Original subtitles

So we learned about stateful widgets, the other type of widget you can create in Flutter which differs

from the stateless widget in that it can manage internal data.

Well you could do that in a stateless widget too, you can do that in any Dart class after all

but in a stateful widget if you manage that internal data actually in a state object then which is connected

to the stateful widget, you can update this in a way that it's picked up by Flutter and that UI also

updates.

That's nice,

now let's dive a little bit deeper into Dart at this point,

let me show you one alternative syntax here which I would recommend using because it is a common pattern

and a common practice and you'll see it in other official examples and we'll also use it throughout the

rest of the course. In Dart,

you can have variables on classes like question index and you call these variables properties then and

you can have functions and classes and they are called methods then.

Now that's not new.

Now in Dart, you also can put your code into more than just one file,

we only used one file thus far but actually, I'm already importing something from that material.dart

file which is exposed by the Flutter package.

Now as a default in Dart, every file is its own enclosed ecosystem you could say, it's own so-called

library which can be a confusing name because we have that lib folder and it makes up a a bigger library

but in the end, every file in that folder is its own mini library.

These files can still work together with the help of import statements but you can control what can

be imported and what can be accessed in another file

and for example, you might want to restrict that this state object can be used and changed from inside

another file.

Now we only have that file here, right

but we'll soon add more files and then we want to make sure that we can maybe use our widget in other

files because we are for example also using the MaterialApp or the scaffold widgets which we get from

material.dart but in these widgets, we might not want to grant access to all the properties that are

in there or to this class, we might not want to grant access at all from inside other files because the

only thing that should use this class is the MyApp class here which needs the MyAppState class to

create that state.

Now of course, we as a developer, we are writing our own code and we can simply not use that class anywhere

else and we're fine

but if you are working in a bigger team or if you are sharing your application with other developers, then

you might want to put some protection into place so that the MyAppState can't be used or manipulated

from outside this file because if it could be manipulated from outside, then maybe you could change

question index from inside another file and that wouldn't be picked up by that widget correctly and your

app might therefore get out of sync or you might introduce weird bugs. Now to inform Dart that this

MyAappState class should only be usable from inside the main.dart file,

so from inside the file where it's defined, you add a leading underscore in front of the name and of course

you also have to add that in all the places where you use that name.

Now this might look like a cosmetic thing but actually, that leading underscore is a special syntax

in Dart and it turns this class which normally is a public class, which means it can be used in

any file which imports the main.dart file, this turns it from a public into a private class and now

MyAppState is a class that can only be used inside the main.dart file,

so in our case only in that MyApp class here. And that's what I want because since the state works together

with this class, it should really only be usable inside the same file

where the MyApp class lives.

So that is a common syntax you see

and on the same page, this variable and this method should also only be accessible from inside that file.

Hence we can add an underscore in front of question index and then also again in all the places where

we reference question index to signal that this should be a private property of this class here,

which means we can't access it from anywhere else in our project and let us do the same for this answerQuestion

function or method as it is then called. Let's add an underscore in front of it and then also

add an underscore here. Again, this is basically a convention I'm following here,

however it's not just a cosmetic change, it really has an impact,

it is a syntax understood by Dart.

It doesn't change the way our app works right now but now, we really ensure that MyAppState and all

the properties and methods in there can't be used from anywhere else

than inside this file and that is really important.