Dynamically determine content-type of body from HTTP header and set body reader
This function is used for the writefunction
option in a curl HTTP request.
The idea is that we read the header of the HTTP response
and when our code determines that the header is complete
(the presence of a blank line), it examines the contents
of the header and finds a Content-Type field.
It uses the value of this to determine the nature of the
body of the HTTP response and dynamically (re)sets the reader
for the curl handle appropriately. If the content is binary,
it collects the content into a raw vector;
if it is text, it sets the appropriate character encoding
and collects the content into a character vector.
This function is like basicTextGatherer
but behaves dynamically by determining how to read the content
based on the header of the HTTP response.
This function returns a list of functions that are used
to update and query a shared state across calls.
dynCurlReader(curl = getCurlHandle(), txt = character(), max = NA,
value = NULL, verbose = FALSE, binary = NA, baseURL = NA,
isHTTP = NA, encoding = NA)curl |
the curl handle to be used for the request. It is
essential that this handle be used in the low-level call to
|
txt |
initial value of the text. This is almost always an empty character vector. |
max |
the maximum number of characters to read. This is almost
always |
value |
a function that can be specified which will be used to
convert the body of the response from text or raw in a customized
manner,
e.g. uncompress a gzip body. This can als be done explicitly
with a call |
verbose |
a logical value indicating whether messages about progress and operations are written on the console as the header and body are processed. |
binary |
a logical value indicating whether the caller knows
whether the resulting content is binary ( |
baseURL |
the URL of the request which can be used to follow links to other URLs that are described relative to this. |
isHTTP |
a logical value indicating whether the request/download uses
HTTP or not. If this is |
encoding |
a string that allows the caller to specify and override the encoding of the result. This is used to convert text returned from the server. |
A list with 5 elements all of which are functions. These are
update |
the function that does the actual reading/processing of the content that libcurl passes to it from the header and the body. This is the work-horse of the reader. |
value |
a function to get the body of the response |
header |
a function to get the content of the HTPP header |
reset |
a function to reset the internal contents which allows the same reader to be re-used in subsequent HTTP requests |
curl |
accessor function for the curl handle specified in the call to create this dynamic reader object. |
This list has the S3 class vector
c("DynamicRCurlTextHandler", "RCurlTextHandler", "RCurlCallbackFunction")
Duncan Temple Lang
libcurl http://curl.haxx.se
# Each of these examples can be done with getURLContent().
# These are here just to illustrate the dynamic reader.
if(url.exists("http://www.omegahat.net/Rcartogram/demo.jpg")) withAutoprint({
header = dynCurlReader()
curlPerform(url = "http://www.omegahat.net/Rcartogram/demo.jpg",
headerfunction = header$update, curl = header$curl())
class( header$value() )
length( header$value() )
})
if(url.exists("http://www.omegahat.net/dd.gz")) withAutoprint({
# gzip example.
header = dynCurlReader()
curlPerform(url = "http://www.omegahat.net/dd.gz",
headerfunction = header$update, curl = header$curl())
class( header$value() )
length( header$value() )
if (getRversion() >= "4")
cat(memDecompress(header$value(), asChar = TRUE))
## or cat(Rcompression::gunzip(header$value()))
})
# Character encoding example
## Not run:
header = dynCurlReader()
curlPerform(url = "http://www.razorvine.net/test/utf8form/formaccepter.sn",
postfields = c(text = "ABC", outputencoding = "UTF-8"),
verbose = TRUE,
writefunction = header$update, curl = header$curl())
class( header$value() )
Encoding( header$value() )
## End(Not run)Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.