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Hi.
My named is Amit,@paradoxed on Twitter.

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So today we're gonna talk about
delegation.

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Delegation is a very common design pattern
used all over when programming an iOS app.

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If you've done any of our courses like the
Blog Reader project or the Ribbit

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app, you've noticed that within the table

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View Controller, you have the table view
delegate.

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And then if you've used a scroll view or a
text field,

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you've probably used a delegate to go
along with that.

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So we've used delegate all over the places
but we've

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never specifically addressed what is the
delegate, a design pattern.

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Now a delegate or a protocol is basically
something that defines

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a bunch of methods that can be implemented
by any class.

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It's the most common design pattern and it
means, to delegate control.

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So you

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are basically giving over control and
having

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that class implement the functionality of
that method.

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So let's take a look at how you can

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define a protocol, and then implement that
in a class.

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Alright, I've created a sample app over

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here, and, basically it's a single view
application.

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Let me run the app.

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And what you'll see is that there's a
button here.

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Which says, press me, and then a text is
displayed on the text label.

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So the way this app is designed is that if
I go to the main story board you won't see

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anything because it's just a simple view,
so there is

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our view controller and it has the main
view in it.

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And let me go to the ViewController.m and
you'll see that

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it has a property called THCustomView and

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all I do is add this custom view as a sub
view to my

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main view.
Now this custom view, let's head on over

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to customview.h, as you can see the
interface, all it has

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is a method called set text where you can
set the text of the

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label in the custom view.

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Now, the, this is assuming that you have
created this custom view

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that you want to reuse it in multiple
places throughout your application.

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And you do not want to give direct access
to the button or the label.

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You just want the person using the custom
view to set the

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text and then you want to control the look
and feel and

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the color and all the other attributes of
the controls that are housed

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within this custom view.
Now of course, as a contrived example,

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I'm just trying to drive home the point
that if you create a complex view

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that can be reused throughout your
application or you're creating a third

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party component so that other people can
use it or opensource

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this component.

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And you don't want to give completely
control over all the controls within that

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custom view, then what happens is that
when a button is pressed.

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Now, it's assuming that you, you can, of
course, expose the button press method

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and set target actions to other view
controllers, but the way this custom

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control is designed is that you have a
button press method with that, within the

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custom view, so it's setting itself a text
when the button is pressed.

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Now, inside the THView controller.m, I
actually

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want to receive that button pressed event.

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I want to know when that button was
pressed within the custom

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view, so that I can go ahead and call that
set text method.

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I can simply set the text, but that
doesn't help me because

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I want to set the text only if the button
has been pressed.

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Now how do, how does the custom view

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instruct the THviewController that the
button was pressed.

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Right now there's no way of doing that
unless we expose the button

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as a public property.
Right now it's a private property.

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Both of these are private properties.

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But let's say you've created a, a custom

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view and you don't wanna expose these
things

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as, public properties because you don't
want them

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to manipulate the look and feel of your
control.

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So, in the init frame, basically, we are
setting the frame for the

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button and adding it as a sub-view and the
same thing we're doing

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for the label.

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Now the only way we can do this is if we
create a delegate object that

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delegates the responsibility of the
button-press method to its delegate.

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So the only way we can do that is if we
create a delegate object within our

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custom view that would delegate the
responsibility of the button

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press method to its delegate.
So how do we go

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about doing that?
Well let's take a look, so, the first

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thing I would need in my interface is a
delegate object.

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So we'll say at property nonatomic, and
this would be weak, this is the

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perfect example of using a weak property
you will see that in a little

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bit why it's a weak property and not very
strong.

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So, I have defined this delegate which is
of type id.

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We, we don't specify exactly what this is,
because we

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don't know what kind of delegate it's
going to be.

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It could be a view controller, it could be
another view, it

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could be a table view controller, it could
be a scroll view.

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Anything could be a

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delegate of this custom view.

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We don't know what could be the delegate,
that's why it's an id, which means that

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the type of this object is going to be
determined at run time.

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So, the next thing we need to do is define
a protocol.

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Now a protocol as its, as the name states,
is, is

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a set of methods that can be implemented
by a delegate.

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So the delegate needs some sort of
structure, the

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delegate just can't be like, well I am
going

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to do x, y and z, no, the delegate

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needs some set up instructions that it can
implement.

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So we need to define what those
instructions look like.

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So you need to declare the instructions,
which is a

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protocol, and then that protocol is
implemented by your delegate.

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So we will declare our protocol right here
in our THCustomView.

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So it's after the interface
implementation.

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So, as you can see, after the end, @end
over here.

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So we need to give our protocol a name.
So, the protocol names are basically the

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name of the class that needs a protocol
followed by the suffix delegate.

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Now, it's usually a protocol on NSObject,
so we'll leave that as is.

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And then we define our methods.

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Now similarly, as you define methods in
your @interface, you

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can have class methods or you can have
instance methods.

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So I'll create a instance method called
buttonPressed.

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Now mind you, this can be called anything
you want.

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I'm simply calling it buttonPressed
because we're

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delegating that button pressed action to
our delegate.

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Alright, so now that we've defined our
protocol, it's always

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good practice to define what kind of
delegate this is.

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So, as you can see in these angle brackets

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over here, you specified NSObject, which
means that the

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protocol is of type NF, NSObject.
Similarly, you can mention

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that this delegate should only be of type
THView custom delegate.

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You don't want the other class just
implementing any kind of delegate

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and hoping that this will work, cuz
otherwise you'll get a run-time error.

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At least when you put

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this here, you will get a compiler warning
if that delegate hasn't been specified.

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We'll come to it when we actually define
the delegate in the view controller.

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So you'll get a better understanding of
why I'm doing this.

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Now of course, we're getting an error

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here saying that, cannot find protocol
declaration.

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That's because our protocol is defined
after

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the@Interface, so one way we can prevent
that

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error is just cause copying the @protocol
ThCustomViewDelegate.

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Just as we define an @class, which is a
forward declaration, that

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hey, this is a class that's going to be
implemented in the future.

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Similarly we can specify a forward
declaration for our protocol.

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So basically we're telling the compiler
that this is a forward

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declaration for a protocol and we're gonna
declare it later on somewhere.

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And later on, in this case, is right after
our @interface declaration.

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So, here we have actually created our own,
very own protocol.

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Now you've must have seen these protocols
with

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table view controllers or scroll views or
text fields.

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There are so many delicates defined for
you in the UIKit framework.

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And here we are defining our very own
delegate.

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So let's go ahead and see how we can
implement this delegate.

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So, when we go back to our
THViewController.m, and

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here in our self.customView, we can refer
to that delegate property.

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So .customView.delegate

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and here we can just specify self.

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Because we're saying we are going to be
the delegate for the custom view.

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We're gonna take over the responsibility
of that button

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press method, or any other methods the
protocol provides.

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So, as you can see here, I get a compiler
warning and if I click here it says

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assigning id THCustomViewDelicate from
incompatible type, so that's

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why we specify the THCustomViewDelegate in
angle brackets.

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Because it's saying like, are you really
haven't implemented this delegate.

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And by implementing, we have to go to the

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interface definition of our view
controllers.

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So let's go to the .h file.

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And here is where we specify
THCustomViewDelegate.

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So that's where we need to specify it and
as you can see, we get an error saying

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that it cannot find the protocol.
So let's Import our THCustomView.

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So that error will go away.

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And so now when we go back to

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our implementation, we don't get the error
anymore.

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You know, it, it's satisfied that you're
gonna be implementing this delegate.

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Now, we get another warning here, saying

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that method, buttonPressed in protocol not
implemented.

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That's because we have to implement the
methods required

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by our protocol.

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So, here we'll go and define the button
press method.

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So we'll say self.customView set text.

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And let's set the text, we'll say button
pressed

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view controller.

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So, are we done?

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Well, let's run our application and see if
this works,

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and it won't work but let me show you
anyway.

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So it says button pressed custom view, and

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it should really be saying, button pressed
view controller.

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Let me remove this space here so that
work's consistent.

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So why doesn't it work?

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We've set our delegates, we've set our
button pressed methods.

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Well.

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All that is fine.
We've defined our protocol and

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we've set the delegate, but somewhere in
our THCustomView implementation,

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we need to instruct it that it needs to
delegate that buttonPressed method.

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We aren't delegating that responsibility
anywhere.

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So, as you can see, in this buttonPressed
method, we're simply setting the text.

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So this is the last piece of the puzzle
where we check our delegate object,

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so we'll say, if self.delegate

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responds to selector, so here we're
checking to see

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if our delegate actually has this method.
Does it have this

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method implemented, buttonPressed and if
it does,

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then let's delegate that buttonPressed
method, to the method of our delegate.

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So there is another method, called perform
selector.

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So if you were passing an argument, you
could use perform selector with object.

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In our case, we are not passing an
argument so

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we will just say perform selector and then
add selector, buttonPressed.

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So basically what this is doing here is
performing this method of our delegate.

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And in this case, our delegate is our view
controller, could it be something else?

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Sure, it could be any other class.
And I will say else,

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so if it doesn't respond to the
buttonPressed

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method, then we're just gonna do our
default action.

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You don't have to do this but I just
wanted to show

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that you can have default behavior or you
could have delegated behavior.

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So now if we run our application and I hit
Press Me,

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you'll notice that it says, button
pressed: view controller, as it should.

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So going back to our code, one last thing
that you want to note is

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that in our protocol we can define some of
the methods as optional.

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As you notice that when we didn't define
any of these methods

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as optional, the compiler complains saying

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that we hadn't implemented the
buttonPressed method.

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So, how do you define it as optional?
Well, you can

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just simply put the keyword @optional and
basically it'll say whatever

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methods follow this @optional definition
these methods will be optional.

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So now if we go back to our view
controller implementation and I remove the

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implementation of the buttonPressed
method, you'll notice

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that I don't get a compiler warning
anymore.

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And if I run my application,

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it's simply going to use the default
behavior.

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Basically, buttonPressed custom view, so
that buttonPressed custom view

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is coming from my custom view and not the
delegate.

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So there you have it.

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This is what delegates are all about.

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They're about delegating control or
responsibility to another class.

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Now this doesn't have to be between views

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or controls, this could be between view
controllers or

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your custom class.

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So, don't think that you're restricted to

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just UIView subclasses when creating a
delegate.