feat(mappings): show leaf node in preview window

This revision introduces new plugin mappings for showing the current
leaf node in a preview window, through the 'preview' and
'preview:jump' actions available for the 'file://' scheme.

Documentation has been updated accordingly.

Author: brandon1024

autoload/fern/scheme/file/mapping.vim

@@ -13,6 +13,8 @@ function! fern#scheme#file#mapping#init(disable_default_mappings) abort
   nnoremap <buffer><silent> <Plug>(fern-action-trash=)    :<C-u>call <SID>call_without_guard('trash')<CR>
   nnoremap <buffer><silent> <Plug>(fern-action-remove)    :<C-u>call <SID>call('remove')<CR>
   nnoremap <buffer><silent> <Plug>(fern-action-remove=)   :<C-u>call <SID>call_without_guard('remove')<CR>
+  nnoremap <buffer><silent> <Plug>(fern-action-preview)   :<C-u>call <SID>call('preview', '')<CR>
+  nnoremap <buffer><silent> <Plug>(fern-action-preview:jump) :<C-u>call <SID>call('preview', 'jump')<CR>
 
   if !a:disable_default_mappings
     nmap <buffer><nowait> N <Plug>(fern-action-new-file)
@@ -205,6 +207,28 @@ function! s:map_remove(helper) abort
         \.then({ -> a:helper.sync.echo(printf('%d items are removed', len(ps))) })
 endfunction
 
+function! s:map_preview(helper, mode) abort
+  let node = a:helper.sync.get_cursor_node()
+  if node is# v:null
+    return s:Promise.reject('cursor node is not visible')
+  endif
+  if node.status isnot# a:helper.STATUS_NONE
+    return s:Promise.reject('cannot preview a directory')
+  endif
+
+  try
+    execute "noautocmd bo pedit " .. node._path
+
+    if a:mode ==# 'jump'
+      wincmd P
+    endif
+
+    return s:Promise.resolve()
+  catch
+    return s:Promise.reject(v:exception)
+  endtry
+endfunction
+
 function! s:new_file(helper, name) abort
   let node = a:helper.sync.get_cursor_node()
   let node = node.status isnot# a:helper.STATUS_EXPANDED ? node.__owner : node

doc/fern.txt

@@ -1037,6 +1037,7 @@ GLOBAL							*fern-mapping-global*
 	      \ <Plug>(fern-action-open:edit)
 	      \ <Plug>(fern-action-open:edit-or-tabedit)
 <
+
 *<Plug>(fern-action-open)*
 	An alias to "open:edit" action. Users can overwrite this mapping to
 	change the default behavior of "open" action like:
@@ -1045,6 +1046,7 @@ GLOBAL							*fern-mapping-global*
 	      \ <Plug>(fern-action-open)
 	      \ <Plug>(fern-action-open:select)
 <
+
 *<Plug>(fern-action-diff:select)*
 *<Plug>(fern-action-diff:select:vert)*
 	Open a first marked node through "window selector"
@@ -1252,7 +1254,7 @@ The following mappings/actions are only available on file:// scheme.
 	nmap <buffer>
 	      \ <Plug>(my-trash)
 	      \ <Plug>(fern-action-trash=)y<CR>
-
+<
 *<Plug>(fern-action-remove)*
 *<Plug>(fern-action-remove=)*
 	Open a prompt to ask if fern can DELETE the cursor node or marked
@@ -1265,7 +1267,7 @@ The following mappings/actions are only available on file:// scheme.
 	nmap <buffer>
 	      \ <Plug>(my-trash)
 	      \ <Plug>(fern-action-trash=)y<CR>
-
+<
 *<Plug>(fern-action-cd:root)*
 *<Plug>(fern-action-lcd:root)*
 *<Plug>(fern-action-tcd:root)*
@@ -1372,6 +1374,11 @@ The following mappings/actions are only available on file:// scheme.
 	Yank the node path. In FILE scheme, |<Plug>(fern-action-yank)| is
 	aliased to this mapping.
 
+*<Plug>(fern-action-preview)*
+*<Plug>(fern-action-preview:jump)*
+	Show the cursor node in the |preview-window|, optionally moving
+	the cursor to that window.
+
 -----------------------------------------------------------------------------
 DICT							*fern-mapping-dict*