text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
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The way we've generally handled
this is by actually binding it | 2,801.57 | 7.12 |
at creation time. | 2,808.69 | 2.54 |
And so there's a
shorthand whereby we can | 2,811.23 | 10.79 |
say when we create this
increment function automatically | 2,822.02 | 4.5 |
bind it to this class. | 2,826.52 | 2.01 |
And so this is actually a new
addition to the ECMAScript standard | 2,828.53 | 6.33 |
where this is called
class properties whereby | 2,834.86 | 2.34 |
inline as we're declaring
this class we can also | 2,837.2 | 2.97 |
create properties that should
be added during the constructor. | 2,840.17 | 3.7 |
And so this is syntactically
the same as doing this. | 2,843.87 | 6.41 |
Having the increment function
defined as we did before. | 2,850.28 | 9.79 |
And in the constructor
when this is created | 2,860.07 | 2.02 |
doing this.Increment equals
this.Increment.bind whatever | 2,862.09 | 9.4 |
the this context we want to be. | 2,871.49 | 2.29 |
And in this case it's this. | 2,873.78 | 3.06 |
Or in other words, it's
the same as just doing | 2,876.84 | 2.16 |
this.Increment is equal to the anonymous
function that we defined down there. | 2,879 | 4.01 |
Whatever. | 2,887.03 | 2.58 |
And so that's just, rather than having
to write everything in the constructor, | 2,889.61 | 5.56 |
we can just use this
shorthand down here which | 2,895.17 | 7.68 |
is just generally the preferred
way because it's easier to read. | 2,902.85 | 6.164 |
And now we've gone ahead and
created this timer that runs. | 2,909.014 | 2.416 |
And so you can see that
the numbers are going up. | 2,911.43 | 3.277 |
And the reason that this is happening
is because we created this increment | 2,914.707 | 3.083 |
function. | 2,917.79 | 0.75 |
We correctly bound it to the
this that we wanted to bind it to | 2,918.54 | 3.177 |
and then we said, hey,
component when you're | 2,921.717 | 1.833 |
done mounting set up this timer, set up
on an interval of 1,000 milliseconds. | 2,923.55 | 5.32 |
So every second call
this increment function. | 2,928.87 | 2.522 |
And what does that
increment function do? | 2,931.392 | 1.708 |
Well it updates the state to be
the previous states count plus 1 | 2,933.1 | 3.5 |
and then down in the render
we render this.state.count. | 2,936.6 | 3.3 |
And so you see every second the
state gets updated to a new number, | 2,939.9 | 4.17 |
and the new number is shown there. | 2,944.07 | 2.46 |
And so we never had to manually say
hey, run this code when you mount. | 2,946.53 | 9.15 |
We just created this method called
component.DidMount and React handles | 2,955.68 | 5.28 |
automatically invoking that for
you after the component mounted. | 2,960.96 | 2.82 |
And it's the same as the constructor. | 2,967.59 | 2.08 |
We never had to manually
invoke the constructor. | 2,969.67 | 2.08 |
It just gets called automatically when
a class instance is getting created? | 2,971.75 | 3.65 |
Any questions on the mount cycle or
the mount processor I should say? | 2,978.22 | 5.25 |
Great so now let's talk
about the update cycle. | 2,991.01 | 4.23 |
And so, just like in
the mounting process, | 2,995.24 | 2.17 |
there is a bunch of lifecycle hooks
that got called for you automatically. | 2,997.41 | 3.35 |
There are also a bunch
of lifecycle hooks | 3,000.76 | 1.708 |
that get called automatically every
single time we want to re-render. | 3,002.468 | 4.762 |
And so the first thing
that happens is component | 3,007.23 | 2.38 |
will receive props which
takes the next props. | 3,009.61 | 3.54 |
And so say you had
something in your state | 3,013.15 | 2.1 |
that really depended on
what the props were set to, | 3,015.25 | 3.744 |
you can actually use this function
to update any of those state fields | 3,018.994 | 2.916 |
that rely on the props. | 3,021.91 | 1.51 |
And you do that by
calling this .setstate. | 3,023.42 | 2.9 |
Next is this thing called
shouldComponentUpdate, | 3,026.32 | 3.15 |
which takes the next
props and the next date, | 3,029.47 | 2.94 |
and here you can compare change
values and decide whether or not | 3,032.41 | 3.54 |
you want that component to render. | 3,035.95 | 1.47 |
And you can actually stop
the update cycle here. | 3,037.42 | 4.14 |
And so this is a good optimization. | 3,044.26 | 2.1 |
So say you have a very
complicated component | 3,046.36 | 2.14 |
that takes a really long time
to render you don't necessarily | 3,048.5 | 3.54 |
want it to render every
single time you get a new prop | 3,052.04 | 3.18 |
because it might be that the
new prop doesn't actually | 3,055.22 | 2.25 |
change anything that's shown. | 3,057.47 | 2.04 |
And so you could use this
method to stop it early. | 3,059.51 | 4.02 |
But that adds a lot of
complexity to your app | 3,063.53 | 1.89 |
and there's almost always
a premature optimization. | 3,065.42 | 3.45 |
The next happens render, we
know exactly what happens there. | 3,068.87 | 4.02 |
And last we have this thing
called ComponentDidUpdate, | 3,072.89 | 3.06 |
whereby you can do anything
that isn't needed for the UI, | 3,075.95 | 3.06 |
like network requests,
which is basically | 3,079.01 | 2.25 |
the analog for ComponentDidMount. | 3,081.26 | 2.69 |
And so let's see an
example for this update. | 3,083.95 | 3.17 |
So say rather than just
rendering this text we actually | 3,087.12 | 8.78 |
pass this count to another function. | 3,095.9 | 3.16 |
So let's have this thing called a
count, which takes as a propped account. | 3,099.06 | 10.29 |
And then let's create
this class called count. | 3,114.47 | 3.05 |
And then here, let's first
just render the text. | 3,122.61 | 3.315 |
And so now we're back to where
we started, where we basically | 3,132.16 | 10.8 |
have some text that gets rendered
based on this.PropsAccount, | 3,142.96 | 4.73 |
and let's style it to be larger. | 3,147.69 | 2.87 |
And so now we're basically back
to exactly where we started. | 3,158.201 | 2.499 |
We have this app which
automatically increments | 3,160.7 | 4.7 |
and then it passes whatever its
state count is as the count prop | 3,165.4 | 3.51 |
to this other class that we call count. | 3,168.91 | 4.22 |
And then this component
we basically just take | 3,173.13 | 2.35 |
that prop and render that text. | 3,175.48 | 4.11 |
But say we actually only wanted
to update on odd numbers. | 3,179.59 | 5.94 |
So say we want to create a new-- | 3,185.53 | 4.1 |
or say we want to have this
be called CountEvennumbers. | 3,189.63 | 5.39 |
And so, in this example
here, how might we say, | 3,203.29 | 4.79 |
hey don't actually update
unless your number is even? | 3,208.08 | 5.97 |
Right now every single time it
receives a new prop it's updating. | 3,214.05 | 5.02 |
But say we only wanted to count even
numbers, what's some strategy that we | 3,219.07 | 4.07 |
may use in order to skip the
rendering for odd numbers | 3,223.14 | 3.33 |
and only render on even numbers? | 3,226.47 | 1.59 |
AUDIENCE: You could use
[INAUDIBLE] before an update. | 3,234.78 | 3.85 |
SPEAKER 1: Yeah, so we have this
thing called shouldComponentUpdate, | 3,238.63 | 3.38 |
which takes the next props. | 3,242.01 | 1.17 |
And so we have the ability
to look at the next props | 3,243.18 | 2.19 |
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