (![self trySomethingWithError:&error]) {
  // Handle Error
}

Not Preferred:

NSError *error;
[self trySomethingWithError:&error];
if (error) {
  // Handle Error
}

Some of Apple’s APIs write garbage values to the error parameter (if non-NULL) in successful cases, so switching on the error can cause false negatives (and subsequently crash).

Code Organization

Use #pragma mark - to categorize methods in functional groupings and protocol/delegate implementations following this general structure.

#pragma mark - Lifecycle

- (instancetype)init {}
- (void)dealloc {}
- (void)viewDidLoad {}
- (void)viewWillAppear:(BOOL)animated {}
- (void)didReceiveMemoryWarning {}

#pragma mark - Accessors

- (void)setCustomProperty:(id)value {}
- (id)customProperty {}

#pragma mark - IBActions

- (IBAction)submitData:(id)sender {}

#pragma mark - Public

- (void)publicMethod {}

#pragma mark - Private

- (void)privateMethod {}

#pragma mark - Protocol conformance
#pragma mark - UITextFieldDelegate
#pragma mark - UITableViewDataSource
#pragma mark - UITableViewDelegate

#pragma mark - NSCopying

- (id)copyWithZone:(NSZone *)zone {}

#pragma mark - NSObject

- (NSString *)description {}

Singletons

Singleton objects should use a thread-safe pattern for creating their shared instance.

+ (instancetype)sharedInstance {
  static id sharedInstance = nil;

  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    sharedInstance = [[self alloc] init];
  });

  return sharedInstance;
}

This will prevent possible and sometimes prolific crashes.

PS.This style guide is based on raywenderlich/objective-c-style-guide