Rethink Async with RXJS

Senior UI Engineer at
About Me
bittersweetryan
ranklam@netﬂix.com

Before We Begin
Be KIND to those of us with OCD and…

Before We Begin
Change those Dropbox icons to B&W!

Reactive Programming
Java…
Groovy…
and JavaScript
At Netﬂix do a lot of RX…
.NET…

What if?
The iterator pattern and observer met at a bar, !
fell in love, got married, and had a baby?

What if?
This baby would allow you to use the same code to !
respond to events and asynchronous operations?

Observables
What to events and async functions have in common?

Observables
As a mouse moves, doesn’t it just emit coordinates?
Tracking Mouse Movements

Observables
Tracking Mouse Movements
1s 2s
Mousedown
0s
Mouseup
{x : 200, y: 521} {x : 240, y: 552} {x : 210, y: 602} {x : 199, y: 579}
{x : 221, y: 530} {x : 218, y: 570} {x : 200, y: 599}

Observables
var mouseMoves =
Observable.fromEvent( mouseDown )
.takeUntil( mouseUp )
.map( function( mouseEvent ){
return {
x : mouseEvent.clientX,
y : mouseEvent.clientY
};
} ) ;
!
mouseMoves.subscribe( function( cords ){
//do stuff for each mouse move cord
});
Mouse Moves Observable

Observables
Observables Everywhere!
• Events!
• AJAX Requests!
• Node Functions!
• Arrays!
• Promises!
• And more….

Observables
Why not another Async approach?
• Replayable!
• Composable!
• Cancellable!
• Cache-able!
• Shareable!
• Can emit multiple value-able

Functional Review!
(With an RX twist)

Functional Review
Map
Reduce
Filter
Zip

var searchResultsSets =
keyups.
map( function( key ){
return Observable.getJSON('/search?' +
input.value)
});
Functional Review
Map - Transforms data

var searchResultsSets = keyups.
filter( function ( key ){
return input.value.length > 1;
}).
map( function( key ){
return Observable.getJSON('/search?' +
input.value)
);
Functional Review
Filter - Narrows Collections

var html =
searchResultsSets.
reduce(function( prev,curr ) {
return prev + '<li>' + curr.name + '</li>';
},'' );
Functional Review
Reduce - Turns a collection into a single value
Initial prev value.

Functional Review
Zip - Combines two collections
var movies = [ 'Super Troopers', 'Pulp Fiction',
'Fargo' ] ;
var boxArts =[ '/cdn/23212/120x80',
'/cdn/73212/120x80','/cdn/99212/120x80' ] ;
!
var withArt = Observable.
zip(movies, boxarts, function(movie, boxart){
return {title : movie, boxart : boxart};
});
!
//[{title : 'Super Troo…', boxart : '/cdn…'},
// {title : 'Pulp Fict…', boxart : '/cdn…' },
// {title : 'Fargo', boxart : 'cdn…' } ]

Functional Review
Observable Data Streams Are Like Crazy Straws
keyPresses Observable
subscribe
throttle
ﬁlter
map
distinctUntilChanged
reduce

Thinking Functionally
Replace loops with map.
searchResults = Observable.
getJSON(‘/people?’ + input.value);
!
searchResults.
forEach( function( reps ){
var names = [];
for( var i = 1; i < resp; i++ ){
var name = data[i].fName + ' ' +
data[i].lName;
names.push( name );
}
});

Replace loops with map and reduce.
searchResults = Observable.
getJSON(‘/people?’ + input.value).
map( function( data ){
return data.fName + data.lName;
} );
!
searchResults.forEach( function( resp ){
//resp will now be the names array
})

Replace if’s with ﬁlters.
var keyPresses = O.fromEvent( el, 'keyup' )
!
keyPresses.forEach( function( e ){
if( e.which === keys.enter ){ //=> no!
//do something
}
});

Replace if’s with ﬁlters.
var enterPresses = O.fromEvent( el, 'keyup' )
.filter( function( e ){
return e.which && e.which === keys.enter;
});
!
enterPresses.forEach( function( e ){
//do something
});

Don’t put too much in a single stream.
var submits = O.fromEvent(input,'keypresses').
throttle().
map( function( e ){
return e.which
} ).
filter( function( key ){
return key === keys.enter || keys.escape;
}).
map().
reduce().
...
Smaller streams are OK.

var keys =
Observable.fromEvent(input,'keypresses').
throttle().
map( function( e ){
return e.which
} );
!
var enters = keys.filter( function( key ) ){
return e.which === keys.enter;
}
!
var escapes = keys.filter( function( key ) ){
Don’t put too much in a single stream.
Smaller streams are OK.

Flattening Patterns!
managing concurrency!

Observables = Events Over Time
Key presses over time…
.5s 1s 1.5s 2s 2.5s 3s
B R E A K IJ <- N G B A D

1s 2s 3s 4s 5s 6s
BR
BRE
BREAK
BREAKJ
BREAKING
BREAKING BA
BREAKING BAD
Ajax requests over time…

The Three Musketeers
Goofy as merge
Donald as concat
Mickey as switchLatest
Starring

Flattening Patterns
merge - combines items in a collection as each item arrives
concat - combines collections in the order they arrived
switchLatest - switches to the latest collection that !
arrives

Merge
1s 2s
http://jsbin.com/wehusi/13/edit
data
data
data
data

Concat
1s 2s
http://jsbin.com/fejod/4/edit
!
!
data
data
data
data

SwitchLatest
!
1s 2s
data
data
data
data

Building Animated
AutoComplete!
Putting Everything Together

Animated Autocomplete
SearchBBrBreBreaBreak

Simple Widget, High Complexity
• Respond to key presses
• Send off Ajax requests
• Animate out when search results become invalid
• Animate in when new search results come in
• Don’t show old results
• Make sure one animation is ﬁnished before starting  
another

var keyups =
Observable.
fromEvent( searchInput, 'keypress');

var searchResultsSets =
keyups.
filter( function ( e ){
}).
map( function( e ){
return Observable.
getJSON('/search?' + input.value);
}).
switchLatest();

var animateOuts =
keyups.
map(function( resultSet ){
return animateOut(resultsDiv);
});
!
var animateIns =
searchResultsSets.
map( function( resultSet ){
return Observable.
of(resultsSet).
concat(animateIn(resultsDiv));
});

In Conclusion!
!
ranklam@netﬂix.com 
@bittersweetryan!

In Conclusion
• Observables are a POWERFUL abstraction!
• Requires a bit of mental rewiring!
• The RX API is HUGE, take baby steps!
• Merging strategies are the key coordinating async!
• Compose streams of data from small streams

To Find Out More
• http://github.com/jhusain/learnrx!
• https://github.com/Reactive-Extensions/RxJS/tree/master/doc!
• Chat with me

Thank You.!
!
ranklam@netﬂix.com 
@bittersweetryan!

Rethink Async with RXJS

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