What do I need?

• @required

  • Mac OS X 10.6+
  • XCode 4.2+

• @optional

  • Device with iOS 4.0+
  • Paid iOS Program Membership

Objective-C

• Typed
• Object oriented, inspired by Smalltalk
• Superset of C. Keep calm...

Methods Syntax

• Calling a method

// java
anObject.aMethod(); 

   
    // obj-c
    [anObject aMethod]; 

Methods Syntax

• Calling a method

// java
anObject.aMethod().anotherMethod(); 

   
    // obj-c
    [[anObject aMethod] anotherMethod];

Methods Syntax

• Calling a method

// java
array.add(0,object); 

   
    // obj-c
    [array insertObject:object atIndex:0];

Interesting stuff

• Protocols
  • Very similar to the interface concept in Java.
    • You can define required and optional methods and properties.
• Categories
  • Allows extending a class' behavior...
  • Or even overriding it :-)
  • No subclassing needed.

Interesting stuff

• Blocks
  • Similar to closures in Ruby or Javascript.
• Properties

   
        @property (retain) NSString *name;
        ...
        object.name = @"Slim Shady";
        NSLog(@"My name is %@",object.name);

but...

Explicit Memory Management

Explicit Memory Management

• Reference counting
• How?
  • Increase the object's reference counter when you start using it.
  • Decrease the object's reference counter when you finish using it.
  • When the counter reaches 0, it will be automatically deallocated.

Explicit Memory Management

• How do I increase the object's reference counter?

  • Use retain: [array retain];
  • Use init constructors: [[NSArray alloc] init].

• How do I decrease the object's reference counter?

  • Use release: [array release];
  • Use autorelease: [array autorelease];

Explicit Memory Management

• Example

        // Instanciates and increases reference counter
        NSOperationQueue *q = [[NSOperationQueue alloc] init];

        // uses object normally
        [q addOperationWithBlock:^{
            // do something in background
        }];

        // decreases reference counter
        [q release];

Explicit Memory Management

• What is autorelease ?
  • It adds the object to the autorelease pool, to get released later.
    • The AppKit and UIKit frameworks automatically create a pool at the beginning of each event-loop iteration, such as a mouse down event or a tap, and drain it at the end.
  • So you can still use the object a bit more, but be careful.

Objects

Objects

• Code separated in two parts
  • @interface and @implementation
• Public objects must have their interface in ObjectName.h and the implementation in ObjectName.m.
• If you need private objects, you should define both parts in your .m file.

Tweet.h

    #import "User.h"

    @interface Tweet
    
    @property (nonatomic, retain) User *author;
    @property (nonatomic, retain) NSDate *date;
    @property (nonatomic, retain) NSString *text;

    - (Tweet *) initWithAuthor:(User *)author
                                        text:(NSString *)text;
    
    + (NSArray *) getTweetsByAuthor:(User *)author; 

    @end

Tweet.m

    #import "Tweet.h"

    @implementation Tweet
    
    @synthesize author;
    @synthesize date, text;

    // release all properties
    - (void) dealloc 
    {
        self.author = nil; // no release here?
        self.date = nil; // we'll talk about that
        self.text = nil; // later 
        
        [super dealloc];    
    }
    
    ...

Tweet.m

    ...

    - (Tweet *) initWithAuthor:(User *)aUser 
                                       text:(NSString *)aText 
    {
        if (self = [super init]) {
            self.author = aUser;
            self.text = aText;
        }
        return self;    
    }

    + (NSArray *) getTweetsByAuthor:(User *)author 
    {
        // retrieve tweets ...  
    }

    @end

Defining properties

    @property (nonatomic, retain) NSString *text;

• Will generate accesors (getter and setter)
  • dot notation:
    • tweet.text
  • accesor methods:

            - (NSString *) text;
            - (void) setText:(NSString *)text;

Defining properties

• @interface

  • @property (attributes) type name;

    • attributes most used:
      • nonatomic: synthesized accessors won't provide robust access to properties in a multithreaded environment.
      • retain: generated setter will retain the new value (and release the previous one). Use this for objects.
      • assign: generated setter will only assign the new value. Use this for scalar types.

Defining properties

• @implementation

        @synthesize name;
    

Defining properties

• If using retain attribute, retains and releases are made by the generated accesors.
  • So even if you are managing memory manually, they will do it for you.
    • Which explains the self.property = nil in the previous dealloc method.

Defining properties

    @property (nonatomic, retain) NSString *text;

    - (void)setText:(NSString *)newText {
        [newText retain]; // because of retain
        [text release]; // because of retain
        text = newText;
    }
    
    - (NSString *)text {
        return text;
    }

Defining methods

• Class methods start with + sign

    + (NSArray *) getTweetsByAuthor:(User *)author; 

• Instance methods start with - sign

    - (Tweet *) initWithText:(NSString *)text;

Defining methods

• Again, method definition in @interface.
• and method implementation in @implementation.

Defining methods

• Method attributes:
  • (NSArray *), (NSUInteger), (int)...
  • If it is an object, it needs *
  • If it is an scalar, it doesn't need anything.

Constructors

• Naming convention
  • Constructor with no arguments: init
  • Constructor with arguments

   
             initWithArg1:(type1)arg1 
                           arg2:(type2)arg2

                                ...

Constructors

• @implementation

   
        - (Tweet *) initWithAuthor:(User *)aUser
                                            text:(NSString *)aText 
        {
            if (self = [super init]) { // always
                // your code here
                self.author = aUser;
                self.text = aText;
            }
            return self; // always
        }

Constructors

• Using them:

   
        Tweet *t = [[Tweet alloc] 
                            initWithAuthor:aUser
                                             text:@"Lorem ipsum"];

Further reading

• Cocoa Dev Central: Learn Objective-C
• Apple Docs: Learning Objective-C

Enough

Introduction

• Most apps have lists.
• Most apps consume web services.

Creating a list: The View

• Lists are built with table views.
• Table views are composed by sections.
  • Each section is composed by rows.
• indexPath object will let you access them:
  • indexPath.section
  • indexPath.row
• Table views have two styles: plain and grouped.

Creating a list: The Controller

• Two key concepts about the tableView:
  • dataSource
  • delegate
• Both are protocols your table view controller must implement.

Creating a list: The Controller

• dataSource
  • Provides the info needed to construct and populate the table.
  • UITableViewDataSource Protocol Reference

Creating a list: The Controller

• delegate
  • Allows the table view controller to manage selections, further table view configuration and other actions.
  • UITableViewDelegate Protocol Reference

Creating a web service consumer

• Two main kinds of response formats: XML and JSON.
• There are third-party libraries for both of them:
  • XML: XMLReader
  • JSON: JSONKit, SBJson...

The Model

• User.h

    @interface User : NSObject
    
    @property (nonatomic, retain) NSString *username;
    
    - (User *) initWithUsername:(NSString *)username;
    + (User *) userWithUsername:(NSString *)username;
    
    + (NSArray *) getUsers;
    
    @end

The Model

• User.m

    @implementation User
    @synthesize username;
    
    - (void) dealloc {
        self.username = nil;
    
        [super dealloc];
    }
    
    - (User *) initWithUsername:(NSString *)aUsername {
        if (self = [super init]) {
            self.username = aUsername;
        }
        return self;
    }   ...

The Model

• User.m

    ...

    + (User *) userWithUsername:(NSString *)username {
        return [[[User alloc] initWithUsername:username] 
            autorelease];
    }

    @end

The Model

• Tweet.h

    #import "User.h"
    
    @interface Tweet : NSObject
    
    @property (nonatomic, retain) User *author;
    @property (nonatomic, retain) NSString *text;
    @property (nonatomic, retain) NSDate *date;
    
    
    - (Tweet *) initWithAuthor:(User *)author 
                                        text:(NSString *)text
                                        date:(NSDate *)date;
    
    + (NSArray *) getTweetsByAuthor:(User *)author;
    
    
    @end

The Model

• Tweet.m

    @implementation Tweet
    @synthesize author, text, date;
    
    - (void) dealloc 
    {
        self.author = nil;
        self.text = nil;
        self.date = nil;
    
        [super dealloc];
    }
    
    ...

The Model

• Tweet.m

    ...
    
    - (Tweet *) initWithAuthor:(User *)aUser 
                                        text:(NSString *)aText 
                                        date:(NSDate *)aDate 
    {
        if (self = [super init]) {
            self.author = aUser;
            self.text = aText;
            self.date = aDate;
        }
        return self;
    }
    
    @end

The Model

• getTweetsByAuthor: obtains tweets from twitter api

  • http://api.twitter.com/statuses/usertimeline.json? includerts=true&screen_name=%@ &page=%d&count=%d

• Parsing JSON

    // obtain response from the source url
    NSString *response = [NSString stringWithContentsOfURL:
                                                     [NSURL URLWithString:url] 
                                      encoding:NSUTF8StringEncoding 
                                           error:&error];
    
    ...         

The Model

• Parsing JSON

    ...
    
    NSArray *parsedResponse = [response objectFromJSONString];
    for (NSDictionary *parsedTweet in parsedResponse)
    {
        NSDate *date = [formatter dateFromString:
            [parsedTweet objectForKey:@"created_at"]];
    
        NSString *text = [[parsedTweet objectForKey:@"text"] 
            stringByTrimmingCharactersInSet:
            [NSCharacterSet whitespaceAndNewlineCharacterSet]];
    }
    

The User Controller

• Has @property NSArray *users.
• Loads the users when the view is loaded.
• Implements UITableViewDataSource to populate the table.
• Implements UITableViewDelegate to add behavior.

The User Controller

• UITableViewDataSource
  • numberOfSections:

          1
    

  • numberOfRowsInSection:

          [self.users count]
    

  • cellForRowAtIndexPath:

          [self.users objectAtIndex:indexPath.row]
    

The User Controller

• UITableViewDelegate
  • didSelectRowAtIndexPath:
    • Opens tweets view for

        [self.users objectAtIndex:indexPath.row]
      

The Tweets Controller

• Has @property User *user and NSArray *tweets.
• Loads tweets when the view is loaded.
• Implements dataSource and delegate in the same way than the User Controller.

Putting this all together

• iOS apps have, at least, one window (UIWindow)
• Windows have a rootViewController property.
  • It is the app's main view controller.

Putting this all together

• Something like window.rootViewController = usersViewController would be ok...
  • But if we need navigation between views, it is not a good approach.

Putting this all together

• UINavigationController

    UsersViewController *usersViewController = [[UsersViewController alloc] 
                                                            initWithNibName:@"UsersViewController" bundle:nil];
    
    self.window.rootViewController = [[[UINavigationController alloc] 
                                            initWithRootViewController:usersViewController] 
                                            autorelease];
   
   [usersViewController release];

UI thread

• All your code runs in the main thread by default.
• The UI runs in the main thread.
• You should leave the UI thread as free as possible.
• Network connections block the main thread.
  • Any code can block the main thread, indeed.

NSOperations & Queues

NSOperations & Queues

• With NSOperations and NSOperationQueues you can easily run code in background.
  • And even easier if you use blocks :)

NSOperations & Queues

    self.loading = YES;
    [self.tableView reloadData];
    
    NSOperationQueue *q = [[NSOperationQueue alloc] init];
    [q addOperationWithBlock:^{
        NSArray *tweetsByAuthor = [Tweet getTweetsByAuthor:
                                                                        self.user];
    
        // refresh the view.    
        // this is related with the UI, so it has to run
        // in main thread.
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            self.loading = NO;
            self.tweets = tweetsByAuthor;
            [self.tableView reloadData];
        }];
    }];
    [q release];

What have we learned?

• We've learned the basics of Objective-c.
• We've learned to manage memory correctly.
• We've learned to make a simple table view that consumes data from an API.

What do we have to learn?

• Using iVars.
• Using categories.
• Unit testing.
• Reading data from a file.
• Persisting data.
• ...