Now we're going to create an iOS application that uses notifications

to drive a simple user interface.

We'll start with an existing storyboard containing a view

with two buttons and a label.

Two buttons, red and

blue, will drive changes to the UI label using notifications.

Obviously this is not a typical way you set up a GUI to work.

But this is a good educational exercise for learning more about notifications.

So we've already established through Interface Builder IB

actions and an IB outlet for Motif label.

So this is the IB action.

These two are the IB action implementations for

the two buttons and we're going to drive everything on our user interface

through this view controller file.

So, the first thing we do is create

Constance for our notification names.

This is always a good idea when you're doing with NSStrings because

they are prone to typos and Xcode can't help you auto correct a string.

So we have RedButtonNotif and

[SOUND] the blue, BlueButtonNotif.

[SOUND] So the next thing we'll

do is post notifications within

our IB action methods when

the buttons are pressed.

So NS notification Center,

default center, post notification name.

And for the blue button, this will be blue notif.

And object will be nil.

And, similarly, for the red button,

NSNotificationCenter, postNotification.

This time it's RED_NOTIF Then within viewDidLoad,

we'll set up the blocks that will be listening for

those notification names.

So first [SOUND] we'll get a pointer to default center.

[SOUND] And then, we'll send a message to the center to addObserver and

we want the version of addObserver that takes block as a parameter.

And we are looking for, first of all, the Red_Notif.

And we're not concerned with the objects.

Don't really care about the queue.

And In the block, we'll be setting a text for the UI label.

So self notiflable.text

equals Got Red Notif.

Then we'll do the same thing for blue.

Send a message to the notification center, this time for the blue notification.

Object is nil, queue is nil and then for the block,

we'll be making the change to the notification

label text to indicate that we've got blue notification.

So the mechanics of how this is supposed to work is that when a button is pressed

a notification is posted, the name of which depends on the button.

And these blocks that have been registered as

observers for those notifications will make the change to b y label.

So, can run this and see how things go.

So here we have red and blue buttons.

Push blue, it says we got a blue notif, push red, red notif, blue notif.

And we look down here in the log, we see in addition to our red and

blue notifications, there's all these other notifications coming through.

And this is because in the AppDelegate,

we left the block that is listening for all notifications.

So, you see that, not only are there many notifications posted when an application

first starts but iOS continues to use notifications to communicate internally.