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Everybody else is gone this is Caleb with Dev slopes and we're going to be revisiting a project from

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earlier in this section and it's the one where we built this calculator app OK we made an actual project

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here.

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We have a view controller of course we have some incomes for various things.

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And then in our model section we have what's called a calculator manager or I guess I should say a calculation

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manager.

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Now the interesting thing is is that this works very similarly to what is called a view model.

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And a view model is essentially a layer.

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Well you're familiar with model view controller and you know what each of those are individually Model

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View view model is another paradigm where there is of course models.

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There are controllers and then there are what are called view models and a view model is basically responsible

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for converting the data objects from the model and then presenting them in a easily displayable way.

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And that's basically what we're doing here is we've basically created a view model accidentally.

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This handles all of the logic that otherwise would need to be written in the view controller but we're

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abstracting it out of view controller and into a much more manageable layer called a view model so we're

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actually going to go ahead and rename this calculator calculator or view model.

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All right.

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And of course if we're going to be renaming this we need to rename everything so look for a calculation

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manager and we're going to replace that with calculator view model.

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OK.

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And click.

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All and it's going to go ahead and replace all of those and we should be good in here there isn't another

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instance of calculator if you model.

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But if I try to build this we're going to get a lot of problems particularly in the calculator View

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Controller and in our tests.

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So we're going to go ahead and do the same.

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Find and Replace find calculation manager and replace with calculator view model and actually the process

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that we use to do this.

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Creating an instance wups shoot well we're going to go back and put lowercase C's here I'm going to

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copy this anywhere that we are using that instance we're going to replace it.

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Of course copying and pasting is never the best option but it is an option.

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So we have a calculator view model here.

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Let's actually rename this file to Kelk Q. later or view model Swift.

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This is also calculator view modeled out Swift.

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And there we go.

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Very cool.

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Now if we look around make sure that we've done everything the right way.

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And if we come in here we can go ahead and type calculator view model tests.

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OK.

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I'm going to rename the file that as well.

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And I'm going to name this commented file name the same thing.

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Now of course calculation manager no longer exists but calculator view model does exist.

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And so this is going to be a set of tests to actually test our calculator view model to make sure that

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it works the same way as before and you know what I don't actually see any other instances here so I'm

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going to build it and see if it all builds successfully which it does.

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And so a view model allows us to essentially separate all of our logic like we have here.

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All of all of the things that basically convert the values perform the calculations.

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Basically what works with the data and creates it into a presentable way we can access through our instance

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of the view model so we can basically just create an instance of it and then call all of our logic on

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our view model which is very cool.

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So let's go ahead and let's build and run our tests.

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Let's make sure that they still run properly and work to make sure every single test passes and OK one

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test failed.

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So what is up with that.

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OK so it's saying zero is not equal to 25.

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We must have changed something here.

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Let's take a look.

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So when we push a number button it appends it to the array.

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Oh but it never converts them back into a string and sets the labels so let's go ahead and type value

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label dot text and that should be equal to our view model dot string of five values because if you remember

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that basically takes our array which when we append we add 1 to the array then it converted into a string

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so it can be displayed in the label.

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That's super weird OK.

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So let's try running those tests again.

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This is why testing is super helpful because it helps you to catch these weird problems that sometimes

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you might accidentally introduce if you were to accidentally delete a line of code and check it out.

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It says all test succeeded.

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Let's go to the test navigator here and everything works.

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Our view model is successfully instantiated and this is how you use model view view model to test view

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controllers.

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It makes it much easier to actually test the logic which you want to test.

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So anyway this is Caleb with Devis Loeb's dot com let's head on over to our exercise.