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📄 A quick reference cheat sheet for common, high level topics in MulleObjC.
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README.md

README.md

Preface

This is an edited version of the "Objective-C Cheat Sheet" adapted for mulle-objc. I took out everything that is not applicable to mulle-objc and rewrote some parts to fit the current state of the art. Visit the original site iwasrobbed/Objective-C-CheatSheet for the original author's introduction and more information. The list of contributors can be found here.

MulleObjC Cheat Sheet

This is not meant to be a beginner's guide or a detailed discussion about Objective-C; it is meant to be a quick reference to common, high level topics.

Table of Contents

Commenting

Comments should be used to organize code and to provide extra information for future refactoring or for other developers who might be reading your code. Comments are ignored by the compiler so they do not increase the compiled program size.

Two ways of commenting:

// This is an inline comment

/* This is a block comment
   and it can span multiple lines. */

// You can also use it to comment out code
/*
- (SomeOtherClass *)doWork
{
    // Implement this
}
*/

To comment out many lines of code, you can also use

#if 0
whatever
#endif

Using pragma to organize your code:

#pragma mark - Use pragma mark to logically organize your code

// Declare some methods or variables here

#pragma mark - They also show up nicely in the properties/methods list in Xcode

// Declare some more methods or variables here

Back to top

Data Types

Size

Permissible sizes of data types are determined by how many bytes of memory are allocated for that specific type and whether it's a 32-bit or 64-bit environment. In a 32-bit environment, long is given 4 bytes, which equates to a total range of 2^(4*8) (with 8 bits in a byte) or 4294967295. In a 64-bit environment, long is given 8 bytes, which equates to 2^(8*8) or 1.84467440737096e19.

For a complete guide to 64-bit changes, please see the transition document.

C Primitives

Note: Objective-C inherits all of the C language primitive types and then adds a few extras.

Void

void is C's empty data type. It is most commonly used to specify the return type for functions that don't return anything.

Integers

Integers can be signed or unsigned. When signed, they can be either positive or negative and when unsigned, they can only be positive. Example: When declaring an unsigned int, the range of allowable integer values for a 32-bit compiler will shift from -2147483648 to +2147483647 to instead be 0 to +4294967295.

Integer types with their accompanying byte sizes:

// Char (1 byte for both 32-bit and 64-bit)
unsigned char anUnsignedChar = 255;
NSLog(@"char size: %zu", sizeof(char));

// Short (2 bytes for both 32-bit and 64-bit)
short aShort = -32768;
unsigned short anUnsignedShort = 65535;
NSLog(@"short size: %zu", sizeof(short));

// Integer (4 bytes for both 32-bit and 64-bit)
int anInt = -2147483648;
unsigned int anUnsignedInt = 4294967295;
NSLog(@"int size: %zu", sizeof(int));

// Long (4 bytes for 32-bit, 8 bytes for 64-bit)
long aLong = -9223372036854775808; // 64-bit
unsigned long anUnsignedLong = 18446744073709551615; // 64-bit
NSLog(@"long size: %zu", sizeof(long));

// Long Long (8 bytes for both 32-bit and 64-bit)
long long aLongLong = -9223372036854775808;
unsigned long long anUnsignedLongLong = 18446744073709551615;
NSLog(@"long long size: %zu", sizeof(long long));

Fixed width integer types with their accompanying byte sizes as the variable names:

// Exact integer types
int8_t aOneByteInt = 127;
uint8_t aOneByteUnsignedInt = 255;
int16_t aTwoByteInt = 32767;
uint16_t aTwoByteUnsignedInt = 65535;
int32_t aFourByteInt = 2147483647;
uint32_t aFourByteUnsignedInt = 4294967295;
int64_t anEightByteInt = 9223372036854775807;
uint64_t anEightByteUnsignedInt = 18446744073709551615;

Floating Point

Floats are always implicitly signed.

// Single precision floating-point (4 bytes for both 32-bit and 64-bit)
float aFloat = 72.0345f;
NSLog(@"float size: %zu", sizeof(float));

// Double precision floating-point (8 bytes for both 32-bit and 64-bit)
double aDouble = -72.0345f;
NSLog(@"double size: %zu", sizeof(double));

// Extended precision floating-point (16 bytes for both 32-bit and 64-bit)
long double aLongDouble = 72.0345e7L;
NSLog(@"long double size: %zu", sizeof(long double));

Objective-C Primitives

id : Known as the anonymous or dynamic object type, it can store a reference to any type of object with no need to specify a pointer symbol.

id delegate = self->delegate;

Class : Used to denote an object's class and can be used for introspection of objects.

Class aClass = [NSArray class];

Method : Used to denote a method and can be used for swizzling methods.

Method aMethod = [aClass instanceMethodForSelector:aSelector);

SEL : Used to specify a selector which is compiler-assigned code that identifies a method name.

SEL someSelector = @selector(someMethodName);

IMP : Used to point to the memory address of the start of a method. You will probably never need to use this.

IMP theImplementation = [self methodForSelector:someSelector];

BOOL : Used to specify a boolean type where 0 is considered NO (false) and 1 is considered YES (true). Any nil object is also considered to be NO so there is no need to perform an equality check with nil (e.g. just write if (someObject) not if (someObject != nil)).

// Boolean
BOOL isBool = YES; // Or NO

nil : Used to specify a null object pointer. When classes are first initialized, all properties of the class are set to 0 which means they point to nil.

Objective-C also has a number of other types such as NSInteger, NSUInteger, NSRange etc.

Enum & Bitmask Types

Enumeration types can be defined a number of different ways:

enum {
    CellStyleDefault,
    CellStyleValue1,
    CellStyleValue2,
    CellStyleSubtitle
};

typedef enum {
    CellStyleDefault,
    CellStyleValue1,
    CellStyleValue2,
    CellStyleSubtitle
} CellStyle;

Working with Bitmasks

// Set bits (only valid if it makes sense that your status may have many of the bitmask values)
BitMask status = OptionNone;
status |= OptionBottom;
status != OptionTop;

// Toggle bit
status ^= OptionTop;

// Set single bit to zero
status &= ! OptionBottom;

// Check if it matches a certain bit
if (status & OptionTop) {
    [self doSometing];
}

Casting to Data Types

Sometimes it is necessary to cast an id or different type into a specific class or data type. Examples of this would be casting from a float to an int or from a NSObject to a subclass such as NSArray.

Casting non-object data types:

// Format: nonObjectType variableName = (nonObjectType)variableNameToCastFrom;
int anInt = (int) anAnonymouslyTypedNonObjectOrDifferentDataType;

Casting object data types:

// Format: ClassNameOrObjectType *variableName = (ClassNameOrObjectType *)variableNameToCastFrom;
NSArray *array;

array = nil;
if( [someObject isKindOfClass:[NSArray class]])
   array = (NSArray *) someParameter;

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Constants

#define defines a macro which replaces all references with the actual constant value contents before compilation starts, instead of being a memory pointer to that constant value. You can also use const to create read only memory. But the use of const is generally avoided in mulle-objc, as it adds complexity with negligble gain. For integer constants that fit into an int you will often find anonymous enums handy.

Constants can be defined as:

// Format: type constantName = value;
NSString *ShortDateFormat = @"MM/dd/yyyy";

// Format: #define constantName value
#define ShortDateFormat @"MM/dd/yyyy"

// Format: enum {  MyIntConstant = value };
enum {  MyMagicIntNumber = 1848 };

To make the constant available to external classes, you must also add it to the header .h file:

extern NSString * ShortDateFormat;

If you know that a constant will only be available within it's containing implementation .m file, specify it as:

static NSString *ShortDateFormat = @"MM/dd/yyyy";

A static variable declared within a method retains its value between invocations. This is only useful for constants and should be avoided. Do not use static variables for singletons or such.

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Operators

Arithmetic Operators

Operator Purpose
 + Addition
 - Subtraction
* Multiplication
/ Division
% Modulo

Relational and Equality Operators

Operator Purpose
== Equal to
!= Not equal to
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to

Logical Operators

Operator Purpose
! NOT
&& Logical AND
&#124;&#124; Logical OR

Compound Assignment Operators

Operator Purpose
+= Addition
-= Subtraction
*= Multiplication
/= Division
%= Modulo
&= Bitwise AND
&#124;= Bitwise Inclusive OR
&#126;= Bitwise Complement
^= Exclusive OR
<<= Shift Left
>>= Shift Right

Increment/Decrement Operators

Operator Purpose
++ Addition
-- Subtraction

Bitwise Operators

Operator Purpose
& Bitwise AND
&#124; Bitwise Inclusive OR
^ Exclusive OR
&#126; Unary complement (bit inversion)
<< Shift Left
>> Shift Right

Other operators

Operator Purpose
() Cast
? : Conditional
& Memory Address
* Pointer

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Declaring Classes

Classes are declared using two files: a header (.h) file and an implementation (.m) file.

The header file should contain (in this order):

  • Any needed #import statements or forward @class declarations
  • Any protocol declarations
  • An @interface declaration specifying which class you're inheriting from
  • All publicly accessible variables, properties and methods

The implementation file should contain (in this order):

  • Any needed #import statements
  • An anonymous category, or class extension, for any private variables or properties
  • An @implementation declaration specifying the class
  • All public and private methods

Example:

MyClass.h

#import "import.h"

// import other classes in same project
#import "SomeClass.h"

// Used instead of #import to forward declare a class in property return types, etc.
@class SomeOtherClass;

// Place all global constants at the top
extern NSString *ErrorDomain;

// Format: YourClassName : ClassThatYouAreInheritingFrom
@interface MyClass : SomeClass

// Public properties first
@property (readonly, retain) SomeClass *someProperty;

// Then class methods
+ (instancetype) myClassMethod;

// Then instance methods
- (SomeOtherClass *) doWork;

@end

MyClass.m

#import "MyClass.h"
#import "SomeOtherClass.h"

#import "import-private.h"


// Declare any constants at the top
NSString *ErrorDomain = @"com.myIncredibleApp.errors";
static NSString *ShortDateFormat = @"MM/dd/yyyy";


@implementation MyClass

// Use #pragma mark - statements to logically organize your code
#pragma mark - Class Methods

+ (instancetype) myClassMethod
{
   return [[[MyClass alloc] init] autorelease];
}

#pragma mark - Init & Dealloc methods

- (id) init
{
   // don't call super if superclass is NSObject
   self = [super init];
   if( ! self)
      return( self);

   // Initialize any properties or setup code here
   _someProperty = [NSObject new];  // don't autorelease
   return( self);
}

// Finalize method should remove any observers or readonly properties
- (void) finalize
{
   // clear readonly properties
   [_someProperty autorelease];
   _someProperty = nil;

   // will clear normal properties
   [super finalize];
}

// Dealloc method should always follow init method
- (void) dealloc
{
    // Release, not autorelease any remaining instance variables

    [super dealloc];
}

#pragma mark - Instance Methods

- (SomeOtherClass *)doWork
{
    // Implement this
}

@end

Instantiation

When you want to create a new instance of a class, you use the syntax:

MyClass *myClass = [MyClass instantiatedObject];

This is a shortcut for [[MyClass new] autorelease] which in turn is a shortcut for:

MyClass *myClass = [[[MyClass alloc] init] autorelease];

The alloc class method returns a pointer to memory in a newly allocated block. The allocated memory is large enough to store an instance of the class and meta-information, such as the isa pointer that preceeds that memory block. The allocated instance memory contains zeros except. isa, contained in the meta-infomation, is automatically initialized to point to the class object that allocated the memory and enables the instance to receive messages such as init that are used to complete initialization.

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Preprocessor Directives

This section needs a lot of love, so please feel free to improve upon it!

Directive Purpose
#define Used to define constants or macros that are replaced by the compiler at runtime
#elif An else if conditional statement
#else An else conditional statement
#endif An end if conditional statement
#error Used to flag an error line in code
#if An if conditional statement
#ifdef An if defined conditional statement
#ifndef An if not defined conditional statement
#import Imports a header file. This directive is identical to #include, except that it won't include the same file more than once
#include Includes a header file
#pragma Used for commenting code or inhibiting compiler warnings
#undef Used to undefine and redefine macros
#warning Used to flag a warning line in code

Special operator

The special operator defined is used in #if and #elif expressions to test whether a certain name is defined as a macro.

defined is useful when you wish to test more than one macro for existence at once. For example, #if defined(__IPHONE_8_0) || defined(__MAC_10_9) would succeed if either of the names __IPHONE_8_0 or __MAC_10_9 is defined as a macro.

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Compiler Directives

Also see the literals section.

Classes and Protocols

Directive Purpose
@class Declares the names of classes defined elsewhere
@end Ends the declaration/definition of a class, category, or protocol
@implementation Begins the definition of a class or category
@interface Begins the declaration of a class or category interface
@optional Declares the methods following the directive as optional. Classes implementing this protocol can decide whether to implement an optional method or not and should first check if the method is implemented before sending it a message.
@protocol Begins the declaration of a formal protocol
@required Declares the methods following the directive as required (default)

Properties

Directive Purpose
@dynamic Tells the compiler that the setter/getter methods are not implemented by the class itself but somewhere else, like the superclass
@property Declares a property with a backing instance variable
@synthesize Synthesizes a property and allows the compiler to generate setters/getters for the backing instance variable

Errors

Directive Purpose
@catch Specifies what to do if an exception was thrown in the @try block
@finally Specifies code that runs whether an exception occurred or not
@throw Throws an exception
@try Specifies a block of code to attempt

Visibility of Instance Variables

Directive Purpose
@private Limits the scope of instance variables specified below it to the class that declares it
@protected Limits the scope of instance variables specified below it to declaring and inheriting classes
@public Removes restrictions on the scope of instance variables specified below it

The default is @protected, so there is no need to explicitly specify this.

Others

Directive Purpose
@autoreleasepool Replaces (and is 6 times faster than) the NSAutoreleasePool class
@compatibility_alias Allows you to define an alias name for an existing class.
@defs(classname) Yields the internal data structure of classname instances
@encode(spec) Yields a character string (char *) that encodes the type structure of spec
@import Imports a module and autolinks its framework (currently for Apple frameworks only)
@protocol(name) Returns the given protocol (a selector of type PROTOCOL)
@selector(method) Returns the compiled selector that identifies a method

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Literals

Literals are compiler directives which provide a shorthand notation for creating common objects.

Syntax What it does
@"string" Returns an NSString object
@() Dynamically evaluates the boxed expression and returns the appropriate object literal based on its value
@28, @3.14 Returns an NSNumber object initialized with an appropriate class constructor, depending on the value
@[] Returns an NSArray object
@{} Returns an NSDictionary object

Caveats

Similar to NSString literals, collection objects made via literal arrays and dictionaries are immutable. Instead, you will have to make a mutable copy after making the immutable dictionary or array. Additionally, you cannot have static initializers like you can with NSString.

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Methods

Declaration Syntax

For methods without a return type, use void:

// Does not return anything or take any arguments
- (void) someMethod;

+ precedes declarations of class methods:

// Call on a class (e.g. [MyClass someClassMethod]);
+ (void )someClassMethod;

- precedes declarations of class instance methods:

// Called on an instance of a class (e.g. [[NSString alloc] init]);
- (void) someClassInstanceMethod;

Method arguments are declared after colons : and the method signature should describe the argument type:

// Does something with an NSObject argument
- (void) doWorkWithObject:(NSObject *)object;

Argument and return types are declared using type casting syntax:

// Returns an NSString object for the given NSObject arguments
- (NSString *) stringFromObject:(NSObject *)object
             andSomeOtherObject:(NSObject *)otherObject;

Calling Methods

Methods are called using bracket syntax: [self someMethod]; or [self sometMethodWithObject:object];

self is a reference to the method's containing class. The self variable is present in all Objective-C methods and it is one of two hidden arguments passed to code that implements a method, the other being _cmd, which identifies the received message.

At times, it is necessary to call a method in the superclass using [super someMethod];.

Under the hood, method calls are turned into a optimization-level dependent variation of one of these two C functions:

void  *mulle_objc_object_call( id self, SEL _cmd, void *_param);
void  *mulle_objc_object_call( id self, SEL _cmd, void *_param, mulle_objc_superid_t superid);

Testing Selectors

If you'd like to test if a class responds to a certain selector before you send it (and possibly crash), you can do so with:

if ([someClassOrInstance respondsToSelector:@selector(someMethodName)])
{
    // Call the selector or do something here
}

This pattern is common when you have a delegate implemented and need to test for methods declared as @optional before calling them on the delegate object.

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Properties and Variables

Declaring a property allows you to maintain a reference to an object within a class or to pass objects between classes.

Public properties are declared in the header (.h) file:

@interface MyClass : NSObject

@property (retain) NSString *fullName;

@end

The LLVM compiler automatically synthesizes all properties so there is no longer a need to explicitly write @synthesize statements for properties anymore. When a property is synthesized, accessors are created which allow you to set and get the value of a property.

Even though you may not see them since they are created at build time, a getter/setter pair can be shown as:

- (NSString *) fullName
{
    return( _fullName);
}
- (void) setFullName:(NSString *) aValue
{
   // possibly other code, depending on attributes. This shows `retain`
   [_fullName autorelease];
   _fullName = [aValue retain];
}

You can overrride the getter and setter of a property to create customized behavior, or even use this pattern to create transient properties such as:

- (NSString *) fullName
{
    return( [NSString stringWithFormat:@"%@ %@", [self firstName], [self lastName]]);
}

Properties are always backed by an instance variable with a leading underscore, so creating a property called firstName will have a backing instance variable with the name _firstName. You only access that private instance variable if you override the getter/setter or if you need to setup the instance variable in the class init method.

Property Attributes

When a property is specified, it is given the syntax:

@property SomeClass *someProperty;

// Or
@property (xxx) SomeClass *someProperty;

where xxx can be a combination of:

Type What it does
copy Creates an immutable copy of the object upon assignment and is typically used for creating an immutable version of a mutable object. Use this if you need the value of the object as it is at this moment, and you don't want that value to reflect any future changes made by other owners of the object.
assign Generates a setter which assigns the value directly to the instance variable, rather than copying or retaining it. This is typically used for creating properties for primitive types (float, int, BOOL, etc).
retain This is the default and is required when the attribute is a pointer to an object. The automatically generated setter will retain (i.e. increment the retain count of) the object and keep the object alive until released.
readonly This only generates a getter method so it won't allow the property to be changed via the setter method.
readwrite This is the default and generates both a setter and a getter for the property. Often times, a readonly property will be publicly defined and then a readwrite for the same property name will be privately redefined to allow mutation of the property value within that class only.
getter=method Used to specify a different name for the property's getter method. This is typically done for boolean properties (e.g. getter=isFinished)
setter=method Used to specify a different name for the property's setter method. (e.g. setter=setProjectAsFinished)

Accessing Properties

Properties can be accessed with the bracket syntax:

[self myProperty];

Local Variables

Local variables exist only within the scope of a method.

- (void) doWork
{
   NSString *localStringVariable = @"Some local string variable.";

   [self doSomethingWithString:localStringVariable];
}

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Naming Conventions

The general rule of thumb: Clarity and brevity are both important, but clarity should never be sacrificed for brevity.

Methods and Properties

These both use camelCase where the first letter of the first word is lowercase and the first letter of each additional word is capitalized.

Class names and Protocols

These both use CapitalCase where the first letter of every word is capitalized.

Methods

These should use verbs if they perform some action (e.g. performInBackground). You should be able to infer what is happening, what arguments a method takes, or what is being returned just by reading a method signature.

Example:

// Correct
- (Cell *) cellForRow:(NSUInteger) row
{
    // Code
}

// Incorrect (not expressive enough)
- (Cell *) cell:(NSUInteger) row
{
    // Code
}

Properties, Instance Variables and Local Variables

When using properties, instance variables are internally created with a preceeding underscore, so myVariableName is created as _myVariableName.

To keep local variables seperate from instance variables, you should not prefix them with underscores. Conversely you should also prefix your non-property instance variables with an underscore.

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Control Statements

Objective-C uses all of the same control statements that other languages have:

If-Else If-Else

if (someTestCondition)
{
    // Code to execute if the condition is true
}
else
   if (someOtherTestCondition)
   {
       // Code to execute if the other test condition is true
   } else
   {
       // Code to execute if the prior conditions are false
   }

Ternary Operators

The shorthand notation for an if-else statement is a ternary operator of the form: someTestCondition ? doIfTrue : doIfFalse;

Example:

- (NSString *) stringForTrueOrFalse:(BOOL) trueOrFalse
{
    return trueOrFalse ? @"True" : @"False";
}

For Loops

NSInteger   i;

for( i = 0; i < totalCount; i++)
{
    // Code to execute while i < totalCount
}

Fast Enumeration

Person  *person;

for( person in arrayOfPeople)
{
    // Code to execute each time
}

where arrayOfPeople can be any object that conforms to the NSFastEnumeration protocol. NSArray and NSSet enumerate over their objects, and NSDictionary enumerates over keys.

While Loop

while( someTextCondition)
{
   // Code to execute while the condition is true
}

Do While Loop

do
{
    // Code to execute while the condition is true
}
while (someTestCondition);

Switch

Switch statements are often used in place of if statements if there is a need to test if a certain variable matches the value of another constant or variable. For example, you may want to test if an error code integer you received matches an existing constant value or if it's a new error code.

switch (errorStatusCode)
{
case ServerErrorCode:
    // Code to execute if it matches
   break;

case NetworkErrorCode:
case WifiErrorCode:
case SystemErrorCode:
   // Code to execute if it matches
   break;

default:
   // Code to execute if nothing else matched
   break;
}

Note: switch statements are fallthrough: when control reaches the matched case (or default block if nothing matches), it continues execution of the next statements in the source code (including default) until the break statement or the end of switch is reached. This also allows multiple values to match the same point without any special syntax; they are just listed with empty bodies.

Exiting Loops

  • return : Stops execution and returns to the calling function. It can also be used to return a value from a method.
  • break : Used to stop execution of a loop.

Some people prefer enumeration methods that have special BOOL variables (e.g. BOOL stop) that are used to stop loop execution. Setting that variable to YES within the loop is similar to calling break. But the effect is not immediate.

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Extending Classes

There are a few different ways to extend a class in Objective-C, with some approaches being much easier than others.

Approach Difficulty Purpose
Inheritance Easy Used if all you want to do is inherit behavior from another class, such as NSObject
Category Easy Used if all you need to do is add additional methods to that class. If you also need to add instance variables to an existing class using a category, you can fake this by using associative references.
Delegation Easy Used to allow one class to react to changes in or influence behavior of another class while minimizing coupling.
Subclass Can be difficult Used if you need to add methods and properties to an existing class or if you want to inherit behavior from an existing class. Some classes are not designed to be subclassed.
Swizzle Can be difficult Swizzling allows you to replace a method in an existing class with one of your own making. This approach can lead to a lot of unexpected behavior, so it should be used very sparingly.

Inheritance

Inheritance essentially allows you to create concrete subclasses, which typically have specialized behavior, while inheriting all methods and properties from a superclass which are specified as @public or @protected within the superclass' header file.

Looking through any framework or open source project, you can see the use of inheritance to not only get behavior for free, but to also consolidate code and allow it to be easily reused. Examples of this approach can be seen with any of the mutable framework classes such as NSMutableString which is a subclass of NSString.

In Objective-C, all objects have much of their behavior defined by the NSObject class through the act of class inheritance. Without inheritance, you would have to implement common methods like object or class equality checks on your own and you'd end up with a lot of duplicate code across classes.

// MyClass inherits all behavior from the NSObject class
@interface MyClass : NSObject

// Class implementation

@end

Inheritance inherently creates coupling between classes, so be sure to take this into consideration.

Categories

Categories are a very useful and easy way of extending classes, especially when you don't have access to the original source code (such as for Cocoa Touch frameworks and classes). A category can be declared for any class and any methods that you declare in a category will be available to all instances of the original class, as well as any subclasses of the original class. At runtime, there's no difference between a method added by a category and one that is implemented by the original class.

Categories are also useful to:

  • Declare informal protocols
  • Group related methods similar to having multiple classes
  • Break up a large class implementation into multiple categories, which helps with incremental compilation
  • Easily configure a class differently for different applications

Implementation

Categories are named with the format: ClassYouAreExtending + DescriptorForWhatYouAreAdding

As an example, let's say that we need to add a new method to a PNGImage class (and all subclasses) that allows us to easily crop instances of that class to a certain height. You'd then need to create a header/implementation file pair named PNGImage+HeightCrop with the following implementation:

PNGImage+HeightCrop.h

@interface PNGImage( HeightCrop)

- (PNGImage *) croppedImageToHeight:(float) height;

@end

PNGImage+ResizeCrop.m

#import "PNGImage+HeightCrop.h"

@implementation PNGImage( HeightCrop)

- (PNGImage *) croppedImageToHeight:(float) height
{
    // Implementation code here
}

You could then call these methods on any instances of PNGImage or it's subclasses such as:

PNGImage *croppedImage = [bobRossImage croppedImageToHeight:1024.0];

Naming Conflicts

Because the methods declared in a category are added to an existing class, you should be very careful about method names. If the name of a method declared in a category is the same as a method in the original class, the method in the category overrides the original class. If it overrides a method in another category, the behavior is undefined as long as the dependency is not explicitly declared with MULLE_OBJC_DEPENDS_ON_CATEGORY. For proper overriding of a method in a class you do not own, you must be sure that the method is defined, where you expect it to be defined.

e.g.

MULLE_OBJC_DEPENDS_ON_CATEGORY( Foo, OverrideMe);

TODO: Show how to write an assert to test that the method to override is indeed in the expected place.

Delegation

Delegation is basically a way to allow one class to react to changes made in another class or to influence the behavior of another class while minimizing coupling between the two. Delegates are often associated with Protocols, so this is covered first:

Conforming to an Existing Protocol

To conform to an existing protocol, import the header file that contains the protocol declaration (not necessary for framework classes). Then, insert the protocol name within < > symbols and separate multiple protocols by a comma. Both options shown below will work just fine, but I prefer to keep them in the header file since it feels cleaner to me.

Option 1: In your .h file:

#import "LocationManager.h"

@interface MyController : Controller <LocationManagerDelegate>

@end

Creating Your Own Protocol

To create your own protocol for other classes to conform to, follow this syntax:

LocationManager.h

#import "import.h"

// Declare your protocol and decide which methods are required/optional
// for the delegate class to implement
@protocol LocationManagerDelegate <NSObject>
- (void)didAcquireLocation:(Location *)location;
- (void)didFailToAcquireLocationWithError:(NSError *)error;
@optional
- (void)didFindLocationName:(NSString *)locationName;
@end

@interface LocationManager : NSObject

// Create a property to store a reference to a delegate (delegates
// aren't retained)
@property(assign) id <LocationManagerDelegate> delegate;

// Implement any other methods here

@end

When we declare the @protocol named LocationManagerDelegate, all methods are defaulted to being @required so it's not necessary to explicitly state this. However, if you want certain methods to be @optional for conforming classes to implement, you must state this.

Adding The Delegate Property

Now we can declare a property, appropriately called delegate, which references the LocationManagerDelegate protocol.

Adding Default Implementations to Your Own Protocol

TODO: Talk about protocolclasses here a bit.

Sending Delegate Messages

In the example above, LocationManager.h declares some methods that the class which is acting as the delegate must implement. Within LocationManager.m itself, you could implement these a few different ways, but we'll just show two cases: a) required methods; b) optional methods.

Required Methods

- (void)updateLocation
{
    // Perform some work

    // When ready, notify the delegate method
    [delegate didAcquireLocation:locationObject];
}

Optional Methods

- (void) reverseGeoCode
{
    // Perform some work

    // When ready, notify the delegate method
    if ([delegate respondsToSelector:@selector( didFindLocationName:)])
    {
        [delegate didFindLocationName:locationName];
    }
}

The only difference between @required and @optional methods is that you should always check if the referenced delegate implemented an optional method before calling it on that class.

Implementing Delegate Methods

To implement a delegate method, just conform to the protocol like was discussed earlier, and then write it as if it was a normal method:

MyViewController.m

- (void) didFindLocationName:(NSString *) locationName
{
    NSLog(@"We found a location with the name %@", locationName);
}

Subclassing

Subclassing is essentially the same as inheritance, but you would typically create the subclass to either

  • Override a method or property implementation in the superclass; or
  • Create specialized behavior for the subclass (e.g. Toyota is a subclass of Car... it still has tires, an engine, etc, but it has additional custom behavior that uniquely makes it a Toyota)

Many design patterns, such as categories and delegation, exist so that you don't have to subclass another class.

Other times, classes like NSObject are designed to be easily subclassed. The general rule of thumb is to subclass another class only if you can satisfy the Liskov substitution principle:

If S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program.

Example

Let's assume that we want to model cars. All cars have similar behavior and characteristics, so let's put some of that in a superclass called Car.

Car.h

#import "import.h"

@interface Car : NSObject

@property( retain) NSString *make;
@property( retain) NSString *model;
@property( assign) NSInteger year;

- (void) startEngine;
- (void) pressGasPedal;
- (void) pressBrakePedal;

@end

Car.m

#import "Car.h"

#import "import-private.h"


@implementation Car

- (void) startEngine
{
   NSLog( @"Starting the engine.");
}

- (void) pressGasPedal
{
   NSLog( @"Accelerating...");
}

- (void) pressBrakePedal
{
   NSLog( @"Decelerating...");
}

@end

Now when we want to spawn off new car makes and models with unique characteristics, we use the Car superclass as a starting point and then add custom behavior in the subclass.

Toyota.h

#import "Car.h"


@interface Toyota : Car

- (void) preventAccident;

@end

Toyota.m

#import "Toyota.h"

#import "import-private.h"


@implementation Toyota

- (void) startEngine
{
   // Perform custom start sequence, different from the superclass

   NSLog( @"Starting the engine.");
}

- (void) preventAccident
{
   [self pressBrakePedal];

   [self deployAirbags];
}

- (void) deployAirbags
{
   NSLog( @"Deploying the airbags.");
}

@end

Even though pressBrakePedal is declared in the Car class, that method is still accessible in the Toyota class due to inheritance.

Designated Initializers

Often times, classes implement designated class initializers to allow for easy instantiation. If you override the main designated initializer for a class or provide a new designated initializer, it's important to ensure that you also override all other designated initializers so they use your new implementation instead of the superclass version. If you forget to do so and someone calls one of the secondary designated initializers on your subclass, they will get behavior from the superclass instead.

// The new designated initializer for this class
- (instancetype) initWithFullName:(NSString *) fullName
{
    if( self = [super init])
    {
        _fullName = fullName;
        [self commonSetup];
    }
    return( self);
}

// Provide a sensible default for other initializers
- (instancetype) init
{
    return [self initWithFullName:@"Default User"];
}

If you'd rather someone not use a default initializer for some rare case, you should throw an exception and provide them with an alternative solution:

- (instancetype) init
{
   [NSException raise:NSInvalidArgumentException
               format:@"%s Using the %@ initializer directly is not supported. Use %@ instead.", __PRETTY_FUNCTION__, NSStringFromSelector(@selector(init)), NSStringFromSelector(@selector(initWithFrame:))];
    return nil;
}

Overriding Methods

If you're subclassing another class to override a method within that class, you must be a little cautious. If you want to maintain the same behavior as the superclass, but just modify it slightly, you can call super within the override like this:

- (void) myMethod
{
    // You don't have to call super as the first method

    [super myMethod];

    // Provide your additional custom behavior here
}

If you don't want any of the superclass's behavior for the overridden method, simply leave out that call to super, but be careful that there aren't any memory or object lifecycle consequences for doing so.

Additionally, if the superclass has primitive methods upon which other derived methods are implemented, you must ensure that you override all necessary primitive methods necessary for the derived methods to work properly.

There is no shortcut to reading the documentation or the code of the to-be-subclassed class here.

Caveats

Certain classes don't lend themselves well to being easily subclassed and therefore subclassing is discouraged in those cases. An example of this is when trying to subclass a class cluster such as NSString or NSNumber. Class clusters have quite a few private classes within them. And these may change each version. So it's difficult to ensure that you have overridden all of the primitive methods and designated initializers within the class cluster properly.

Swizzling

As is often the case, clarity is better than cleverness. As a general rule, it's typically better to work around a bug in a method implementation than it is to replace the method by using method swizzling. The reason being that other people using your code might not realize that you replaced the method implementation and then they are stuck wondering why a method isn't responding with the default behavior.

For this reason, we don't discuss swizzling here but you are welcome to read up on it here.

Swizzling is more an issue, when you have a closed source vendor, whose bugs you have to fix in order to ship your product. With MulleObjC you have the sources.

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Error Handling

Errors are typically handled three different ways: assertions, exceptions, and recoverable errors. In the case of assertions and exceptions, they should only be used in rare cases since crashing your app is obviously not a great user experience.

Assertions

Assertions are used when you want to ensure that a value is what it is supposed to be. If it's not the correct value, you force exit the app.

NSAssert(someCondition, @"The condition was false, so we are exiting the app.");

Important: Do not call functions with side effects in the condition parameter of this macro. The condition parameter is not evaluated when assertions are disabled, so if you call functions with side effects, those functions may never get called when you build the project in a non-debug configuration.

Exceptions

Exceptions are only used for programming or unexpected runtime errors. Examples: attempting to access the 6th element of an array with 5 elements (out-of-bounds access), attempting to mutate immutable objects, or sending an invalid message to an object. You usually take care of these sorts of errors with exceptions when an application is being created rather than at runtime.

An example of this might be if you have a library which requires an API key to use.

// Check for an empty API key
- (void) assertAPIKey
{
    if (! [[self apiKey] length])
        [NSException raise:@"Forecastr"
                    format:@"Your Forecast.io API key must be populated before you can access the API.", nil];
}

Try-Catch

If you're worried that a block of code is going to throw an exception, you can wrap it in a try-catch block but keep in mind that this has slight performance implications

@try
{
    // The code to try here
}
@catch( NSException *exception)
{
    // Handle the caught exception
}
@finally
{
    // Execute code here that would run after both the @try and @catch blocks
}

Using exceptions as an error reporting mechanism is generally discouraged.

Recoverable Errors

Many times, methods will return an error condition. To access the full error explanation, you access the current NSError object. This object is contained in a thread-local variable analogous to errno:

   if( ! [NSFileHandle fileHandleForReadingAtPath:location])
     NSLog(@"Failed because: %@", [NSError mulleCurrentError]);

Creating Your Own Errors

It's also possible to create your own NSError objects to return in methods.

// Error domain & enums
NSString *MyErrorDomain = @"de.wasauchimmer.errors";
enum MyErrorType
{
    CachedItemNotFound,
    CacheNotEnabled
};

@implementation Cacher

- (id) checkForecastCacheForURLString:(NSString *) urlString
{
   NSError   *error;
   id        cachedItem;

   cachedItem = [[self cache] objectForKey:urlString];
   if( cachedItem)
     return( cachedItem);

   error = [NSError errorWithDomain:MyErrorDomain
                               code:CachedItemNotFound
                           userInfo:nil];
   [NSError mulleSetCurrentError:error];
   return( cachedItem);
}

@end

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Loose coupling

Passing information between classes through methods calls is what one calls tightly coupled. Delegation is a way to loosely couple classes, but there are also a few more possibilities shown here .

NSNotificationCenter

Notifications are broadcast messages that are used to decouple classes and establish anonymous communication between objects at runtime. Notifications may be posted by any number of objects and received by any number of objects thus enabling one-to-many and many-to-many relationships between objects.

Note: Notifications are sent synchronously so if your observer method takes a long time to return, you are essentially prevently the notification from being sent to other observing objects.

Registering Observers

You can register to be notified when a certain event has happened, including system notifications, such as when a NSBundle has been loaded.

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector( bundleDidLoad):
                                             name:NSBundleDidLoadNotification
                                           object:self];

When the NSBundleDidLoadNotification notification is broadcast by the OS framework, the bundleDidLoad: will be called by NSNotificationCenter and an object will be sent along with the notification that could contain data.

A possible implementation of the bundleDidLoad: method could be:

- (void) bundleDidLoad:(NSNotification *) notification
{
    // Optional check to make sure the method was called from the notification
    NSLog( @"Bundle load: %@", [notification object])
}

Removing Observers

It's important to remove yourself as an observer before the class is deallocated, otherwise NSNotificationCenter will attempt to call a method on a deallocated class and a crash will ensue. This should be done in -finalize:

- (void) finalize
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
    [super finalize];
}

Posting Notifications

You can also create and post your own notifications. It's best practice to keep notification names in constants so that you don't accidentally misspell one of the notification names and sit there trying to figure out why the notification wasn't sent/received.

Naming notifications:

Notifications are identified by NSString objects whose names are composed in this way:

[Name of associated class] + [Did | Will] + [UniquePartOfName] + Notification

Declare a string constant, using the notification name as the string's value:

// Remember to put the extern of this in the header file
NSString *OMGFactoryLookAtMyObjectNotification = @"OMGFactoryLookAtMyObjectNotification";

Post notification:

[[NSNotificationCenter defaultCenter] postNotificationName:OMGFactoryLookAtMyObjectNotification
                                                    object:self];

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User Defaults

User defaults are basically a way of storing simple preference values which can be saved and restored across app launches. It is not meant to be used as a data storage layer, like Core Data or sqlite.

Storing Values

NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
[userDefaults setValue:@"Some value" forKey:@"SomeUserPreference"];

Retrieving Values

NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
id someValue = [userDefaults valueForKey:@"SomeUserPreference"];

There are also other convenience methods on NSUserDefaults instances such as boolForKey:, stringForKey:, etc.

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Common Patterns

Singletons

Singleton's are a special kind of class where only one instance of the class exists for the current process. They are a convenient way to share data between different parts of an app without creating global variables or having to pass the data around manually, but they should be used sparingly since they often create tighter coupling between classes.

To turn a class into a singleton, you adopt the protocol MulleObjCSingleton.

@interface MyClass < MulleObjCSingleton>
@end

By that you automatically gain a + (instancetype) sharedInstance method.

If the above code were placed within MyClass, then you would get a reference to that singleton class in another class with the following code:

MyClass *myClass = [MyClass sharedInstance];
[myClass doSomething];
NSLog(@"Property value is %@", myClass.someProperty);

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