pre-view/CS50-raw · Datasets at Hugging Face

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

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