Lonely Planet Objective-C Style Guide

This guide is forked from The New York Times' Objective-C Style Guide

This guide expires on 11 July 2014. Please update this expiry date as part of making any changes to guide.

This style guide outlines the coding conventions of the iOS team at Lonely Planet. We welcome your feedback in issues, pull requests, and via Slack.

Introduction

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

Dot-Notation Syntax

Dot-notation should always be used for accessing and mutating properties. Bracket notation is preferred in all other instances.

For example:

view.backgroundColor = [UIColor orangeColor];
[UIApplication sharedApplication].delegate;

Not:

[view setBackgroundColor:[UIColor orangeColor]];
UIApplication.sharedApplication.delegate;

Spacing

Indent with one tab per indentation level. This should be the default setting in Xcode.
Method braces and other braces (if/else/switch/while/for etc.) always open and close on a new line.

For example:

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.
@synthesize and @dynamic should each be declared on new lines in the implementation.

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.

For example:

if (!error)
{
    return success;
}

Not:

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.

For example:

result = a > b ? x : y;

Not:

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

Error handling

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

For example:

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

Not:

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 subsequent crashes).

Methods

In method signatures, there should be a space after the scope (-/+ symbol). There should be a space between the method segments.

For Example:

- (void)setExampleText:(NSString *)text image:(UIImage *)image;

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.

Property definitions should be used in place of naked instance variables whenever possible. Direct instance variable access 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.

For example:

@interface NYTSection: NSObject

@property (nonatomic) NSString *headline;

@end

Not:

@interface NYTSection : NSObject {
    NSString *headline;
}

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.

For example:

UIButton *settingsButton;

Not

UIButton *setBut;

A three-letter prefix should always be used for class names and constants, however may be omitted for Core Data entity names. Constants should be camel-case with all words capitalized and prefixed by the related class name for clarity. Three-letter prefixes should be created on a per-project basis, and used for that project. For instance, Postcards uses the PCA prefix. Code that is intended to be used in a cross-app manner (e.g. code that's part of a private CocoaPod) should be prefixed with LPM.

For example:

static const NSTimeInterval NYTArticleViewControllerNavigationFadeAnimationDuration = 0.3;

Not:

static const NSTimeInterval fadetime = 1.7;

Properties should be camel-case with the leading word being lowercase. If Xcode can automatically synthesize the variable, then let it. Otherwise, in order to be consistent, the backing instance variables for these properties should be camel-case with the leading word being lowercase and a leading underscore. This is the same format as Xcode's default synthesis.

For example:

@synthesize descriptiveVariableName = _descriptiveVariableName;

Not:

id varnm;

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.. Local variables should not contain underscores.

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. This does not apply to those comments used to generate documentation.

init and dealloc

dealloc methods should be placed at the top of the implementation, directly after the @synthesize and @dynamic statements. init should be placed directly below the dealloc methods of any class.

init methods should be structured like this:

- (instancetype)init
{
    self = [super init]; // or call the designated initalizer
    if (self)
    {
        // Custom initialization
    }

    return self;
}

Literals

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

For example:

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

Not:

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 *buildingZIPCode = [NSNumber numberWithInteger:10018];

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.

For example:

CGRect frame = self.view.frame;

CGFloat x = CGRectGetMinX(frame);
CGFloat y = CGRectGetMinY(frame);
CGFloat width = CGRectGetWidth(frame);
CGFloat height = CGRectGetHeight(frame);

Not:

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;

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.

For example:

static NSString * const NYTAboutViewControllerCompanyName = @"The New York Times Company";

static const CGFloat NYTImageThumbnailHeight = 50.0;

Not:

#define CompanyName @"The New York Times Company"

#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()

Example:

typedef NS_ENUM(NSInteger, NYTAdRequestState) {
    NYTAdRequestStateInactive,
    NYTAdRequestStateLoading
};

For more info, see this NSHipster article.

Private Properties and Methods

Private properties should be declared in class extensions (anonymous categories) in the implementation file of a class. Named categories (such as LPMPrivate or private) should never be used unless extending another class.

For example:

@interface NYTAdvertisement ()

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

@end

Not:

@interface NYTAdvertisement (Private)

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

@end

Private methods no longer need to be declared, and can simply be implemented. However, for clarity, all private methods should be implemented beneath a #pragma mark - Private Methods to clearly distinguish them from overidden methods of a superclass.

Image Naming and the Asset Catalog

Image names should be named consistently to preserve organization and developer sanity. They should be named as one camel case string with a description of their purpose, followed by the un-prefixed name of the class or property they are customizing (if there is one), followed by a further description of color and/or placement, and finally their state.

For example:

RefreshBarButtonItem / RefreshBarButtonItem@2x and RefreshBarButtonItemSelected / RefreshBarButtonItemSelected@2x
ArticleNavigationBarWhite / ArticleNavigationBarWhite@2x and ArticleNavigationBarBlackSelected / ArticleNavigationBarBlackSelected@2x.

Always use the Assets Catalog for images, grouping all images for a given use (e.g. RefreshBarButtonItem / RefreshBarButtonItem@2x and RefreshBarButtonItemSelected / RefreshBarButtonItemSelected@2x) in a given Image Set. More info about Asset Catalogs is available here.

Booleans

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.

For a BOOL, here are two examples:

if (isAwesome)
{
	// Do something
}

or:

if (![someObject boolValue])
{
	// Do something
}

Not:

if ([someObject boolValue] == NO)
{
	// Do something
}

And NEVER:

if (isAwesome == YES)
{
	// Do something
}

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

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

Text and example taken from the Cocoa Naming Guidelines.

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.

Xcode project

The physical files should be kept in sync with the Xcode project files in order to avoid file sprawl. Any Xcode groups created should be reflected by folders in the filesystem. Code should be grouped not only by type, but also by feature for greater clarity, and folders and their contents should be alphabetized for easier reading.

TODO: Point to a canonical example project

When possible, always turn on "Treat Warnings as Errors" in the target's Build Settings and enable as many additional warnings as possible. If you need to ignore a specific warning, use Clang's pragma feature.

Core Data Model Files

The current core data model file should not contain a version specific identifier. When adding new model versions, the current model file should be renamed to include the current version of the app, and a new model file added without any version specific identifier. This strategy makes it simple to identify which model versions belong to which versions of app releases.

Other Objective-C Style Guides

Here are some other guides with different approaches to certain things described above; let these be a reference point for conversation on the above.

Google
GitHub
Adium
Sam Soffes
CocoaDevCentral
Luke Redpath
Marcus Zarra

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README.md

README.md

Lonely Planet Objective-C Style Guide

Introduction

Table of Contents

Dot-Notation Syntax

Spacing

Conditionals

Ternary Operator

Error handling

Methods

Variables

Naming

Underscores

Comments

init and dealloc

Literals

CGRect Functions

Constants

Enumerated Types

Private Properties and Methods

Image Naming and the Asset Catalog

Booleans

Singletons

Xcode project

Core Data Model Files

Other Objective-C Style Guides

Files

README.md

Latest commit

History

README.md

File metadata and controls

Lonely Planet Objective-C Style Guide

Introduction

Table of Contents

Dot-Notation Syntax

Spacing

Conditionals

Ternary Operator

Error handling

Methods

Variables

Naming

Underscores

Comments

init and dealloc

Literals

CGRect Functions

Constants

Enumerated Types

Private Properties and Methods

Image Naming and the Asset Catalog

Booleans

Singletons

Xcode project

Core Data Model Files

Other Objective-C Style Guides