The official Objective-C style guide.
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Introduction
The reason we made this style guide was so that we could keep the code in our books, tutorials, and starter kits nice and consistent - even though we have many different authors working on the books.
This style guide is different from other Objective-C style guides you may see, because the focus is centered on readability for print and the web. Many of the decisions were made with an eye toward conserving space for print, easy legibility, and tutorial writing.
Credits
The creation of this style guide was a collaborative effort from various raywenderlich.com team members under the direction of Nicholas Waynik. The team includes: Soheil Moayedi Azarpour, Ricardo Rendon Cepeda, Tony Dahbura, Colin Eberhardt, Matt Galloway, Greg Heo, Matthijs Hollemans, Christopher LaPollo, Saul Mora, Andy Pereira, Mic Pringle, Pietro Rea, Cesare Rocchi, Marin Todorov, Nicholas Waynik, and Ray Wenderlich
We would like to thank the creators of the New York Times and Robots & Pencils' Objective-C Style Guides. These two style guides provided a solid starting point for this guide to be created and based upon.
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:
Table of Contents
Language
US English should be used.
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Code Organization
Use
#pragma mark - to categorize methods in functional groupings and protocol/delegate implementations following this general structure.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/whileetc.) always open on the same line as the statement but close on a new line.
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- 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,
@synthesizeand@dynamicshould 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.
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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.
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.
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A three letter prefix should always be used for class names and constants, however may be omitted for Core Data entity names. For any official raywenderlich.com books, starter kits, or tutorials, the prefix 'RWT' should be used.
Constants should be camel-case with all words capitalized and prefixed by the related class name for clarity.
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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.
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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.Preferred:
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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:
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Property Attributes
Property attributes should be explicitly listed, and will help new programmers when reading the code. The order of properties should be storage then atomicity, which is consistent with automatically generated code when connecting UI elements from Interface Builder.
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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:
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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.
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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:
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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.Preferred:
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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()For Example:
You can also make explicit value assignments (showing older k-style constant definition):
Older k-style constant definitions should be avoided unless writing CoreFoundation C code (unlikely).
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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.
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.
When using an enumerated type for a switch, 'default' is not needed. For example:
Private Properties
Private properties should be declared in class extensions (anonymous categories) in the implementation file of a class. Named categories (such as
RWTPrivate or private) should never be used unless extending another class. The Anonymous category can be shared/exposed for testing using the <headerfile>+Private.h file naming convention.For Example:
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.
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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: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.
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or
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.
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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'.
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.
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.
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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.Preferred:
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Error handling
When methods return an error parameter by reference, switch on the returned value, not the error variable.
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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).
Singletons
Singleton objects should use a thread-safe pattern for creating their shared instance.
This will prevent possible and sometimes prolific crashes.
Line Breaks
Line breaks are an important topic since this style guide is focused for print and online readability.
For example:
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).
Smiley Face
Smiley faces are a very prominent style feature of the raywenderlich.com site! It is very important to have the correct smile signifying the immense amount of happiness and excitement for the coding topic. The end square bracket is used because it represents the largest smile able to be captured using ascii art. A half-hearted smile is represented if an end parenthesis is used, and thus not preferred.
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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.