Flutter 12 - Dealing with Errors Video Lecture | Flutter: Build beautiful native apps in record time - App Development

everybody it's Brian welcome to the 12th
flutter tutorial we're going to continue
our journey learning Dart
now one thing you should know is that we
skip something here we went from control
flow statements of classes we skipped us
exemptions and there's a reason why I
did that
a lot of tutorials out there we'll talk
about exemptions and say exemptions or
classes but we'll cover classes later
which means you are completely mystified
and confused by exemptions the whole
time
I wanted to reverse and talk about
classes first that way during this
tutorial there should be zero
misconceptions about what's going on
under the hood and we can actually work
with the advanced code rather than
jumping back and forth so without
further ado whoops exemptions dart
provides an exemption and error types
and there's a distinction between the
two that you really need to understand
here so the exemption class is basically
it's information that we are conveying
where the error class is an error object
that represents the actual failure that
we should have avoided so that's kind of
the distinction between the two and dart
has a little bit of I should say little
bit a couple predefined subtypes
and you can actually build your own
which we're going to do in this tutorial
to do that not just build your own but
to just handle errors in general you
have to know how to throw errors and how
to catch errors think of it like playing
with a ball you ever play you know with
just like a baseball or football little
kid you throw it and catch it you have
to understand that program execution is
very linear meaning if at any time it
breaks your program will stop I'm sure
you've seen programs that have
completely died alright so what we're
gonna do first is we're going to say
we're going to catch an error now
usually when you catch something it's
not a great thing right so we're going
to say test a and then we're going to
print and say
we're gonna do intentionally make an
error here just because I want to show
you what happens now if you see this you
can tell that somebody ran out of coffee
there was some issue going on not only
is this not even a variable but it's
undefined completely we don't even have
a test class in here anywhere so let's
just run this and see what happens first
off you can see the optional application
is really trying to warn us hey bad
things are gonna happen if we do this
but if we actually just run it you'll
see that boom whole thing just explodes
and life as we know it has you know
ceased to exist
no top-level getter or setter tests
declare the entire application just hard
crashed we can look at the stack trace
here and we can see you know tracing it
back up to here this is the reason why
no such method error now that's a key
word right there err we were just
talking about exceptions and errors and
the error is the actual thing that
happened so really what's going on under
the hood here is there is no such method
and that's you know kind of preceded by
the error name here so that's a typical
format for errors you'll be able to see
you know division by zero error you know
catastrophic failure error blah blah
blah there's any type of error you can
imagine but that's really the naming
convention and we're gonna make a custom
error before we do all that we're gonna
actually learn how to catch these things
so first thing we're going to do is I
just want to print some things out here
oops
this may look a little weird at first
but just trust me on this
whoops
all right so this probably looks a
little bit crazy we're going to go over
this in depth because it's kind of the
bread and butter of this tutorial here
and then we are just gonna say done so
we know this is gonna crash because it's
got the same code in here what we're
going to do is we're gonna wrap it
around what's called a try catch finally
block
so there's try here's catch and here's
finally these do three distinct things
try we'll try to do something for
example try to run a marathon try to get
a new job try to find a wife or a
husband or whatever you're doing try to
make a cake catch means that an error
was thrown and we need to catch that
think of it like a big fishing net it's
catching the problem and then finally
finally does not care what happened up
here finally it's just off doing its own
thing saying okay I don't care what you
are doing I want to do it my way so if
you want to execute code and regardless
of whether or not there was an error you
would put it down and finally so let's
run this you can see sure enough
starting trying error no such method
error same error and then we've got our
error message with the rest of it and
then tried calling tests and then
finally we're done and you could
actually just you know get this right
here
so if we just don't care about that air
okay air don't care so we're just gonna
keep going on and our program finished
with exit code zero which means that it
did not crash it ran fully it did
exactly what we expected it to do and
life is good some things you should know
about this though it is very bad
programming to make what's called a
silent error meaning nothing happened
here because if that error became
catastrophic it could potentially crash
your program so it's always good to
either log or let the user know or do
something that's why randomly sometimes
you're new using a program you'll see a
message saying error occurred blah blah
blah and you're like I don't care leave
me alone I just want to do my work but
that's the programmers way of telling
you hey something under the hood went
boom and you should be aware of this so
that is how you catch an error now we're
going to actually raise an error and
this is actually a little funky if
you've never done it before
alright so we're going to do our try
catch block here
if you're wondering what this e is the e
is an automatic variable it's actually a
representation it's the error class or
the actual I'm sorry not the air class
the exemption class that's in memory and
we're gonna show how to build one of
those here in a little bit but right now
I just want to flesh this bad boy out so
bear with me here all right so we've got
our try-catch block in all of its glory
and we're just going to do some simple
logic here we're gonna say if a not
equal b then throw throws a special
keyword and then do wow that's bad when
you misspelled ooh I'm just I'm still in
a turkey coma
yesterday was Thanksgiving I'm just like
not with it all right so we're going to
catch the air and right now this is
what's called a silent exemption because
we're not doing anything with this so we
want to actually fix this problem here
something went boom and we want to just
because I actually print out our error
message and print complete completer is
anyone a word computer let's actually
just while we're in here fix the problem
Wow that wasn't playing around when I
said I was gonna make a lot of errors in
this tutorial alright so we're going to
geez there's even another one up here
alright so we're going to call this out
and let's just comment this guy out here
so we know that because those numbers
are not equal that is going to throw
this error now if you look at that
you're gonna stop and say Brian that's
not even an exemption class that's just
a string you're absolutely right and
dart you can throw any type of object
you want that will be I think under the
hood it gets wrapped into their
exemption class and bundled up nicely
and converted into a message I think all
it really needs is a two string function
under the hood
alright so let's run this to find out
what happens and it says something went
boom not the same so that is how you
throw an error now why would you want to
ever create an error I mean why are you
intentionally doing this it's simple
sometimes you want to know that
something bad happened and you cannot
continue execution for example let's say
you're trying to open a file but that
file does not exist you would throw a
file not found exception or a file not
found error and say hey boom file not
found a big ugly message pops up on the
screen and the user goes oh crap wrong
file name so one thing you should also
know is that you can actually cascade
error messages so let's make a void
internal and we're just going to say
throw note program just doesn't feel
like working today
and in here we are going to just call
the internal now let's see what happens
here let's comment this out now when we
run this boom it crashed so we're
calling out err
as you can see here outer is calling
internal internal actually throws the
air so you don't actually have to catch
the air where you're throwing it that is
where some tricky programming comes in
because you never know where an error is
actually going to come from so that is
how you would actually catch that
program failed what I'm getting at here
is sometimes you'll use classes and
libraries and things that other people
have built and if an error occurs or an
error is thrown you may have to actually
catch those so you may not know that the
error could potentially exist but you're
gonna have to deal with it at some point
and that right there can get to some
real headaches and that's actually one
of the biggest problem our biggest
reasons why programs crash it's you rely
on somebody else's code you're not fully
comprehending what's going on under the
hood and then problems ensue alright so
let's just say we want to create our own
err object here so we want to implement
the exemption class and we want to just
kind of flush this out
say final string want a message and
we're gonna make this optional just
because we're gonna override this
whoopsie fat-fingered that one very
simple I mean very easy you can make
this extremely complex we've had our
conversation about classes and you know
flow control all that you can do a whole
lot of stuff in here like maybe a custom
thing you'd want to do is dump this to
an error log maybe you would want to
pipe it out to a website maybe you would
want to take corrective action and fix
something or pump something out to a
database table but for this example
because we haven't covered most of that
we're just gonna make it very simple
very easy to work with so we have our
color air close that close that
so now we're going to import our color
error and we are going to make one
horrendous ugly looking piece of code
here now I did promise we would cover an
um so let's actually without further ado
here oops we're going to make an enum
and we're going to call it color there's
a reason why I didn't make an entire
tutorial just on an dumps because they
aren't just dead simple so an enum is
basically a enumeration of something an
enumeration of values it's a zero-based
index so this is 0 1 2 and it's super
simple to work with and we're gonna say
void cats because I like cats then we're
gonna say color let's call this color
notice how the upper and lowercase are
two distinct things and we're going to
say try and then we're as a catch and
finally
now here we're gonna actually check
we're going to say if color not equal
color dot and then notice here's our
enumerated values and the values is
actually in there as a function you can
get a list of colors we've talked about
generic programming a little bit at a
high level it's going to be probably
actually the next tutorial but you can
see how we have any list of colors much
like in the past we've had a list of
strings or integers so if the cat is not
black then we're going to say throw new
whoops cat err why are you not picking
that up here oops it's not cat here geez
color err told you Thanksgiving it got
me all messed up must be black because
black cats are pretty awesome some
people think they're bad luck and I had
black cat for years and years he was
probably the best cat I ever had and
then we are just going to leave it at
that
we're going to say print
so if we do a brown cat why not
everybody likes brown cats run this you
can see that unknown error must be black
and probably would have done that so we
can see that nothing really happened
here it's actually just Pat the cat
whoops let's run this again you can see
how pet the cat never gets printed
because we had the unknown error of must
be black now this is a problem why is
this an unknown error we actually know
it's an error because well we threw it
so what we're gonna do here is we're
going to add another called
on oops we're going to say on color
error and what we're doing here whoops
we missed a little bit of verbage here
we need to do something with that so on
color error
catch there we go we're gonna catch the
exemption and then we're going to say
prints you selected the wrong color so
what we're really doing here is we're
saying hey we want to know that Brian is
horrible at spelling we want to know
what type of error and so you can
actually create your own error types
here using error classes and let's
actually make another error here just
because and we'll say cat a and we'll do
our faithful little error here
there's no cat type therefore you know
we know it's gonna extremely explode
here and that would be a programming
error anyway so so you selected the
wrong color let's actually make this
black now our little program should work
meaning it should pet the cat but then
we've got this guy right here let's see
what happens
bang suddenly we get an unknown error so
that is a good example of a couple
things a how to use an enumeration or an
enumerator be polymorphic programming we
see that this algorithm does two
distinctly different things based on the
input and how to use custom error
handling it can get much much more
in-depth than this you can make just you
know dozens and hundreds of these
typically what I do is I will write a
function and then in every function
especially production-quality code will
immediately be wrapped with a try catch
finally and then I will put my specific
error cases exactly how you see in this
tutorial that way if I expect you know
like file not found or disk space full
or something like that I know exactly
what happened and then I always have to
catch all at the end that way if
something completely out of left field
happens my program doesn't crash it just
says hey unknown error occurred so that
in a nutshell yes error handling I hope
you found this educational entertaining
be sure to check out my website boy
drums comm for the source code for this
and all other tutorials the lobby album
github in case you're wondering I
actually gotten quite a few questions
about that I put a lot of tutorials out
here and when you click the link it
actually you know puts them in as
tutorials so I've got like cute Java
Python flutter blah blah blah if you
just crack it open you'll see the
lessons are in there these are all sub
modules so you can go right into the sub
module and get the actual code if
there's no code it'll just have a readme
file I try to keep it in there that way
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that's it thanks for watching