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\name{Media-class}
\docType{class}
\alias{class:Media}
\alias{Media-class}
\alias{class:application/*}
\alias{application/*-class}
\alias{class:audio/*}
\alias{audio/*-class}
\alias{class:image/*}
\alias{image/*-class}
\alias{class:message/*}
\alias{message/*-class}
\alias{class:model/*}
\alias{model/*-class}
\alias{class:multipart/*}
\alias{multipart/*-class}
\alias{class:text/*}
\alias{text/*-class}
\alias{class:video/*}
\alias{video/*-class}
\alias{class:text/plain}
\alias{text/plain-class}
\alias{class:text/html}
\alias{text/html-class}
\alias{class:text/csv}
\alias{text/csv-class}
\alias{class:application/xml}
\alias{application/xml-class}
\alias{class:application/json}
\alias{application/json-class}
% Utilities:
\alias{mediaCoercionTable}
\alias{length,NullMedia-method}
% Coercion:
\alias{mediaTarget}
\alias{mediaTarget,application/json-method}
\alias{mediaTarget,application/xml-method}
\alias{mediaTarget,text/html-method}
\alias{mediaTarget,text/csv-method}
\alias{mediaTarget,text/*-method}
\alias{mediaTarget,NullMedia-method}
\alias{coerce,application/json,list-method}
\alias{coerce,data.frame,text/csv-method}
\alias{coerce,application/xml,XMLAbstractNode-method}
\alias{coerce,text/csv,data.frame-method}
\alias{coerce,ANY,Media-method}
\alias{coerce,list,application/json-method}
\alias{coerce,data.frame,Media-method}
% Show:
\alias{show,Media-method}
\title{Media}
\description{
The \code{Media} object represents a value identified by a URI. There
is a \code{Media} subclass for each media type, such as
\dQuote{text/csv} or \dQuote{application/xml}. Coercion methods (see
\code{\link{setAs}}) define mappings between \code{Media} subclasses
and \R objects. The user does not usually need to access this
functionality directly.
}
\section{Type conversion}{
Each \code{Media} subclass may be converted to/from one or more \R
types. The mappings are established by \code{\link{setAs}}.
The following bi-directional mappings are built into the package:
\tabular{ll}{
\code{application/xml}, \code{text/html} \tab
\code{\link[XML:XMLAbstractNode-class]{XMLAbstractNode}} \cr
\code{application/json} \tab \code{list} \cr
\code{text/csv} \tab \code{data.frame} \cr
\code{text/*} \tab \code{character}
}
But call \code{mediaCoercionTable} to see what is defined in the
current session.
The \code{\link{as}} function is the canonical interface to converting
media to \R objects. It (usefully) requires that the user specify the
target \R type. For convenience, the \code{mediaTarget} generic
returns the default R type for a given \code{Media} object.
To support a new media type, one should define a \code{Media} subclass
with the same name as the media type (\code{application/xml}), a
corresponding \code{mediaTarget} method, and all relevant
\code{coerce} methods. See the \code{Media} class hierarchy to
determine where the new type fits.
}
\section{Helpers}{
\itemize{
\item{\code{mediaCoercionTable()}: Returns a character matrix with
columns \dQuote{from} and \dQuote{to}, indicating the available
coercions of media types to/from \R objects.
}
}
}
\examples{
txt <- '{"json":{"rocks":true}}'
json <- as(txt, "application/json")
as(json, mediaTarget(json))
}
\author{ Michael Lawrence }
\keyword{methods}
\keyword{classes}
|
