Wonderful Objective-C style guide.

This style guide outlines the OSX and iOS coding conventions that I like to follow. The conventions focus on readability by restoring some tried and tested formatting conventions that have somehow given way to overzealously compacted code over recent years. The irony is that today our computers are faster than ever and our optimizing compilers are awesome. No performance is lost in adding extra newlines and allocating additional local variables but efficiency gains can be tremendous.

This guide is more comprehensive and also more specific than the individual guides from which I drew my inspiration. I believe it is pretty feature complete right now and ready to be applied to your own code or to that of your organization.

I welcome your feedback in issues and pull requests.

Introduction

As with any coding style guide, this one has been created to not only gain a high level of consistency between disparate bodies of code but also to vastly improve legibility. While the former goal is important because it alleviates the engineer from having to adjust to differing stylistic conventions, the latter is critical when it comes down to the arduous task of scanning someone else's code in search of a time-tested pattern or an otherwise obvious error.

The application and enforcement of a style guide is not about doing things the right way or the wrong way, it's about agreeing on doing things the same way.

Credits

To get here, we leveraged the work of The official raywenderlich.com Objective-C style guide, The New York Times Objective-C Style Guide and the Objective-C Cheat Sheet. Many thanks to the contributors to those documents.

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

Tools

To assist with the application of the conventions defined herein, I have created an uncrustify config file (".uncrustify.cfg") that, for the most part, can automatically reformat your code for you and can be used in conjunction with the BBUncrustifyPlugin-Xcode plugin. My suggestion is to drop that config file into your project's top directory so that the formatting style is maintained along side the project itself.

NOTE: By default, uncrustify does not look for a .uncrustify.cfg file in your project directory. You can specify that location when using uncrustify from the command line with the "-c" flag. If using the Xcode plugin mentioned below you will need to make sure to build and install my forked version until 2.0.4 has been released.

Why Uncrustify and not Clang-Format?

Let's just get this out of the way right now. While clang-format has come along quite nicely, when it comes down to configurability uncrustify wins by a great margin. This is especially true for auto formatting objective-c code. Believe me, I started with writing a clang-format config file to support this project, but I quickly learned it wasn't possible to specify all of the formatting requirements.

BBUncrustifyPlugin-Xcode

To make it easier to use the supplied uncrustify config file, I suggest installing the excellent BBUncrustifyPlugin-Xcode so you can reformat code live from within Xcode. You can install BBUncrustifyPlugin-Xcode manually or via the Alcatraz Xcode package manager.

Once BBUncrustifyPlugin-Xcode has been installed, follow these steps:

Restart Xcode.
Configure BBUncrustifyPlugin:

Edit->Format Code->BBUncrustifyPlugin Preferences...

As seen here:

Uncrustify 0.61-snapshot

The current distributed version of uncrustify has some shortcomings when it comes to formatting objective-c code properly, specifically messages and blocks. I have been working on fixes. :)

While the BBUncrustifyPlugin-Xcode comes with its own build of uncrustify, I suggest you download my snapshot build and install it in place of the binary used by the Xcode plugin. You should then use the ".uncrustify-061.cfg" config file (make sure you rename it to just ".uncrustify.cfg").

Download my latest uncrustify snapshot:

uncrustify-0.61-snapshot.zip

Language
Project Organization
Code Organization
Line Wrapping (Code Width)
Spacing
Comments
Documentation
- Class Documentation
- Constant Documentation
- Property Documentation
- Class Method Documentation
- Instance Method Documentation
Naming
- Naming Conventions for Methods and Variables
- Naming Conventions for Constants and Macros
- Naming Conventions for Enumerated Types
- Underscores
Methods
Variables
Property Attributes
Dot-Notation Syntax
Literals
Constants
Enumerated Types
Switch Statements and Case Label Blocks
Private Properties
Booleans
Conditionals
- Conditional Expressions
- Ternary Operator
Return Statements
Init Methods
Class Constructor Methods
CGRect Functions
Golden Path
Error handling
Singletons
Line Breaks
Xcode Project
Other Objective-C Style Guides

Language

US English should be used.

Preferred:

UIColor *myColor = [UIColor whiteColor];

Not Preferred:

UIColor *myColour = [UIColor whiteColor];

Project Organization

Apple provides two methods for organizing code within projects:

Groups
Folder references

When adding folders to an Xcode project, you will be given the option to select how to treat folders. The default is to create Groups.

The problem with using Groups is that the organization is purely internal to Xcode; that is, if you look at the file structure on disk you will see that there is no organization whatsoever. As an alternative, you could use Folder references.

Both techniques have their pros and cons. A better tactic is to use Groups with references.

Refer to the following recommended folder structure for an OSX project:

Your_Project
  |-- AppDelegate.h
  |-- AppDelegate.m
  |-- Images.xcassets
  |-- MainMenu.xib
  |-- Supporting Files
  |-- Models
  |-- Views
  |-- Controllers
  |-- Stores
  |-- Helpers

Following initial project creation, add each of the folders by right-clicking on your project folder and selecting New Group. After each folder has been added, select one and view its settings in the Identity Inspector. Click on the gray folder icon that appears next to the Group name, under the Location dropdown menu. Click on the New Folder button and create a new folder with the same name as the group, then click on the Choose button.

Add new files of each type to each respective folder.

Moving existing files into the right folder

If you have already create files in the project and now need to move them, you can move them at the same time as when you create a containing folder. For instance, select all of your Views and the right click. Select New Group from Selection then name the folder accordingly. Next, select the folder and view its settings in the Identity Inspector. Click on the gray folder icon that appears next to the Group name, under the Location dropdown menu. Click on the New Folder button and create a new folder with the same name as the group, then click on the Choose button.

The final step is to move the files on disk into their respective folders.

NOTE: If you are using git you must use the git mv command to move these files into place, instead of any other method (mv, OSX Finder, etc).

Code Organization

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

#pragma mark - Class Methods

+ (ClassObject)classWithDefaults;
+ (ClassObject)convertToJSON;
+ (ClassObject)convertToXML;

#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 {}

Line Wrapping (Code Width)

Although really big computer displays are the norm these days as they help boost the productivity of engineers, that productivity gain is bound to trend downward if a large portion of that screen real estate must be dedicated to a single code window because of excessively long lines. While automated wrapping may at first appear to be a solution, that feature tosses out all of the gains achieved from proper styling...and therefore efficiency is similarly lost. The better solution is to continue to restrict line width.

Line width is currently restricted to 80 columns. Xcode should be configured to display a Page Guide at 80 columns to assist with manual formatting:

Preferences->Text Editing->Page Guide at column:

As seen here:

Objective-C is a verbose language. Selectors can be very long. The recommendation is to use an Xcode plugin along with a formatting configuration to facilitate the reformatting of code to comply with this guide.

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 a new line and 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
}

In conditional statements, braces appear on the same line as an else or else if branching key word. The conditional statement is less readable when constructed as follows:

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.

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.

Documentation

Header files should include completed documentation tags to support Doxygen auto-generation via the Doxywrite tool. In general, JavaDoc style tags should be used. Documentation for a public interface will be generated from header files only. Include documentation for a private interface in the implementation file.

The following items should be documented to facilitiate developer on-boarding, development of new functionality, and the ongoing maintenance cycle:

Classes
Prototype
Constants
Properties
Class Methods
Instance Methods

Class Documentation

-- **EXAMPLE PENDING** --

Constant Documentation

/**
 *  @memberof PDFReaderConfig
 *  Default value for bookmarksEnabled: TRUE
 */
extern const BOOL kPDFReaderDefaultBookmarksEnabled;

Property Documentation

/**
 *  Enable/disable bookmark support.
 *
 *  @see kPDFReaderDefaultBookmarksEnabled
 */
@property (nonatomic, readwrite, unsafe_unretained, getter=isBookmarksEnabled)
    BOOL bookmarksEnabled;

Class Method Documentation

/**
 * Returns the shared PDFReaderConfig instance, creating it if necessary.
 *
 * @return The shared PDFReaderConfig instance.
 */
+ (instancetype)sharedConfig;

Instance Method Documentation

/**
 *  Initializes and returns a newly allocated PDFReaderViewController object.
 *
 *  @param object Reference to an initialized PDFReaderDocument
 *
 *  @return Initialized class instance or nil on failure
 *
 *  @throws "<MissingArguments>" When object is nil
 *  @throws "<WrongType>" When object is not a reference to a PDFReaderDocument
 *    object
 *
 *  @remark Designated initializer.
 */
- (instancetype)initWithReaderDocument:(PDFReaderDocument *)object;

/**
 *  Update bookmark state for each page in the PDFReaderDocument's bookmarks 
 *    list.
 */
- (void)updateToolbarBookmarkIcon;

Naming

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

Naming Conventions for Methods and Variables

Long, descriptive method and variable names are good as they support the concept of self-documenting code. Therefore, while clarity and brevity are both important, clarity should never be sacrificed for brevity.

Preferred:

UIButton *settingsButton;
NSString *pageTitle = @"My Title";
int pageCounter = 0;

Not Preferred:

UIButton *setBut;
NSString *string = @"My Title";
int c = 0;

Naming Conventions for Constants and Macros

The following naming patterns may at first appear overly complex but the intention is to support a quick way for an engineer to determine the type (i.e. constant or macro), the origin, as well as the location of the item's declaration. The pattern breaks down like this:

tPRE_Space_Name

Where:

Element	Definition	Value	Example
t	type = constant	k*	kMIX_MyClass_DefaultTitle
	type = macro	m*	mBUZ_MyClass_doubleIt
PRE	three-letter prefix	XXX	kXXX_MyClass_MenuBarHeight
Space	unique namespace within tPRE	WindowSize	kXXX_MyClass_WindowSize
Name	unique name within tPRE_Space	BorderColor	kZRE_MyClass_BorderColor
		isIphone	mZBB_MyClass_isIphone

The type (t) element must either be a "k" (for constant) or an "m" (for macro). Other fields are to be defined by the developer.

Constant names should be camel-case with all words capitalized and prefixed by the letter "k" and a three-letter prefix followed by an underscore, followed by a specific namespace identifier (either the library name, class name, or app name), followed by another underscore which is then followed by the camel-case constant name (leading with an uppercase letter).

Preferred:

static const NSInteger kMIX_PDFReader_DefaultPagingViews = 3;

Not Preferred:

static NSTimeInterval const fadetime = 1.7;

Macro names should be camel-case with all words capitalized and prefixed by the letter "m" and a three-letter prefix followed by an underscore, followed by a specific namespace identifier (either the library name, class name, or app name), followed by another underscore which is then followed by the camel-case function name (leading with a lowercase letter).

Preferred

#define mMIX_MacroLib_rgb(r,g,b) [UIColor colorWithRed:r/255.0f green:g/255.0f blue:b/255.0f alpha:1.0f]

Not Preferred

#define RGBColor(r,g,b) [UIColor colorWithRed:r/255.0f green:g/255.0f blue:b/255.0f alpha:1.0f]

Property names should be camel-case with the leading word being lowercase.

Instance variable names 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.

Preferred:

@property (nonatomic, readwrite, strong) NSString *descriptiveVariableName;

// if/when needed
@synthesize descriptiveVariableName = _descriptiveVariableName;

Not Preferred:

id varnm;
...
@property (strong) id varnm;
...
@synthesize varnm;

Naming Conventions for Enumerated Types

Enumerated Type names should be camel-case with all words capitalized and prefixed by the letter "e" and a three-letter prefix followed by an underscore, followed by a specific namespace identifier (either the library name, class name, or app name), followed by another underscore which is then followed by the camel-case constant name (leading with an uppercase letter).

Preferred:

typedef enum eMIX_UtilityLib_PlayerStateType : NSInteger eMIX_UtilityLib_PlayerStateType;
enum eMIX_UtilityLib_PlayerStateType : NSInteger {
  PlayerStateOff,
  PlayerStatePlaying,
  PlayerStatePaused
};

// using the NS_ENUM macro (preferred)
typedef NS_ENUM(NSInteger, eMIX_UtilityLib_PlayerStateType) {
  PlayerStateOff,
  PlayerStatePlaying,
  PlayerStatePaused
};

Not Preferred:

enum {
    PlayerStateOff,
    PlayerStatePlaying,
    PlayerStatePaused
};

typedef NSInteger PlayerState;

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..

These are some notable exceptions to this rule:

Setup and Tear down: init and dealloc
Overriding accessors (getters/setters)
Archiving activities: e.g. NSCoding Protocol's encodeWithCoder and initWithCoder

Whenever the object is in an unstable state (e.g. during setup and tear down) you should access the backing instance variable (i.e. _variableName) directly to avoid any potential side effects of the getters/setters. For one opinion on this practice, see: Don’t Message self in Objective-C init (or dealloc).

Whenever you elect to override a getter/setter to provide a custom implementation (instead of the one provided by synthesis) you will need to access the backing instance variables directly to avoid a runtime loop.

Whenever you are attempting to archive and unarchive an object, you are likely intending to capture and restore the exact values of the internal data members; therefore, in those cases you should access th instance variables directly to avoid setting/restoring values that would otherwise be mutated through getters/setters.

Finally, 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.

Preferred:

- (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;

Not Preferred:

-(void)setT:(NSString *)text i:(UIImage *)image;
- (void)sendAction:(SEL)aSelector :(id)anObject :(BOOL)flag;
- (id)taggedView:(NSInteger)tag;
- (instancetype)initWithWidth:(CGFloat)width andHeight:(CGFloat)height;
- (instancetype)initWith:(int)width and:(int)height;  // Never do this.

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.

Preferred:

@interface MyAppTutorial : NSObject

@property (strong, nonatomic) NSString *tutorialName;

@end

Not Preferred:

@interface MyAppTutorial : 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, accessibility (readonly, readwrite), followed by storage. Yes, we include all three attributes every time.

This ordering varies from the generated code produced by Xcode's templates, but it increases legibility by enforcing a consistent visual pattern: most @property statements will begin with the same attributes (nonatomic, readwrite) which pushes the differences between adjacent statements to the right.

Preferred:

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

Not Preferred:

@property (nonatomic, readwrite, strong) NSString *tutorialName;

Dot-Notation Syntax

Dot notation syntax is purely a convenient wrapper around accessor method calls. When you use dot notation, the property is still accessed or changed using the underlying getter and setter methods (auto-generated via synthesis or not). Read more here.

Example:

- (void)updateMyBackroundColor
{
  // dot notation...
  self.bgColor = [UIColor blueColor];
  
  // is same as sending setBgColor message to self...
  [self setBgColor:[UIColor blueColor]];
}

It is because the getter and setter methods are still the underlying implementation involved that dot notation (and explicit messages to self) should generally be avoided within init and dealloc methods as discussed here. In these methods, access data members directly via the _variableName.

Example:

- (instancetype)init
{
  self = [super init];
  if (self)
  {
    _bgColor = [UIColor whiteColor];
  }

  return self;
}

Outside of init and dealloc, dot-notation should always be used for accessing and mutating properties as it makes code more concise.

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 (declared with the const keyword) 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 to conduct an error-prone find and replace. Constants should never be declared using the #define preprocessor directive as that pattern circumvents type checking; you may, however, use #define when declaring a macro.

Preferred:

// Format: type const constantName = value;
NSString * const kMIX_MyClassShortDateFormat = @"MM/dd/yyyy";

// Macro: #define constantName value
\#define mMIX_MacroLib_isIPad (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)

Not Preferred:

\#define CompanyName @"MyCompany"
\#define thumbnailHeight 2

To make a constant available to external classes, you must also add it to the header .h file:

extern NSString * const kMIX_MyClass_ShortDateFormat;

If the constant is to be used only within the implementation file in which it is declared, narrow its scope with the static keyword.

static NSString * const kMIX_MyClass_ShortDateFormat = @"MM/dd/yyyy";

A static variable declared within a method retains its value between invocations. This can be useful when declaring a singleton or creating custom setters/getters for a property.

Enumerated Types

When declaring enumerated types, 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()

Preferred

// using the NS_ENUM macro (preferred)
typedef NS_ENUM(NSInteger, eMIX_UtilityLib_PlayerStateType) {
  PlayerStateOff,
  PlayerStatePlaying,
  PlayerStatePaused
};

// with explicit value assignments
typedef NS_ENUM(NSInteger, eMIX_UtilityLib_PlayerStateType) {
  PlayerStateOff = 0,
  PlayerStatePlaying = 1,
  PlayerStatePaused = 2
};

Not Preferred:

// Apple Modern Objective-C style, provides type checking, enum type declared inline...
typedef enum _eMIX_UtilityLib_PlayerStateType : NSUInteger {
    PlayerStateOff,
    PlayerStatePlaying,
    PlayerStatePaused
} eMIX_UtilityLib_PlayerStateType;

// Apple Modern Objective-C style, provides type checking, enum type declared seperately...
enum _eMIX_UtilityLib_PlayerStateType : NSInteger {
  PlayerStateOff,
  PlayerStatePlaying,
  PlayerStatePaused
};
typedef enum _eMIX_UtilityLib_PlayerStateType : NSInteger eMIX_UtilityLib_PlayerStateType;

// Apple legacy style enum (32bit/64bit portability, no formal relationship between type and enum)...
enum {
    PlayerStateOff,
    PlayerStatePlaying,
    PlayerStatePaused
};
typedef NSInteger PlayerState;

// Apple legacy style enum (no 32bit/64bit portability)...
typedef enum {
    PlayerStateOff,
    PlayerStatePlaying,
    PlayerStatePaused
} PlayerState;
// NOTE: Implies...
// typedef int PlayerState;

// Generic C-style enum (no 32bit/64bit portability)...
typedef enum {
    PlayerStateOff,
    PlayerStatePlaying,
    PlayerStatePaused
} PlayerState;

Switch Statements and Case Label Blocks

Braces for switch statements begin on a new line and close on a new line as mentioned in Spacing above. Beware that braces are usually not required for case label blocks but may be enforced by the compiler and are definitely required when declaring variables within a case statement in order to define scope; therefore, when a case contains more than one line, braces should always be added.

Note: With regard to case blocks with braces, this is one situation where braces will begin on the same line as the case: label but will still end on a new line.

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

There are times when the same code can be used for multiple cases, and a fall-through should be used. A fall-through is the removal of the break statement for a case thus allowing the flow of execution to pass to the next case value. A fall-through should be commented for coding clarity.

switch (condition) 
{
  case 1:
    // ** fall-through! **
    
  case 2:
    // code executed for values 1 and 2
    break;
    
  default: 
    // ...
    break;
}

When using an enumerated type for a switch, default is not needed. For example:

kMIX_LeftMenuTopItemType menuType = kMIX_LeftMenuTopItemMain;

switch (menuType) {
  case kMIX_LeftMenuTopItemMain:
    // ...
    break;
  case kMIX_LeftMenuTopItemShows:
    // ...
    break;
  case kMIX_LeftMenuTopItemSchedule:
    // ...
    break;
}

Private Properties

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

For Example:

@interface MyAppDetailViewController ()

@property (nonatomic, readwrite, strong) GADBannerView *googleAdView;
@property (nonatomic, readwrite, strong) ADBannerView *iAdView;
@property (nonatomic, readwrite, 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. This pattern may at first appear to be result in unnatural file names but when glancing at a directory listing it will result in a natural grouping of obviously related files.

Examples:

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.

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.

Preferred:

if (someObject)
{
  // code...
}

if (![anotherObject boolValue]) 
{
  // code...
}

Not Preferred:

if (someObject == nil)
if ([anotherObject boolValue] == NO)
if (isAwesome == YES)     // Never do this.
if (isAwesome == true)    // Never do this.

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 (nonatomic, readwrite, unsafe_unretained, 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 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;

Conditional Expressions

The expression evaluated in a conditional statement should generally consist of variables and not method calls. It's okay to declare temporary local variables ahead of the conditional. Because sending messages to Objective-C methods is often ridiculously verbose, calling the methods directly within the conditional expression often makes it hard to visually parse what exactly it is that is being tested. Collect your evaluation criteria, and then evaluate in the expression.

Preferred:

BOOL continuousPlayEnabled = [[MediaAppPrefs sharedInstance] continuousPlay];
MediaAppTrack *nextMediaTrack = [MediaAppPlayer nextTrack];
if (continuousPlayEnabled && nextMediaTrack)
{
  // play the next song
}

Not Preferred:

if ([[MediaAppPrefs sharedInstance] continuousPlay] && [MediaAppPlayer nextTrack])
{
  // play the next song
}

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. Parentheses are always added for improved readability.

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;

result = isHorizontal ? x : y;

Return Statements

The expression evaluated in a return statement should generally consist of variables and not complex method calls. It should always be immediately obvious as to what the returned value represents and how it has been derived. Forming a return statement with an expression that includes a call to one or more methods with complex selectors is never a good idea. As with Conditional Expressions, collect your evaluation criteria, and then evaluate in the expression.

Preferred:

- (BOOL) playNext
{
  BOOL continuousPlayEnabled = [[MediaAppPrefs sharedInstance] continuousPlay];
  MediaAppTrack *nextMediaTrack = [MediaAppPlayer nextTrack];
  
  return (continuousPlayEnabled && nextMediaTrack);
}

Not Preferred:

- (BOOL) playNext
{
  return ([[MediaAppPrefs sharedInstance] continuousPlay] && [MediaAppPlayer nextTrack]);
}

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;
}

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

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:(MyAppAirplaneType)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 };

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. Bail out and return early. 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
  }
}

Error handling

As mentioned in Apple's developer documentation, when a method returns an NSError object by reference it's important to always test the return value of the method call itself to determine whether an error has occurred.

If an error has occurred, the value of the NSError object will generally refer to a valid NSError object and the method will return NO; however, if no error has occurred, the NSError object may or may not be set to nil.

Preferred:

NSError *error;
BOOL success;
success = [self trySomethingWithError:&error];

if (!success) {
  // Handle Error
  // e.g.
  NSLog(@"Something bad just happened: %@", error);
}

Not Preferred:

NSError *error;

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

When an error is encountered your app should recover or present the user with the details (and possibly a solution). Leverage the information returned by the NSError object to determine the most appropriate course of action.

Singletons

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

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

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

  return sharedInstance;
}

This will prevent possible and sometimes prolific crashes.

An optional addition to the above standard pattern is the following:

- (void)dealloc {

    // implement -dealloc & remove abort() when refactoring for
    // non-singleton use.
    abort();
}

The idea behind adding the dealloc() method is to provide one more defensive measure since a dealloc message should never be sent to a Singleton in the first place. The logic is explained in a StackOverflow thread.

Line Breaks

Line breaks are an important topic since this style guide is focused for print and online readability.

For example:

self.productsRequest = [[SKProductsRequest alloc] initWithProductIdentifiers:productIdentifiers];

A long line of code like this should be carried on to the second line adhering to this style guide's Spacing section (two spaces).

self.productsRequest = [[SKProductsRequest alloc] 
  initWithProductIdentifiers:productIdentifiers];

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.

Other Objective-C Style Guides

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

Robots & Pencils
New York Times
Google
GitHub
Adium
Sam Soffes
CocoaDevCentral
Luke Redpath
Marcus Zarra

Files

README.md

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README.md

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Wonderful Objective-C style guide.

Introduction

Credits

Background

Tools

Why Uncrustify and not Clang-Format?

BBUncrustifyPlugin-Xcode

Uncrustify 0.61-snapshot

Table of Contents

Language

Project Organization

Moving existing files into the right folder

Code Organization

Line Wrapping (Code Width)

Spacing

Comments

Documentation

Naming

Naming Conventions for Methods and Variables

Naming Conventions for Constants and Macros

Naming Conventions for Enumerated Types

Underscores

Methods

Variables

Property Attributes

Dot-Notation Syntax

Literals

Constants

Enumerated Types

Switch Statements and Case Label Blocks

Private Properties

Image Naming

Booleans

Conditionals

Conditional Expressions

Ternary Operator

Return Statements

Init Methods

Class Constructor Methods

CGRect Functions

Golden Path

Error handling

Singletons

Line Breaks

Xcode project

Other Objective-C Style Guides