text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
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and decide whether or
not we want to update, | 3,245.37 | 2.77 |
and if we return false from this
function, the update cycle is over. | 3,248.14 | 3.74 |
It won't render. | 3,251.88 | 2.2 |
And so if in this function we wanted
to check if that count were odd | 3,254.08 | 6.26 |
we can abort early. | 3,260.34 | 2.05 |
And so let's actually do that. | 3,262.39 | 1.61 |
So let's have this method
called shouldComponentUpdate | 3,264 | 5.784 |
and first let's just return false. | 3,269.784 | 1.416 |
Now what's going to happen here? | 3,273.857 | 1.333 |
Nothing. | 3,277.91 | 0.88 |
We can see that it's receiving stuff. | 3,278.79 | 1.74 |
So see down here in the logs
we're receiving this thing called | 3,288.46 | 4.94 |
updating every second. | 3,293.4 | 2.83 |
But the UI is just stuck
at 0, it never updates | 3,296.23 | 2.592 |
and that's because we're saying
hey, should the component update? | 3,298.822 | 2.708 |
No we shouldn't. | 3,301.53 | 1.59 |
And so React is saying,
OK, then we're not | 3,303.12 | 1.98 |
going to call this render method
and nothing changes over here. | 3,305.1 | 3.92 |
And so how may we change this is such
that it only counts the even numbers? | 3,309.02 | 5.65 |
Well we can say return, check to see if
the next Props.count is divisible by 2. | 3,319.74 | 16.17 |
And, if it returns true, that
means we're an odd number. | 3,335.91 | 11.81 |
But what we want is whether
we're an even number. | 3,347.72 | 2.89 |
And so we can actually just say,
oh, return the inverse of this. | 3,350.61 | 3.104 |
And now it's only going to
count those even numbers. | 3,353.714 | 2.166 |
Does that make sense? | 3,361.456 | 0.874 |
And so say we wanted to do
something like send a request | 3,366.4 | 4.075 |
or server every single
time this UI updated, | 3,370.475 | 5.675 |
we wouldn't really want to
do that in the render cycle, | 3,376.15 | 2.31 |
we wouldn't really want to do
it in shouldComponentUpdate. | 3,378.46 | 4.6 |
We can actually do that
in ComponentDidUpdate. | 3,383.06 | 2.198 |
And so we can actually do for
now let's just console.log. | 3,388.32 | 3.18 |
We can say, hey, we updated it
and we created a new number. | 3,397.13 | 4.63 |
And as you see, it's only getting
called for those even numbers. | 3,401.76 | 4.4 |
Because first shouldComponentUpdate
gets called and then | 3,406.16 | 3.81 |
it renders and then
ComponentDidUpdate gets called. | 3,409.97 | 3.05 |
And, since we exit early
on all odd numbers, | 3,413.02 | 3.1 |
only the even numbers will reach the
render cycle and ComponentDidUpdate. | 3,416.12 | 4.29 |
And so what you see logged
are only those even numbers. | 3,420.41 | 2.49 |
So does this concept of updating
and methods that automatically | 3,430.21 | 5.61 |
get invoked makes sense to everyone? | 3,435.82 | 3.324 |
AUDIENCE: Could you only
do this kind of stuff | 3,439.144 | 2.39 |
with [INAUDIBLE] and not
the purely functional ones? | 3,441.534 | 2.56 |
SPEAKER 1: Yeah. | 3,444.094 | 0.666 |
So the question was, do these
methods exist on the functional ones? | 3,444.76 | 3.93 |
And they don't because, say
this functional components | 3,448.69 | 4.2 |
are literally just functions. | 3,452.89 | 2.37 |
They take props and they return a node. | 3,455.26 | 2.07 |
And so since they're functions,
all they do are get invoked. | 3,457.33 | 3.24 |
You pass them props and
then you get a node back. | 3,460.57 | 2.452 |
And in order to have
these methods you actually | 3,463.022 | 1.958 |
have to have this
concept of an instance. | 3,464.98 | 5.67 |
And so React components have
instances, and these instances | 3,470.65 | 3.15 |
are tracked throughout their lifecycle. | 3,473.8 | 1.92 |
And since these are class
instances React well | 3,475.72 | 4.59 |
go ahead and invoke
those methods for you. | 3,480.31 | 3.54 |
And so the stateless
functional components | 3,483.85 | 2.254 |
are just functions that
get invoked and do not | 3,486.104 | 1.916 |
have these class methods, whereas
the React components not only have | 3,488.02 | 3.96 |
these methods but the methods also
get invoked automatically for you. | 3,491.98 | 3.14 |
But yeah great question. | 3,495.12 | 1 |
So that's the update cycle. | 3,500.36 | 2.43 |
And then lastly we have what's
called the unmount cycle. | 3,502.79 | 3.566 |
There's only one thing that
happens in the unmount cycle. | 3,506.356 | 2.374 |
You have componentWillUnmount which
gives you a chance to clean up. | 3,508.73 | 4.42 |
What do I mean by clean up? | 3,513.15 | 1.13 |
Well, you can remove any event listeners
if you had them in React, mostly Web. | 3,514.28 | 5.91 |
You can invalidate any network
requests that you have out. | 3,520.19 | 2.67 |
Or lastly you should clear
any timeouts or intervals. | 3,522.86 | 3 |
And so there's actually some bugs that
you can create if you're not careful. | 3,525.86 | 6.1 |
So let's actually revert
this back to being | 3,531.96 | 8.86 |
just an app that shows that count. | 3,540.82 | 6.43 |
So now we have again that
number being incremented. | 3,555.81 | 3.61 |
And this class called app keeps track
of its own number and is updating. | 3,567.75 | 6.52 |
But say, let's actually
call this counter | 3,574.27 | 5.03 |
and create this thing called app. | 3,579.3 | 2.1 |
And in this, let's actually
just show the component. | 3,590.59 | 2.752 |
So now we have basically
the same exact thing. | 3,615.49 | 2.03 |
So we have this thing
called class app which | 3,617.52 | 2.87 |
just returns an instance of counter. | 3,620.39 | 2.54 |
And counter is what we've
been working on thus far. | 3,622.93 | 2.44 |
It creates an interval and we'll
keep track of that interval | 3,625.37 | 2.64 |
and keep counting up. | 3,628.01 | 2.21 |
But say we actually have this button
that will toggle whether or not | 3,630.22 | 3.79 |
this counter is shown. | 3,634.01 | 1.14 |
And so let's have this
thing called state | 3,635.15 | 13.81 |
and let's have this
flag called show counter | 3,648.96 | 3.037 |
and let's initialize it to true. | 3,651.997 | 1.333 |
And then down here we
can do something like, | 3,657.27 | 4.8 |
if you should show the
counter then return this. | 3,662.07 | 9.25 |
Else let's just return an empty view. | 3,676.96 | 2.356 |
And so now this is basically
the exact same thing | 3,687.75 | 2.76 |
because we have no way to change whether
or not we want to show the counter. | 3,690.51 | 4.65 |
But say we did, so let's
create toggleCounter. | 3,695.16 | 2.61 |
So show counter. | 3,708.04 | 2.17 |
It should be the opposite of what
the previous state show counter was. | 3,710.21 | 3.07 |
And let's also have a button
that will toggle this counter. | 3,716.14 | 17.34 |
And so now we have a button
where if we click it, | 3,743.37 | 3.582 |
we can just toggle whether
or not that counter shows. | 3,746.952 | 2.208 |
And there's actually
a pretty bad bug here. | 3,761.53 | 2.726 |
And that is that when we
toggled this counter-- | 3,767.61 | 3.512 |
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