luasocket/doc/callback.html

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<title>LuaSocket: Network support for the Lua language</title>
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<tr><td align=center valign=top>Network support for the Lua language
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<h2 id=stream>Callbacks</h2>
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<p>
HTTP, FTP, and SMTP transfers sometimes involve large amounts of
information. Sometimes an application needs to generate outgoing data
in real time, or needs to process incoming information as it is being
received. To address these problems, LuaSocket allows HTTP and SMTP message
bodies and FTP file contents to be streamed through the
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callback mechanism outlined below.
</p>
<p>
Instead of returning the received contents
as a big to the Lua application, the library allows the user to
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provide a <em>receive callback</em> that will be called with successive
chunks of data, as the data becomes available. Conversely, the <em>send
callback</em> mechanism can be used when the application wants to incrementally provide LuaSocket with the data to be sent.
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</p>
<!-- receive +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<p class=name id=receive>
<b>receive_cb(</b>chunk, err<b>)</b>
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</p>
<p class=description>
A receive callback will be repeatedly called by
LuaSocket whenever new data is available. Each time it is called, the
callback receives successive chunks of downloaded data.
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</p>
<p class=parameters>
<tt>Chunk</tt> contains the latest downloaded chunk of data.
When the transmission is over, the function is called with an
empty string (i.e.&nbsp;<tt>""</tt>) in <tt>chunk</tt>.
If an error occurs, the function receives a <b><tt>nil</tt></b>
<tt>chunk</tt> and an error message in <tt>err</tt>.
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</p>
<p class=return>
The callback can abort transmission by returning <b><tt>nil</tt></b> as its first
return value, and an optional error message as the
second return value. To continue receiving
data, the function should return non-<b><tt>nil</tt></b> as its first return
value. Optionally, in this case, it may return a
second return value, with a new callback function to take its place.
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</p>
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<p class=name id=send>
<b>send_cb()</b>
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</p>
<p class=description>
A send callback will be called whenever LuaSocket
needs more data to be sent.
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</p>
<p class=return>
Each time the callback is called, it should return the next chunk of data. It
can optionally return, as it's second return value, a new callback to replace
itself. The callback can abort the process at any time by returning
<b><tt>nil</tt></b> followed by an optional error message.
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</p>
<!-- predefined +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<h2 id=predefined>Predefined Callbacks</h2>
<p>
The Callback module provides several predefined callbacks to
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perform the most common input/output operations. Callbacks are provided
that send and receive contents of files and strings. Furthermore,
composition functions are provided to chain callbacks with filters, such as
the filters defined in the <a href=mime.html>MIME</a> module.
Together, these two modules provide a powerful interface to send and
receive information.
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</p>
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<p class=name id=done>
socket.callback.<b>done()</b>
</p>
<p class=description>
This function creates and returns a callback that successfully terminates
the transmission.
</p>
<!-- fail +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<p class=name id=fail>
socket.callback.<b>fail(</b>message<b>)</b>
</p>
<p class=description>
This function creates and returns a callback that aborts the
transmission with a given error <tt>message</tt>.
</p>
<!-- receive.concat +++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<p class=name id=receive.concat>
socket.callback.<b>receive.concat(</b>cat<b>)</b>
</p>
<p class=description>
This function creates a receive callback that stores whatever it receives
into a concat object. When done, the application can get the contents
received as a single string, directly from the concat object.
</p>
<p class=parameters>
<tt>Cat</tt> is the target concat object, or <b><tt>nil</tt></b>.
If <tt>cat</tt> is <b><tt>nil</tt></b>, the function creates its own
concat object.
</p>
<p class=return>
The function returns a receive callback for the file, and the concat object
that will be used to store the received contents.
</p>
<!-- receive.file +++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<p class=name id=receive.file>
socket.callback.<b>receive.file(</b>file, io_err<b>)</b>
</p>
<p class=description>
This function creates a receive callback that stores whatever it receives
into a file. When done, the callback closes the file.
</p>
<p class=parameters>
<tt>File</tt> is a file handle opened for writing. If <tt>file</tt> is
<b><tt>nil</tt></b>, <tt>io_err</tt> can contain an error message. In this
case, the function returns a callback that just aborts
transmission with the error message.
</p>
<p class=return>
The function returns a receive callback for the file.
</p>
<p class=note>
Note: This function is designed so that it directly accepts the return
values of Lua's IO <tt>io.open</tt> library function.
</p>
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<p class=name id=receive.chain>
socket.callback.<b>receive.chain(</b>filter, receive_cb<b>)</b>
</p>
<p class=description>
This function creates a receive callback that passes all received data
through a filter, before handing it to a given receive callback.
</p>
<p class=parameters>
<tt>Cat</tt> is the target concat object, or <b><tt>nil</tt></b>.
If <tt>cat</tt> is <b><tt>nil</tt></b>, the function creates its own
concat object.
</p>
<p class=return>
The function returns a receive callback for the file, and the concat object
that will be used to store the received contents.
</p>
<p class=note>
Note: Several filters are defined in the <a href=mime.html>MIME</a>
module. Below is an example that creates a receive callback that
creates a string from the received contents, after decoding the
Quoted-Printable transfer content encoding.
</p>
<pre class=example>
string_cb, concat = socket.callback.receive.concat()
receive_cb = socket.callback.receive.chain(
socket.mime.decode("quoted-printable"),
string_cb
)
</pre>
<p class=note>
The call to <tt>callback.chain</tt> creates a chained
receive callback that decodes data using the
<tt><a href=mime.html#decode>mime.decode</a></tt>
Quoted-Printable MIME filter and
hands the decoded data to a concat receive callback.
The concatenated decoded data can be retrieved later
from the associated concat object.
</p>
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<!-- send.file ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<p class=name id=send.file>
socket.callback.<b>send.file(</b>file, io_err<b>)</b>
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</p>
<p class=description>
This function creates a send callback that returns the contents
of a file, chunk by chunk, until the file has been entirely sent.
When done, the callback closes the file.
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</p>
<p class=parameters>
<tt>File</tt> is a file handle opened for reading. If <tt>file</tt> is
<b><tt>nil</tt></b>, <tt>io_err</tt> can contain an error message. In this
case, the function returns a callback that just aborts
transmission with the error message.
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</p>
<p class=return>
The function returns a send callback for the file.
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</p>
<p class=note>
Note: This function is designed so that it directly accepts the return
values of Lua's IO <tt>io.open</tt> library function.
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</p>
<!-- send.string ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<p class=name id=send.string>
socket.callback.<b>send.string(</b>str, err<b>)</b>
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</p>
<p class=description>
This function creates a send callback that will send
the contents of a string.
</p>
<p class=parameters>
<tt>Str</tt> is the string to be sent.
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</p>
<p class=return>
It returns a send callback for the string,
or <b><tt>nil</tt></b> if <tt>str</tt> is <b><tt>nil</tt></b>.
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</p>
<!-- send.chain ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
<p class=name id=send.chain>
socket.callback.<b>send.chain(</b>send_cb, filter<b>)</b>
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</p>
<p class=description>
This function creates a send callback that will filter
all the data it receives from another send callback, before
sending it through.
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</p>
<p class=parameters>
<tt>Send_cb</tt> is the send callback to be chained to <tt>filter</tt>.
</p>
<p class=return>
Returns a callback chained with the filter.
</p>
<p class=note>
Note: Several filters are defined in the <a href=mime.html>MIME</a>
module. Below is an example that creates a send callback that sends
a file's contents encoded in the Base64 transfer content encoding.
</p>
<pre class=example>
send_cb = socket.callback.send.chain(
socket.callback.send.file(io.open("input.bin"))
socket.mime.chain(
socket.mime.encode("base64"),
socket.mime.wrap("base64")
)
)
</pre>
<p class=note>
The call to <a href=mime.html#chain><tt>mime.chain</tt></a>
creates a chained filter that encodes it's input and then breaks it
into lines. The call to <tt>callback.chain</tt> creates a chained
send callback that reads the file from disk and passes it through the
filter before sending it.
</p>
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<small>
Last modified by Diego Nehab on <br>
Sat Aug 9 01:00:41 PDT 2003
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