Westfield Labs Objective-C Style Guide

This style guide outlines the coding conventions of the iOS team at Westfield Labs. We welcome your feedback in issues and pull requests. Also, we’re hiring.

Originally forked from The New York Times Objective-C Style Guide.

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

Table of Contents

• Dot Notation Syntax
• Spacing
• Conditionals
  • Ternary Operator
• Error handling
• Methods
• Line breaks
• Variables
• Naming
  • Categories
• Comments
• Init & Dealloc
• Literals
• CGRect Functions
• Constants
• Enumerated Types
• Bitmasks
• Private Properties
• Booleans
• Singletons
• Imports
• Protocols
• Xcode Project

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.
• 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.

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 be used to separate functionality (though often this can indicate an opportunity to split the method into several, smaller methods). In methods with long or verbose names, a single line of whitespace may be used to provide visual separation before the method’s body.
• @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 named 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 subsequently crash).

Methods

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

For example:

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

The order of method signatures on header files should always have the class methods preceding the instance methods and include an empty line between both types of methods.

For example:

+ (instancetype)sharedManager;

- (void)getRequestWithURL:(NSURL *)URL; 
- (void)getRequestWithURLString:(NSString *)URLString; 

Line Breaks

You can break lines in the middle of a method declaration or call, as long as the new line is aligned on colons:

For example:

- (void)setReallyLongMethodNameThatDoesntEventFitOnScreenExampleText:(NSString *)text 
                           reallyLongSecondParameterThatIsNamedImage:(UIImage *)image;
                                                               
                                                               
[self setReallyLongMethodNameThatDoesntEventFitOnScreenExampleText:@"Text"
                         reallyLongSecondParameterThatIsNamedImage:nil];
                                               

However, you can't break lines on a method call if one of the parameter is an inline block, to avoid long identation.

For example:

NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request progress:nil destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
    NSURL *documentsDirectoryURL = [[NSFileManager defaultManager] URLForDirectory:NSDocumentDirectory inDomain:NSUserDomainMask appropriateForURL:nil create:NO error:nil];
    return [documentsDirectoryURL URLByAppendingPathComponent:[response suggestedFilename]];
} completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
    NSLog(@"File downloaded to: %@", filePath);
}];

Not:

NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request
                                                                 progress:nil
                                                              destination:^NSURL *(NSURL *targetPath, NSURLResponse *response) {
                                                                  NSURL *documentsDirectoryURL = [[NSFileManager defaultManager] URLForDirectory:NSDocumentDirectory inDomain:NSUserDomainMask appropriateForURL:nil create:NO error:nil];
                                                                  return [documentsDirectoryURL URLByAppendingPathComponent:[response suggestedFilename]];
                                                              }
                                                        completionHandler:^(NSURLResponse *response, NSURL *filePath, NSError *error) {
                                                            NSLog(@"File downloaded to: %@", filePath);
                                                        }];

Note that you also shouldn't break line before an end curly brace and the next parameter.

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).

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;
}

Properties Qualifiers

Properties should always have qualifiers (nonatomic or atomic and weak, strong or copy). The use of assign is discouraged, since it's the only option for primitive types (and weak should be used instead for an object type).

readwrite should also be ommited for the same reason as assign (unless if you're overriding a property behavior and it's really needed).

copy should be used for types that have a mutable variant, such as NSString, NSDictionary and NSMutableDictionary.

The order of the attributes must be (nonatomic/atomic), (weak/strong/copy), (readonly), (nonnull/null).

For example:

@property (nonatomic, copy) NSString *headline;
@property (nonatomic, copy, readonly) NSString *title;
@property (nonatomic) NSInteger currentPage;
@property (nonatomic, weak) IBOutlet UIButton *privacyPolicyButton;
@property (nonatomic, weak, nullable) id<UITableViewDelegate> delegate;
@property (nonatomic, weak) id<UITableViewDelegate> delegate;

Not:

@property (nonatomic, strong) NSString *headerText;
@property (nonatomic, readwrite) CGFloat imageHeight;
@property NSInteger currentPage;
@property (nonatomic) NSArray *URLs;
@property (nonatomic, strong) IBOutlet UIButton *contactButton;
@property (weak, nonatomic) IBOutlet UIButton *submitButton;

Variable Qualifiers

When it comes to the variable qualifiers introduced with ARC, the qualifier (__strong, __weak, __unsafe_unretained, __autoreleasing) should be placed before the variable type, e.g., __weak NSString *text.

Types

Platform-dependent primitive types should be avoided:

• NSInteger or NSUInteger instead of long or int
• CGFloat instead of float or double

For calling C-functions that receive a float or double, you should check if there's a version that takes a CGFloat implemented in CGFloatType. For example:

CGFloat roundedWidth = roundCGFloat(width);

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;
static const NSTimeInterval kFadeAnimationDuration = 0.5;

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 (NSStringEncodingDetection)

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.

Xcode's default file comment header with a Copyright notice should be avoided, except when using 3rd party code that for some reason wasn't imported via CocoaPods.

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];

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:

static const NSTimeInterval kFadeAnimationDuration = 0.5;

#define CompanyName @"The New York Times Company"

#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
};

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

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). Comparing to NO is also discouraged.

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 (someNumber.boolValue == NO) // Prefer !someNumber.boolValue

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

The imports are automatically organized by a build phase. The expected behavior can be seen below.

Note: For modules use the @import syntax. @import should be used whenever possible.

@import CoreGraphics;
@import UIKit;

#import <AFNetworking/AFNetworking.h>

#import "NYTProfileViewController.h"
#import "NYTButton.h"
#import "NYTUserView.h"

The imports are grouped by kinds:

1. Module imports (@import Something;) sorted alphabetically
2. Library imports (#import <SomeLibrary/Something.h>) sorted alphabetically
3. Local import (#import "Something.h) sorted alphabetically. If the file is an implementation file (.m), the first import is its header.

There should be an empty line between two different groups.

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.