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Objective C Style Guide

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Tim Anglade edited this page Aug 18, 2014 · 17 revisions

THE DOCUMENTATION FOR REALM IS AVAILABLE AT HTTP://REALM.IO

Note: Realm uses a variation on the NY Times styleguide. We only add, not modify. Take a look at History for an overview of these additions.

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:

Table of Contents

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 separate functionality, but often there should probably be new methods.
  • @synthesize and @dynamic should each be declared on new lines in the implementation.
  • Trim whitespace on whitespace-only lines. Xcode has a setting under "Text Editing"->"Automatically trim trailing whitespace"->"Including whitespace-only lines" that helps enforce this rule.

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 instance 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 (-/+ symbol). There should be a space between the method segments.

For Example:

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

The beginning brace should have it's own line.

For Example:

- (void)setExampleText:(NSString *)text image:(UIImage *)image 
{    
    // Code begins here...
}

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.

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;

@end

Not:

@interface NYTSection : NSObject 
{
    NSString *headline;
}

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.

For example:

static const NSTimeInterval NYTArticleViewControllerNavigationFadeAnimationDuration = 0.3;

Not:

static const NSTimeInterval fadetime = 1.7;

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;

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.

Documentation

Internal documentation meant only for developers reading the source should use the // syntax.

Long-form documentation should be done via markdown files under docs, not inline with the code.

We use appledoc to generate our class documentation from inline comments.

Please add & maintain an overview at the top of every class:

/**
 
 <ClassName> does blah and bleh
 
 Some paragraph (indented by 1 space)
    
    Some code sample (idented by 1 space + 1 tab, with a newline at the top & bottom)
 
 Another paragraph with **bold** and *italics*.
 CamelCase words get automatically lined to the corresponding class in our docs.
 For plural versions, use: ClassName(s) 
 
 @warning A warming of some sort. Always at the bottom
 @warning You can have multiple warnings
 @bug A known bug or temporary limitation
 */

Please group methods & properties by Task, and order all members of a task according to its importance (likelihood it will be necessary or looked for). Order tasks according to discovery order (e.g. you must be able to write to a structure first before you can query or read anything from it!) Task names should begin with a verbs and not use class names, e.g. “Reading from a Table”, not “RLMTable Read Operations”.
You can create a Task by adding the following separator between groups of methods & properties. All properties & methods should be grouped under a Task, or they should not generate any docs (i.e. be publicly hidden and assumed never to be used by any external developers).

/**---------------------------------------------------------------------------------------
 *  @name Writing & Updating Objects in a Table
 *  ---------------------------------------------------------------------------------------
 */

Please document all methods & properties that are meant to be used by developers. You should use class names in those descriptions. All sentences should begin with a capital letter and end with a period. VV-Documenter-Xcode can help generate the necessary comment blocks. Please be sure to configure it to begin all lines with a whitespace instead of its default.

Example for properties:

/**
 Number of objects in this RLMTable.
 */
@property (nonatomic, readonly) NSUInteger rowCount;

Example for methods:

/**
 One sentence describing what the method does in English; this will be the description of the function. Indented by one space.

 Any further paragraphs will be displayed under “discussion”. Also indented by one space.
 
    A code sample, indented by 1 space + 1 tab. Optional if the method is simple to use
 
 @warning A warning, usually displayed at the bottom.
 @bug A bug or temporary limitation. Keep it grouped with other bugs or warnings, with an empty line above and below.
 
 @param paramName 1 or 2 sentence description of the param. Include the type it accepts if unclear.
 @param param2Name Keep params grouped together, with an empty line above and below.
 
 @return 1 or 2 sentence description of what is returned. Include the type if (id).
 */

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 initalizer
    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:

#define CompanyName @"The New York Times Company"

#define thumbnailHeight 2

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()

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. 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;

@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.

For example:

  • 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

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.

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 alloc] init];
   });

   return sharedInstance;
}

This will prevent possible and sometimes prolific crashes.

License

All files should include the following header:

////////////////////////////////////////////////////////////////////////////
//
// Copyright 2014 Realm Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
////////////////////////////////////////////////////////////////////////////

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.

Use the Github Objective-C .gitignore file on new projects.

Project organization should be set to “Realm, Inc.”

For sample apps, use io.realm.RealmExample as the bundle identifier

Line Length

Lines of code should strive to be under 100 characters wide. Xcode has a setting under "Text Editing"->"Page guide at column" that helps remember this rule. When lines exceed this maximum length, it should be broken up into multiple lines, by using colon alignment where applicable.

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