Objective C Style Guide

Back to Library

This style guide was forked from the NYTimes Objective-C Style Guide (LICENSE).

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:

Table of Contents

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 using 4 spaces. Never indent with tabs. Be sure to set this preference in Xcode.

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

  • Method braces always open on a newline after the method name. This practice helps readability with long method names that have parameters spread across several lines. It also aids with IDE code folding.

For example:

- (void)doSomethingWithUser:(User *)user { 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 be used to separate functionality (though often this can indicate an opportunity to split the method into several, smaller methods).

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.

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 named variables.

For example:

result = a > b ? x : y;

Not:

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

Omitted middle operand

x ?: y is shorthand for x ? x : y. This GCC extension is convenient for assigning default or fallback values.

For example:

UIColor *color = self.titleColor ?: [UIColor greenColor];

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 subsequently crash).

Additional reading:

Methods

In method signatures, there should be a space after the scope (- or + symbol). There should be a newline between method arguments, unless it harms readability. Typically, multiple similar method invocations in a row will look better if kept on a single line.

For example:

- (void)encodeWithCoder:(NSCoder *)encoder { [encoder encodeInteger:self.identifier forKey:PSKEntryIDKey]; [encoder encodeBool:self.isPublic forKey:PSKEntryIsPublicKey]; [encoder encodeObject:self.content forKey:PSKEntryContentKey]; [encoder encodeObject:self.media forKey:PSKEntryMediaKey]; }

Not:

- (void)encodeWithCoder:(NSCoder *)encoder { [encoder encodeInteger:self.identifier forKey:PSKEntryIDKey]; [encoder encodeBool:self.isPublic forKey:PSKEntryIsPublicKey]; [encoder encodeObject:self.content forKey:PSKEntryContentKey]; [encoder encodeObject:self.media forKey:PSKEntryMediaKey]; }

Variables

Variables should be named descriptively, with the variable’s name clearly communicating what the variable is and pertinent information a programmer needs to use that value properly.

For example:

  • NSString *title: It is reasonable to assume a “title” is a string.

  • NSString *titleHTML: This indicates a title that may contain HTML which needs parsing for display. “HTML” is needed for a programmer to use this variable effectively.

  • NSAttributedString *titleAttributedString: A title, already formatted for display. AttributedString hints that this value is not just a vanilla title, and adding it could be a reasonable choice depending on context.

  • NSDate *now: No further clarification is needed.

  • NSDate *lastModifiedDate: Simply lastModified can be ambiguous; depending on context, one could reasonably assume it is one of a few different types.

  • NSURL *URL vs. NSString *URLString: In situations when a value can reasonably be represented by different classes, it is often useful to disambiguate in the variable’s name.

  • NSString *releaseDateString: Another example where a value could be represented by another class, and the name can help disambiguate.

Single letter variable names should be avoided except as simple counter variables in loops.

Asterisks indicating a type is a pointer should be “attached to” the variable name. For example, NSString *text not NSString* text or NSString * text, except in the case of constants (NSString * const NYTConstantString).

Capitalize abbreviations (HTTP, URL, JSON, …). For example, NSError *JSONError instead of NSError *jsonError.

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, see Apple’s docs on using accessor methods in initializer methods and dealloc.

For example:

@interface NYTSection : NSObject   @property (nonatomic, copy) 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 (e.g., NYT) 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. A two letter prefix (e.g., NS) is reserved for use by Apple.

For example:

static const NSTimeInterval NYTArticleViewControllerNavigationFadeAnimationDuration = 0.3;

Not:

static const NSTimeInterval fadetime = 1.7;

Properties and local variables should be camel-case with the leading word being lowercase.

Instance variables should be camel-case with the leading word being lowercase, and should be prefixed with an underscore. This is consistent with instance variables synthesized automatically by LLVM. If LLVM can synthesize the variable automatically, then let it.

For example:

@synthesize descriptiveVariableName = _descriptiveVariableName;

Not:

id varnm;

Categories

Categories may be used to concisely segment functionality and should be named to describe that functionality.

For example:

@interface UIViewController (NYTMediaPlaying) @interface NSString (NSStringEncodingDetectio

Not:

@interface NYTAdvertisement (private) @interface NSString (NYTAdditions

Methods and properties added in categories should be named with an app- or organization-specific prefix. This avoids unintentionally overriding an existing method, and it reduces the chance of two categories from different libraries adding a method of the same name. (The Objective-C runtime doesn’t specify which method will be called in the latter case, which can lead to unintended effects.)

For example:

@interface NSArray (NYTAccessors) - (id)nyt_objectOrNilAtIndex:(NSUInteger)index; @end

Not:

@interface NSArray (NYTAccessors) - (id)objectOrNilAtIndex:(NSUInteger)index; @end

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.

Organization

Group methods under #pragma mark - sections for the class that first defined the methods.

For example:

#pragma mark - NSObject

  • -init

  • -description

  • -dealloc

#pragma mark - UIViewController

  • -viewDidLoad:

  • -viewDidAppear:

If there is a need for subgrouping, you can do #pragma mark - UIViewController (Rotation). However, large groups or large numbers of groups might suggest a logical refactoring.

Denote protocol conformance with angle brackets: #pragma mark - <UIScrollViewDelegate>

Alphabetize lists of related files/folders/variables or arrange them in some logical format.

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 initializer 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];

Format Specifiers

Prefer using %@. Wrap primitive values (float, double, long), in @(). For example, NSLog(@"Speed: %@ MPH", @(user.speed)), instead of NSLog(@"Speed: %f MPH", user.speed).

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

Use constants 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 = @"Originate";   static const CGFloat NYTImageThumbnailHeight = 50.0;

Not:

#define CompanyName @"Originate"   #define thumbnailHeight 2

Enumerated Types

When using enums, use the new fixed underlying type specification, which provides stronger type checking and code completion. The SDK includes a macro to facilitate and encourage use of fixed underlying types: NS_ENUM().

Example:

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

Bitmasks

When working with bitmasks, use the NS_OPTIONS macro.

Example:

typedef NS_OPTIONS(NSUInteger, NYTAdCategory) { NYTAdCategoryAutos = 1 << 0, NYTAdCategoryJobs = 1 << 1, NYTAdCategoryRealState = 1 << 2, NYTAdCategoryTechnology = 1 << 3 };

Properties

Property specifiers should be in this order:

@property (nonatomic|atomic, weak|strong|assign|copy, readwrite|readonly, nullable|nonnull

Private Properties

Private properties should be declared in class extensions (anonymous categories) in the implementation file of a class.

For example:

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

Image Naming

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.

Images that are used for a similar purpose should be grouped in respective groups in an Images folder or Asset Catalog.

Booleans

Never compare something directly to YES, because YES is defined as 1, and a BOOL in Objective-C is a CHAR type that is 8 bits long (so a value of 11111110 will return NO if compared to YES).

For an object pointer:

if (!someObject) { }   if (someObject == nil) { }

For a BOOL value:

if (isAwesome) if (!someNumber.boolValue

Not:

if (isAwesome == YES) // Never do this.

If the name of a BOOL property is expressed as an adjective, the property’s name can omit the is prefix but should specify the conventional name for the getter.

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 class] alloc] init]; });   return sharedInstance; }

This will prevent possible and sometimes frequent crashes.

Imports

For modules use the @import syntax. Xcode templates might still use the older #import <UIKit/UIKit.h>, so be sure to change those.

@import UIKit; @import OriginateHTTP;

Protocols

In a delegate or data source protocol, the first parameter to each method should be the object sending the message.

This helps disambiguate in cases when an object is the delegate for multiple similarly-typed objects, and it helps clarify intent to readers of a class implementing these delegate methods.

For example:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath;

Not:

- (void)didSelectTableRowAtIndexPath:(NSIndexPath *)indexPath;

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.

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.

Modern Features

Lightweight Generics

Take advantage of Objective-C’s lightweight generics starting in Xcode 7.

For example:

@property (nonatomic, copy) NSArray<NSDate *> *dates;

Not:

@property (nonatomic, copy) NSArray *dates;

Nullability annotations

Nullability annotations help with Swift interoperability.

Other Objective-C Style Guides

If ours doesn’t fit your tastes, have a look at some other style guides:

Back to Library