Flutter 8 - Class Constructors Video Lecture | Flutter: Build beautiful native apps in record time - App Development

everybody this is Brian and welcome to
the eighth tutorial on flutter and dart
we're gonna go ahead and continue our
conversation on classes and let's
actually call this class - well it's
called classes - there we go
and while that's churning away we're
going to look at what we're going to
cover today is called constructors what
our constructors well whenever you have
a class and you create a new instance of
it something has to happen in the
background and that's what we're going
to cover today that background is called
a constructor it's when the object is
actually constructed in memory and to
kind of solidify that main point here
what we're gonna do is go out here and
we're gonna say new house that dirt and
I'm kind of doing this unscripted
meaning I do not have my notes I'm just
kind of typing so there probably be some
errors and maybe some goof ups and we're
gonna keep this class very simple
because we're not focused on the class
itself but rather the constructor so
with equals zero and length yeah all
right so we've got that and we need to
import house dart and we're going to say
house house 1 equal new house so we've
done this before we know what this is
it's just going to make a new house
what's going on under the hood here is
it's looking at your blueprint and
saying what do I need to do and then
it's going out and making a new instance
of this and let me turn my music down I
can't even hear myself think here I have
to keep music on the background or my
headset shuts off it's kind of like an
annoying feature but anyways so it's
saying what do I need to do here now it
knows what it needs to do by way of
what's called a constructor
and every class has what's called a
default constructor meaning it just
magically exists and if you're wondering
where it is it's right here boom so
somewhere in there under the hood on ODU
is a function that looks like this and
it probably just says return would be my
guess now all that is it's called the
default constructor but it doesn't exist
in a manner that you can see it it's
under the hood and you'll never access
it
it exists solely for the compiler now if
you want to actually play around with it
you can add it in constructed so what
happens now is when we run this you'll
see it suddenly says boom constructed
where's that getting called we haven't
changed anything in our main so it's
saying all right here's the blueprint we
want to use here's the name of the
variable we want to use make a new
instance of that and use this
constructor in this case it's saying use
the default constructor you can tell us
a default because there is a you know
parentheses with nothing in it that's
called the default constructor to
solidify that we're going to make a yet
another one and it's going to say
default constructor already defined
meaning it knows that's the default
because it's just an empty set of
parentheses okay that's great what do
you need a constructor for what's the
whole purpose of this thing constructor
says hey what if you want to initialize
a set of variables before you actually
use them in this case let's say we want
to use the width and the length we don't
want those to be 0 so whenever a new
house is created we want to
automatically set those variables and we
want to force whoever's using this class
to set those variables so we're going to
say at W and L
and we are going to say with equal W
length equal L and we're going to print
these out yeah fat-fingered that one all
right so when we go to run this now you
guessed it it's gonna crash bang why no
constructor house with matching
arguments declared in class home
whatever you see that your first thing
should be look for the new keyword and
then you'll see the error right next to
it it says two required arguments but
zero found well how do we know what
those arguments are by looking at the
constructor we have two integers we have
to do deal with here so let's say we
want it to be 5 by 9 bang so now
suddenly we have a width of 5 and a
length of 9 we can actually take that a
step further and make this you guessed
it optional let's say this is 5 let's
say this is 9 well let's go back to our
program here and notice how we've got to
actually name them out now doing this
for illustrative purposes and we want to
say we want the width to actually be 6
instead of 5 and it should be 6 by 9
there we go so that in a nutshell is how
to deal with the constructor and why you
need one now special note dart does not
have at least I haven't found the
concept of ad constructor and what a
Deconstructor is is when the class is
actually being destroyed under the hood
there's this thing called garbage
collection and garbage collection is a
very complex topic but it's very easy to
understand when you are creating
variables right here the use of the new
keyword you are putting that out on
memory
and you're putting it on something
called the heap under the hood it may
actually be somewhere else but the heap
is just a giant free store memory think
of it like a giant warehouse you're
basically opening the door of the
warehouse and throwing a house in it and
saying here store this somewhere I don't
care where the problem is when your
program dies or should get shut off or
the user closes it or whatever that
house still sits out memory so you need
to actually delete that garbage
collection takes care of all that mess
for you all you have to do is say I want
a new house and I want you to put it
somewhere I don't care where and let me
access it when I need it and then when
your program is closed or if that
variable goes out of scope let's say we
have a function up here let's move this
up here whoops and that's called test
does the same thing however something
drastically different is happening in
the background here what we're going to
say and I'm just going to put in
comments here and then flagged for
garbage collection that's what's really
going on in the background here so when
this test function is called it's going
to go up and it's going to say create a
new house bang then it's going to call a
constructor whatever the constructor may
be then in the background it goes out of
scope what happens is everything that
was created in the scope is now flagged
for garbage collection what that means
is it is no longer being used by the
program and the program thinks it's safe
to delete it out of memory now if you've
ever worked with older languages like C
and C++ if you try to access something
that doesn't actually exist in memory
your whole program just goes kaboom and
crashes garbage collection takes all
that headache away meaning you don't
have to worry about it you just have to
create it it'll take care of it and then
when you no longer using it anymore
it'll get rid of it for you if your
program crashes and dies an ugly death
it'll
you know all that memory for you you
don't have to worry about it so why does
all this matter to you because all of
this is being done under the hood now
there's a special case what if and just
follow me here you want to actually do
something with this and then close it
out
you really should and there's probably a
million other ways to do this
dispose your class meaning and this is
very similar actually built in I think
and some other languages kill me now you
would call that before the end here so
you would say you may be tempted to do
something like that now why would you do
this let's say you opened a file or a
database or network connection you would
want to actually close it here so you
would say close all open connections or
close all the things that would be a
good way to do it but remember under the
hood garbage collection is being called
so if you forget to do any of this it's
going to happen automatically and you
don't have to worry about it that's a
very nice concept there are more
advanced topics to it but that in a
nutshell is constructors and
deconstructors I hope you found this
educational entertaining and thank you
for watching if you're so inclined you
can get the source code for this and
other tutorials out on my website just
go out tutorials tutorials click on the
tutorials link and go out to github and
I will have all the code out there thank
you