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in this video I'm going to start the

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process of converting our calculator app

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to use a view model the calculator app

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that we've created earlier in the course

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is a very simple one so it'll be very

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easy to see how the view model

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fits into the overall architecture we'll

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see how to create and use a view model

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but more importantly what that view

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model can do for us and I just want to

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make it clear that this is a very simple

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example and you almost certainly

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wouldn't use a view model in this app in

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practice so they'll be clear I'm not

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suggesting that you use view models all

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the time we're converting this app to

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demonstrate how a view model separates

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the logic from the user interface UI so

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our main activities going to do nothing

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more than display the UI to the user and

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respond to events like buttons being

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tapped it responded to those events by

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calling functions in the view model and

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it's actually up to the view model as to

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what happens when those buttons are

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tapped the activity will observe the

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view model using live data so that it

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can reflect any changes in the UI that's

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all that's going to do display data call

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view model functions when UI events

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occur in update itself when data changes

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everything else will be handled by the

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view model in more complex apps there

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might perhaps be a business logic layer

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and a data base but we're taking baby

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steps and starting slowly here at the

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moment alright so as you can see on

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screen I've got the calculator app

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opened and this is the calculator app

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from earlier in the course we're going

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to start converting that to use view

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model you can download the project from

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the resources for the last video in the

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calculator section if you don't have it

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available locally on your computer now

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if you get problems opening the projects

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from the resources try the files in

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validate cases restart option other

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words up here if you happen to get an

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error file builder that caches restart

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and go ahead and click on invalidate and

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restart and you may also need to rebuild

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the project but in my case I've got it

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working okay and as it turns out before

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we start there are a few suggestions

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from Android studio in main activity if

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I score I don't have a look here you can

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see here we've got a suggestion there

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for Android studio regarding a lambda

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right him

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it actually says where lambda arguments

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should be moved out of parentheses and

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to do that I can just come over here

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click on the light bulb and there's an

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option there to move at land or argument

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out of parentheses I'm going to click on

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that that removes that first warning

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these lint checks are revolving all the

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time so don't be surprised when you come

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back to a project and find suggestions

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and warnings that weren't there before

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this was the case here now that we've

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changed that first one there's a

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suggestion also over here if I move out

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of view that the parameter view is never

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used and could be renamed or if we have

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a look at that warning again could be

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renamed to and underscore doing that

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tells the compiler that the parameter

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isn't used and also allows it to

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optimize the code sometimes but if we

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come over there and click on the light

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bulb there's an option over there for

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another option remove single lambda

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parameter declaration so I'm going to

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click on that to accept that suggestion

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as well alright so that's no turn it up

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we've no longer get anyone so you've got

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a green tick now in the top right-hand

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corner of that class so we now need to

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create a new Kotlin file class let's do

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that so I'm going to right click on the

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calculator package new Kotlin file class

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and we're going to call this one

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calculator view model okay there's a

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view model and the class declaration

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line will be class calculator view model

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and we need to extend the view model

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class to create the view model for this

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app so it's : view model and then

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parentheses then left or right curly

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braces if you were prompted make sure

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you import the class from Android arch

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that life cycle don't view model but you

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can see in line three the imports happen

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for me automatically all right so now

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that we've done that the next step is to

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move everything out of the main activity

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class that's not related to the UI so

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any processing and all state will be

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moved to the view model what I say all

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state I mean all state that's not

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related to the UI the activity will

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still take care of restoring the

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contents of edit text widgets following

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following a rotation for example

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but that's handled by Android anyway

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along the way as we do this we're going

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to get a lot of errors but that's good

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because it highlights the bits of code

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that we would be looking at now all that

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we should be looking at rather so

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opening up our main activity you can see

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right at the top here we've got a couple

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of variables destroying the operand and

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the pending operation now they're

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functional not UI variables so I'm going

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to cut them out of here

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along with the comment that we added to

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that effect and we're gonna paste them

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at the top of the calculator view model

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class okay back to our calculator your

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model and I'm going to paste it directly

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in the class there for our calculator

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view model and obviously now that I've

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done that as expected

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we've got a load of errors now we're

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going to go through this process of

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fixing those so looking now at the

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various functions that are in this class

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an obvious function that's got nothing

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to do with their UI is this perform

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operation function if you recall that

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takes care of the calculating and we can

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freely move that into the view model so

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I'm going to cut this function entirely

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out of main activity and then we'll

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paste it into the calculator view model

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there cutting that out calculate a view

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model I'm going to paste that in now

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that when you do that you get this

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option about accepting or is it okay to

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accept the synthetic imports we don't

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want to do that so I'm gonna click on

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cancel here instead just make a bit of

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space so this is battaglia remember that

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the view model mustn't hold any

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reference to the activity referring to

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the activities widget is a definite

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no-no which is the reason we've

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cancelled that if you did happen to

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accept the imports by mistake don't

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worry we can't delete them later once we

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change the code or you can just do an

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undo and repeat the operation but for

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now you saw that I didn't accept those

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synthetic imports so go back to main

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activity again you can see that there's

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no errors in the unsaved instance state

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and on restore instance state functions

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we're not interested in saving State now

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that's the job of the view model so I'm

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going to delete these two functions

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entirely

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and once we do that we go back up to the

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top of the file we've got these three

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constants here now then longer being

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used is there the bundle keys so for

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that reason we can now delete those

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entirely now we're really breaking

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things at this point because we never

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got errors in both classes both

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mainactivity and calculator view model

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showing errors but that's actually quite

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normal when you perform a significant

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refactor to an application like we're

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doing right now let's move over now and

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focus on the calculator view model class

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now in our perform operation function

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the last two lines are referring to

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widgets in the activity now that's for

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bidness we know to access widgets

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directly so we need to change those

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lines to use local variables instead and

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I'll do that in a moment now these are

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the this is the code with errors on

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lines of 37 and 38 when we come to

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change a widget reference to a variable

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it's obvious then that the widget isn't

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going to be updated these two lines

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update the result in the display if we

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don't update the widget then obviously

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we need some other way of showing the

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result back in the UI this is where the

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Android live data class comes in we saw

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that mentioned in the documentation in

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the previous video and if you could I

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have a few which I'm just gonna bring up

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in the browser now you can see here that

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says live data is an observable data

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holder class so that means that our

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activity can observe it and react when

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it changes live data is also life cycle

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aware it won't attempt to notify an

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observer about changes unless the

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observer is active so how to read

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through this document and also follow

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the various links once we've got this

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calculator app working because it

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describes how activities are

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automatically and subscribed when they

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get destroyed for example we don't have

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to concern ourselves with unsubscribing

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manually and we'll also get

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automatically subscribed again if the

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activity is recreated as a result of a

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configuration change now if we scroll

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down a little bit down here you'll be

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able to see that there's a section here

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called working with live data objects

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this will make more sense when you've

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seen them used in our app so I do

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suggest you come back to this later and

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most of what we're going to do is

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covered here

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the section down here create live data

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objects show some sample code to create

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a live data object this is still Java

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code but Google are updating all their

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examples there is something wrong with

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that example though we'll see what that

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is when we've checked out the next

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section I want to look at so scrolling a

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little bit further we're going to get to

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the update live data object section this

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one here update live data objects this

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talks about or which reduces another

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class this is the mutable live data

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class in fact scrolling up a bit if we

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come up and look at our code example

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there you can see here that in current

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name is to clear this immutable live

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data type so a live data object doesn't

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have any functions for updating it you

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have to use a mutable live data for that

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and that's actually what's wrong with

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this example code here the mutable live

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data object in current name is public or

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more specifically the get current name

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get a method that exposes it is public

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and returns a mutable live data object

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so that means there's nothing to prevent

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the UI from updating it this snippet

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shows basic usage not necessarily best

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practice the activity could call the set

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value function to change M current name

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it shouldn't the whole point of this

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exercise is to separate the UI from the

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apps logic but there's nothing at the

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moment to prevent it with this example

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code so when you go to the trouble of

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finding these documents read them

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carefully even if some of it doesn't

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make sense it'll be there in the back of

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your mind you'll be surprised how often

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something that doesn't make sense

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suddenly does when it's time to need it

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and actually use it alright so I

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apologize for all the scrolling but I'll

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just scroll back down now to the update

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live data object section here it's

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talking about the set value and post

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value methods of the mutable live data

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object he used these to modify their

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values will be using set value but if

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the code that's changing them is running

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on a separate thread in an async task

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for example then you'd use post value

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instead of course we're using Kotlin not

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Java so we'll refer

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to the value and post value attributes

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instead of calling this setters the

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final sentence of that or in that first

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paragraph is why I suggest reading the

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entire document even if you think you've

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found what you need further up it

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actually says there that up here usually

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mutable live data was used in the view

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model and then the view model only

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exposes immutable law of data objects to

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the observers so we're going to start

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off exposing the mutable live data

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objects and then we'll change the code

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to only expose live data which the

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activity can't change alright so back to

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our code so at the moment we've got two

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references to two widgets so we need to

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replace those with live data objects so

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let's actually declare the two objects

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first so below the private fire pending

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operation line move into eval result is

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equal to it's going to be mutable live

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data of top string and let the

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parenthesis on the end I'm going to do

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this same for new numbers had Val new

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number equals it's going to also be a

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mutable live data or type string there

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and we can have the parentheses again so

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now that we've done that we can replace

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these two widget references down here on

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line 41 and 42 with our mutable live

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data objects so I've used the same names

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so the change is trivial instead of

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setting the text all we need to do is

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set the value instead of result dot set

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text we're going to change that to

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result dot value equals and get rid of

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the parentheses and operand 1 dot 2

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string and on the next line it's going

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to be a new number and set up text it's

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going to be value equals and I'll remove

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the parentheses and leave the double

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quotes in there now if you had some

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synthetic imports at the top of the file

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in the input section they should now

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have been removed if not though you

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could safely delete them we're using

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these variables now not references to

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the activities widgets as you can see I

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haven't got any of those synthetic

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imports showing anymore

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all right so let's end the video here in

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the next video we've still got some work

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to do because we've still got some

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errors showing in main activity so let's

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work on cleaning the main activity class

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up in the next video