Objective-C A Beginner's Dive (with notes)


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Objective-C is a Smalltalk-like Object-Oriented layer on top of the C language. It is the official language of OS X and iOS. Assuming you have a background in Object-Oriented Programming and a basic understanding of the C language or syntax, this talk will cover everything you need to know about Objective-C. By the end of the talk, you will understand how to make and use your own objects, the Foundation Framework and the data structures it provides, and the Objective-C specific language constructs and syntactic-sugar.

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Objective-C A Beginner's Dive (with notes)

  1. 1. Objective-C A Beginner’s DiveSaturday, March 9, 13
  2. 2. I Assume • A Background in Java or C++ • An understanding of OOP • A basic understanding of CSaturday, March 9, 13
  3. 3. What is Objective-C? • Superset of C • Smalltalk style Object-Oriented • The Official Language for iOS and OS XSaturday, March 9, 13
  4. 4. Cool! But before we dive in...Saturday, March 9, 13
  5. 5. Hello World! // // main.m #include <stdio.h> #include <Foundation/Foundation.h> int main(int argc, const char **argv) { NSString *message = @"Hello World!"; printf("%sn", [message cString]); return 0; }Saturday, March 9, 13The main() program runs.Make a string object called message with the value “Hello World!”Print message to the screen followed by a newline character.End the program.
  6. 6. Hello World! • Objective-C String Literal • @”Hello World!” // // main.m • C Function #include <stdio.h> #include <Foundation/Foundation.h> • printf() int main(int argc, const char **argv) { NSString *message = @"Hello World!"; • main() printf("%sn", [message cString]); return 0; • } Objective-C message • [message cString]Saturday, March 9, 13This shows that you can have C code along-side Objective-C. (well, actually, it’s visa-versa)This shows some Objective-C @ symbol action.This shows an Objective-C message using those fancy square brackets.
  7. 7. Ok. Now, on to the teaching!Saturday, March 9, 13
  8. 8. Objective-C Features • C Functions • C Structs • C Unions • C Pointers • C Arrays • C Everything ElseSaturday, March 9, 13You can do all the C you want in your Objective-C. It’s called a superset for a reason!Fun fact! C++ is not a superset. It changes some stuff...
  9. 9. Objective-C Features • Objects • Data and Collections • Methods • Object Literals • Inheritance • Object Subscripts • Properties • Forin loops • Protocols • Blocks and Closures • Categories • ARCSaturday, March 9, 13Objective-C adds on to C all this Object-Oriented stuff on top.ARC - Automatic Reference Counting
  10. 10. You Will Learn About • Making and Using Objects • Using Foundation Framework Data Types • Automatic Reference Counting (ARC)Saturday, March 9, 13
  11. 11. Writing Objects A Quick Run-ThroughSaturday, March 9, 13
  12. 12. Objects • Interface • like Java’s Interface, but every object has one • placed in the header file • Implementation • must fill minimum requirements of Interface • placed in the implementation fileSaturday, March 9, 13
  13. 13. Example Object // // // MyObject.m // MyObject.h // Implementation // Interface #import “MyObject.h” #import <Foundation/Foundation.h> @implementation MyObject @interface MyObject : NSObject { int _myInt; // method implementations go here } @end // method declarations go here @endSaturday, March 9, 13NSObject is the superclass. NSObject is the root of all Foundation Framework objects.Instance Variables can be listed within the optional curly brackets after the interface opening.Instance Variables can be declared in either the Implementation, Interface, or both.
  14. 14. Objects (More Info) • Objects have Instance Variables (ivars) • No Class variables, use C static globals • No enforced “public” and “private” • Object Instances can be of type: •id •Class *Saturday, March 9, 13id is the generic object type. All objects are of type id.If you want to be more specific, objects can be represented as pointers to their class.
  15. 15. Object Methods • Class Method or Instance Method • Variables are “named” in the method’s signature (fancy word for method name) • Default return and variable type is idSaturday, March 9, 13
  16. 16. Format of a Method +/- (return type)methodName; +/- (return type)methodWithVar:(var type)var; +/- (return type)methodWithVar1:(var type)var1 ! ! ! ! ! ! ! ! var2:(var type)var2;Saturday, March 9, 13The + denotes a class method, where the - denotes an instance method.The default return type is id, the generic Objective-C Object.The names of these methods are methodName, methodWithVar:, and methodWithVar1:var2:.When pronounced, there is a slight pause at each colon.
  17. 17. Method Examples // // // MyObject.m // MyObject.h // Implementation // Interface #import “MyObject.h” #import <Foundation/Foundation.h> @implementation MyObject @interface MyObject : NSObject { int _myInt; // setter } - (void)setMyInt:(int)myInt { _myInt = myInt; // setter } - (void)setMyInt:(int)myInt; // getter // getter - (int)myInt { - (int)myInt; return _myInt; } @end @endSaturday, March 9, 13Here are two instance methods in our object.These methods are following the standard style for accessors and mutators in Objective-C.Getters are methods whose name match the variable in question, and setters begin with theword set.
  18. 18. Object Method Calling • Not like C, C++, or Java • Based on Smalltalk message passing • The Square Brackets [] are your friend! [object method]; [object methodWithVar:value]; [object methodWithVar1:val1 ! ! ! ! ! ! ! ! var2:val2];Saturday, March 9, 13If you need to make your method call multi-line, the general convention is to line up thecolons.Xcode generally lines them up for you with the tab key.
  19. 19. Testing Responsiveness to a Selector • The name of a method is its Selector SEL mySelector = selector(myMethodWithParameter:) • Every object inherits respondsToSelector: • Takes the selector to test for • Returns YES when the object can respond to that method • Returns NO when the object cannot respond to that methodSaturday, March 9, 13
  20. 20. Object Constructor • Not a special method (unlike Java) • Just an instance method to set up the Object’s Instance Variables • Generally named initSaturday, March 9, 13
  21. 21. Generic Constructor - (id)init { if (self = [super init]) { // initialize variables } return self; }Saturday, March 9, 13This code calls the superclass’s init method and sets the self variable to its result.If there is a problem in the initialization, [super init] will return 0, which would cause the ifstatement to skip to the return at the bottom of the method, and the method would return 0.Otherwise, [super init] will return the pointer to the object being initialized, which will beassigned to self, and the if statement will execute the code within, finally returning thepointer to self at the end of the method.
  22. 22. Constructor Example // default constructor - (id)init { // calls my custom constructor return [self initWithInt:0]; } // custom constructor - (id)initWithInt:(int)myInt { if (self = [super init]) { _myInt = myInt; } return self; }Saturday, March 9, 13
  23. 23. Inheritance • Single Inheritance from Objects • Method Overloading Supported • Superclass defined in the Interface • Super class referenced with superSaturday, March 9, 13
  24. 24. Inheritance Example // // // MyObject.m // MyObject.h // Implementation // Interface #import “MyObject.h” #import <Foundation/Foundation.h> @implementation MyObject @interface MyObject : NSObject { int _myInt; - (id)init { } if (self = [super init]) { _myInt = 5; - (id)init; } return self; @end } @endSaturday, March 9, 13Here we inherit init from our superclass NSObject.We override init with our own definition.We then call our superclass’s implementation of init using the super keyword.
  25. 25. Properties • Syntactic sugar for variable, accessor, and mutator declarations all-in-one • Properties are declared in the Interface and synthesized in the Implementation • Let you use “dot syntax” with getters and setters ! myObjectInstance.myInt = 5; ! int x = myObjectInstance.myInt;Saturday, March 9, 13
  26. 26. Property Example // // // NSString+Reverse.m // NSString+Reverse.h // Category // Category #import "NSString.h" #import <Foundation/Foundation.h> @implementation MyObject @interface MyObject : NSObject // creates variable _myInt @property (assign, nonatomic) int myInt; // creates getter myInt // creates setter setMyInt: @end @synthesize myInt = _myInt; @endSaturday, March 9, 13This will create an Instance Variable named _myInt with a getter and setter method.
  27. 27. More on Properties • Attributes strong weak copy assign readonly atomic nonatomic • @synthesize vs @dynamic !synthesize variable = nameOfIvar; @ • Default ivar name for variable is _variable • Custom getters and setters getter = myGetter, setter = mySetter:Saturday, March 9, 13strong - holds a retained reference to the assigned object. the assigned object will not bereleased and deallocated until self is deallocated or the variable is reassigned.weak - if the assigned object is ever released, the variable is set to nil.copy - the variable is set to a copy of the assigned object.assign - used with non-object types. copies the literal value into the variable.readonly - do not generate a setter for this property.readwrite - generate a setter for the property. this is the default.atomic - this is the default. synchronize getting and setting the variable.nonatomic - considered faster than atomic.@synthesize - generates the getter and setter methods for you. Any you write having thesame name override any generated. Synthesize is assumed the default by the compiler andcreates an Instance Variable named with an underscore.@dynamic - no automatic generation of getters and setters. The compiler assumes these willbe added at runtime. Mostly used when using CoreData.
  28. 28. Protocols • Like an Objective-C Interface • Similar to Java Interfaces • Can implement multiple Protocols • Protocols can ‘inherit’ other ProtocolsSaturday, March 9, 13
  29. 29. Example Protocol // // // MyProtocol.h // MyObject.h // Protocol // Interface #import <Foundation/Foundation.h> #import <Foundation/Foundation.h> #import "MyProtocol.h" @protocol MyProtocol <NSObject> @interface MyObject : NSObject <MyProtocol> @property (assign, nonatomic) int myInt; @end - (NSString *)stringMyInt; @endSaturday, March 9, 13MyProtocol “inherits” NSObject, and MyObject implements MyProtocol.This means that whatever implements MyPrototcol must also implement all the functionalityof NSObject.
  30. 30. Categories • The ability to add new methods to an Object • Changes apply to all instances of the object • Overwrites any methods that already exist in the class • Cannot add Properties and ivars • Can be dangerousSaturday, March 9, 13
  31. 31. Example Category // // // NSString+Reverse.m // NSString+Reverse.h // Category // Category #import "NSString+Reverse.h" #import <Foundation/Foundation.h> #import <stdlib.h> @interface NSString (Reverse) @implementation NSString (Reverse) - (NSString *)reverse; - (NSString *)reverse { int length = [self length]; @end char *newString = ! ! calloc(length+1, sizeof(char)); int current = 0; const char *cstr = [self cString]; for (int i=length-1; i >= 0; i--) { newString[current] = cstr[i]; current++; } NSString *new = ! ! [NSString stringWithCString:newString]; free(newString); return new; } @endSaturday, March 9, 13This is a category on NSString that adds the method reverse, which returns a reversed string.Now, all implementations of NSString have the method reverse.If NSString already had a reverse method, it is no longer accessible.If another category is added that also has a reverse method, this method implementation willno longer be accessible.Notice the naming convention of the category Interface and Implementation files.
  32. 32. Class Extension • Like Categories, but with the ability to add ivars and Properties • Implementations of methods required in the main Implementation block • Can be used to declare “private” methodsSaturday, March 9, 13In Objective-C, as in C, Public and Private information isn’t compiler managed, but rather,determined by what you as the developer let others see.If you place it in your header file, expect people to see it and look at it. The header file is theforward facing side of your code.If it’s not in your header file, don’t expect anyone to know it exists. Libraries are generallybinaries with header files, implementation files are for the developer’s eyes only (generally).Technically, a spy who knows the implementation could still use a “private” method, but youwouldn’t expect anyone to do that.
  33. 33. Class Extension Example // // MyObject.m // Class Extension @interface MyObject () @property (assign) int myPrivateInt; @end // Implementation @implementation MyObject @synthesize myPrivateInt = _myPrivateInt; // more implementation here @endSaturday, March 9, 13Class Extensions are generally declared within the implementation file before the@implementation block.
  34. 34. Ok. Now, lets take a look at MyObjectSaturday, March 9, 13
  35. 35. MyObject // // // MyObject.m // MyObject.h #import "MyObject.h" #import <Foundation/Foundation.h> #import "MyProtocol.h" @interface MyObject () @property (assign, nonatomic) int myPrivateInt; @interface MyObject : NSObject <MyProtocol> @end @property (assign, nonatomic) int myInt; @implementation MyObject // default constructor @synthesize myInt = _myInt; - (id)init; @synthesize myPrivateInt = _myPrivateInt; // custom constructor - (id)init { - (id)initWithInt:(int)myInt; return [self initWithInt:0]; } @end - (id)initWithInt:(int)myInt { if (self = [super init]) { _myInt = myInt; ! ! _myPrivateInt = 5; } return self; } - (NSString *)stringMyInt { return [NSString stringWithFormat:@"%d", _myInt]; } @endSaturday, March 9, 13Here we have an object that holds a public myInt variable and an ‘private’ myPrivateIntvariable.Both ivars are accessible through the property ‘dot’ syntax and their getter/setter methods.MyObject fulfills the MyProtocol protocol by implementing the stringMyInt method.The @synthesize statements here were technically unnecessary since they are default.
  36. 36. Great But how do I use it?Saturday, March 9, 13
  37. 37. Using MyObject MyObject *obj = [[MyObject alloc] init]; obj.myInt = 5; NSLog(@"%in", obj.myInt); MyObject *other = [MyObject alloc]; other = [other initWithInt:5]; [other setMyInt:10]; NSLog(@"%in", [obj myInt]);Saturday, March 9, 13Remember: an object instance is a pointer to an instance of its class.
  38. 38. Using MyObject • alloc class method • use of init and initWithInt: methods MyObject *obj = [[MyObject alloc] init]; obj.myInt = 5; • NSLog(@"%in", obj.myInt); splitting up the alloc and init method calls MyObject *other = [MyObject alloc]; other = [other initWithInt:5]; • use of dot syntax and [other setMyInt:10]; NSLog(@"%in", [obj myInt]); generated methods • NSLog() for printing messages to the consoleSaturday, March 9, 13alloc allocates a new object for you.init (and initWithInt:) initialize that new object for you.These methods together are like the new keyword in Java and C++.(there is a new class method that automatically calls init after alloc, but it’s generallyconsidered better to call alloc and init instead)The different init methods go to show that init is not super special.The call to alloc and init don’t need to happen on the same line, but you should always usethe result of the init method as your object.NSLog() is a C Function that takes an NSString and printf style arguments (including %@ toprint an object’s description method which returns an NSString). It prints a message to theconsole.
  39. 39. One More Thing Imports and Forward DeclarationsSaturday, March 9, 13
  40. 40. Imports • Objective-C introduces the #import Preprocessor Directive #import "MyHeaderFile.h" #import <SystemHeader.h> • Guarantees the file is only included once • Better Practice than C’s #includeSaturday, March 9, 13
  41. 41. Forward Declarations • A Promise to the Compiler that a Class or Protocol will exist at Compile Time @class PromisedObject; @protocol PromisedProtocol; • Minimizes the amount of #includesSaturday, March 9, 13
  42. 42. Objective-C Data What you get with the Foundation FrameworkSaturday, March 9, 13
  43. 43. nil • Represents the absence of an object • Messages passed to nil return nil • nil is for objects, NULL is for C pointersSaturday, March 9, 13Unlike Java and C++, there is nothing wrong with calling a method on a nil object.There’s no real difference between nil and NULL, but it’s bad convention to use them“inappropriately”
  44. 44. YES and NO • Boolean type BOOL for “true” and “false” • Use BOOL with YES and NO • Dont use _Bool or bool with true and false from stdbool.hSaturday, March 9, 13Yet again, there’s no significant difference between C’s _Bool and bool and Objective-C’sBOOL, it’s just better practice to use the Objective-C stuff with Objective-C code.BOOL is #define’d as an unsigned char, whereas _Bool is a typedef for an int and guaranteedto be only 1 or 0.
  45. 45. NSNumber • Object Representation for Integers, Booleans, Floats, Doubles, Characters, etc. • Object Literal Syntax @1 @1.0 @YES @NO @(1+2) @aSaturday, March 9, 13
  46. 46. NSString • Immutable string • NSMutableString for mutable strings • Object Literal Syntax @"Hello World!" @("Hello World!")Saturday, March 9, 13Immutable means that the data defining the string cannot be modified.Mutable means modifiable.
  47. 47. NSArray • Immutable object array • NSMutableArray for mutable arrays • Object Literal Syntax @[object1, object2, ..., objectN]; • Object Subscripting Syntax array[0] = object; id object = array[0];Saturday, March 9, 13The array can only hold Objective-C objects.
  48. 48. NSDictionary • Immutable object dictionary • NSMutableDictionary for mutable • Object Literal Syntax @{key1:value1, key2:value2, ...}; • Object Subscripting Syntax dictionary[key] = object; id object = dictionary[key];Saturday, March 9, 13Both the key and the value must be an Objective-C object.
  49. 49. Implementing Array Subscripting • Accessing Objects objectAtIndexedSubscript: • Setting Objects setObject:atIndexedSubscript: • The index subscript must be an integralSaturday, March 9, 13The indexed subscript is generally an NSUInteger (an unsigned long), but can be any integertype.
  50. 50. Implementing Dictionary Subscripting • Accessing Object values with key Objects objectForKeyedSubscript: • Setting Object values with key Objects setObject:forKeyedSubscript:Saturday, March 9, 13
  51. 51. Forin Loops • Loop over the contents of a collection • Foundation NSDictionary NSArray Framework collections • Any implementation of the NSFastEnumeration Protocol • Any Subclass of NSEnumeratorSaturday, March 9, 13NSDictionary will loop through the keys in the dictionary, not the values.I would recommend that if you wanted to make a collection, make a subclass ofNSEnumerator (or use an enumerator), it’s easier...
  52. 52. Forin Loops Example NSArray *array = @[@1, @2, @3, @4, @5]; for (NSNumber *num in array) { NSLog(@"%@", num); } for (id num in [array reverseObjectEnumerator]) { NSLog(@"%@", num); }Saturday, March 9, 13an array can only hold objects.the first for loop will loop through the array from index 0 to index 4.the second for loop will loop through the array from index 4 to index 0.
  53. 53. Objective-C Blocks • Functions you can declare within functions • Can close-over variables for later use • Can pass block functions around like dataSaturday, March 9, 13
  54. 54. Block Syntax • Similar to C Function Pointer Syntax return_type (^name)(parameter types) = ! ^(parameter list) { ! ! // do stuff ! }; return_type (^name)() = ^{ ! ! // do stuff ! ! // takes no parameters ! };Saturday, March 9, 13
  55. 55. Block Example int multiplier = 5; The block itself has read int (^mult)(int) = ^(int num){ return num * multiplier; access to variables defined }; in the lexical scope at the int num = mult(5); // num = 25 creation of the block. __block int number = 0; The block can close over void (^increment)() = ^{ number++; outside variables and }; modify their values by increment(); // number = 1 declaring the variable with increment(); // number = 2 the __block modifier.Saturday, March 9, 13
  56. 56. Type-Safe Enums • Enums that you can declare the type they enumerate over • int, char, unsigned char, NSUInteger, ... • Syntax similar to Object Subclassing typedef enum MyEnum : NSUInteger { A, B, C } MyEnum; enum MyUnsignedCharEnum : unsigned char { FIRST, SECOND, THIRD }; typedef enum MyUnsignedCharEnum MyUnsignedCharEnum; typedef enum : NSUInteger { ONE, TWO, THREE } AnotherEnum;Saturday, March 9, 13
  57. 57. Automatic Reference Counting What it is, and what it means to youSaturday, March 9, 13
  58. 58. Referencing Counting • Nearly-Manual Memory Management • Objects have a counter showing how many references are using them • Retain Objects when you receive them [object retain]; • Release Objects when you’re done using them [object release]; • Objects deallocate themselves when their retain count reaches 0Saturday, March 9, 13Objects are created with a retain count of 1, so you don’t need to retain objects you justcreated.
  59. 59. Automatic Reference Counting (ARC) • Almost Compile Time Garbage Collection • Retain and Release messages are added at compile time by the compiler • ARC manages when to do this for you • strong variables are Retained when assigned • weak variables are not Retained on assignment and are zeroed out when deallocatedSaturday, March 9, 13
  60. 60. Retain Cycle • Two Objects Reference Each Other • Their retain counts can never reach 0 • Set one of the references to weak to prevent one of the objects from retaining the other and causing a cycleSaturday, March 9, 13
  61. 61. So there you have it Objective-C thrown at youSaturday, March 9, 13
  62. 62. Useful References • Programming with Objective-C http://developer.apple.com/library/ios/#documentation/cocoa/conceptual/ ProgrammingWithObjectiveC/Introduction/Introduction.html http://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/ ProgrammingWithObjectiveC/ProgrammingWithObjectiveC.pdf • Objective-C Cheat Sheet and Quick Reference http://cdn5.raywenderlich.com/downloads/RW-Objective-C-Cheatsheet-v1_2.pdf • Coding Guidelines for Cocoa http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CodingGuidelines/ CodingGuidelines.html http://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CodingGuidelines/ CodingGuidelines.pdf • Ry’s Objective-C Tutorial http://rypress.com/tutorials/objective-c/index.htmlSaturday, March 9, 13Programming with Objective-C and Coding Guidelines for Cocoa are Apple documents.Ry’s Objective-C Tutorial is a nice looking piece of documentation.