# EventStream
[]
(http://ci.testling.com/dominictarr/event-stream)
[Streams](http://nodejs.org/api/strems.html "Stream") are node's 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`.
`event-stream` creates
[0.8 streams](https://github.com/joyent/node/blob/v0.8/doc/api/stream.markdown)
, which are compatible with [0.10 streams](http://nodejs.org/api/stream.html "Stream")
>NOTE: I shall use the term "through stream" to refer to a stream that is writable and readable.
###[simple example](https://github.com/dominictarr/event-stream/blob/master/examples/pretty.js):
``` js
//pretty.js
if(!module.parent) {
var es = require('event-stream')
es.pipeline( //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 ...
``` bash
curl -sS registry.npmjs.org/event-stream | node pretty.js
```
[node Stream documentation](http://nodejs.org/api/stream.html)
## through (write?, end?)
Reemits data synchronously. Easy way to create syncronous through streams.
Pass in an optional `write` and `end` methods. They will be called in the
context of the stream. Use `this.pause()` and `this.resume()` to manage flow.
Check `this.paused` to see current flow state. (write always returns `!this.paused`)
this function is the basis for most of the syncronous streams in `event-stream`.
``` js
es.through(function write(data) {
this.emit('data', data)
//this.pause()
},
function end () { //optional
this.emit('end')
})
```
##map (asyncFunction)
Create a through stream from an asyncronous function.
``` js
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.
## mapSync (syncFunction)
Same as `map`, but the callback is called synchronously. Based on `es.through`
## split (matcher)
Break up a stream and reassemble it so that each line is a chunk. matcher may be a `String`, or a `RegExp`
Example, read every line in a file ...
``` js
es.pipeline(
fs.createReadStream(file, {flags: 'r'}),
es.split(),
es.map(function (line, cb) {
//do something with the line
cb(null, line)
})
)
```
`split` takes the same arguments as `string.split` except it defaults to '\n' instead of ',', and the optional `limit` paremeter is ignored.
[String#split](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String/split)
## join (separator)
create a through stream that emits `separator` between each chunk, just like Array#join.
(for legacy reasons, if you pass a callback instead of a string, join is a synonym for `es.wait`)
## replace (from, to)
Replace all occurences of `from` with `to`. `from` may be a `String` or a `RegExp`.
Works just like `string.split(from).join(to)`, but streaming.
## parse
Convienience function for parsing JSON chunks. For newline separated JSON,
use with `es.split`
``` js
fs.createReadStream(filename)
.pipe(es.split()) //defaults to lines.
.pipe(es.parse())
```
## stringify
convert javascript objects into lines of text. The text will have whitespace escaped and have a `\n` appended, so it will be compatible with `es.parse`
``` js
objectStream
.pipe(es.stringify())
.pipe(fs.createWriteStream(filename))
```
##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.
``` js
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`.
``` js
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.
``` js
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)
```
## pipeline (stream1,...,streamN)
Turn a pipeline into a single stream. `pipeline` returns a stream that writes to the first stream
and reads from the last stream.
Listening for 'error' will recieve errors from all streams inside the pipe.
> `connect` is an alias for `pipeline`.
``` js
es.pipeline( //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 !
)
```
## pause ()
A stream that buffers all chunks when paused.
``` js
var ps = es.pause()
ps.pause() //buffer the stream, also do not allow 'end'
ps.resume() //allow chunks through
```
## duplex (writeStream, readStream)
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 `pipeline` and `child`.)
``` js
var grep = cp.exec('grep Stream')
es.duplex(grep.stdin, grep.stdout)
```
## child (child_process)
Create a through stream from a child process ...
``` js
var cp = require('child_process')
es.child(cp.exec('grep Stream')) // a through stream
```
## wait (callback)
waits for stream to emit 'end'.
joins chunks of a stream into a single string.
takes an optional callback, which will be passed the
complete string when it receives the 'end' event.
also, emits a single 'data' event.
``` js
readStream.pipe(es.join(function (err, text) {
// have complete text here.
}))
```