Original subtitles

0 Now in the last few lessons we've been refactoring our code and making it simpler, easier to read and 1

more succinct. And in the process we've learned a lot of the fundamentals of Dart that enables us to do 2

this such as enums or const, final parsing function and a whole lot of programming knowledge. 3

But in this lesson we're going to get back to our design and we're going to build out the next card. 4

Now the next card is going to be a slider and we're gonna be using a Flutter slider but we're going 5

to customize it to our liking. And in the process we're going to learn a lot about customizing Flutter 6

widgets. 7

So first things first, let's head over to our next reusable card, which is right here. And at the moment 8

it doesn't really do anything. It's completely empty. 9

So let's give it a cardChild. The cardChild property is going to hold a number of things. Because we 10

have quite a few things that are going to be stacked vertically, 11

it makes sense to create a column widget. And it's going to have a bunch of children, the first of which 12

is going to be a text widget. And our text is simply going to say the word height because this is going 13

to be the height input. And I would really love for that text to have the same style as the male and 14

female text here. 15

So the same font size, the same color and the same font. And at the moment that style is coming from a 16

constant that's defined in our icon_content.dart file. And it doesn't really make sense to reach into 17

the icon content file to be able to define a label that's on my input page. 18

It seems like that would get really messy. 19

So this is a good time to talk about creating a dedicated constants file. As we create our design and 20

our user interface, it's very frequent that you might want to share designs or share settings across 21

different screens. 22

So instead of having it live inside a particular file or a particular component, it makes sense to create 23

a new file that is going to be a Dart file and it's going to be called constants. 24

And this is going to hold all of the things that are constant across our app. 25

So that is going to include this labelTextStyle for example. So I'm going to cut it out of here and paste 26

it into here instead. 27

And of course whenever we're using material components we have to import our material library. 28

Now in addition to these text styles, I can also add in all of my color constants from over here. And we 29

can paste that into this file instead. 30

And now I have all of my constants together in one place and I can refer to it from any of my files 31

simply by importing the constants. 32

So at the moment you can see that we have some errors here because it doesn't know about labelTextStyle 33

anymore since we moved it out. 34

But as soon as I hit import and I import my constants.dart file, then that error goes away and it's 35

able to see into here and see what text style it should apply to that label. 36

Now similarly on my input page, I've got errors all over the place because it doesn't know about this 37

activeCardcolor and inactiveCardColor. 38

So let's go ahead and import our constants into this file as well. 39

So now it's a good time to mention that the naming convention for these constants usually start out 40

with a K. 41

So we can refactor our code to meet that style. So we can right click on each of our constants and we 42

can go to refactor and rename which is our friend that is going to search for all the places where we 43

use this constant and rename it all in one go. 44

So instead of calling it bottomContainerHeight, the convention is to start with a K, K for constant 45

because C is used for a lot of other things. 46

So K for constant and then we're going to start off with a capital B. 47

So K constant, bottomContainerHeight we're going to refactor that and now you can see everywhere where 48

we use that, it's now been refactored automatically. 49

So I'm just gonna go ahead and do the same for all my other constant. And from now on when I'm creating 50

new constants in my constants.dart, I'll be using that naming convention. 51

All right. 52

So here are all my constants renamed and they're going to be renamed everywhere where they're used. 53

Now why did I go through all that trouble of renaming all my constants? 54

Well whenever you're trying to pick out a color or a text style from your constants file, you start out 55

by typing K. And now Android Studio is intelligent enough to show you all of the relevant constants 56

that you might want to use at this point. So you can see that there's kBottomContainerColor, kBottom 57

ContainerHeight and there's also constants that Flutter has defined such as the kMaterialEdges, k 58

MaterialListPadding, 59

so this way all the constants can be searched easily in one go. 60

So let's change that back to kActiveCardColor. And every time you're looking for a constant you get 61

that dropdown list to select from, which is much easier than trying to remember what the name of your 62

constants at the top of the file is especially once this list gets a bit longer and your app gets more 63

complex. 64

All right. 65

So back to our reusableCard which now has a text widget and I'm now able to give it a additional style 66

which is going to be the kLabelTextStyle, 67

so I can simply select that. 68

And now you should be able to see in the app we've got the same styling for that height text. But our 69

card has now resized to the size of its child. 70

So remember that the way that containers work is that if they have no children, they'll try to size themselves 71

to be as large as the parent allows, 72

and in this case it was stretched to the full width of the column. But once it receives the child, it 73

will try to resize itself to fit the size of the child. 74

So that's why my card has now shrunken in width. 75

But this is relatively easy to fix. 76

We can simply go to our main column at the very top and we can change it's crossAxisAlignment to our 77

friend which is crossAxisAlignment.stretch. And now it will stretch all of its children to take 78

up the full width of the column no matter the size of the child. In addition to the height label, I also 79

want to add a value for the selected height so that when the user toggles the slider it will change 80

as they move it around. 81

So I now need to add a number and a bit of text that says cm or centimeter. In order to do that, 82

I'm going to add a row under my text widget right here. So my row widget is going to have just two children 83

and both of them are going to be text widgets because one of them is going to display the height that's 84

selected and the second one is just going to display the word cm. My text widget is going to contain 85

some numbers. 86

So let's say it starts out with maybe the string 180. And then it's going to have some styling. And I 87

want this to be super large 88

so my textile is going to have a font size of maybe 50 and it's also going to have a super super heavy 89

font weight. So I'm going to set it as probably w900 which is the heaviest weight possible. 90

The heavyweight champion of my font. 91

So now if we take a look at our design, you can see that the number is pretty much as thick as my icons 92

and I quite like that consistency. So I'm probably going to be using this number style across the other numbers 93

that are on the screen, 94

so for example when I create the weight card or the age card. So it makes sense to turn this into a constant. 95

Pause the video and create a constant from this text style and replace all of this code here. 96

All right. 97

So of course we're going to create our constants in our brand new constants.dart file. 98

We're going to start out with the keyword const 99

and we're going to name it using the K convention. 100

So this is going to be k and I'm going to call it numberTextStyle. 101

You can call yours anything you wish. 102

And I'm going to replace it with what I have here which is my text style. 103

And over here I'm going to call it NumberTextStyle. 104

And now that should refer to our file over here. 105

And I'm going to paste in that text style I had from before. 106

So now I can reuse this one and this one across a lot of my design components. 107

And it keeps my code relatively simple and straightforward on my main page. Now that I've created my 108

number, 109

let's go ahead and add the second text widget in my row which is going to simply just say cm. 110

Now if you're from the States and you want to have imperial as your unit then feel free to change the 111

numbers or the units as you wish. 112

Now, that cm is going to have the same text style as my male, female or height labels. 113

Simple enough, I can simply tap into the style property, add my k for constant and select my label text style. 114

So now I have 180 and I have my cm. 115

Now you'll notice that everything is kind of all over the place at the moment. My height label is all 116

squished right at the top 117

and my number row is all the way to the left. 118

And it doesn't look very good. 119

So let's bring all of our components into the center of our column by changing the mainAxisAlignment 120

of our column to center. 121

And this should bring everything down into the middle of the column. 122

Now let's also make our row centered along the main axis 123

so it brings it right into the middle, by going over to our row and changing our mainAxisAlignment 124

to center as well. 125

And now everything's at least in the middle. 126

But there's one other thing that I would really like to have, which is for the cm or the centimeter 127

or whatever units you chose, to be on the same baseline as my number here. 128

So that way it looks like it's actually a 180 cm rather than sort of having the unit floating around 129

somewhere in the middle. 130

And you can see in the final design that's exactly what I have and it looks a lot nicer when it's aligned 131

all to the same baseline. 132

Now we can do this quite easily by simply adding a crossAxisAlignment to our row. 133

So remember that the main axis is along the length of the row and the cross axis is along the short 134

side, 135

so the width of the row. 136

And we're going to set our crossAxisAlignment to one of the options which sounds very very much like 137

what we want, which is .baseline. 138

And in this case, it's going to place the children along the cross axis such that their base lines match. 139

Let's go ahead and select that. 140

Now at this point if we hit save and we tried to look at our design, you'll see that it will break. 141

And the reason it tells us seems kind of cryptic. It's saying if we are going to change our cross axis 142

alignment to baseline, then we actually have to have a text baseline property set to something and it 143

can't be null. 144

How do I know this? 145

Well you can simply take this error message from our run panel and pass it through our friend which 146

has Google of course. And we can search to see if this has already occurred for somebody else. 147

So very often when you get error codes with Flutter, you can search for it in Google and it'll come 148

up in Stack Overflow or in the actual Flutter GitHub repository issue area. And here somebody actually 149

from the Flutter team has provided the solution here saying that you have to specify the textBaseline 150

argument if you want the baseline to be aligned, so that they know which baseline to line against and 151

maybe in the future, they're going to plan on making that message better. 152

And this is really one of the great things about the Flutter community. It's in development but your 153

input and also all the other developers experiences are contributing to the final product. 154

And day by day fixes will be made and changes will happen and it's just going to get better. 155

So let's try and implement that. 156

So they're talking about something called a textBaseline argument. 157

So where we have our crossAxisAlignment for our row, if we go ahead we can see that if we click on 158

CONTROL + J or CONTROL + Q on Windows, you can see that one of its properties is exactly that text 159

base line. 160

So let's try and set it so that our crossAxisAlignment.baseline will work. 161

So let's add textBaseline and then we can start typing TextBaseline with a capital T. And we can 162

now see that we have two options either alphabetic which is for alphabetic characters or ideographic 163

which is for basically graphical characters. 164

So we want alphabetic and now if we hit save, you can see that our error goes away and our row is now 165

beautifully aligned across the baseline. Pretty neat. 166

Now if you're working on any sort of development be it web or mobile or Flutter, you'll inevitably encounter 167

errors and warnings and problems. But just know that there's a strong community behind you who will be 168

able to help you out and you are not alone. 169

And in most cases the things that you've experienced, other people have experienced too and they've probably 170

come across a solution already. All right. 171

So we've solved our labels and our numbers, and it's now time to add a slider. Now Flutter has a widget 172

called a slider and it will implement a material design slider. And if we search for it, you can see that 173

this is roughly what it looks like. It's got a small round thumb which you can move around and it's got 174

one active side and one inactive side. 175

And when you click on it, it's got this sort of outline that pops up. And that looks very similar to what 176

we want to implement in our design. 177

So let's go ahead and implement it so that we can create this design. Now right below where I have my 178

row ending here, 179

I'm going to add a slider widget. And the slider widget has a number of properties such as a onChanged 180

callback. 181

So it's expecting a method that takes a double as an input and outputs void. 182

So whenever you see an arrow as one of the properties, you always know that this is expecting a function. 183

And it can also detect when a change starts or when the change ends. 184

So that means when the user starts dragging and ends dragging, you can also set the minimum position 185

and the maximum position. 186

So maybe if you're setting a temperature, you'll go from 0 to 30, but maybe for other users your scale 187

will be different so you can determine that. 188

And then there's the number of divisions you might want, a label for the slider and then there's the 189

active color and the inactive color. 190

So the active color, the documentation says, is the color to use for the proportion of the slider track 191

that's active. 192

And if you click on it, it gives you a little bit more detail. 193

And it shows that the active side is actually the side between the thumb, 194

so the circle in the middle, and the minimum value. 195

So basically that's going to be the color on the left here. 196

And then the inactive color is the one on the right. 197

So let's go ahead and implement some of these properties so that we can bring our slider to life. So 198

first things first, we're going to provide a value. Now, 199

the value of our slider is going to be the height that the user selected. 200

And we also want it to show up inside here, in this number. 201

So let's go ahead and create another property for our class that's going to be a integer and it's 202

going to be called height. And we'll start it off with a value of 180. 203

Now this can't be final or constant because it's going to change all the time and every time the user 204

slides on that slider. Instead of hardcoding 180 in here, we're going to be putting the value of the 205

height in here. 206

But because the text widget needs a string as the data, we can't simply just put a number in here. And 207

it tells us that type it can't be assigned to type string. 208

So let's go ahead and convert it by writing height.toString. 209

So now we have a string that's converted from our number and you can always convert numbers to strings 210

or ints to doubles. But if you want to do the opposite way then it's a little bit more tricky because 211

A B and C don't map directly to a number right? 212

But this is easy to do. 213

So now for our slider, the value is going to be our height. And we're going to get another incompatible 214

issue here. 215

It's going to tell us that the argument type int can't be assigned to the parameter type double, and 216

that's because the value is expecting a double to go in there as the value. 217

So what we can do is we can use again our trusty .notation to change it to a double. 218

So again super easy conversion and easily doable. 219

So now our height is going to be displayed as the value of the slider. 220

So that's the starting value basically. Now, 221

next we're going to define the scale for our height and most people are not zero centimetres tall hopefully. 222

I guess unless you're like a single celled organism or something, but they're probably not going to be 223

using our app. 224

So let's start off with a minimum value of maybe let's say 120, and maybe a max value of say, I don't know, 220 225

right? 226

So because we're in centimeters, we're talking about 1 meter 20 and 2 meter 20. You can obviously change 227

your scale if this doesn't make sense at all. 228

If all the people that you live around are like three meters tall, then feel free to change the scale. 229

Again these are things that work really well as constants. 230

So it might make sense to add it into our constants over here. 231

But for now I'm just going to leave it in here to make it easy to see how our slider is being built 232

out. 233

But feel free to create a constant and you'll be able to easily hit up one file and update those constants 234

if needed. 235

So you can see that our slider is still being highlighted as yellow, 236

and whenever that happens you also get a line in the right telling you what the warning is about. 237

You also get a little yellow box telling you there's problems or there's warnings found. 238

So in this case if we hover over it, it tells us that the parameter or the property onChanged is required. 239

So at the moment, we're still missing one more property which is the onChanged. 240

And you can see that just as I was typing over here, that it's telling us that it expect a function something 241

that takes a double as an input and returns void. 242

So we have to pass in a function into the slider just as we did for our reusableCard. In our onChanged, 243

instead of just having an anonymous function with no inputs, it actually expects a double value in there. 244

And that value is going to be the new value of the slider. 245

So you can see that in the docs it tells us that the onChanged is called during a drag when the user is 246

selecting a new value by dragging. 247

And if we click on it to see more information, you can see that the slider will pass the new value to 248

the callback and we will be able to tap into that new value inside the anonymous function. 249

So this is how they're recommending us to use the onChanged property. 250

So let's go ahead and do that. In our onChanged property, it's going to take a double. 251

And we're going to call it newValue. 252

And this is going to be the value that will be passed to us from the slider when the slider gets changed 253

by the user. 254

And then inside the curly braces here, we'll be able to tap into that newValue and we'll be able to use 255

it for our purposes of changing the height. 256

Now if we go ahead and hit save and when we run it, we can see it vaguely on screen but it's so dark 257

that we actually can't differentiate it from the background. 258

So let's go ahead and add some color to it. 259

Let's add an activeColor which is going to be a custom color as usual. 260

And in this case it's going to be 0xFFEB1555, and this will turn the left side pink and let's 261

go ahead and add a color for the right side as well. 262

So the inactiveColor and this is going to be 0xFF8D8E98. 263

So now we have a gray on the right and a pink on the left. 264

Now at this stage something very curious happens. When I click onto the handle of my slider and I drag 265

it around, because we're going to print the new value that the slider is at, we're going to see it inside 266

our console. 267

So you'll see the value change and I can go all the way down to the very bottom which is 120 and go 268

back up to 220. 269

But the slider itself is not really doing anything on screen right? It's not animating, it's not moving 270

at all. It looks like it's completely stuck. If it wasn't for all of these readings that I'm getting by 271

printing the new value, I would have thought that my slider is completely not working. 272

Why is this? 273

Well if we head into the documentation and we look at the onChanged property, you can see that it says 274

that the slider passes the new value to the callback. 275

So when I move my toggle around, it will pass the new value into my callback through this input here. 276

So I get access to that new value which is being printed inside my console. But it does not actually 277

change the state of my slider until the parent widget rebuilds the slider with the new value. 278

So how do we rebuild the parents slider with the new value? 279

Well the callback, 280

so everything in between these curly braces, should update the status of the parent using set state. 281

So the parent gets rebuilt in the way that they've shown here. 282

So for example here in their onChanged, they have a new value being passed into this callback here and 283

then that new value is used to update the value of the slider which is right here. 284

So this is how we're able to update the appearance of the slider. 285

Let's give it a go in our code as well and actually get our slider moving and animating. Instead of printing 286

the new value, 287

we're going to set the height property which is a mutable or a variable property that we have in here. 288

And it's also an integer which starts out at 180. 289

Once the slider is changed, we're going to set the height to equal the new value. 290

But the problem is that new value is a double 291

but height is an integer. 292

So let's go ahead and round it up to an integer so that can be turned into the nearest whole number. 293

And then all we have to do is call set state and make sure that the state of our slider gets updated 294

based on this height. 295

Now if we hit save and we check out our handiwork, then you can see our slider's moving and our text which 296

displays the height also, is also updating. 297

Perfect. 298

We've got all of our functionality of our slider card now enabled.