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G'day everyone, welcome back. The last video finished with a challenge,

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to define a callback interface and use it to successfully edit tasks.

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There are often different ways to do things and your solution may look a bit different to mine,

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but it should be roughly along the same lines,

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I'll start by defining the interface in
MainActivityFragment.

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Then we'll call the activity's onTaskEdit function in onEditClick.

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That's all the changes we have to make to MainActivityFragment,

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but there's another change that we should make. We should make sure the activity really does implement the interface.

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That causes our code to crash straight away, rather than appearing to work until the user taps a button.

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If you can't even run your app without it crashing,
you'll spot there's a problem straight away.

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I'll demonstrate that after making the change.

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We can't get a reference to the activity before onAttach,

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so that's where I'll add the code.

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Okay, to see how this can help to spot
problems more easily, I'll run the app.

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Remember that we haven't made
MainActivity implement the interface yet.

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As soon as the app runs, it crashes.

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The message in the log is
"MainActivity must implement onTaskEdit",

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which makes it very easy for the programmer to work out what's wrong, and of course to fix it.

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That leads nicely onto the next step in the challenge,
implementing the interface in MainActivity.

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We'll start by declaring that it does implement the interface.

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I'll add the necessary function after onOptionsItemsSelected, before the taskEditRequest function.

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Remember, control I will let Android Studio generate the stub for us,

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and we can replace the TODO.

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It doesn't have a lot to do, because all
the work's already being performed by taskEditRequest.

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You may be wondering why we didn't just rename taskEditRequest, to onTaskEdit.

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That would work, but it's not a good idea

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taskEditRequest accepts a nullable task type, which it has to have when we call it from the plus icon on the menu.

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When we're adding a new task,
we pass null for the task argument.

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If we called the function that takes a nullable type from our onClickListener,

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we'd lose the null type safety that Kotlin provides.

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A bug in our code could result in the user
adding a new task when they wanted to edit an existing one.

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This way, if we do try to pass a null to onTaskEdit,

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the compiler will complain, and we'll detect
the problem straight away.

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Ok does it work.

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I'll edit task 5. I'll change the description
so that it reads fifth as a word, instead of as a number,

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and I'll change the sort order to 7.

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Okay, we save and return to the list
and we can see that task 5 has been updated.

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It's got the new description and it's moved further down the list,

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so the new sort order's taken effect. So that looks like it's working.

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What if we change the task name?
I'll edit task 1 and change the name to task 10.

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Then I save, and tasks 1's now task 10.

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We can tell it's the same task by the description,

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and that's one of the reasons that we've been boring in our
choice of names and descriptions.

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At some point, you'll want to test the app with more realistic task names and descriptions,

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including making sure that those max length values we used are working.

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You'll also want to check the display, to see how it looks

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when names that use the max length number of characters are entered.

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But when you're still developing, it's much easier to verify that things are working as you expect,

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if you follow some sort of pattern, like this.

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We can also tell what the ID should be from the name,

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our new task 10 should have an ID of 1 in the database,

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and that can be useful if strange things happen,
and you need to work out what's going on.

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So that's the edit button working and the end of the challenge.

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Remember, you may not have done things exactly the same way as I did,

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but if your app works and you can successfully edit tasks,
then well done.

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I'll stop this video here.

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In the next video, we'll look at deleting tasks when the delete button's tapped. See you then.