Flutter 9 - Extending Classes Video Lecture | Flutter: Build beautiful native apps in record time - App Development

everybody this is Brian welcome to the
drumroll where are we we are on the 9th
flutter tutorial we're gonna continue
our conversation on classes here did
have some interesting feedback from this
guy here president president one dart is
dead why are you doing Tut's on this uh
actually I don't think dart is dead I
think darts actually just getting
started I think it's very new and varied
in its infancy and I'm hoping Google
grows it quite a bit
so that is why kind sir I am doing
tutorials on this so we're gonna
continue our conversation about classes
if you're trying to follow along with
the official documentation we are on
extending a class so extending a class
use extends to create a sub class and
super to refer to the super class what
in the heck does that mean we're going
to cover that over the next three
tutorials and I'm going to do a little
bit here I'm gonna actually cover how to
extend the class abstract classes and
mix-ins
but we're gonna have to kind of break it
up to a few tutorials so let's just get
going with this one here and let's do
next yeah gotta just my office chair
here what's up with that
and while that is calculating the
meaning of life to everything I'm going
to actually just this chair man it's
like driving me nuts
all right so what we're gonna look at
here is classes and inheritance and you
may have heard of inheritance before in
other languages so the first thing we're
going to do here is we're going to make
a new dart file we'll call it mammal and
in mammal we're gonna make a class and
we're gonna say bull as backbone and
we're gonna make another bowl as hair
because the mammal has to have a
backbone and hair there is our beautiful
mammal class in all of its glory so what
we want to do now is actually make
another one and we're gonna call this
feline for all your cat lovers out there
we're going to import the mammal class
and want to say class feline and this is
where the extends keyword comes in and
extends mammal so what have we actually
done here if you've not familiar at all
with inheritance and what I'm talking
about here is we're extending this class
it used to be the keyword used to be
another languages inherit or it was just
a semicolon so what we're doing is we're
taking the mammal class and actually
adding to it or extending it so we're
gonna say pool has claws true because
most cats have claws and then we're just
gonna make this void test I'm gonna say
assert you notice how intellisense is
smart enough to know what's going on
here has claws has backbone has hair so
you can see the other properties here so
we're just going to assert that this has
a backbone and we're going to assert
that it has hair now what a certain does
is it actually throws an error message
if this tries anything and it doesn't
evaluate to true so that's just a test
function we're gonna leave in there for
now but what we're going to do is take
this even further and show you that you
can do some really cool things with
inheritance here so now we're going to
take our feline class that we made we're
making a class and call it lion and
we're going to say avoid roar and let's
just do this so tomorrow is Thanksgiving
while I'm typing here and I'm actually
kind of nervous because I'm cooking a
turkey for the first time yeah it's
gonna be interesting here some people
have told me to cook the turkey upside
down some people have told me to not
stuff it some people have told me to
stuff it some people told me to do all
of these crazy things one person told me
to get like a surgical syringe and
actually inject things into it I'm
guessing that's how you actually like
calm down your guests you just like
inject like some sort of sedative into
the turkey all right so what we're doing
here is we've got our test and we've got
our roar function now let's actually
test this whole thing out let's get rid
of this
and let's say lion Simba why not
actually you spell Simba oh well equal
new lion and we're gonna say Simba dot
and you notice how BAM it has all these
other properties here so we can actually
call tests we can actually make it roar
although I don't think Simba would roar
but come on keyboard you can do it I
believe in you and he's roaring and the
test ran we can actually go back here
and this is uh this is a tricky thing
about inheritance let's say back in the
mammal class here we flipped those two
faults then we're gonna say not all
mammals have here because I'm changing
everything well you can see how felines
test is not going to just explode Lions
test is now just going to explode so
let's run this and see what happens
and you can see unhandled exemption
failed assertion meaning our assert
statement evaluated defaults has here is
not true and it's saying line seven
number twelve so there's seven twelve
BAM that's where our issue is right here
and then we can kind of do this you can
see how it's in lion test Dada Dada what
we can actually try here is play around
with what's called the superclass and we
can say super whoops dot test so what is
this actually doing super actually grabs
the class that you're extending in this
case feline and it's going to call test
here in feline
so let's actually do this print testing
in feline
yeah let's do this print testing in Lion
that way we know exactly where this is
coming from here and you can see how its
testing in Lion testing in feline
because this is being called but then
we're calling super test which is
actually calling in feline
now this assert function is actually
failing right here saying bla bla bla
line 912 is not true and we can see how
it's calling feline test right here and
it gives you the path to the file I
think yep you can actually click on it
takes you right to it so you can follow
this is called a trail here if you will
or a stack trace you can see that it's
being called here it's being called here
and finally boom that's where the air is
being called so that is the tricky part
about inheritance is if you make
something and other people inherit
classes and extend them you can really
break a lot of things and so you have to
be a little careful and you have to
almost not trust what you're extending
and that is where overrides come in
right so we should just put over right
there so we know what we're doing here
this is just a declaration telling us
that we're overriding this what we could
actually do is we can actually put a
setter in here and set that to true but
I'm not going to mess around with it I
want to leave this as is so you
understand the some of the pitfalls that
can happen when you're doing inheritance
so to kind of recap here we have our
lion class the line parrots from feline
feline actually inherits from mammal and
let's flip this back to true that way if
somebody downloads the code it's not
gonna have an error and I don't get 100
hate mails so what is mammal really
doing here mammal is considered the
route superclass if you want meaning
it's not extending anything else it's
just the bay
class if you will all of these
properties then flow up through all of
the other things that are inheriting it
so that is inheritance in a nutshell
short simple to the point but a very
powerful feature you can do a lot of
cool things with this but you can also
paint yourself into a corner so
definitely structure your programs well
that's it
thank you for watching I hope you found
this educational entertaining if you're
looking for the source code for this and
other tutorials going to my website boy
drums calm you got to get hub and
download all the source also there is a
toy drums Facebook group with 1700 other
programmers out there that can
definitely help you if you have any
issues