Objective-C Style Guide

Credits

This styleguide is base on raywenderlich.com Objective-C Style Guide .

Background

Here are some of the documents from Apple that informed the style guide. If something isn't mentioned here, it's probably covered in great detail in one of these:

The Objective-C Programming Language
Cocoa Fundamentals Guide
Coding Guidelines for Cocoa
iOS App Programming Guide

Language

US English should be used.

UIColor *myColor = [UIColor whiteColor];

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 - Custom 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 {}

Spacing

Indent using 2 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 and other braces (if/else/switch/while etc.) always open on the same line as the statement but close on a new line.

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, but often there should probably be new methods.
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.

// blocks are easily readable
[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.

Naming

Long, descriptive method and variable names are good.

UIButton *settingsButton;

A three letter prefix 'MWM' should be used for class names and constants.

Constants should be camel-case with all words capitalized and prefixed by the related class name for clarity.

static NSTimeInterval const MWMTransitionAnimationDuration = 0.3;

Properties should be camel-case with the leading word being lowercase. Use auto-synthesis for properties rather than manual @synthesize statements unless you have good reason.

@property(strong, nonatomic) NSString *descriptiveVariableName;

Underscores

When using properties, instance variables should always be accessed and mutated using self.. This means that all properties will be visually distinct, as they will all be prefaced with self..

An exception to this: inside initializers, the backing instance variable (i.e. _variableName) should be used directly to avoid any potential side effects of the getters/setters.

Local variables should not contain underscores.

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.

- (void)setExampleText:(NSString *)text image:(UIImage *)image;
- (void)sendAction:(SEL)aSelector to:(id)anObject forAllCells:(BOOL)flag;
- (id)viewWithTag:(NSInteger)tag;
- (instancetype)initWithWidth:(CGFloat)width height:(CGFloat)height;

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.

Direct access to instance variables that 'back' properties should be avoided except in initializer methods (init, initWithCoder:, etc…), dealloc methods and within custom setters and getters. For more information on using Accessor Methods in Initializer Methods and dealloc, see here.

@interface MWMUser : NSObject

@property(copy, nonatomic) NSString *userName;

@end

Property Attributes

Property attributes should be explicitly listed, and will help new programmers when reading the code.

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

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.

@property(copy, nonatomic) NSString *userName;

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.

NSInteger arrayCount = [self.array count];
view.backgroundColor = [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.

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

static NSString * const MWMApplicationName = @"maps.me";

static CGFloat const MWMButtonWidth = 50.0;

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()

typedef NS_ENUM(NSUInteger, MWMMyPositionMode) {
  MWMMyPositionModePendingPosition,
  MWMMyPositionModeNotFollowNoPosition,
  MWMMyPositionModeNotFollow,
  MWMMyPositionModeFollow,
  MWMMyPositionModeFollowAndRotate
};

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

typedef NS_ENUM(NSUInteger, DurationInHours) {
  One = 1,
  Two = 2,
  Six = 6,
  Twelve = 12,
  Day = 24
};

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

Case Statements

Braces are not required for case statements, unless enforced by the complier.
When a case contains more than one line, braces should be added.

switch (condition) {
  case 1:
    // ...
    break;
  case 2: {
    // ...
    // Multi-line example using braces
    break;
  }
  case 3:
    // ...
    break;
  default:
    // ...
    break;
}

Private Properties

Private properties should be declared in class extensions (anonymous categories) in the implementation file of a class. Named categories (such as MWMPrivate or private) should never be used unless extending another class. The Anonymous category can be shared/exposed for testing using the +Private.h file naming convention.

@interface MWMPlacePageViewController ()

@property(strong, nonatomic) GADBannerView *googleAdView;
@property(strong, nonatomic) ADBannerView *iAdView;
@property(strong, nonatomic) UIWebView *adXWebView;

@end

Booleans

Objective-C uses YES and NO. Therefore true and false should only be used for CoreFoundation, C or C++ code. Since nil resolves to NO it is unnecessary to compare it in conditions. Never compare something directly to YES, because YES is defined to 1 and a BOOL can be up to 8 bits.

This allows for more consistency across files and greater visual clarity.

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

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.

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.

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

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

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);

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.

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

  //Do something important
}

Error handling

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

NSError *error;
if (![self trySomethingWithError:&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).

Line Breaks

Line length should not exceed 120 characters.

For example:

[MWMPushNotifications application:application didReceiveRemoteNotification:userInfo fetchCompletionHandler:completionHandler];

If a method declaration/call does not fit on a single line, put each parameter on its own line

[MWMPushNotifications application:application
     didReceiveRemoteNotification:userInfo
           fetchCompletionHandler:completionHandler];

Objective-C and C++

In general, C++ should be avoided in iOS-specific code. C++ can only be used when you need to call our Core API. In this case you should create some adapter classes which will work with both Objective-C and Swift. If .mm file contains C++ class, all code inside that class should be formatted according to C++ coding style, even if it contains Objective-C code lines.

C++ can't be used in Objective-C header files.

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OBJC_STYLE.md

OBJC_STYLE.md

Objective-C Style Guide

Credits

Background

Language

Code Organization

Spacing

Comments

Naming

Underscores

Methods

Variables

Property Attributes

Dot-Notation Syntax

Literals

Constants

Enumerated Types

Case Statements

Private Properties

Booleans

Conditionals

Ternary Operator

Init Methods

Class Constructor Methods

CGRect Functions

Golden Path

Error handling

Line Breaks

Objective-C and C++

Files

OBJC_STYLE.md

Latest commit

History

OBJC_STYLE.md

File metadata and controls

Objective-C Style Guide

Credits

Background

Language

Code Organization

Spacing

Comments

Naming

Underscores

Methods

Variables

Property Attributes

Dot-Notation Syntax

Literals

Constants

Enumerated Types

Case Statements

Private Properties

Booleans

Conditionals

Ternary Operator

Init Methods

Class Constructor Methods

CGRect Functions

Golden Path

Error handling

Line Breaks

Objective-C and C++