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Now, in the last lesson, we've managed to brighten up our categor's view controller by assigning some
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random colors to the background of the cells.
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And we're persisting this data, so that every single time we open up our app or we add a new category,
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the color stays consistent for each of the categories.
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So the next thing I want to do is I want to go to the items or the TodoListViewController and I
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want to create a slightly different effect.
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I want to create a gradient effect so that, maybe, at the top, we've got one color say, blue, right? And it's
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light blue.
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And then through a gradient, all the items get progressively darker and darker and darker, and it'll look
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like a nice gradient transition.
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So let's try and see if we can figure out how to do that.
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Now, back in the Chameleon Framework Documentation, you can see that they have some methods that allow
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you to color a shade lighter or darker, so that will probably help us to achieve our goals.
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So let's go into our TodoListViewController and go to the place where it makes the most sense
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to change the background color of the cell which is, of course, inside cellForRowAt indexPath again.
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Now, the place where we change the cell properties here is inside this "if-let" and we change the
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textLabel's text
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and we're also going to change the cell's background color.
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So we're going to set the background color depending on the current indexPath. If the indexPath is,
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say, 0, like the first cell, then we're not going to darken the color by very much. But if the indexPath
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is much higher, say, at, you know, the tenth row, then we're going to darken it a lot more.
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So it makes sense if we can change that darkening percentage based on our item number in all of the
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todoItems.
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So, say, if we had 10 todoItems, then we want the first item to be darkened by 10 percent.
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The fifth item to be darkened by 50 percent and the last item to be darkened by, maybe, say, 100 percent.
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So that way we get a nice gradient transition from the top item all the way down to the bottom item.
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And we can start doing that by setting the sales background color to--
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Let's just pick one from Chameleon Framework, maybe something like Flat Sky Blue, right?
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It looks pretty nice.
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So we can say that cell's backgroundColor = flatSkyBlue. And in order to get this color out of
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Chameleon Framework, we, of course, have to remember to import it.
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So there's a FlatSkyBlue. And we're going to say darken
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this color by a percentage. And the percentage is going to take a CGFloat,
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so that's a floating point or decimal number.
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And the way that this works is it takes a number between 0 and 1, where one is basically 100 percent
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and 0 is zero percent, and everything in between is your percentage as a CGFloat orCore Graphics floating
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point number.
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So the percentage that we want to darken our background color depends on two things.
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One is which item are we currently trying to color up.
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And the second thing is how many items are they in our todoList.
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So let's delete this placeholder and give ourselves a bit of space in order to write out the maths.
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So the percentage that we're going to darken our cell is going to be the current indexPath.row.
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Say, if we're currently on the fifth row, right? Let's just imagine one particular situation, rather than
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trying to plan for all the scenarios. We're currently on row number, number five, right?
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And there's a total of 10 items in todoItems. So, now we're on 5 out of 10.
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So we want to darken our background color by, basically, 50 percent, so close to 0.5,
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and we can get that number by dividing the indexPath.row by the total number of items, right?
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So todoItems.count.
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Now, indexPath.row is going to be an integer
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and todoItems.count is also going to be an integer.
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So integer divided by integer,
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that's perfectly valid.
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But we need to get it out as a CGFloat in order to satisfy the requirements of this particular parameter.
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So we can cast it into a CGFloat
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or you might think we can. This line of code will make sense to the largest number of people. But I'm
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going to show you what happens when we do this
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and you might already be able to predict if you are good with the logic, and you've been keeping up with
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what I've been saying. But no matter,
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this line makes sense to us, right?
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Take the current indexPath.row which is five, divided by 10,
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you get 0.5.
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And 0.5 works as a floating point percentage to darken our FlatSkyBlue color.
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Now, all of this code, remember, gets called for every single cell in the tableView.
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So when we are on row number one out of 10 items, then this is going to be 0.1.
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And so we're only going to darken our backgroundColor by 10 percent.
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Now, the problem as Xcode is suggesting to us, is that this darkoen byPercentage method returns an optional
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UIColor, but this needs a definite UIColor.
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So what can we do?
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Well, we can, of course, use our handy "if-let," and you can see that we are now building a optional binding
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pyramid. And this pattern, unfortunately, is a bit too common in Swift.
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So if let colour equal all of this,
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so let's cut it and paste it.
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Now, we can, of course, shorten this if we don't want all of that extra commenting.
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You can see that we can actually make it all fit into one line. At this point, it's telling us that your
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optionally chaining todoItems here using this question mark.
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So this is saying get the count if todoItems is not nil.
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Now, we know that todoItems can't be nil
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inside this current if-let because it's only going to get called if todoItems an indexPath.row
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is not nil.
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So this is a place where it's actually safe to simply force unwrap, force unwrap todoItems.
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So if like let colour equal this darker color, then what we should do is we should set the cell's 
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backgroundColor to this color.
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And now, if we run our app, you'll see something slightly peculiar happened.
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So let's go into our Home category and let's add some items, shall we?
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Buy Food. Eat food.
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Enjoy Food.
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I'm getting a peek into my mindset here.
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All right, so--
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Oh, there's still the separators over here, right?
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So let's get rid of the separators by putting it inside viewDidLoad, tableView.separateStyle
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= .none.
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So we'll run that change a little bit later on.
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But the more pertinent problem is, why on earth do we have the same colors, even though we told our code
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to darken this FlatSkyBlue color by a percentage depending on which row we're on.
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So we're expecting a gradient here,
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right?
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That would be nice.
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So here is a fundamental logical mathematical issue with type conversions.
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So I'm going to print two things here.
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I'm going to print, say, "version 1" and version 1 is going to be what we've got here.
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indexPath.row / todoItems.count, and then the result of that division turned into
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a CGFloat.
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And the second version or "version 2" is going to be slightly different.
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What if we first turn indexPath.row into a CGFloat by casting it, and then after we've turned
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it into a float, do we divide it by todoItems.count,
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And because we can only do division between similar types,
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right,
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as it says, "Binary operator / cannot be applied to different types,"
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then we're also going to convert todoItems.count into a CGFloat as well.
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So the important difference between version 1 and version 2, is that version 1
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we're doing whole number divided by a whole number, and the result gets turned into a CGFloat,
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version 2 is whole number gets turned into a CGFloat, whole number gets turned into a CGFloat,
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then CGFloats are divided by CGFloats.
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So let's go ahead and hit run and see what we get printed out for every single row in our table view.
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So let's, again, go into Home where we've already got three items,
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and let's take a look over here.
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So version 1 for the first row, indexPath is 0, 0 divided by 3 is still going to be 0,
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so both of them are 0.0.
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Now, the differences come in the second row where the index top row is equal to 1. 1 divided by 3 is a
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third or 0.3 recurring.
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But that only gets shown in our second version, not our first version. Our first version is still 0.
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Now why is that?
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Well, when you divide a whole number by a whole number, the output here gets rounded into a whole number,
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and then that rounded whole number gets turned into a floating point.
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So you can try it yourself inside Swift Playgrounds, but this is, essentially, the reason why our code
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does not work.
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And this is a very, very simple reason why sometimes when you work with floats and doubles, and trying to
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use variables that are integers, you might get unexpected problems.
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So this is something that happens to a lot of people and we're simply going to replace that inside here
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where it says byPercentage.
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Okay. So, now I'm going to comment out my print statements because we no longer need them. And I'm going
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to run our app
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and we are going to hope for the best.
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All right.
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So let's go inside Home, and dun-dud-dun,
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Isn't that cool?
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And as we add more items--
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I don't know. "Eat more food."
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"I'm so hungry."
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"Feed me."
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You can see the gradient starts building up, and depending on which road we're on, it will darken it by
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the percentage determined by which row it is in the total number of items.
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So this is looking pretty neat, right?
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Now, there's just one problem, and the problem is that this part gets so dark that we can't actually see
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the text anymore.
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Yeah, it looks pretty neat and it's pretty.
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But as we always say, you should choose function over form,
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right?
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Unless we can have both.
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So what do we need to happen in order for our app to work well?
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Well, we will need the text for the darker backgrounds to be white instead of black, and the text for
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the lighter backgrounds to be black instead of white,
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so how can we do this?
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Well, we can either Google for it and look through custom solutions or why don't we just see if Chameleon
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Framework already has a method that does it.
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If it doesn't, we'll build a custom solution. But if it does, let's not reinvent the wheel.
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And if you scroll through that documentation, you'll come across this neat little thing called contrasting
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text. And it means that you no longer have to worry about the text
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standing out from the background color because it's going to take care of it for you.
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So let's try and figure out where it allows us to do that.
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So I'm just going to go for command F to find contrasting text. Contrasting Text. Contrasting colors.
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Okay. So this contrasting color feature returns either a dark color or light color depending on what
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the Chameleon algorithm believes is a better choice.
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And we can use it by using UIColor(contrastingBlackOrWhiteColorOn:UIColor, isFlat,
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true or false,
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or we can just use the shorthand contrast color passing in the current background color and specifying
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whether it should be flat or not flat.
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So let's go ahead and implement that in our code.
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So inside the same place where we did the optional binding, where we specify the background color, we're
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going to change the cell's textLabel.textColor. And that textColor is going to be equal to 
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ContrastColorOf backgroundColor.
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So we're going to pass in our backgroundColor that we generated up here and return a flat color or not
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a flat color,
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we're going to set equal
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true.
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And now let's run our app and see the magic happen.
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Nice, right?
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Looks pretty sweet.
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Now, you can see that they're all white because the flat sky blue is a little bit dark of a color by
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itself, and the white stands out quite nicely against it.
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So this algorithm behind the scenes is trying to figure out whether it's white or black is the more
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optimal choice.
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So if we went for a slightly lighter color from the palette, instead of flat sky blue, say, something like, I don't know,
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a flat white,
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right?
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Let's try that one, FlatWhite.
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This is making me want coffee now.
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Okay, so let's go ahead and hit run. And you can see that, automatically, the lighter colors at the top were
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given black text colors, and the darker colors at the bottom were given white text colors.
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So it's pretty neat, right? Now, it's all very well and good
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specifying a color and getting it darkened by percentage.
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Wouldn't it be nice if you could carry through the color of the category, so the user will select Home
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and this will be a gradient of green colors similar to our Home color.
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So what you'll need to do is to, somehow, change this to the color that we persisted using Realm.
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So pause the video and try and see if you can complete this challenge.
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All right.
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So this one shouldn't be too hard because if you remember we passed in the current category using the
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selectedCategory property, and this is a optional category that contains all the pieces of data in here,
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so the name of the category, as well as the color of the category as a string.
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So if we go in here and replace this FlatWhite using something like UIColor, using hexString,
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and the string is going to come from our selectedCategory.colour property.
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Now, we've got some problems here because if you click on the error, it says that selectedCategory is--
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It says that--it tells us that selectedCategory,
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the first little highlight, is an optional category and it's not unwrapped.
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The other problem is that the output or the return of this initializer create a UIColor using
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hexString is also a optional UIColor, and it's also not unwrapped.
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So there's two ways of solving this.
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You can either go down a pretty steep if-let pyramid,
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if let color equals this, 
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then if let blah-blah-blah.
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And then if let, the next one,
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then if let, the next one, unwrapping each property as you go along.
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But let's go back to first principles
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where inside this if let
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at the moment, and checking to see if todoItems at indexPath.row is or is not nil.
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Now, if this is not nil, then that means that we manage to have items inside todoItems, then that means
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todoItem exists. And todoItems comes from our selectedCategory which must mean that selectCategory
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can't be nil either.
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So sometimes having these warnings forces you, as a human developer, to think and see if this is safe
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to be unwrapped.
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Now we've unwrapped selectedCategory.
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The next thing we need to tackle is, what if we can't actually get a color from this hextString?
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What if I just entered some piece of, you know, mumbo-jumbo of a hexString, and you can't actually get a
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hex color?
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And this is a problem you always have when you allow strings because strings can be free form.
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It doesn't have to conform to a particular format.
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Then I could have put in any old string in here.
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And of course, you can't just generate a color from any string.
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That's not how it works.
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It has to be a valid hexString.
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Now, we don't have any validation for that.
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But what we can say is if this is nil, then don't go forwards with the next method,
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i.e., darkened by percentage.
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So we're saying optionally chain this.
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So if this is not nil, i.e., we got a valid hex code creating a valid UIColor and it's not nil, then go
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forwards and try to darken it. Because if we try to darken a nil value, then this is where our app is going
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to crash.
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So we're using optional chaining to make sure that all of this is not nil before we continue with our
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code block.
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So let's run our app and see if we manage to carry forwards that color from the category to our
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TodoListViewController.
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All right.
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So here's our Home category which is green and if we click on it-- Oh, look. Isn't that nice?
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We've got a gradient of green items
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and we've got text that's contrasting with the background color, and it looks pretty neat to me at least.
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So did you manage to get that to work?
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If so, then great.
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If not, then you might need to watch some of the past videos
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a few more times.
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All right.
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In the next lesson, we're going to tidy everything up and we're going to change our nav bar to make it
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reflect the colors that we've got in our
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TodoListViewController.
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So all of that and more, we'll see you on the next lesson.