Swift Error Handling - Do; Try; Catch - iOS Interview Question Series Video Lecture | Learn and Understand: Objective C Tutorial (Hindi) - Web Development

what's up everybody we're back with
another video in my iOS interview
question series and today we're going to
be talking about error handling and more
specifically the due try-catch syntax
we're going to talk about when to use it
how to use it what it is all that stuff
let's dive in alright so in order to
show off the do try-catch syntax and how
to use that we're going to use a pretty
common example and that is logging into
an app now forgive me I didn't go ahead
and build a fully functional you know
login screen that is connected to a
back-end so we are going to kind of
fudge some stuff a little bit but you'll
get the idea so as usual I'll walk you
through my starter project the
storyboard here is pretty
self-explanatory just have an email text
field password text field and a login
button I do have some helper methods so
I have a string extension that is a
regular expression to validate an email
this is to make sure the email is in a
valid format so in case you write just
you know Shawn Allen at Gmail you know
without the com this will catch that
kind of stuff feel free to steal this if
you like
and my other helper is for just showing
alerts just so I don't have to type this
code over and over again you'll see this
in action later and then going to our
view controller just have the outlets
for my text fields and then it's going
to clean it up a little bit and then I
have my viewdidload method and then I
have the login button
tapped IB action so that is a quick
rundown of the project but before we get
into all that let me show you a quick
example of a function that requires do
try-catch now the key here is that some
functions will do what's called throws
let me show you it real quick let's say
we're trying to convert an image into
data so we would say let image data
equal data and then we initialize that
with contents of and then URL now here's
the key see how this says data contents
of URL and it says it throws here's
another example of a function that
throws anytime you see a function that
throws you're going to be required to do
a try and most likely a do try cache
block so this is the key here is looking
for this keyword throws alright so let's
go ahead and pick that function here and
let's go ahead and just do a URL you
know file path with string and then my
file path what this does if you're not
familiar with this little bit of code
this is basically if you have an image
at a file path in your app this will
just convert that image into data so you
can use it in other ways this is just my
example of a function that Apple uses
that throws so we can show the do try
catch block after we get through this
quick example I'm gonna do a custom
example so you'll really understand it
after that but this is kind of like the
intro so you see I get two errors here
one just says I'm not using the
variables so let's ignore that one and
click on the two and see here it says I
can call throw but it's not marked with
try and the air is not handled so if you
choose a function that throws Xcode will
actually not let you proceed unless you
handle it with a do try catch block so
let's go ahead and implement that again
this will just be a quick intro and
we'll dive into it more later so you
want to start with do open and closed
brackets then we're going to go ahead
and cut and paste this into into the do
block now we have to preface this with
trot here over here so what we're trying
to do is we're we're trying to get data
contents of URL that's what we're going
to try and then if that try fails then
we go into the catch block so it's
actually very similar conceptually to
like an if-else statement so one way to
look at this would be you're telling it
to try this little bit of code if it's
successful you know now you have the
image data variable you can you know do
whatever you need to do with image data
because you have it so that as long as
you're in this block that means the try
it was a successful try so to speak
everything worked out fine if the try is
unsuccessful and you throw some errors
that is when you go down into this catch
block and here's where you need to
handle the air appropriately so in this
case you would just do you know maybe
you would want to print error dot
localized description and all this
warning is because I'm not actually
using this variable yet so let's go
ahead and actually run this to see what
happen to console okay now as you can
see I just reuse my login button tapped
function so I'm just going to tap this
again and you'll see the error pop up
and it went into the catch block because
I printed the error localized
description says the file you know my
file path because this is obviously not
a correct file path and something I just
made up couldn't be open because there's
no such
so in real life if that happened now
you'd probably actually want to show you
know a pop up show show pop up with air
this way the user you know gets a
description of what actually happened in
what's going on you don't want to just
print to the console so okay that was
just a real quick down and dirty
introduction of the do try catch again
we're gonna go through a more in-depth
example here in a second that's more
custom using an enum with error cases
but before I gave a more complex example
I wanted to give you guys this
introduction so you at least had an idea
of where things were going so again the
key here is this function here this data
contents of URL it is a function that
does what's called throws and any
function that throws you have to wrap in
and do try catch block with a caveat you
can just do a try sometimes we won't get
into that right now but just just know
that if a function throws you're gonna
have to handle the air somehow most
likely in a do try catch block so again
you wrap it in this do block this part
right here is if the try was successful
and what's in the catch block is if it
was unsuccessful and you handled that so
that is the basics let's dive into a
more complex example so in this more
complex example we're going to handle a
couple of special air cases that have to
do with our login screen in order to do
that we're gonna put our error cases in
an enum so let's go ahead and type that
out real quick alright so here's my you
know you see I have a called it's called
login error it's conforming to the error
protocol and then I have the different
cases and obviously when you're filling
out a form there are more cases but
these are just some simple ones I wanted
to demonstrate for you so we have the
incomplete form this is when you know
they forget to type in an email or
password invalid email is for like I
mentioned before when their email just
isn't right like they forget the dot-com
at the end or they forget the @ symbol
and an incorrect password length as you
know sometimes you have to have a
password that's eight characters long so
this is handles that error case so again
just a couple of the typical cases
you'll handle during the login process
so now let's go ahead and create our
login function that's going to throw an
error remember I said this is the key if
a function throws that's when you have
to handle it with a do try catch block
so we're gonna go func login and then
here's the keyword throws and then you
do your normal open and closed brackets
so now we've designated our login
function that it throws and we know
bori that uses this function is going to
have to handle the air so if you're
wondering when you would want to use
throws on a function I like said if
you're writing a function and you know
it's gonna be able to throw a couple
different errors you want to handle that
that's probably a good case for that if
your function doesn't throw any errors
then you don't use throws so I'm gonna
create some variables real quick that's
gonna make the rest of the function
easier to read and so forth so you know
text field text and then let password
goal password text field dot txt again
these are just convenience variables you
don't necessarily have to do these now
like I said I didn't build a fully wired
login screen that connects to a back-end
or anything like that so this is kind of
where I've to fudge it a little bit so
right here let's just you know pretend
this is great code that logs in my user
it really is amazing okay so that is my
code that logs in the user again forgive
me for fudging that but before we log in
the user we want to check for all these
errors that we just talked about up here
on lines 13 through 17 all these login
errors so let's go ahead and start doing
that so the first thing we want to check
for is this incomplete form and what
this really means is that you know is
the password text field and email text
field empty so let's check for that so
if you know it's empty or password that
is empty and again this email and
password variable is a convenience
variables I just created out right above
it so if this is in fact the case if
email is empty or password is empty this
is where we want to throw an error and
the error we want to throw is login
error dot incomplete form you see here's
our three errors in our enum
so we're gonna throw a log in incomplete
form and then you're gonna see how this
all ties together when we actually call
the login function in log in button
tapped up here on line 26 and 28 we'll
get to that that's the second half so
that's the first check we want to do
we're checking to make sure our email
and password fields actually have
something in them if that isn't the case
let's go ahead and make sure the email
is valid so if email dot is valid and
again this is valid property that I
added is part of my extension on email
which let me show you here
remember this is the helper function I
talked about it's using a regular
expression to make sure the format of
the email is correct again to make sure
there's an @ symbol make sure they
actually have the comm etc again feel
free to steal that from the source code
if you want it's very useful I use it in
pretty much all my projects so if email
is valid email actually we need to put
the bank operator in front of that
because we want to see if it's not valid
so if email is not valid then we want to
throw login error dot invalid email
makes sense right and then the last one
the check is we want to make sure our
password is of the correct length so if
password dot okay I'm in Swift three
right now so floors coming out right
around the corner insta before you can
just do password dot count but since
we're still in Swift three characters
dot count
so again if you're watching this in
Swift for you don't need password
characters that count just D password
count is less than eight let's say our
password requires eight characters if
that is the case you guessed it we're
gonna go ahead and throw login error dot
incorrect password length and then if
all these checks pass then we're gonna
execute my amazing code to login the
user okay so a quick recap here on lines
thirty to forty nine here we have our
login function and again the key here is
it throws so now I'm gonna have to
handle it in the do try catch block and
then you're gonna see we're gonna have
to catch all the different error types
so the different error types are you
know this login incomplete form login
error invalid email and then login error
incorrect password length so let's go
and call our login function in login
button tapped so when the login button
is tapped we're gonna go ahead and call
the login function now again Xcode is
gonna yell at us hopefully yep and like
I said if you designate your your
function with throws here Mexico's not
gonna let you not handle the air that's
why you want to only use throws when you
for sure want your errors handled it's a
very dangerous might be a strong word
but you know use caution when using this
function so as you can see we get the
air can call throw but it's not marked
with try and the error is not handled so
basically Xcode is saying yo dude handle
this error let's do that so again we
just go ahead and wrap it in the du and
then we want to try
cool and then we want to catch and then
like you saw before I just had the one
catch block and this was kind of like a
catch-all for the heirs however in the
case of our login you saw we had three
different you know login types
so again just it's very similar to an if
statement so this is kind of like a
else--if so now we want to catch login
air dot incomplete form when if if that
is in case the air that it catches what
do we want to do we want to show an
alert specific to that air again
remember I showed you this alert helper
at the beginning this is just so I don't
have to type this over and over again
and you'll see how this comes in handy
and again feel free to steal this I use
this all the time too so I just do alert
dot show basic and then the title here
I'll just put in complete incomplete
form and then the message I will be
please fill out both email and password
fields and the view controller that is
presenting this is going to be self it's
going to be this view controller so that
was catching the incomplete form error
and I'm gonna go in fast-forward the
typing for the next two and we'll review
it
alright let's review our do try catch
block so again because the function
login throws we have to handle it with
do try catch so what are we doing we're
trying login so if login was successful
this is actually probably where we would
you know transition to next screen
because it was successful but again
everything in this block right here is
what happens when the try is successful
and then all the catch blocks are when
the try is not successful and then you
can either only use one catch block to
have like a catch all like a general
error or if you have specific errors you
can do what I'm doing here in catching
the specific errors and in handling them
appropriately which will see an action
when I run the app here in just a bit
because we're just about done so these
are all very similar like I said I'm
just catching the invalid email so again
this is it the format of the email is
invalid and then I'm just showing an
alert with the title invalid email
format in the message please make sure
you format your email correctly and then
down here with the incorrect password
length again just showing an alert with
the title password too short and then
you know password should be at least
eight characters and finally the catch
block does have to have a final catch
that's not specific to an error kind of
a catch-all if you will that's why this
error message is pretty generic you know
just as unable to login there was an
error when attempting to login so again
here's an example of handing some pretty
typical login errors let's go ahead and
run the app and see this in action okay
so I got my app up here let's test the
first one like I'm not gonna enter
anything empty so hit login and then you
get the pop-up incomplete form please
fill out the email and password fields
okay that's exactly what we expected now
let's just type you know Shawn allen dev
at gmail no com we'll just forget about
it and here's my password login and I'll
email format you know so you see we're
getting the correct errors because we
handled them with the specific error
cases let's do let's go ahead and add
the com so now I have a valid email and
then they should do a password of three
characters because that's obviously too
short hit login
invalid email format look I missed that
but my air is caught it I put a space
for some reason up here in Shawn allen
dev so if I get rid of space now my
email format is correct but my password
is too short
so I should get password too short
password should be at least 8 characters
and then now if my password is a
characters nothing is going to happen
because none of my errors got caught as
you can see I'm tapping it nothing
happened no error pop-ups this is where
my magical code down here is getting
executed because everything went fine so
there you go that's the basics of the do
try catch syntax hopefully if you get
this question in an interview you'll
crush it if you have any further
questions about it go ahead and leave a
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