Autoclose buffer occupying Vim's preview window when new buffer opens in preview window

I use Unite.vim frequently to scroll through lists of files in Vim. Sometimes I use Unite's auto-preview feature to have each file open in the preview window as I scroll over it. If I scroll through a large number of files, this leaves me with a bunch of open buffers that I do not need. How can I make the buffer currently occupying the preview window close when a new buffer is opened into the preview window?

I was able to solve this problem by looking through the Unite source, copying the preview function, and making the modification I needed directly on this function to do a vertical open of the preview window:

function! s:preview.func(candidate)
  let buflisted = buflisted(
        \ unite#util#escape_file_searching(
        \ a:candidate.action__path))
  if filereadable(a:candidate.action__path)
    " If execute this command, unite.vim will be affected by events.
    noautocmd silent execute 'vert pedit!'  " added vert
          \ fnameescape(a:candidate.action__path)
    call SetWidthToHalfScreen()  " added to resize preview window
    let prev_winnr = winnr('#')
    let winnr = winnr()
    wincmd P
    doautoall BufRead
    setlocal nomodified
    execute prev_winnr.'wincmd w'
    execute winnr.'wincmd w'
  endif
  if !buflisted
    call unite#add_previewed_buffer_list(
        \ bufnr(unite#util#escape_file_searching(
        \       a:candidate.action__path)))
  endif
endfunction
call unite#custom#action("openable", "preview", s:preview)
call unite#custom#action("file", "preview", s:preview)

function! SetWidthToHalfScreen()
  let diff = 100 - winwidth(winnr())
  exec "vert resize " . (diff > 0 ? '+' : '-') . abs(diff)
endfunction