How to systematically change the behavior of knitr \ label {} to add hyperlinks

Question

I want to change the behavior knitrwhen it creates an environment figurein LaTeXto invoke a different command LaTeXthan \label{}, for example \alabel{}, where I define \alabelto run \label{foo}, as well \hypertarget{foo}{}as using a package hyperref LaTeX. I am doing this to create a URL in a web browser to get to a specific place in a document .pdfbuilt with pdflatexfor example. http://.../my.pdf#nameddest=foo.

How can I either override \label{}or fix the extra digit \hypertarget{same label used by \label{}in the figures?

This is in the context of the file .Rnw. I want the anchor to appear inside the figure for optimal positioning of the cursor when jumping into the.pdf` document figure.

UPDATE

In rethinking this, I believe that it’s better not to generate the bindings hypertarget, but to write a function Rthat parses the file LaTeX auxto get the link page number ( \newlabel), to generate the necessary URL to the file pdf. In a file .Rnwor .RmdI can call this function from a sentence to insert a calculated URL.

UPDATE

I finally decided to go with the excellent @werner method, which works flawlessly. For everyone who is interested in an approach Rthat does not require use hypertarget, here is the code LaTeXneeded to configure it - this handles the case when the physical page numbers do not match the logical page of the number (for example, using logical numbers such as the chapter number - the page in the chapter.

% Creates .pag file mapping absolute page numbers to logical page
% numbers; works with R function latexRef

\newwrite\pgfile
\immediate\openout\pgfile=\jobname .pag
\newcounter{abspage}
\setcounter{abspage}{0}

\useackage{everypage}
\AddEverypageHook{%
  \addtocounter{abspage}{1}
  \immediate\write\pgfile{\thepage, \theabspage}%
}
\AtEndDocument{\clearpage\immediate\closeout\pgfile}

R, .aux, .pag:

## Create hyperlink to appropriate physical page in a pdf document
## created by pdflatex given the .aux and .pag file.  Absolute and
## named page numbers are store in the .pag file created by hslide.sty

latexRef <- function(label, base, name, path='doc/',
                     blogpath='/home/harrelfe/R/blog/blogdown/static/doc/',
                     lang=c('markdown', 'latex')) {
  lang <- match.arg(lang)
  aux <- paste0(blogpath, base, '.aux')
  if(! file.exists(aux))
    stop(paste('no file named', aux))
  path <- paste0(path, base, '.pdf')
  pag  <- paste0(blogpath, base, '.pag')
  pagemap <- NULL
  if(file.exists(pag)) {
    p <- read.table(pag, sep=',')
    pagemap        <- trimws(p[[2]])
    names(pagemap) <- trimws(p[[1]])
  }

  r <- readLines(aux)
  w <- paste0('\\\\newlabel\\{', label, '\\}')
  i <- grepl(w, r)
  if(! any(i)) stop(paste('no label =', label))
  r <- r[i][1]
  r <- gsub('\\{', ',', r)
  r <- gsub('\\}', ',', r)
  x <- scan(text=r, sep=',', what=character(0), quiet=TRUE)
  section <- x[5]
  if(section != '') section <- paste0(' Section ', section)
  page    <- trimws(x[7])
  if(length(pagemap)) page <- pagemap[page]
  url     <- paste0('http://fharrell.com/', path, '#page=', page)
  switch(lang,
         markdown = paste0('[', name, section, '](', url, ')'),
         latex    = paste0('\\href{', url, '}{', name, section, '}')
         )
  }

Answer 1

LaTeX \label \label -and- \hypertarget :

---
title: 'A title'
author: 'An author'
date: 'A date'
output: 
  pdf_document:
    keep_tex: true
header-includes:
  - \AtBeginDocument{
      \let\oldlabel\label
      \renewcommand{\label}[1]{\oldlabel{#1}\hypertarget{#1}{}}
    }
---

See Figure \ref{fig:figure}.

\begin{figure}
  \caption{A figure caption}
  \label{fig:figure}
\end{figure}