Execute methods asynchronously using NSOperation

Sometimes we want to run some methods in background, for example a service request.
Cocoa has a class ready for execute asynchronous methods, it's NSOperation. NSOperation is a key value coding and key value observing compilant, this means that you can observe some properties to perform different actions in your application, for example, if it finished executing (isExecuting).

1. //We init a new queue
2. NSOperationQueue *queue = [NSOperationQueue new];
4. //Create a new operation
5. NSInvocationOperation *operation = [NSInvocationOperation alloc] initWithTarget:self selector:@selector(getAge:) object:@"Nicolas"];
7. //Add operation to the queue
8. [queue addOperation:operation];

Right here we are creating a new queue and adding an operation to it, this way the callback is in the queue and will be called asynchronously.
So we need to declare the callback

1. -(void)getAge:(id)sender
2. {
3.    //Do magic
4. }

In this case sender will be an (NSString *) object with value "Nicolas", but you can send whatever object you want as a parameter to the callback.

How to use NSUserDefaults

NSUserDefaults is a useful class used to store data in the device, you can save and access information quickly.

1. //First we get the singleton initialization for NSUserDefaults
2. NSUserDefaults *myPreferences = [NSUserDefaults standardUserDefaults];
4. // We are trying to save our volume value
5. [myPreferences  setDouble:0.9 forKey:@"Volume"];
7. //We will force to synchronize
8. [myPreferences  synchronize];

Notice that in this case we have saved our volume value which is a double, but we can store integer, string, even an object to NSUserDefaults class.

So now we can call synchronize to synch all the values that we have been set. Synchronize method is not necessary because it is called during a period of time, but you can call it to ensure that your data is synched.

So now if we need to get the value of our volume, we can just write:

1. //We use our NSUserDefaults object to get the Volume
2. double volume = [myPreferences doubleForKey@"Volume"];

And that's all, we can store some values in our device using this simple class.

Working with NSNotifications

Notifications are a great way to communicate with all the application, you can send just a message or add an object to the notification too.
The way it works is very simple, you need to post a notification with a name, for example if i want to let the project know my name, i could write :

1. //Post notification
2. [[NSNotificationCenter defaultCenter] postNotificationName:@"myNameIs"];

Right there you can see that NSNotificationCenter class has a defaultCenter. Commonly defaultCenter provides a singleton design pattern which has exactly one instance.
This way you'll use always the same object at memory, but how could the application knows my name if i'm not sending it?

You can also add an object to the postNotification method, for example in this case:

1. //Now we add an object...
2. [[NSNotificationCenter defaultCenter] postNotificationName:@"myNameIs" object:@"Nicolas Vidal"];

Gotcha! Now all the project should know my name, but... we need a way to listen to this notification.

So let's see that another ViewController is telling himself... "Which is his name?", so here come's the magic.

In the ViewController event, "ViewDidLoad" we must add an observer to that notification and a callback to receive that object.
So why ViewDidLoad? Because this event is called just when the View is loaded, remember that we need to add just ONE observer for each class/controller , and then remove our observer when we not need it
anymore.

1. //Listen to notification
2. [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(knowHisName:) name:@"myNameIs" object:nil] ;

That way we need to implement the selector this way :

1. -(void)knowHisName:(NSNotification *) nameNotification
2. {
3.    NSString *hisName = [nameNotification object];
4.    NSLog("%@",hisName);
5. }

We could observe that the NSNotification has an object, the same that we sent in postNotification, pretty simple, huh?

Now we need to remove the observer to not listen anymore to my notification :

1. //Removing notification
2. [[NSNotificationCenter defaultCenter] removeObserver:self name:@"myNameIs" object:nil];

That's all... now you can send messages through all the application with this amazing class.

Introduction to the Interface Builder

The interface builder is a great tool for designing the user interface (user interface) of our application.
From XCode 4, Interface Builder is integrated into XCode and can be used with ".xib" files.
XIB, short for Mac OS X Interface Builder, are XML files that contain the positions, types of elements and controls that contains the view you are creating.

The controls are part of the xib and must be IBOutlet type, so if we want to create a button in our view, we need to create it inside @interface.

1. @interface xibTestViewController : UIViewController
2. {
3.       IBOutlet UIButton *_customButton;
4. }

In case of events, for example when you press the button (event TouchUpInside:), action must be IBAction. Tapping the button will call an event like this:

1. //Touch event
2. -(IBAction) customButtonTouched:(id)sender;

After writing this in the interface, we can go to ".xib" file and drag a button to the view, as we see in the picture

Pressing right button we will see that it will list the events you can call, and also the ability to reference the IBOutlet.
Remember that the IBOutlet will serve to change control properties, such as text, size, among others.


We will see something like this:

If we hold down on the circle at the right of each event, and drag on "File's Owner", we will be able to see all IBOutlet controls that are in the interface, in this case, the buttons.

This is a quick introduction to the interface builder, I will post later more advanced articles on it.