Cumulate text across callbacks (from an HTTP response)
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.
basicTextGatherer 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.
debugGatherer can be used with the debugfunction
libcurl option in a call and the associated update
function is called whenever libcurl has information
about the header, data and general messages about the
request.
These functions return a list of functions.
Each time one calls basicTextGatherer or
debugGatherer, 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.
We use an S3 class named RCurlCallbackFunction to indicate
that the collection of funcions can be used as a callback.
The update function is the one that is actually used
as the callback function in the CURL option.
The value function can be invoked to get the current
state that has been accumulated by the
update 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 reset function in the collection
permits this.
multiTextGatherer is used when we are downloading multiple
URIs concurrently in a single libcurl operation. This merely
uses the tools of basicTextGatherer applied to each of
several URIs. See getURIAsynchronous.
basicTextGatherer(txt = character(), max = NA, value = NULL,
.mapUnicode = TRUE)
multiTextGatherer(uris, binary = rep(NA, length(uris)))
debugGatherer()txt |
an initial character vector to start things. We allow this to be specified so that one can initialize the content. |
max |
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! |
uris |
for |
value |
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. |
.mapUnicode |
a logical value that controls whether the resulting text is processed to map components of the form \uxxxx to their appropriate Unicode representation. |
binary |
a logical vector that indicates which URIs yield binary content |
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 update component from this function.
Both the basicTextGatherer and debugGatherer
functions return an object of class
RCurlCallbackFunction.
basicTextGatherer extends this with the class
RCurlTextHandler
and
debugGatherer extends this with the class
RCurlDebugHandler.
Each of these has the same basic structure,
being a list of 3 functions.
update |
the function that is called with the text from the callback routine and which processes this text by accumulating it into a vector |
value |
a function that returns the text cumulated across the
callbacks. This takes an argument |
reset |
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. |
multiTextGatherer returns a list with an element corresponding
to each URI. Each element is an object obtained by calling
basicTextGatherer, i.e. a collection of 3 functions with
shared state.
Duncan Temple Lang
Curl homepage http://curl.haxx.se
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)Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.