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EventStream

Streams are nodes best and most misunderstood idea, and EventStream is a toolkit to make creating and working with streams easy.

Normally, streams are only used of IO,
but in event stream we send all kinds of objects down the pipe.
If your application's input and output are streams,
shouldn't the throughput be a stream too?

The EventStream functions resemble the array functions,
because Streams are like Arrays, but laid out in time, rather than in memory.

All the event-stream functions return instances of Stream.

Stream API docs: nodejs.org/api/streams

NOTE: I shall use the term "through stream" to refer to a stream that is writable and readable.

###simple example:


//pretty.js

if(!module.parent) {
  var es = require('event-stream')
  es.connect(                         //connect streams together with `pipe`
    process.openStdin(),              //open stdin
    es.split(),                       //split stream to break on newlines
    es.map(function (data, callback) {//turn this async function into a stream
      callback(null
        , inspect(JSON.parse(data)))  //render it nicely
    }),
    process.stdout                    // pipe it to stdout !
    )
  }

run it ...

curl -sS registry.npmjs.org/event-stream | node pretty.js

test are in event-stream_tests

node Stream documentation

##map (asyncFunction)

Create a through stream from an asyncronous function. 

var es = require('event-stream')

es.map(function (data, callback) {
  //transform data
  // ...
  callback(null, data)
})

Each map MUST call the callback. It may callback with data, with an error or with no arguments,

  • callback() drop this data.
    this makes the map work like filter,
    note:callback(null,null) is not the same, and will emit null

  • callback(null, newData) turn data into newData

  • callback(error) emit an error for this item.

Note: if a callback is not called, map will think that it is still being processed,
every call must be answered or the stream will not know when to end.

Also, if the callback is called more than once, every call but the first will be ignored.

##readable (asyncFunction)

create a readable stream (that respects pause) from an async function.
while the stream is not paused,
the function will be polled with (count, callback),
and this will be the readable stream.


es.readable(function (count, callback) {
  if(streamHasEnded)
    return this.emit('end')

  //...

  this.emit('data', data) //use this way to emit multiple chunks per call.

  callback() // you MUST always call the callback eventually.
             // the function will not be called again until you do this.
})

you can also pass the data and the error to the callback.
you may only call the callback once.
calling the same callback more than once will have no effect.

##readArray (array)

Create a readable stream from an Array.

Just emit each item as a data event, respecting pause and resume.

  var es = require('event-stream')
    , reader = es.readArray([1,2,3])

  reader.pipe(...)

writeArray (callback)

create a writeable stream from a callback,
all data events are stored in an array, which is passed to the callback when the stream ends.

  var es = require('event-stream')
    , reader = es.readArray([1, 2, 3])
    , writer = es.writeArray(function (err, array){
      //array deepEqual [1, 2, 3]
    })

  reader.pipe(writer)

split ()

Break up a stream and reassemble it so that each line is a chunk.

Example, read every line in a file ...

  es.connect(
    fs.createReadStream(file, {flags: 'r'}),
    es.split(),
    es.map(function (line, cb) {
       //do something with the line 
       cb(null, line)
    })
  )

connect (stream1,...,streamN)

Connect multiple Streams together into one stream.
connect will return a Stream. This stream will write to the first stream, and will emit data from the last stream.

Listening for 'error' will recieve errors from all streams inside the pipe.


  es.connect(                         //connect streams together with `pipe`
    process.openStdin(),              //open stdin
    es.split(),                       //split stream to break on newlines
    es.map(function (data, callback) {//turn this async function into a stream
      callback(null
        , inspect(JSON.parse(data)))  //render it nicely
    }),
    process.stdout                    // pipe it to stdout !
    )

gate (isShut=true)

If the gate is shut, buffer the stream.
All calls to write will return false (pause upstream),
and end will not be sent downstream.

If the gate is open, let the stream through.

Named shut instead of close, because close is already kinda meaningful with streams.

Gate is useful for holding off processing a stream until some resource (i.e. a database, or network connection) is ready. 


  var gate = es.gate()

  gate.open() //allow the gate to stream

  gate.close() //buffer the stream, also do not allow 'end'

duplex

Takes a writable stream and a readable stream and makes them appear as a readable writable stream.

It is assumed that the two streams are connected to each other in some way.

(This is used by connect and child.)

  var grep = cp.exec('grep Stream')

  es.duplex(grep.stdin, grep.stdout)

child (child_process)

Create a through stream from a child process ...

  var cp = require('child_process')

  es.child(cp.exec('grep Stream')) // a through stream

pipeable (streamCreatorFunction,...)

The arguments to pipable must be functions that return
instances of Stream or async functions.
(If a function is returned, it will be turned into a Stream
with es.map.)

Here is the first example rewritten to use pipeable.

//examples/pretty_pipeable.js
var inspect = require('util').inspect

if(!module.parent)
  require('event-stream').pipeable(function () {
    return function (data, callback) {
      try {
        data = JSON.parse(data)
      } catch (err) {}              //pass non JSON straight through!
      callback(null, inspect(data))
      }
    })  
  })


curl -sS registry.npmjs.org/event-stream | node pipeable_pretty.js

## or, turn the pipe into a server!

node pipeable_pretty.js --port 4646

curl -sS registry.npmjs.org/event-stream | curl -sSNT- localhost:4646

compatible modules:

almost compatible modules: (1+ these issues)