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<table width="100%" summary="page for basicTextGatherer {RCurl}"><tr><td>basicTextGatherer {RCurl}</td><td style="text-align: right;">R Documentation</td></tr></table>

<h2>Cumulate text across callbacks (from an HTTP response)</h2>

<h3>Description</h3>

<p>These functions create callback functions that can be used
to with the libcurl engine  when it passes information to us
when it is available as part of the HTTP response.
</p>
<p><code>basicTextGatherer</code> is a generator function that returns a closure which is
used to cumulate text provided in callbacks from the libcurl
engine when it reads the response from an HTTP request.
</p>
<p><code>debugGatherer</code> can be used with the <code>debugfunction</code>
libcurl option in a call and the associated <code>update</code>
function is called whenever libcurl has information
about the header, data and general messages about the
request.
</p>
<p>These functions return a list of functions.
Each time one calls <code>basicTextGatherer</code> or
<code>debugGatherer</code>, one gets a new, separate
collection of functions.  However, each
collection of functions (or instance) shares
the variables across the functions and across calls.
This allows them to store data persistently across
the calls without using a global variable.
In this way, we can have multiple instances of the collection
of functions, with each instance updating its own local state
and not interfering with those of the others.
</p>
<p>We use an S3 class named <code>RCurlCallbackFunction</code> to indicate
that the collection of funcions can be used as a callback.
The <code>update</code> function is the one that is actually used
as the callback function in the CURL option.
The <code>value</code> function can be invoked to get the current
state that has been accumulated by the
<code>update</code> function.  This is typically used
when the request is complete.
One can reuse the same collection of functions across
different requests. The information will be cumulated.
Sometimes it is convenient to reuse the object but
reset the state to its original empty value, as it had
been created afresh. The <code>reset</code> function in the collection
permits this.
</p>
<p><code>multiTextGatherer</code> is used when we are downloading multiple
URIs concurrently in a single libcurl operation.  This merely
uses the tools of <code>basicTextGatherer</code> applied to each of
several URIs. See <code><a href="getURIAsynchronous.html">getURIAsynchronous</a></code>.
</p>

<h3>Usage</h3>

<pre>
basicTextGatherer(txt = character(), max = NA, value = NULL,
                    .mapUnicode = TRUE)
multiTextGatherer(uris, binary = rep(NA, length(uris)))
debugGatherer()
</pre>

<h3>Arguments</h3>

<table summary="R argblock">
<tr valign="top"><td><code>txt</code></td>
<td>
<p>an initial character vector to start things.
We allow this to be specified so that one can initialize
the content. 
</p>
</td></tr>
<tr valign="top"><td><code>max</code></td>
<td>
<p>if specified as an integer this controls  the total number
of characters that will be read.  If more are read, the function
tells libcurl to stop!</p>
</td></tr>
<tr valign="top"><td><code>uris</code></td>
<td>
<p>for <code>multiTextGatherer</code>, this is either the number
or the names of the uris being downloaded and for which we
need a separate writer function.
</p>
</td></tr>
<tr valign="top"><td><code>value</code></td>
<td>
<p>if specified, a function that is called when retrieving
the text usually after the completion of the request and the
processing of the response. This function can be used to convert the
result into a different format, e.g. parse an XML document,
read values from table in the text.</p>
</td></tr>
<tr valign="top"><td><code>.mapUnicode</code></td>
<td>
<p>a logical value that controls whether the resulting
text is processed to map components of the form \uxxxx to their
appropriate Unicode representation.</p>
</td></tr>
<tr valign="top"><td><code>binary</code></td>
<td>
<p>a logical vector that indicates which URIs yield binary content</p>
</td></tr>
</table>

<h3>Details</h3>

<p>This is called when the libcurl engine finds sufficient
data on the stream from which it is reading the response.
It cumulates these bytes and hands them to a C routine in
this package which calls the actual gathering function (or a suitable
replacement) returned as the <code>update</code> component from this function.
</p>

<h3>Value</h3>

<p>Both the <code>basicTextGatherer</code> and <code>debugGatherer</code>
functions return an object of class
<code>RCurlCallbackFunction</code>.
<code>basicTextGatherer</code> extends this with the class
<code>RCurlTextHandler</code>
and 
<code>debugGatherer</code> extends this with the class
<code>RCurlDebugHandler</code>.
Each of these has the same basic structure,
being a list of 3 functions.
</p>
<table summary="R valueblock">
<tr valign="top"><td><code>update</code></td>
<td>
<p>the function that is called with the text from the
callback routine and which processes this text by accumulating it
into a vector</p>
</td></tr>
<tr valign="top"><td><code>value</code></td>
<td>
<p>a function that returns the text cumulated across the
callbacks. This takes an argument <code>collapse</code> (and additional ones)
that are handed to <code><a href="../../base/html/paste.html">paste</a></code>.
If the value of  <code>collapse</code> is given as <code>NULL</code>,
the vector of elements containing the different text for each
callback is returned. This is convenient when debugging or if one
knows something about the nature of the callbacks, e.g. the regular
size that causes iit to identify records in a natural way.
</p>
</td></tr>
<tr valign="top"><td><code>reset</code></td>
<td>
<p>a function that resets the internal state to its
original, empty value. This can be used to reuse the same object
across requests but to avoid cumulating new input with the material from previous requests.</p>
</td></tr>
</table>
<p><code>multiTextGatherer</code> returns a list with an element corresponding
to each URI. Each element is an object obtained by calling
<code>basicTextGatherer</code>, i.e. a collection of 3 functions with
shared state.
</p>

<h3>Author(s)</h3>

<p>Duncan Temple Lang</p>

<h3>References</h3>

<p>Curl homepage <a href="http://curl.haxx.se">http://curl.haxx.se</a></p>

<p><code><a href="getURL.html">getURL</a></code>
<code><a href="dynCurlReader.html">dynCurlReader</a></code>  
</p>

<h3>Examples</h3>

<pre>
if(url.exists("http://www.omegahat.net/RCurl/index.html")) withAutoprint({
  txt = getURL("http://www.omegahat.net/RCurl/index.html", write = basicTextGatherer())

h = basicTextGatherer()
  txt = getURL("http://www.omegahat.net/RCurl/index.html", write = h$update)
    ## Cumulate across pages.
  txt = getURL("http://www.omegahat.net/index.html", write = h$update)

headers = basicTextGatherer()
  txt = getURL("http://www.omegahat.net/RCurl/index.html",
               header = TRUE, headerfunction = headers$update)

## Now read the headers.
  cat(headers$value())
  headers$reset()

## Debugging callback
  d = debugGatherer()
  x = getURL("http://www.omegahat.net/RCurl/index.html", debugfunction = d$update, verbose = TRUE)
  cat(names(d$value()))
  d$value()[["headerIn"]]

## This hangs on Solaris
  uris = c("http://www.omegahat.net/RCurl/index.html",
           "http://www.omegahat.net/RCurl/philosophy.html")
  g = multiTextGatherer(uris)
  txt = getURIAsynchronous(uris,  write = g)
  names(txt) # no names this way
  nchar(txt)

# Now don't use names for the gatherer elements.
  g = multiTextGatherer(length(uris))
  txt = getURIAsynchronous(uris,  write = g)
  names(txt)
  nchar(txt)
})

## Not run: 
 Sys.setlocale(,"en_US.latin1")
 Sys.setlocale(,"en_US.UTF-8")
 uris = c("http://www.omegahat.net/RCurl/index.html",
          "http://www.omegahat.net/RCurl/philosophy.html")
 g = multiTextGatherer(uris)
 txt = getURIAsynchronous(uris,  write = g)

## End(Not run)
</pre>

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