text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
|---|---|---|
and we'll see later today
how we get that going. | 81.91 | 4.62 |
And so how exactly does this work? | 86.53 | 1.99 |
How are we allowed to write JavaScript
and have it run on mobile devices? | 88.52 | 4.85 |
So first, the JavaScript is bundled. | 93.37 | 1.5 |
So, just like in React, what happens
are a couple of different steps | 94.87 | 4.17 |
where your JavaScript is transpilaged
so going from ES6, ES7, ES-next, | 99.04 | 5.61 |
down to ES5 code, and
it's also minified. | 104.65 | 3.65 |
And so throughout the process you
go from a bunch of different files | 108.3 | 2.86 |
all to one big JavaScript bundle. | 111.16 | 3.81 |
This runs on your phone. | 114.97 | 1.327 |
There are separate threads
for UI layout and JavaScript. | 116.297 | 2.333 |
And so, as we saw, if we're
running JavaScript in the browser | 118.63 | 2.73 |
and that locks up, then nothing works. | 121.36 | 2.899 |
But in React Native there
are actually separate threads | 124.259 | 2.291 |
for the UI layout and JavaScript. | 126.55 | 1.95 |
And these different threads communicate
asynchronously through a bridge. | 131.95 | 5.44 |
And so the JavaScript
thread will request | 137.39 | 2.18 |
UI elements to be shown and
then, like I alluded to earlier, | 139.57 | 4.45 |
the JavaScript thread can be
blocked and the UI still works. | 144.02 | 2.92 |
And so what do I mean by this bridge? | 146.94 | 3.86 |
And so what ends up happening
is there are separate threads. | 150.8 | 3.14 |
So there's one thread for the UI and
there's one thread for JavaScript. | 153.94 | 5.275 |
And so say we're writing React and
we have something like a button | 162.54 | 7.43 |
that we want to be shown. | 169.97 | 1.97 |
Basically what happens is
there's this bridge which will | 171.94 | 7.56 |
communicate between these two threads. | 179.5 | 1.59 |
And so the JavaScript
will basically say, | 181.09 | 2.11 |
hey UI thread I want a view or a button. | 183.2 | 4.49 |
And then UI will basically
say, OK here it is a button. | 194.4 | 4.197 |
And then if I, as a
user, go ahead and press | 198.597 | 1.833 |
that the UI will actually
send over to the JavaScript, | 200.43 | 4.427 |
oh the button was pressed. | 204.857 | 1.083 |
And so this bridge here
is asynchronous, so | 214.32 | 2.64 |
if something happens on the JavaScript
and the JavaScript is actually | 216.96 | 2.94 |
clogged and blocking, the UI
thread can still do its thing | 219.9 | 3.18 |
and I'll show you an
example of that in a second. | 223.08 | 2.62 |
And, conversely, when you
touch something on the UI | 225.7 | 4.25 |
the JavaScript doesn't know
until the bridge basically says, | 229.95 | 3.73 |
hey, this button was pressed and the
UI can talk to the JavaScript that way. | 233.68 | 4.46 |
And so let's see some example where
we can lock up that JavaScript thread | 238.14 | 4.83 |
and not have the UI lock up. | 242.97 | 3.24 |
And so here we are in
the Snack, snack.expo.io, | 246.21 | 5.49 |
and this allows us to run React Native
and have it all happen in the browser. | 251.7 | 4.92 |
And so a few lectures ago we wrote this
blocking method that basically locks up | 256.62 | 6.404 |
the JavaScript thread. | 263.024 | 0.916 |
So we can go ahead and do that again. | 263.94 | 1.61 |
So say we have a method
called block JavaScript. | 265.55 | 4.33 |
And what that does is you can say, we're
going to block and then do this thing, | 269.88 | 9.4 |
so while date.now so we can say,
cons done is five seconds from now. | 279.28 | 9.68 |
And we can say, while it has not
been that much time just do nothing. | 293.9 | 5.31 |
And then when that's done
we can say, OK we're done. | 302.904 | 2.166 |
And so down here, let's
actually create a scroll view-- | 310.78 | 3.07 |
we'll see what that is in a second-- | 313.85 | 1.5 |
and a button. | 320.239 | 0.541 |
And we actually don't care
about any of all that stuff. | 328.999 | 2.291 |
And so, when this
button is clicked, we're | 341.691 | 1.749 |
going to go ahead and
block that JavaScript. | 343.44 | 1.9 |
All right. | 356.61 | 0.98 |
So everybody following? | 361.52 | 1 |
So, so far we have a scroll view
which is basically just a view that | 362.52 | 4.83 |
can scroll back and forth. | 367.35 | 2.08 |
We have a button that says block
JavaScript, and when it's clicked, | 369.43 | 3.95 |
it will run this function
that blocks the JavaScript. | 373.38 | 3.375 |
And so watch what happens
when I click this button. | 376.755 | 3.255 |
Small bug. | 391.24 | 0.99 |
All right here's a chance. | 398.437 | 1.083 |
Who can spot the bug? | 399.52 | 0.874 |
Oh, on press. | 407 | 2.71 |
So on press, it will actually
block the JavaScript. | 409.71 | 2.46 |
And you see how it still is scrolling,
but that button is locked up. | 412.17 | 4.26 |
And then only after five seconds
have passed do the logs come through | 416.43 | 4.68 |
and then the button returns
to its normal state. | 421.11 | 3.15 |
But notice how when it
was stuck and locked, | 424.26 | 2.302 |
when the JavaScript thread
was locked, we still | 426.562 | 1.958 |
had the ability to
scroll back and forth. | 428.52 | 2.85 |
That's because all of the JavaScript
is controlled on one thread | 431.37 | 3.12 |
whereas on the other thread all of the
UI elements still worked as intended. | 434.49 | 4.9 |
And so even though the JavaScript
thread was locked up here, | 439.39 | 2.51 |
it was in this while loop
doing nothing, the UI elements | 441.9 | 4.05 |
were still able to
scroll back and forth. | 445.95 | 2.86 |
And so it's not as big
of a deal if you lock up | 448.81 | 3.59 |
the JavaScript thread in React
Native but it's still a big deal | 452.4 | 3.45 |
because none of your
event handlers will fire. | 455.85 | 3.87 |
So any questions about how React Native
works, how these UI and JavaScript | 462.75 | 5.45 |
threads run separately in the bridge? | 468.2 | 3 |
Great. | 471.2 | 1.86 |
And so what are some of the differences
between React Native and React Web | 473.06 | 3.15 |
which we've been talking about thus far? | 476.21 | 2.157 |
So there are differences
in base components. | 478.367 | 1.833 |
So as you saw in my quick example,
things like scroll view or capital | 480.2 | 3.63 |
button do not exist in web. | 483.83 | 2.205 |
Things like style, the way
that you style elements, | 486.035 | 2.125 |
is slightly different in React
Native and you don't actually | 488.16 | 3.23 |
have browser APIs anymore. | 491.39 | 2.79 |
So things like CSS, animations,
canvas, SVG, things like that | 494.18 | 4.17 |
don't actually exist in React Native. | 498.35 | 2.467 |
But there are actually things
that have been polyfilled. | 500.817 | 2.333 |
So polyfilled is a term that people use
to mean some methods or functions that | 503.15 | 6.9 |
might exist in one environment do not
necessarily exist in all environments. | 510.05 | 3.97 |
And by polyfilling these we can
actually implement them such | 514.02 | 3.92 |
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