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315 lines
8.9 KiB
Markdown
315 lines
8.9 KiB
Markdown
# EventStream
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<img src=https://secure.travis-ci.org/dominictarr/event-stream.png?branch=master>
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[]
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(http://ci.testling.com/dominictarr/event-stream)
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[Streams](http://nodejs.org/api/stream.html "Stream") are node's best and most misunderstood idea, and
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_<em>EventStream</em>_ is a toolkit to make creating and working with streams <em>easy</em>.
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Normally, streams are only used for IO,
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but in event stream we send all kinds of objects down the pipe.
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If your application's <em>input</em> and <em>output</em> are streams,
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shouldn't the <em>throughput</em> be a stream too?
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The *EventStream* functions resemble the array functions,
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because Streams are like Arrays, but laid out in time, rather than in memory.
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<em>All the `event-stream` functions return instances of `Stream`</em>.
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`event-stream` creates
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[0.8 streams](https://github.com/joyent/node/blob/v0.8/doc/api/stream.markdown)
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, which are compatible with [0.10 streams](http://nodejs.org/api/stream.html "Stream").
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>NOTE: I shall use the term <em>"through stream"</em> to refer to a stream that is writable <em>and</em> readable.
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### [simple example](https://github.com/dominictarr/event-stream/blob/master/examples/pretty.js):
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``` js
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//pretty.js
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if(!module.parent) {
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var es = require('event-stream')
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var inspect = require('util').inspect
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process.stdin //connect streams together with `pipe`
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.pipe(es.split()) //split stream to break on newlines
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.pipe(es.map(function (data, cb) { //turn this async function into a stream
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cb(null
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, inspect(JSON.parse(data))) //render it nicely
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}))
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.pipe(process.stdout) // pipe it to stdout !
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}
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```
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run it ...
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``` bash
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curl -sS registry.npmjs.org/event-stream | node pretty.js
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```
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[node Stream documentation](http://nodejs.org/api/stream.html)
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## through (write?, end?)
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Re-emits data synchronously. Easy way to create synchronous through streams.
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Pass in optional `write` and `end` methods. They will be called in the
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context of the stream. Use `this.pause()` and `this.resume()` to manage flow.
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Check `this.paused` to see current flow state. (write always returns `!this.paused`)
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this function is the basis for most of the synchronous streams in `event-stream`.
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``` js
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es.through(function write(data) {
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this.emit('data', data)
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//this.pause()
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},
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function end () { //optional
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this.emit('end')
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})
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```
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## map (asyncFunction)
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Create a through stream from an asynchronous function.
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``` js
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var es = require('event-stream')
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es.map(function (data, callback) {
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//transform data
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// ...
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callback(null, data)
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})
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```
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Each map MUST call the callback. It may callback with data, with an error or with no arguments,
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* `callback()` drop this data.
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this makes the map work like `filter`,
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note:`callback(null,null)` is not the same, and will emit `null`
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* `callback(null, newData)` turn data into newData
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* `callback(error)` emit an error for this item.
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>Note: if a callback is not called, `map` will think that it is still being processed,
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>every call must be answered or the stream will not know when to end.
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>
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>Also, if the callback is called more than once, every call but the first will be ignored.
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## mapSync (syncFunction)
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Same as `map`, but the callback is called synchronously. Based on `es.through`
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## split (matcher)
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Break up a stream and reassemble it so that each line is a chunk. matcher may be a `String`, or a `RegExp`
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Example, read every line in a file ...
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``` js
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fs.createReadStream(file, {flags: 'r'})
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.pipe(es.split())
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.pipe(es.map(function (line, cb) {
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//do something with the line
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cb(null, line)
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}))
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```
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`split` takes the same arguments as `string.split` except it defaults to '\n' instead of ',', and the optional `limit` parameter is ignored.
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[String#split](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/String/split)
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## join (separator)
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Create a through stream that emits `separator` between each chunk, just like Array#join.
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(for legacy reasons, if you pass a callback instead of a string, join is a synonym for `es.wait`)
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## merge (stream1,...,streamN) or merge (streamArray)
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> concat → merge
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Merges streams into one and returns it.
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Incoming data will be emitted as soon it comes into - no ordering will be applied (for example: `data1 data1 data2 data1 data2` - where `data1` and `data2` is data from two streams).
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Counts how many streams were passed to it and emits end only when all streams emitted end.
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```js
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es.merge(
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process.stdout,
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process.stderr
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).pipe(fs.createWriteStream('output.log'));
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```
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It can also take an Array of streams as input like this:
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```js
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es.merge([
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fs.createReadStream('input1.txt'),
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fs.createReadStream('input2.txt')
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]).pipe(fs.createWriteStream('output.log'));
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```
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## replace (from, to)
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Replace all occurrences of `from` with `to`. `from` may be a `String` or a `RegExp`.
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Works just like `string.split(from).join(to)`, but streaming.
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## parse
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Convenience function for parsing JSON chunks. For newline separated JSON,
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use with `es.split`. By default it logs parsing errors by `console.error`;
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for another behaviour, transforms created by `es.parse({error: true})` will
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emit error events for exceptions thrown from `JSON.parse`, unmodified.
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``` js
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fs.createReadStream(filename)
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.pipe(es.split()) //defaults to lines.
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.pipe(es.parse())
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```
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## stringify
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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`
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``` js
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objectStream
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.pipe(es.stringify())
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.pipe(fs.createWriteStream(filename))
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```
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## readable (asyncFunction)
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create a readable stream (that respects pause) from an async function.
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while the stream is not paused,
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the function will be polled with `(count, callback)`,
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and `this` will be the readable stream.
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``` js
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es.readable(function (count, callback) {
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if(streamHasEnded)
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return this.emit('end')
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//...
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this.emit('data', data) //use this way to emit multiple chunks per call.
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callback() // you MUST always call the callback eventually.
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// the function will not be called again until you do this.
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})
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```
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you can also pass the data and the error to the callback.
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you may only call the callback once.
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calling the same callback more than once will have no effect.
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## readArray (array)
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Create a readable stream from an Array.
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Just emit each item as a data event, respecting `pause` and `resume`.
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``` js
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var es = require('event-stream')
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, reader = es.readArray([1,2,3])
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reader.pipe(...)
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```
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If you want the stream behave like a 0.10 stream you will need to wrap it using [`Readable.wrap()`](http://nodejs.org/api/stream.html#stream_readable_wrap_stream) function. Example:
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``` js
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var s = new stream.Readable({objectMode: true}).wrap(es.readArray([1,2,3]));
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```
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## writeArray (callback)
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create a writeable stream from a callback,
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all `data` events are stored in an array, which is passed to the callback when the stream ends.
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``` js
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var es = require('event-stream')
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, reader = es.readArray([1, 2, 3])
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, writer = es.writeArray(function (err, array){
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//array deepEqual [1, 2, 3]
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})
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reader.pipe(writer)
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```
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## pause ()
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A stream that buffers all chunks when paused.
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``` js
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var ps = es.pause()
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ps.pause() //buffer the stream, also do not allow 'end'
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ps.resume() //allow chunks through
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```
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## duplex (writeStream, readStream)
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Takes a writable stream and a readable stream and makes them appear as a readable writable stream.
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It is assumed that the two streams are connected to each other in some way.
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(This is used by `pipeline` and `child`.)
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``` js
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var grep = cp.exec('grep Stream')
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es.duplex(grep.stdin, grep.stdout)
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```
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## child (child_process)
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Create a through stream from a child process ...
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``` js
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var cp = require('child_process')
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es.child(cp.exec('grep Stream')) // a through stream
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```
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## wait (callback)
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waits for stream to emit 'end'.
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joins chunks of a stream into a single string or buffer.
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takes an optional callback, which will be passed the
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complete string/buffer when it receives the 'end' event.
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also, emits a single 'data' event.
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``` js
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readStream.pipe(es.wait(function (err, body) {
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// have complete text here.
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}))
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```
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# Other Stream Modules
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These modules are not included as a part of *EventStream* but may be
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useful when working with streams.
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## [reduce (syncFunction, initial)](https://github.com/parshap/node-stream-reduce)
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Like `Array.prototype.reduce` but for streams. Given a sync reduce
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function and an initial value it will return a through stream that emits
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a single data event with the reduced value once the input stream ends.
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``` js
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var reduce = require("stream-reduce");
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process.stdin.pipe(reduce(function(acc, data) {
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return acc + data.length;
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}, 0)).on("data", function(length) {
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console.log("stdin size:", length);
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});
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```
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