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(upbeat intro music)

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So this part we are now ready to create

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the ViewModel for our app.

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We want to start here by adding the necessary dependency

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to the build.grade file.

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so lets open our build.grade file,

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the module app, grade file is what we want.

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And what we want to do is add those dependencies.

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As we did for the Calculator app,

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you can copy and paste those same entries

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that you did last time.

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We got those from one of the Android support pages.

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I'm just gonna drop it in here.

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So either way will work.

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So def life cycle_version

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is equal to point type a quote "1.1.1"

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Then on the next line we're going to add a message there.

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The comment should say ViewModel and LiveData

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which is what this next line is going to be all about.

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We'll type implementation and double quotes.

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Android.arch, A R C H.lifecycle

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colon extensions

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colon and a dollar sign

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it's gonna be lifecycle

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in this version which is our variable,

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we've defined in the previous line.

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And closing double quote.

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The other entry we needed to change,

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which again we needed to do for the Calculator app,

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is change J R E to J D K.

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Let's go ahead and do that.

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And at that point we can now sync.

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It goes without saying, that it's important

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to type this exactly correct.

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If you don't type that line correctly you get an error

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when we go to uh, to sync.

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But in my case I've been able to sync it,

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so we're good to go.

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Alright, so let's have a think about

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how this ViewModel will fit into the overall structure

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of our app.

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Hopefully by now you've reviewed the Google document

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that we looked at in an earlier video.

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There's actual a diagram showing the lifecycle

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of a ViewModel in that.

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Let's just bring that up on screen.

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It is worth checking that out.

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And we'll just come down here

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to the lifecycle of a ViewModel,

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click on that.

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And there we can see the diagram that I'm talking about.

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So that diagram shows the activity

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being destroyed and recreated as a result

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of a configuration change.

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But the ViewModel exists

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until the activity is finally destroyed.

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If we make the ViewModel responsible

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for downloading the data

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then the data remains available

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after our configuration change.

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There's no need to download it again.

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And in our case that's exactly what we want.

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So let's actually see how a ViewModel

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will fit into this app.

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So as you can see in this slide,

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the ViewModel is gonna be responsible

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for keeping the feed data.

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So exposes this said data, for a LiveData object.

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Remember that LiveData can store any type of object

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we've used simple Strings and Doubles in the Calculator app,

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but here we're going to be using a LiveData object

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to hold an ArrayList.

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So when the ViewModel needs to get data from a new URL

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it creates a new DownloadData object

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and provides it with the URL

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to download from.

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So once DownloadData has finished downloading

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it creates a ParseApplications object

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to parse the data.

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We've discussed the fact that this isn't the best design

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and then we went and changed the way it worked

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in the Flickr Browser app.

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So it would be easy to separate these two classes

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so the DownloadData sends the raw data

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to the ViewModel.

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Then the ViewModel uses ParseApplications

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to preform the parsing.

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But we're really focusing on the ViewModel at this point.

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So we're not making any other changes at this time.

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DownloadData then calls the Viewodel's

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onDataAvailable function,

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and passes the list of feeds as an argument.

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The ViewModel now has an up to date list

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of the feed data,

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and exposes it through it's LiveData object.

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That's "feedEntries" in this diagram

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MainActivity observes this LiveData,

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and can respond whenever it changes.

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And in this case it responds by

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updating it's applica- it's adapter rather,

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with the new list.

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Now we'll need to make a small change to the Adapter.

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It'll need to provide a function

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that MainActivity can call, to provide the adapter

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with a new list.

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You'll also need to notify the ListView

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that its data has changed.

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We'll make both these changes once we've written

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our ViewModel class.

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Alright, so that's how it all fits together.

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So let's swing back now to Android Studio

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and create a new class, in fact we'll create our ViewModel.

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So I'm gonna go over, and under the top 10 download package,

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new Kotlin File Class

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and we're gonna call this one FeedViewModel

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I'm going to select Kind of Class

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started with a topping, press enter there.

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And I'm gonna start off by adding the usual tag that we had.

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That's private, const, val,

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TAG = FeedViewModel

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and for FeedViewModel we need to extend ViewModel

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and we also need to implement the down loader

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class is Downloader call back interface.

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So let's do that.

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So that's gonna be colon after the class definition.

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And ViewModel Parenthesis, comma,

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and download data,

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.DownloaderCallBack

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so I've got an error at the moment as usual

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because we haven't yet, implemented

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the interface function yet.

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But we'll fix that in a moment.

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First though we need to move some variables,

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from MainActivity into here.

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Let's go and check out MainActivity,

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and I'll just close down some of these other classes

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or at least the greater file of it.

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So I'm looking at our MainActivity class

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to finish it down here.

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We want the DownloadData field,

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that's obvious because MainActivity

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is no longer responsible for downloading.

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But we also want the feedChachedUrl variable

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our ViewModel will decide if the feed needs

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to be downloaded again.

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All MainActivity is going to do is request the feed.

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So let's actually take a copy of this

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select all of the feedCachedUrl

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I'm gonna cut that and paste that

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after the DownloadData definition

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so I can actually copy both of those lines.

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And before I do that though, you might be tempted

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to move feedUrl and feedLimit into the ViewModel as well.

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In this case that's not a good idea.

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The URL that the user's selected

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is part of the interaction with the UI

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so our responsibility for building the URL

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belongs to MainActivity.

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So I'm gonna copy those up,

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or probably should save those two lines

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that's the private var downloadData and the freeCachedUrl

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cut those out of MainActivity

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and go back to our FeedViewModel

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and paste them in there within our class definition.

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And looking back at MainActivity,

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we've now got arrows there or more arrows.

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But don't worry, that'll become right

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as we work on converting the app to user ViewModel.

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Alright so back to FeedViewModel

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we've pasted those two entries in there.

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So the ViewModel has to expose its data,

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and we're gonna use the same technique that we used

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at the end of the calculator conversion.

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So I have a private MutableLiveData object

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and expose the LiveData, to the ViewModel's own.

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Let's go ahead and do that.

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So it's going to be private,

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val feed equals

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it's gonna be MutableLiveData

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and the type there is going to be,

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a list of our FeedEntry objects.

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To list FeedEntry, I need to have parenthesis

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on the end of that.

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And on the next line, we're going to define

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our LiveData objects.

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So val feedEntries,

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colon LiveData

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and the definition, begin as a list of FeedEntry objects.

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List of FeedEntry.

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Then we need to find our get function,

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so get = feed.

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FeedViewModel will update feed,

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and MainActivity will be observing feedEntries.

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Now there's a minor problem with LiveData,

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it's not one I can see Google changing,

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at least not in the short term anyway.

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We've declared our LiveData and MutableLiveData objects,

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as storing a list of feedEntry objects.

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Unfortunately though, that will hold the server till now

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until we initialise it with something,

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there's not really anything else we can return.

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So we're gonna make sure we don't get any errors,

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as a result of returning now,

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by initialising the MutableLiveData object straight away.

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So I'm gonna add a nit block to actually do that.

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So I'll add that just below the definitions there in it.

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Left to right curly braces, and we're gonna type

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feed.post value,

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then in parenthesis, EMPTY capitalised

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_FEED, _LIST.

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Closing in our right parenthesis there.

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So that's worth doing, but you'll still see the

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compiler complaining, we'll deal with that

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when we come to it.

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But what's this EMPTY_FEED_LIST?

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Well the Java Collections class provides

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and empty list for situations like this.

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Rather than creating a new ArrayList

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we can actually use the empty list

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that Collections provides for us.

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That's a bit more efficient than creating objects

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that we are not going to use.

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Another advantage is this empty list is immutable.

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Now if for some strange reason,

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our code tries to add items to it

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we'd get an error.

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So you see the empty list from the Collections class

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makes that code a bit more robust.

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You can't actually mark EMPTY_FEED_LIST as a constant,

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but that's effectively what it is here.

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What we can do though

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is put the definition at the top of the file,

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so that it's available in other parts of the project.

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Let's go ahead and do that.

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So we're gonna do that just below

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the TAG definition online, its 11.

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So that's gonna be val, and in uppercase EMPTY_FEED_LIST.

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And it's a list of FeedEntry objects,

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so list FeedEntry objects, and it's gonna be equal

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to Collections.emptyList.

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You can see the arrow disappears on line 26.

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And just out of interest, if you click on this emptyList

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method call there, because it's Java source code.

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You'll see that it's a top side version

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of the empty, and it's called list constant.

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So it's been cast to top we want,

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which in this case is a list of FeedEntry objects.

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Just above the doc stream for the method,

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there's a definition of EMPTY underscore LIST there.

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You can see there on line 4,443.

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And that's effectively the object

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that we're gonna be getting here.

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That's a public static final object

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which is the Java equivalent of a public const in Kotlin.

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Now let's just close down that Java source.

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Back to our FeedViewModel.

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So the next bit of our code, we need to list

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our MainActivity as the download URL function.

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So let's go and grab that.

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So I'm going to cut that out of the MainActivity class

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and we'll paste that directly in to our FeedViewModel.

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We'll paste it right in there below the init.

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Now this situation, I'm gonna suggest you accept the imports

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even though you can see that one of them there

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is a synthetic import.

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We'll fix that next.

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Let's clean up some of these empty spaces.

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So we need to make a few changes to this function.

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Now the function was private in MainActivity.

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But we need to make it available

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to our ViewModel's owner.

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And also take the opportunity

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to where the logging line one's called.

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And the last thing we want to do is make a change

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to remove the X in the list view

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from the DownloadData constructor.

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So let's remove the private there,

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so the ViewModel owner can access this.

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Gonna add some logging as I mentioned.

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Log.D parenthesis, TAG, comma

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We'll just put downloadUrl,

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and called with url $ feedUrl.

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And as I mentioned we need to remove the second argument

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from the DownloadData constructor.

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And you will often have to tweak functions slightly,

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like we're doing here when you move them into a ViewModel.

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The changes we make here are quite trivial.

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The only ones we had to make,

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were to remove private and delete the reference

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to the ListView.

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We checked the X in the ListView,

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that's a UI component,

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and that doesn't belong to the ViewModel.

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So Android Studio should have removed that synthetic import

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but if you go up and have a look there

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if not you can delete it now

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and you can see we also got the X in the ListView there.

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They're greyed out now, because they are no longer in use.

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I'm going to remove those manually.

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But you might find Android Studio's done

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that for you automatically.

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So the DownloadUrl function at this point

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is doing exactly what it did before

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including checking, to see if it's necessary

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to download the dollar again.

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If the URL hasn't changed, then there's no need

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to perform another download.

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However we did provide the users with a refresh menu item

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and we need some way to indicate

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that the download should be performed again.

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And we can do that by invalidating the FeedCacheUrl.

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So to do that, I'm gonna add a function

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after downloadUrl.

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So fun invalidate we'll call it, parenthesis

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and we'll just type feedCacheUrl

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and set that equal to "INVALIDATE".

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Below that,let's actually get Android Studio

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to generate the required onDataAvailable function

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for our interface.

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OnDataAvailable.

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00:13:13,921 --> 00:13:16,679
And let's actually add some Logging first

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our Log.d will override the to do.

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TAG, onDataAvailable called,

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then we'll type feed.value = data.

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Valueing what's past of this function.

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We'll do Log.d parenthesis, TAG, comma,

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onDataAvailable ends.

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So our only work down there is

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to assign the data to the MutableLiveData object.

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So the final change now is

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to do something that MainActivity's currently doing.

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We've still got a few arrows in MainActivity

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you can see here.

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And we're gonna fix those in the next video,

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when we get this modified app up and actually running.

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00:14:01,098 --> 00:14:02,764
And there aren't really many changes to make

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00:14:02,764 --> 00:14:03,742
as you can see there.

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But one that we will make right now

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is right at the end of MainActivity.

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00:14:07,162 --> 00:14:08,568
If you go right down to the bottom there,

341
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we got this onDestroy function.

342
00:14:11,137 --> 00:14:13,052
So this onDestroy function's cancelling

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00:14:13,052 --> 00:14:14,981
any download that may be going on.

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MainActivity no longer knows about downloading.

345
00:14:17,246 --> 00:14:19,792
So therefore it shouldn't be tempting to cancel it.

346
00:14:19,792 --> 00:14:21,923
We can delete that function from here.

347
00:14:21,923 --> 00:14:24,090
So we're going to do that.

348
00:14:25,986 --> 00:14:28,251
Now this is an extremely important change

349
00:14:28,251 --> 00:14:30,228
it really highlights one of the advantages

350
00:14:30,228 --> 00:14:31,276
of using a ViewModel.

351
00:14:31,276 --> 00:14:33,451
With this function in MainActivity,

352
00:14:33,451 --> 00:14:36,216
consider what happens if the user starts a download

353
00:14:36,216 --> 00:14:37,903
then rotates the device.

354
00:14:37,903 --> 00:14:39,628
When the activity gets destroyed,

355
00:14:39,628 --> 00:14:41,976
this function will cancel the download.

356
00:14:41,976 --> 00:14:44,827
It may have nearly finished, but it will still be cancelled.

357
00:14:44,827 --> 00:14:46,530
When the activity is recreated,

358
00:14:46,530 --> 00:14:48,747
the download will start again.

359
00:14:48,747 --> 00:14:50,540
But now that we've deleted that function,

360
00:14:50,540 --> 00:14:52,993
instead will cancel any pending download

361
00:14:52,993 --> 00:14:56,130
in the ViewModel using its onCleared function.

362
00:14:56,130 --> 00:14:58,607
And I'll get Android Studio to generate the function.

363
00:14:58,607 --> 00:15:00,690
Just go back to that now.

364
00:15:02,871 --> 00:15:05,530
So implement the onCleared function.

365
00:15:05,530 --> 00:15:08,498
You can see that's on the ViewModel there.

366
00:15:08,498 --> 00:15:10,557
And we'll start by removing the super,

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00:15:10,557 --> 00:15:13,640
with a Log.d, parenthesis, TAG comma,

368
00:15:15,102 --> 00:15:17,013
onCleared colon

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00:15:17,013 --> 00:15:19,346
cancelling pending downloads.

370
00:15:22,344 --> 00:15:26,858
Then we'll put downloadData code operator .cancel

371
00:15:26,858 --> 00:15:28,275
parenthesis true.

372
00:15:29,399 --> 00:15:31,122
So remember from the ViewModel lifecycle,

373
00:15:31,122 --> 00:15:32,642
that onCleared's called

374
00:15:32,642 --> 00:15:35,424
when the ViewModel's no longer used.

375
00:15:35,424 --> 00:15:37,582
It's a good place to tidy up any resources,

376
00:15:37,582 --> 00:15:41,245
such as any acing tasks, for example that is selected.

377
00:15:41,245 --> 00:15:44,281
And because the ViewModel survives the configuration change,

378
00:15:44,281 --> 00:15:46,309
the download won't be cancelled

379
00:15:46,309 --> 00:15:48,024
when the user rotates the device.

380
00:15:48,024 --> 00:15:50,487
That's a huge improvement.

381
00:15:50,487 --> 00:15:52,661
Alright so that's our ViewModel finished.

382
00:15:52,661 --> 00:15:53,944
The next video we'll start the process

383
00:15:53,944 --> 00:15:56,453
of modifying MainActivity, and run the app

384
00:15:56,453 --> 00:15:58,705
and check out what we need to do to finish it off.

385
00:15:58,705 --> 00:16:01,133
So I'll see you in the next video.