IPhone Tech Talks/Introduction to Objective-C and Cocoa Touch

From ceejayoz

• Matt Drance (App Framework Evangelist)
• Michael Jurewitz (Development Tools and Performance Evangelist)

Contents 1 iPhone OS 1.1 Sandboxing 1.2 Memory management 2 Objective C 2.1 Calling a method - class and function/message 2.1.1 C++ 2.1.2 Java 2.1.3 Objective-C 2.2 Messaging 2.3 Selectors 2.4 Method dispatch 2.5 Example: Person class 2.6 Initialising instance 2.7 Getting and setting simple type 2.8 Getting and setting objects 3 Memory management 3.1 Autorelease pools 3.2 Example of memory management 3.3 Setter/Getter Pattern 3.4 Properties 3.5 Objective-C Categories 3.6 Objective-C Protocols 4 Cocoa Touch 4.1 Foundation 4.2 Strings 4.3 Arrays 4.3.1 Mutable arrays 4.3.2 Immutable arrays 4.3.3 Dictionaries (aka hashtables in other frameworks) 4.4 File I/O 4.5 NSLog 4.6 Bundles 4.7 Networking 5 UIKit 6 Application Design Patterns 6.1 MVC 6.2 Target-action 6.3 Notifications 6.4 Delegation

[edit] iPhone OS

• Code Signing
• Single app/process environment

[edit] Sandboxing

• frameworks allow access to some non-sandboxed data

[edit] Memory management

• no swap space
• issues memory warning prompting cleanup
• quit app if exceed

[edit] Objective C

C language with Smalltalk-style syntax --> ObjC

C++:

• fast but static
• compile-time decisions, little runtime flexibility

Objective-C:

• dynamic
• runtime decision making
• runtime class extensions

[edit] Calling a method - class and function/message

[edit] C++

someObject->doSomething();

[edit] Java

someObject.doSomething();

[edit] Objective-C

[someObject doSomething];

can be nested!

[edit] Messaging

• [Employee newEmployee];

  - class, method

• [anEmployee setRaise:200];

  - instance of class, method with parameter/argument

• [anEmployee setWork:work withDeadline:today];

  - instance, multiple parameters
  • you know what argument type fits which position, self-documenting

[edit] Selectors

• [Employee newEmployee]

  is 'newEmployee'

• [anEmployee setRaise:200]

  is 'setRaise:'

• [anEmployee setWork:work withDeadline:today]

  is 'setWork:withDeadline'

[edit] Method dispatch

• class hierarchy (single only, not multiple) - Employee -> Person -> NSObject
• NSObject is the root object, defined in Foundation framework
• Employee gets chance to respond to message, then Person, then NSObject
• 'firstName' method
  • in this example, Employee has no firstName method, but Person has a firstName method, so it gets it
• 'description' method
  • defined on NSObject
  • very basic way to print itself out at commandline / memory
  • can override method for debug - output state, etc.
• what happens when no response to message?
  • traverses classes again for opportunity to pass to helper object
  • exception raised if no helper object to forward to
  • exceptions raised only in 'exceptional circumstances' (cute)

[edit] Example: Person class

@interface Person : NSObject { // @interface = 'this is beginning of class defn', Person = class name, NSObject = Person is subclass of NSObject
  NSString *name; // NS = foundation class (NextStep)
  int age;
  NSMutableArray *parents;
 
+  (Person *)personWithName:(NSString *)name; // plus = class method, only send within class
-  (id)initWithName:(NSString *)name; // minus = instance method, only send to instance of class... (id) is return type for method
 
-  (int)age; // getter, no need to put get before, just use name itself
-  (void)setAge:(int)newAge;
 
- (NSArray *)parents;
- (void)setParents:(NSArray *)parents;
@end

#import "Person.h" // #import prevents multiple includes of same header file
@implementation Person
// implementation here
@end

[edit] Initialising instance

- (id)init {
  if (self = [super init]) { // making sure didn't get back invalid object
    name = @"New Person"; // set up instance var (in this case, a string literal - at sign + quoted string)
    parents = [[NSArray alloc] init]; // set up instance var, an array... alloc = allocate space, then initialise with init
  }
  return self; // return instance to whoever asked for it
}

super = send messages to superclass

[edit] Getting and setting simple type

- (int)age {
  return age;
}

- (void)setAge:(int)newAge {
  age = newAge;
}

[edit] Getting and setting objects

- (NSString *)name {
  return name;
}

[edit] Memory management

• runtime tracks object's retain count for entire lifetime of object (starts at 1 on creation)
• need to stick around? send it the retain message (-retain)
• done with it? release it with </code>-release</code>
• once no longer needed by any part of system, cleared out of memory!

MJFoo *myFoo = [[MJFoo alloc] init]; - retain count 1 [myFoo retain]; - retain 2 [myFoo release]; - retain 1 [myFoo release]; - retain 0, cleared from memory

[edit] Autorelease pools

• use -autorelease message
• sends -release at some point in future (delayed)
• used in class convenience methods for temporary objects

• if +alloc or -retain, must -release
• if used convenience method, must -retain to keep

[edit] Example of memory management

comparison of objects

if newName != name, retain newName, release name, and name=newName (returned)

dealloc function - lets you release stuff within class

[edit] Setter/Getter Pattern

• simple but repetitive
• must create code for each getter/setter

[edit] Properties

• quickly and easily create set/get pairs
• specify storage semantics
• easy to override

@property(readwrite) float speed; - attributes, type, name

• attributes
  • readonly/readwrite (mutability)
  • assign/retain/copy (memory management)
  • nonatomic (concurrency)
  • setter= / getter= (API for setter/getter names, ex: bool getter -> isGoingFast)

readwrite is default for @property

@synthesize age, parents;

can create custom/overridden methods

[edit] Objective-C Categories

• mix in custom behaviour
• seamlessly add methods to existing classes
• should use subclass instead to add storage or modify behaviour

uses @implementation, could, for example, add a method to NSString

[edit] Objective-C Protocols

• multiple inheritance via specifications
• way for objects to communicate between each other, identical to Java interfaces
• list of methods, classes can opt-in to conform to protocol by implementing methods and advertising that they comply

uses @protocol MyProtocol

@interface Foo : NSObject <MyProtocol>

used in UITableView for datasourcing

[edit] Cocoa Touch

• two pieces of framework
  • UIKit - event handling, graphics and windowing, text and web management
  • Foundation - object wrappers, strings, data structures, networking and other system services

[edit] Foundation

[edit] Strings

NSString *myString = @"This is a String";

• supports sprintf formatting
• convenience methods in NSString and most of Foundation API - single line stuff like pathComponents (creates array out of /path/to/file), lastPathComponent, stringWithContentsOfFile, etc.
• if you spend 30 lines of code doing something simple, probably missed something!

[edit] Arrays

[edit] Mutable arrays

NSMutableArray *arr = [NSMutableArray arrayWithCapacity:1];
[arr addObject:@"randomString"];

[edit] Immutable arrays

NSArray *arr = [NSArray arrayWithObjects:@"foo", @"bar", @"moof", nil]; // nil required at end! tells where array ends

• convenience methods, like componentsJoinedByString

[edit] Dictionaries (aka hashtables in other frameworks)

• also come in immutable/mutables
• array with keys, essentially
• NSDictionary / NSMutableDictionary

[edit] File I/O

• NSFileManager class handles lots of this stuff
• tmp directory created in root of sandbox, periodically cleared by iPhone
• Library/Caches - saved, not backed up (speeds up sync!!!) - use for non-critical data you don't want to re-download each time
• EVERYTHING ELSE IS BACKED UP

[edit] NSLog

• outputs debug information to debugger console
• supports printf formatting
• also accepts object arguments easily - NSLog(@"Got an array: %@", myArray); (uses description method for pretty format)

[edit] Bundles

• special type of folder
• your application is an example of one
• executables, nib files (user interface), metadata
• accessible via NSBundle class, gives access to static resources (default data, images, sounds)
• application bundle is READ ONLY, new data goes in the sandbox instead
• example: pull a default sqlite database if no user data

[edit] Networking

• standard sockets programming
• CFNetwork, CFSocket (CF = Core Foundation)
• POSIX APIs as well
• NSURLConnection for async requests
• NSXML - xml parser
• Bonjour networking (zero config discovery of peers on same network)

[edit] UIKit

• visual components
• can have third-party ones
• Image views - scaling! resize, aspect ratio, centring, etc.
• Web views

[edit] Application Design Patterns

[edit] MVC

• Model - Foundation API
• View - UIKit Views
• Controller - UIKit View Controllers

[edit] Target-action

• Control has a target and an action
• Target -> Object
• Action -> Selector
• ex. tap button, sends action to target

[edit] Notifications

• Notification centre - looks for observers to send to
• Observer / controller
• movie player, as example, has 'finished loading', 'finished playing' notifications
• NSNotificationCenter

[edit] Delegation

• default image is displayed
• instantiate UIApplication
  • load main nib
  • load main window
• create instance of application delegate
  • UI config
  • first and last chance to act
  • custom class written by you
  • conforms to UIApplicationDelegate
  • hooked up in interface builder

• two key methods to delegation
  • applicationDidFinishLaaunching - load initial UI, saved state
  • applicationWillTerminate - save state and data
    • just a courtesy - DO NOT ASSUME YOU HAVE A LOT OF TIME! quick and easy stuff, important data should be at time of user-entry