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(bright music)

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(typing)

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So we've nearly finished the user interface

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for adding and editing the task details.

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We've still gotta add the code though to save the data.

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Before we do that though,

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let's finish the code to display the fragment.

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At the moment, the user can't go back to the main screen

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without saving the data, which could be annoying

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if they've tapped the plus icon by mistake.

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We need to provide up navigation

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and also decide what we're going to do if the user

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taps the back button whilst they're editing a task.

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If an activity or fragment allows the user to navigate back,

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our users will expect an up button to appear in the toolbar.

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Android takes care of that for us

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if we pass true to the setDisplayAsHomeUpEnabled

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function or the action bar.

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Remember from the life cycle videos

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that a good place to modify the activity's UI

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is the fragment's onActivityCreated function.

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When that's called, the fragment knows that the activity's

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fully created and that it's layout has been inflated.

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So let's make a change there

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in our onActivityCreated function.

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Specifically we're gonna make a change here on line 49.

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I'm gonna type val actionBar equals

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parentheses listener as AppCompatActivity

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closing parentheses period supportActionBar.

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And the next line actionBar question mark

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dot setDisplayHomeAsUpEnabled parentheses true

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so adding that to the fragment code

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rather than setting the up button in the activity

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means that we don't have to worry about calling it

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whenever we display a new fragment.

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So that's pretty straightforward.

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We need a reference to the activity's action bar

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but it might not have one.

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And that's why we're using a safe call

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when calling setDisplayHomeAsUpEnabled.

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Obviously we can't show the up button in the toolbar

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if there isn't a toolbar

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but at least now the code won't crash.

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What's interesting here thought is the way we obtained

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a reference to the support action bar.

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This is a very easy mistake to make but it's wrong.

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So notice that listener is a nullable type.

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OnSaveClicked question mark

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so that means that it can be null.

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If we scroll up and have a look we can see

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we have defined our listener on line 30.

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That's in nullable type,

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we can see OnSaveClicked, question mark

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So that means that it can be null

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and we've seen that fragment's activity isn't

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always available in the fragment life cycle.

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So we're adding this code in the onActivityCreated function

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so it's almost certainly safe to do what we've done here.

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onActivityCreated won't get called

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unless the fragments activity is available.

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Even so, I'd advice against doing this.

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We've cast listener to appCompatActivity

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which it will be if it has an action bar

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but we should've specified the nullable type.

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In other words appCompactActivity question mark.

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As I said though it should be fine in here

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we're unlikely to detach from the activity

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while onActivityCreated is running.

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But the correct code would actually be

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to add a question mark after appCompatActivity

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and outside of the parentheses

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to add a second question mark.

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So I'm making one unnullable type making appCompatActivity

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a nullable type now and also using a safe call

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to support action bar.

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So remember that we're showing general techniques here.

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Although that would almost certainly have been fine

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before I changed it, you may wanna do

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something similar somewhere else.

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Like inside onActivityCreated in other words.

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The thing to take away from this is that Android Studio

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and the Kotlin code checker can only help you so much.

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If you specify a cast to a non nullable type

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then the checker won't warn you to use a safe call.

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If you're casting something that could be null

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make sure you use a nullable type in the cast

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otherwise your code will crash if it attempts to cast null

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to any other type.

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Technically it might be possible for the checker

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to have detected the problem with the cast

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so if you get slightly different behaviour

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and did see a warning, then that's fine,

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things are changing all the time.

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Alright so moving on,

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next we should really make sure that the cast will succeed.

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So let's go ahead and make a change for that.

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So I'm going to type if parentheses

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listener is AppCompatActivity, closing parentheses

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I'm gonna open a code block, left curly brace

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and I left the right curly brace below

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the set call to setDisplayAsHomeUpEnabled

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and then add a line space there.

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Now if you use the fragment from a fragment activity

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for example it won't crash.

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A fragment activity doesn't have to have an action bar

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and without this test that code would have crashed.

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Because one of the reasons for using a fragment

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is so that it can be reused, you shouldn't be making

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assumptions about what will be reusing it.

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Alright so a brief digression next,

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you might be wondering why we can't let Kotlin

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perform a safe cast here.

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So what I might need is I'm gonna duplicate this line.

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I'm gonna comment one out.

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So why can't we just do this

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so val actionBar equals

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I'll just delete the whole, it'll be quicker,

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listener.supportActionBar.

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Why can't we do that?

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Because now we've already checked

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that listener isn't null, haven't we?

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If it is an appCompatActivity then surely it can't be null?

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Well, unfortunately things don't work like that

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in a multi-threaded environment

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and the error message will confirm that

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if I hover over it.

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So my cast to appCompatActivity

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is impossible because the listener is a mutual property

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that could have changed by this time.

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We may know that it can't change between our test

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and when we get to its supportActionBar,

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but there's no way for the compiler to know that.

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The compiler isn't aware of the environment

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that our code's running in,

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it just complies Kotlin code.

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Unless it can determine that a smart cast

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will always succeed, it won't actually allow one.

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And there's been some discussion of this

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at kotlinlang.org, and I have a quick read,

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I'm just gonna quickly bring that up

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on the screen so you've got access to the link.

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So, check out this article and have a bit of a read

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about that if you're interested in some

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of the technical reasons why things work this way.

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So, I'm mentioning this now because there's a trick

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that we can use to get around the problem,

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now go back to our code, so what we can do

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is assign listener to a local val

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and use that instead.

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So, come up here and type val listener

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is equal to listener.

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So now the compiler can be sure

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that the local listener can't be modified

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anywhere else and performs the smart cast.

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And you can see now that listener on line 52

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is highlighted in green to show that a smart cast

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is being performed, if you hover over that

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we can see confirmation of that.

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There's no problem having a local variable

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with the same name as the class variable, by the way.

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The local variable will be used once its declared.

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Alright, so, we're using that technique

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a little later, here we're only accessing

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a single attribute of the listener in one place

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so I prefer to use a nullable type and safe call.

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So that only involves typing two question marks

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rather than the entire line.

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We'll see this technique being used later

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but for now that I'm gonna do is undo those changes.

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And back to how we had it there.

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And again, my preference is to use

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a nullable type and safe call,

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and as you see that involves typing two question marks

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rather than the entire line.

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Alright, so the next video, recap of how an object

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can be of more than one type at the same time.

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See you in the next video.