This style guide outlines the coding conventions of the iOS team at Khan Academy, shared here so it's easy for team members to contribute to our app development. If you feel strongly about changing it, come build things with us and help us make it better!
Thanks to all of our contributors.
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
- Dot-Notation Syntax
- Spacing
- Conditionals
- Error handling
- Methods
- Variables
- Naming
- String Format Specifiers
- Docstrings
- Comments
- Init & Dealloc
- Literals
- CGRect Functions
- Constants
- Enumerated Types
- Private Properties
- Image Naming
- Booleans
- Singletons
- Xcode Project
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;
// Or, for completely aesthetic reasons:
[[UIApplication sharedApplication] delegate];Not:
[view setBackgroundColor:[UIColor orangeColor]];
UIApplication.sharedApplication.delegate;- Indent using 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.
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 separate functionality, but often there should probably be new methods.
@synthesizeand@dynamicshould each be declared on new lines in the implementation.
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;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.
For example:
result = a > b ? x : y;Not:
result = a > b ? x = c > d ? c : d : y;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).
In method signatures, there should be a space after the scope (-/+ symbol). There should be a space between the method segments.
For Example:
- (void)setExampleText:(NSString *)text image:(UIImage *)image;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.
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 on using Accessor Methods in Initializer Methods and dealloc, see here.
For example:
@interface NYTSection: NSObject
@property (nonatomic) NSString *headline;
@endNot:
@interface NYTSection : NSObject {
NSString *headline;
}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.
For example:
static const NSTimeInterval NYTArticleViewControllerNavigationFadeAnimationDuration = 0.3;Not:
static const NSTimeInterval fadetime = 1.7;Properties should be camel-case with the leading word being lowercase. If Xcode can automatically synthesize the variable, then let it. Otherwise, in order to be consistent, the backing instance variables for these properties should be camel-case with the leading word being lowercase and a leading underscore. This is the same format as Xcode's default synthesis.
For example:
@synthesize descriptiveVariableName = _descriptiveVariableName;Not:
id varnm;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.. Local variables should not contain underscores.
When creating static local C functions, internal to a particular implementation, begin that method with a lowercase letter and an underscore. For example:
static CGFloat _lowPassFilter(CGFloat oldValue, CGFloat newValue, CGFloat smoothingFactor)When formatting a string that includes either a NSInteger or NSUInteger value, box the value as an NSNumber. For example:
NSLog(@"There are %@ apples", @([apples count]));On a 32-bit platform, NSInteger and NSUInteger are int and unsigned int, while on a 64-bit platform they are long and unsigned long. Boxing the value ensures that the formatted value is correct, regardless of the platform. Furthermore, it is cleaner than the alternatives of using platform-dependent string format specifiers, or always casting the value to long or unsigned long. (For more details on those approaches, see the string format specifier documentation.)
Also, thanks to the wonders of tagged pointers, if the NSInteger or NSUInteger value is small enough, then no NSNumber instance is actually created.
Finally, some resources on the Internet advocate using %zd as a specifier for NSInteger, and %tu as a specifier for NSUInteger. But those specifiers work "accidentally", and this relation is not fixed in any standard. Consequently, this approach might break in the future, and so we have moved away from it.
All non-trivial methods, interfaces, categories, and protocol declarations should have accompanying comments describing their purpose and how they fit into the larger picture. Docstrings belong in the header file.
We're following the guidelines described here: http://nshipster.com/documentation/
...which point to a couple great examples, including this one and this one. Check these out if you're unsure how to docstring something!
And if you're really lazy and don't want to click links, here's a friendly example:
/**
Instances of `TTTAddressFormatter` create address strings formatted according to a given locale.
For example, addresses in the United States take the form:
Street Address
City State ZIP
Country
Whereas addresses in Japan follow a different convention:
Postal Code
Prefecture Municipality
Street Address
Country
All of the business logic for these rules is handled by`ABCreateStringWithAddressDictionary`, from the `AddressBookUI` framework. `TTTAddressFormatter` acts as a convenient wrapper to this functionality.
*/
@interface TTTAddressFormatter : NSFormatter
/**
Specifies the locale used to format strings. Defaults to the current system locale.
*/
@property (nonatomic, strong) NSLocale *locale;
...
/**
Returns an address string for the specified components formatted with the receiver's locale.
@param street The street address
@param locality The locality (a.k.a. city, municipality, township)
@param region The region (a.k.a. state, prefecture, principality)
@param postalCode The postal code (a.k.a ZIP code)
@param country The country
*/
- (NSString *)stringFromAddressWithStreet:(NSString *)street
locality:(NSString *)locality
region:(NSString *)region
postalCode:(NSString *)postalCode
country:(NSString *)country;
The docstring should describe the function's calling syntax and its semantics, not its implementation.
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.
dealloc methods should be placed at the top of the implementation, directly after the init methods.
init methods should be structured like this:
- (instancetype)init {
self = [super init]; // or call the designated initalizer
if (!self) {
return nil;
}
// Custom initialization
return self;
}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.
NSDictionary key-value pairs and long NSArray values should be placed on new lines, preferably with trailing final commas. In this case, the closing brace or bracket should go on a new line.
For example:
NSArray *names = @[@"Brian", @"Matt", @"Chris", @"Alex", @"Steve", @"Paul"];
NSArray *cultureStatements = @[
@"It's ok to leave (or simply not attend) any meeting that you don't feel is important for you.",
@"Anybody can fix anything.",
@"Shipping beats perfection",
];
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];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 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:
#define CompanyName @"The New York Times Company"
#define thumbnailHeight 2When 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()
Example:
typedef NS_ENUM(NSInteger, NYTAdRequestState) {
NYTAdRequestStateInactive,
NYTAdRequestStateLoading
};Add @required and @optional to all protocol definitions; as much as possible, try to keep all @required methods grouped together at the top and all @optional methods below.
Private properties should be declared in class extensions (anonymous categories) in the implementation file of a class. Named categories (such as NYTPrivate or private) should never be used unless extending another class.
For example:
@interface NYTAdvertisement ()
@property (nonatomic, strong) GADBannerView *googleAdView;
@property (nonatomic, strong) ADBannerView *iAdView;
@property (nonatomic, strong) UIWebView *adXWebView;
@endImage 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@2xandRefreshBarButtonItemSelected/RefreshBarButtonItemSelected@2xArticleNavigationBarWhite/ArticleNavigationBarWhite@2xandArticleNavigationBarBlackSelected/ArticleNavigationBarBlackSelected@2x.
Images that are used for a similar purpose should be grouped in respective groups in an Images folder.
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.
For example:
if (!someObject) {
}Not:
if (someObject == nil) {
}For a BOOL, here are two examples:
if (isAwesome)
if (![someObject boolValue])Not:
if ([someObject boolValue] == NO)
if (isAwesome == YES) // 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 (assign, getter=isEditable) BOOL editable;Text and example taken from the Cocoa Naming Guidelines.
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 alloc] init];
});
return sharedInstance;
}This will prevent possible and sometimes prolific crashes.
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.
If ours doesn't fit your tastes, have a look at some other style guides: