Guidelines for iOS development in use at Spotify
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README.md

Spotify Objective-C Coding Style

Version: 0.9.0

Our general coding conventions at Spotify are documented on an internal wiki, but specifics for Objective-C and Objective-C++ code in the iOS client are documented here.

License

Copyright (c) 2015-2016 Spotify AB.

This work is licensed under a Creative Commons Attribution 4.0 International License.

Table of Contents

  1. Spacing, Lines and Formatting
  2. Brackets
  3. Naming
  4. Comments
  5. Pragma Marks
  6. Constants
  7. Return Early
  8. Initializers
  9. Headers
  10. Nullability
  11. Strings
  12. Dot Notation
  13. Categories

Spacing, Lines and Formatting

Line length

  • Keep your lines within 120 characters width when possible.
    • In Xcode, you can set a page guide in Text Editing in Preferences.

Whitespace

  • Use 4 spaces for indentation and alignment. Do not use tabs.
  • Trailing whitespace is acceptable only on blank lines, but discouraged even there.
    • In Xcode, select "Automatically trim whitespace" and "Including whitespace-only lines" in Text Editing preferences to handle this automatically.
  • Put spaces after commas, and before and after operators.
  • Do not put spaces between parentheses and what they are enclosing.

Example:

foo("bar") 

Not:

foo( "bar" )

Containers

  • Array one-liners are acceptable unless they have too many items or their values are too long.
NSArray *array = @[@"uno", @"dos", @"tres", @"cuatro"];
  • In that case, break them in several lines:
NSArray *array = @[
    @"This is how we do, yeah, chilling, laid back",
    @"Straight stuntin’ yeah we do it like that",
    @"This is how we do, do do do do, this is how we do",
];
  • Dictionary one-liners are reserved for single pairs only:
NSDictionary *dict = @{@"key" : @"highway"};
  • Format it pretty otherwise, leaving a trailing comma after the last item:
NSDictionary *dict = @{
    @"key1" : @"highway",
    @"key2" : @"heart",
};

Brackets

  • Always use brackets for if / for / while / do-while statements. Even if its a one-liner:
if (itsMagic) {
    [self makeItEverlasting];
}
  • Write follow up else clauses after the previous closing bracket on the same line.
if (hasClue) {
    knowExactlyWhatToDo();
} else if (seeItAllSoClear) {
    writeItDown();
} else {
    sing();
    dance();
}

Naming

Comments

  • Use the // style for single line comments or when appending comments in the same line of code.
  • Use the /* */ style for multi-line comments.

Example:

/*
 This is a multi-line comment. The opening and closing markers are on their
 own lines.

 This is a new paragraph in the same block comment.
 */

stop(); // Hammer-time!

// this is a very brief comment.
  • Document all methods and properties in the headers using Xcode’s documentation style. You can select a property or method and use Cmd-/ to see this in action. Make sure to use the available markup tags like @param, @return, etc. (these will be auto-generated for you by Xcode). The /// form is preferred for single line comments, and /** */ for multi-line comments. Use the same formatting as in the example block above.

Pragma Marks

  • Use the pre-processor instruction #pragma mark to mark related groups of methods.

Constants

  • Do not define constants using #define.
  • Publicly (or privately) exposed variables should be constant (trying to assign values will result in compilation error).
extern NSString * const SPTCodeStandardErrorDomain;

Return Early

  • Return early on errors and failed pre-conditions to avoid unnecessary nested brackets and / or unnecessary computations.

Example:

- (void)setFireToTheRain:(id)rain
{
    if ([_rain isEqualTo:rain]) {
        return;
    }

    _rain = rain;
}

Initializers

Example:

- (instancetype)init 
{
    self = [super init];
 
    if (self) {
        // initialize instance variables here
    }
 
    return self;
}

Headers

  • The use of prefix headers has been deprecated. Do not add new code to an existing prefix header.
  • When importing a module use the hash-import variant instead of at-import.
    • Yes: #import <Foundation/Foundation.h>
    • No: @import Foundation;

Nullability

  • Use NS_ASSUME_NONNULL_BEGIN and NS_ASSUME_NONNULL_END in header files, and explicitly add nullable when needed. Example:
#import <Foundation/Foundation.h>

@protocol SPTPlaylist;

NS_ASSUME_NONNULL_BEGIN

typedef void(^SPTSomeBlock)(NSData * _Nullable data, NSError * _Nullable error);

@interface SPTYourClass : NSObject

@property (nonatomic, copy, readonly, nullable) NSString *customTitle;
@property (nonatomic, strong, readonly) id<SPTPlaylist> playlist;

- (nullable instancetype)initWithPlaylist:(id<SPTPlaylist>)playlist
                              customTitle:(nullable NSString *)customTitle;

@end

NS_ASSUME_NONNULL_END

Strings

  • All strings presented to the user should be localized.

Dot Notation

  • Use bracket notation when calling non-accessor methods:
[self doSomething];
  • Use bracket notation when accessing globals through a class method:
[MyClass sharedInstance];
  • Set and access properties using dot notation:
self.myString = @"A string";
  • Except in the init or dealloc methods, always use the ivar directly there:
_myString = nil;

Categories

  • Methods in categories on non-Spotify classes must be prefixed spt_ to avoid name clashes.