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now that we've seen how to use a view

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model and live data in a very simple app

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it's time to look at something a bit

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more complex when we finished the top to

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him downloader app it still had one

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floor we tried to avoid unnecessary yard

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downloads as much as possible but the

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app still downloaded the data again

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after a configuration change such as

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rotating the device at the time I

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suggested that you could use a loader to

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prevent this and said that would be

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covering loaders later in the course

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however in June 2018 Google deprecated

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the loader classes so we're now going to

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be using the view model plus instead

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we've seen that a view model survives a

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configuration change which means it

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shouldn't have to download the data

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again when the device is rotated so in

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this video we'll start the process of

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changing the top 10 downloader to use a

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view model and solve this problem so

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start with the top 10 downloaded project

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open and you can see that I've got that

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open on screen you can download that so

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from the final video in section 7 where

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we created the top 10 download our app

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in this course if you need to so first

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thing I'm looking at main activity you

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can see it's got quite a bit of code

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that's concerned with the UI as we

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scroll down and check out so it needs to

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keep track of the feed limit that the

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user selected in order to make the menu

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correctly reflect their choice now

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because the feed limit is used to

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construct the URL it makes sense to

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leave responsibility for restoring feed

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URL with main activity as well you may

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decide to do things differently but I

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like the fact that the view model isn't

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going to know how to construct the gyro

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it'll be given a fully form link to the

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RSS feed and doesn't have to worry about

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formatting it with different feed limits

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so this separation of responsibilities

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makes it easier to make changes to your

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app without breaking things you could

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use a different feed for example and the

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view model would still work well so

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moving down a bit further

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most of the conveyor code in here is

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concerned as I mentioned with the gyro

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but looking down here we've got this

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function that download gyro well that

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shouldn't be in here because that's not

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a UI function and also scorned a little

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bit further we've looked towards the

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bottom of the class

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we've got this companion object and

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that's the downloaded companion object

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we shouldn't have that in this class

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either well that's a good place to start

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I think this download our companion

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object so what I'm going to do is create

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a new Copland file class let's go ahead

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and do that it's a new Kotlin file class

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we'll call this one download data now

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because we're going to be cutting all

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the code we've got all the code we need

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I won't bother selecting class from the

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drop-down so next we want to go back to

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main activity we're going to cut all the

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code out from the companion object from

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the Declaration right through down to

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the closing right tab curly brace

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leaving just the companion line there

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and I'm going to take the opportunity to

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delete that as well cuz we won't be

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needing that anymore I'm going to paste

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that in the download data class that we

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just created and that's when I got to

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paste it we've got a lump of imports

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that I've actually popped up now in this

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case we don't want to import Android

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widgets on this view so I'm gonna

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unselect that one from the list and

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obviously that's a UI related thing

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which is why we're not including it okay

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okay we've got that you know paste it in

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to our downloaded data class this is a

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basic async class and we you shouldn't

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have to make too many changes to it what

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I'll do you I'll start off by doing is

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pasting or moving the tag to the top

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level and make it a costs let's go ahead

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and do that and that should be Const now

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also we won't need a context for the

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definition here or reference to the

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ListView

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in fact this class shouldn't have any

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knowledge about the UI it's just going

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to download and parse the data now it

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will need to know what created it to

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call it back when the data is available

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and it shouldn't be marked private so

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I'm going to delete that out for now

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change the definition so it's no longer

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private then within the parentheses

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we're going to be using an interface

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method to call back on and we'll write

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that after the declaration but let's in

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print parentheses add private val call

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back :

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I'll call it downloader call back

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leaving everything else s is callback

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will be the object that created the

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download data instance

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and we'll implement a function that we

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can call so we'll get to the interface

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shortly but what we can do now is remove

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the two var declarations up here because

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we're going to replace those in our

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interface with our interface we're also

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going to delete the init block there as

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well all right so I'll go ahead now and

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add our interface so interface and it's

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download a callback then add a left or

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right curly brace there's going to be

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fun we can add a function fun it's going

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to be on data available parentheses data

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: and we're going to find this a list of

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feed entry objects and lists and then a

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diamond and there's going to be feed

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entry alright so that's how it defines

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so we've now got errors in the on post

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execute function when we try to refer to

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things like the adapter in the list view

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again this class isn't concerned about

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the UI it's just you know it just

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downloads and passes the data and sends

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the result back so what I'm going to do

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is replace the code with a call to the

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calling classes on data available

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section let's go ahead and clean that up

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a bit so I'm going to delete this super

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and I believe the other tag entry that

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we had there what we're also going to do

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is only pass the data if result isn't an

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empty string so on the line after the

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VAR pars applications we'll do a test

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for that

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if parentheses result is not empty so

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dot is not empty closing your

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parentheses at a code block then and

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only then when we do a pause and stop

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pars result I could have just copied

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that line there so I'm going to delete

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that now again we're not concerned with

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the UI so we're going to delete these

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two lines here we don't want to want to

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be updating the user interface here

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instead what we're going to do is just

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add that callback as I mentioned so

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callback dot on data available we're

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going to parse applications or paths

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sparse applications dot applications

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looking further down there the doing

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background function is fine but that's

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been making quite a significant change

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to the app structure what I'm going to

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do is rename all the logging there it is

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useful to check that the function has

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been called

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I'll just move that back and at this

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point we're done with the modifications

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for this class but one thing I do want

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to show you and what we need to change

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is when we initially talked about and

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set up this download XML function or

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showing you how simple it was to do that

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I did leave out the error handling we

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had enough to concentrate on at the time

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but now is a good opportunity to add

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that back again so let's go ahead and do

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that this is the download XML function

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so I'm gonna start here by adding things

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to a try block here so it's gonna be try

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left curly brace and I'm gonna indent

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the return there they'll have a closing

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right curly brace then we need to catch

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some exceptions so we'll start with a

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catched ecole and we'll start with

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malformed at URL exception so if you get

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that we're gonna do a log d parentheses

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tag comma download XML colon invalid

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euro and closing double quote +

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e.message so that's our first one next

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what I'm going to do is annoy exception

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so catch e : hi our exception same do

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there with the log log D you could do

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and logged on either but for debug

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purposes I'm going to leave the log D in

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there because that'll become useful a

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few videos time so log D parenthesis tag

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comma and download XML again just home

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IO exception reading data is it getting

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the error message then we'll let our

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plus e dot message on the end again so

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you get more details about what the

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exception was I need to change that

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reference to a lock I see there and the

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third exception we want to catch is a

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security exception e : security

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exception log D parentheses ter comma

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there's gonna be download XML : security

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exception and we'll ask the question

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needs permission question mark then

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double quote + e dot message and what

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you could do there as you can do an e

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dot print stack trace if you wanted to

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and done uppercase e there again

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eat up print stack trace but I'll

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actually comment it out for

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now then one other thing we need to do

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here though because we haven't closed

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that in the Tri we need to return an

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empty string in fact if there was an

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00:09:06,550 --> 00:09:08,290
exception so down here we little

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00:09:08,290 --> 00:09:11,530
returned an empty string and we just

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make a quick note about that turn an

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empty string if there was an exception

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now what I would like to do is tidy up

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these nullable parameters in the two

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functions if we end up passing a null

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string for the URL that's an error in

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our code so I'd rather the app pressures

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quickly so that we can detect the

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00:09:32,350 --> 00:09:34,150
problem in testing so for the doing

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background I'm going to try to remove

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the safe call operator there and also do

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the same for our download XML function

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download XML is our function so we're

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obviously free to modify that one all we

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want that doing background function you

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may remember what's generated for us in

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Android studio used a nullable type but

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our async task if you come up and have a

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look at that again that we've defined on

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line 15 doesn't use nullable types so if

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you're passing multiple values in an

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array then it may well be wise to leave

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the virag parameter as an array of a

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nullable objects and perform an old test

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inside doing background I'm not

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suggesting that you always remove that

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question mark but it is safe to do so

259
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here all right so that's our download

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data class finished in the next video

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we'll swing over and start working on

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our view model so I'll see you in the

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next video