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Stripe iOS Objective-C Style Guide

Ground Rules

Spacing

  • Indent using 4 spaces. No tabs.

  • Avoid starting methods with an empty line

  • There should not be a need to use multiple consecutive empty lines

  • Asterisks should be attached to the variable name NSString *text unless it's NSString * const Text

Variable Naming

  • Lean towards clarity over compactness

  • Avoid single letter variables. Try using idx / jdx instead of i / j in for loops.

  • Acronyms should be all lowercase as a method prefix (ex:url or urlString). Otherwise, they should be all caps when occurring elsewhere in the method name, or as a class name (ex: handleStripeURLCallbackWithURL or STPAPIClient)

  • Internal or private methods and ivars should begin with an _, e.g. - (void)_doPrivateStuff and id _internalVariable. This is not required for private properties which should not include an underscore (this is to distinguish them from their underlying variable which automatically has an _ prefix).

Control Flow

  • Place else if and else on the same line as the preceding closing curly brace:
if (condition) {
    // A
} else if (condition) {
    // B
} else {
    // C
}

  • Always wrap conditional bodies with curly braces

  • Each return statement should be on a separate line for ease of debugging. i.e. do NOT write if (condition) return YES;

  • Use ternary operators sparingly and for simple conditions only:

type = isCard ? @"card" : @"unknown";

type = dictionary[@"type"] ?: @"default";
  • switch statements for enums should contain an entry for each value and avoid using default

Documentation

  • Document using the multi-line syntax in all cases with the content aligned with the first asterisk:
/**
 This is a one line description for a simple method
 */
- (void)title;

/**
 This is a multi-line description for a complicated method

 @param

 @see https://...
 */
- (void)title;
  • Header documentation should wrap lines to 80 characters

Literals

  • Use literals to create immutable instances of NSString, NSDictionary, NSArray, NSNumber:
NSArray *brands = @[@"visa", @"mastercard", @"discover"];

NSDictionary *parameters = @{
                              @"currency": @"usd",
                              @"amount": @1000,
                            };

  • Dictionary colons should be attached to the key

  • Align multi-line literals using default Xcode indentation

Constants

  • Use static constants whenever appropriate. Names should start with a capital letter:
static NSString * const HTTPMethodGET = @"GET";

static const CGFloat ButtonHeight = 100.0;
  • Any public static constants should be prefixed with STP:
static NSString * const STPSDKVersion = @"11.0.0";

Folders

  • We use flat folder structure on disk with some exceptions

  • Save files to the appropriate root level folder. Typical folders include:

    • stripe-ios/Stripe/
    • stripe-ios/Tests/Tests/
    • stripe-ios/Example/Basic Integration/
    • stripe-ios/Example/Non-Card Payment Examples/

  • Save public header files in stripe-ios/Stripe/PublicHeaders/ for Cocoapods compatibility

Design Patterns

Imports

  • Ordering for imports in headers
    • Import system frameworks
    • Import superclasses and protocols sorted alphabetically
    • Use @class for everything else
#import <Foundation/Foundation.h>

#import "STPAPIResponseDecodable.h"
#import "STPBankAccountParams.h"

@class STPAddress, @STPToken;
  • Ordering for imports in implementations
    • Import system frameworks
    • Import corresponding headers
    • Import everything else sorted alphabetically
#import <PassKit/PassKit.h>

#import "STPSource.h"
#import "STPSource+Private.h"

#import "NSDictionary+Stripe.h"
#import "STPSourceOwner.h"
#import "STPSourceReceiver.h"
#import "STPSourceRedirect.h"
#import "STPSourceVerification.h"

Interfaces and Protocols

  • Stick to Xcode default spacing for interfaces, categories, and protocols

  • Always define NS_ASSUME_NON_NULL_BEGIN / NS_ASSUME_NON_NULL_END in headers. NS_ASSUME_NON_NULL_BEGIN / NS_ASSUME_NON_NULL_END should also be used in implementation (.m) files

NS_ASSUME_NON_NULL_BEGIN

@protocol STPSourceProtocol <NSObject>

// ...

@end

// ...

@interface STPSource : NSObject<STPAPIResponseDecodable, STPSourceProtocol>

// ...

@end

// ...

@interface STPSource () <STPInternalAPIResponseDecodable>

// ...

@end

NS_ASSUME_NON_NULL_END
  • Category methods on certain classes need to be prefixed with stp_ to avoid collision:
// NSDictionary+Stripe.h

@interface NSDictionary (Stripe)

- (NSDictionary *)stp_jsonDictionary;

@end

  • Define private properties and methods as class extensions inside the implementation. Ex: STPSource.m.

  • Define internal properties and methods as class extensions inside a +Private.h file. Ex: STPSource+Private.h.

  • Access private properties and methods from test classes by defining a class extension inside the test implementation:

//  STPBankAccountTest.m

@interface STPBankAccount ()

+ (STPBankAccountStatus)statusFromString:(NSString *)string;
+ (NSString *)stringFromStatus:(STPBankAccountStatus)status;

@end

@interface STPBankAccountTest : XCTestCase

@end

@implementation STPBankAccountTest

// ...

@end

Properties

  • Properties should be defined using this syntax:
@property (<nonatomic / atomic>, <weak / copy / strong>, <nullable / _>, <readonly / readwrite>) <class> *<name>;

@property (<nonatomic / atomic>, <assign>, <readonly / readwrite>) <type> <name>;

  • Omit default properties (assign, readwrite, strong)

  • Use copy for classes with mutable counterparts such as NSString, NSArray, NSDictionary

  • Leverage auto property synthesis whenever possible

  • Declare @synthesize and @dynamic on separate lines for shorter diffs

  • Use properties (self.foo) instead of their corresponding instance variables (_foo). Instance variables should only be accessed directly in initializer methods (init, initWithCoder:, etc…), dealloc methods, and within custom getters and setters. For more information, see Apple’s docs on using accessor methods in initializer methods and dealloc..

Init

- (instancetype)init {
    self = [super init];
    if (self) {
        // ...
    }
    return self;
}

Methods

Implementation

  • Do not use #define to define a block of code -- #define code is very difficult to debug

  • Use #pragma mark - <text> and #pragma mark <text> to group methods In large implementation files:

#pragma mark - Button Handlers

#pragma mark - UITableViewDataSource

#pragma mark - UITableViewDelegate