introduce parts metadata.

autoload/airline/extensions/branch.vim

@@ -31,6 +31,6 @@ function! airline#extensions#branch#get_head()
 endfunction
 
 function! airline#extensions#branch#init(ext)
-  let g:airline_parts.branch = 'airline#extensions#branch#get_head'
+  call airline#parts#define_function('branch', 'airline#extensions#branch#get_head')
 endfunction
 

autoload/airline/extensions/bufferline.vim

@@ -14,5 +14,6 @@ function! airline#extensions#bufferline#init(ext)
     let g:bufferline_separator = ' '
   endif
 
-  let g:airline_parts.file = '%{bufferline#refresh_status()}'.bufferline#get_status_string()
+  call airline#parts#define_raw('file', '%{bufferline#refresh_status()}'.bufferline#get_status_string())
 endfunction
+

autoload/airline/extensions/hunks.vim

@@ -54,6 +54,6 @@ function! airline#extensions#hunks#get_hunks()
 endfunction
 
 function! airline#extensions#hunks#init(ext)
-  let g:airline_parts.hunks = 'airline#extensions#hunks#get_hunks'
+  call airline#parts#define_function('hunks', 'airline#extensions#hunks#get_hunks')
 endfunction
 

autoload/airline/parts.vim

@@ -1,14 +1,27 @@
 " MIT License. Copyright (c) 2013 Bailey Ling.
 " vim: et ts=2 sts=2 sw=2
 
-function! airline#parts#append(name)
-  let val = function(a:name)()
-  return empty(val) ? '' : '  '.g:airline_left_alt_sep.' '.val
+let s:parts = {}
+
+function! airline#parts#define(key, config)
+  let s:parts[a:key] = get(s:parts, a:key, {})
+  call extend(s:parts[a:key], a:config)
 endfunction
 
-function! airline#parts#prepend(name)
-  let val = function(a:name)()
-  return empty(val) ? '' : val.' '.g:airline_right_alt_sep.' '
+function! airline#parts#define_function(key, name)
+  call airline#parts#define(a:key, { 'function': a:name })
+endfunction
+
+function! airline#parts#define_text(key, text)
+  call airline#parts#define(a:key, { 'text': a:text })
+endfunction
+
+function! airline#parts#define_raw(key, raw)
+  call airline#parts#define(a:key, { 'raw': a:raw })
+endfunction
+
+function! airline#parts#get(key)
+  return get(s:parts, a:key, {})
 endfunction
 
 function! airline#parts#empty()

autoload/airline/util.vim

@@ -1,39 +1,12 @@
 " MIT License. Copyright (c) 2013 Bailey Ling.
 " vim: et ts=2 sts=2 sw=2
 
-function! s:get_val(part, append)
-  if has_key(g:airline_parts, a:part)
-    let val = g:airline_parts[a:part]
-    if match(val, '%\| ') > -1
-      return val
-    elseif a:append > 0
-      return '%{airline#parts#append("'.val.'")}'
-    elseif a:append < 0
-      return '%{airline#parts#prepend("'.val.'")}'
-    else
-      return '%{'.val.'()}'
-    endif
-  endif
-  return a:part
+function! airline#util#append(text)
+  return empty(a:text) ? '' : '  '.g:airline_left_alt_sep.' '.a:text
 endfunction
 
-function! airline#util#define_left_section(key, parts)
-  if !exists('g:airline_section_{a:key}') && len(a:parts) > 0
-    let g:airline_section_{a:key} = s:get_val(a:parts[0], 0)
-    for i in range(1, len(a:parts) - 1)
-      let g:airline_section_{a:key} .= s:get_val(a:parts[i], 1)
-    endfor
-  endif
-endfunction
-
-function! airline#util#define_right_section(key, parts)
-  if !exists('g:airline_section_{a:key}') && len(a:parts) > 0
-    let g:airline_section_{a:key} = ''
-    for i in range(0, len(a:parts) - 2)
-      let g:airline_section_{a:key} .= s:get_val(a:parts[i], -1)
-    endfor
-    let g:airline_section_{a:key} .= s:get_val(a:parts[-1], 0)
-  endif
+function! airline#util#prepend(text)
+  return empty(a:text) ? '' : a:text.' '.g:airline_right_alt_sep.' '
 endfunction
 
 if v:version >= 704

plugin/airline.vim

@@ -41,16 +41,18 @@ function! s:init()
           \ 'branch': get(g:, 'airline_branch_prefix', get(g:, 'airline_powerline_fonts', 0) ? '' : ''),
           \ }, 'keep')
 
+    call airline#parts#define_function('mode', 'airline#parts#mode')
+    call airline#parts#define_function('iminsert', 'airline#parts#iminsert')
+    call airline#parts#define_function('paste', 'airline#parts#paste')
+    call airline#parts#define('readonly', {
+          \ 'function': 'airline#parts#readonly',
+          \ 'highlight': 'airline_file',
+          \ })
+    call airline#parts#define_raw('file', '%f%m')
+
     call s:check_defined('g:airline_parts', {})
     call extend(g:airline_parts, {
-          \ 'mode': 'airline#parts#mode',
-          \ 'iminsert': 'airline#parts#iminsert',
-          \ 'paste': 'airline#parts#paste',
-          \ 'readonly': '%#airline_file#%{airline#parts#readonly()}',
           \ 'ffenc': '%{printf("%s%s",&fenc,strlen(&ff)>0?"[".&ff."]":"")}',
-          \ 'file': '%f%m',
-          \ 'hunks': 'airline#parts#empty',
-          \ 'branch': 'airline#parts#empty',
           \ 'tagbar': 'airline#parts#empty',
           \ 'syntastic': 'airline#parts#empty',
           \ 'whitespace': 'airline#parts#empty',
@@ -80,14 +82,19 @@ function! s:init()
           \ }, 'keep')
 
     call airline#extensions#load()
-    call airline#util#define_left_section('a', ['mode', 'paste', 'iminsert'])
-    call airline#util#define_left_section('b', ['hunks', 'branch'])
-    call airline#util#define_left_section('c', ['%<', 'file'])
-    call airline#util#define_left_section('gutter', ['readonly', '%='])
-    call airline#util#define_right_section('x', ['tagbar', '%{&filetype}'])
-    call airline#util#define_right_section('y', ['ffenc'])
-    call airline#util#define_right_section('z', ['%3p%% %{g:airline_symbols.linenr} %3l:%3c '])
-    call airline#util#define_right_section('warning', ['syntastic', 'whitespace'])
+
+    if !exists('g:airline_section_a')
+      let g:airline_section_a = airline#section#create_left(['mode', 'paste', 'iminsert'])
+    endif
+    if !exists('g:airline_section_b')
+      let g:airline_section_b = airline#section#create(['hunks', 'branch'])
+    endif
+    let g:airline_section_c = airline#section#create(['%<', 'file'])
+    let g:airline_section_gutter = airline#section#create([' ', 'readonly', '%='])
+    let g:airline_section_x = airline#section#create(['tagbar', '%{&filetype}'])
+    let g:airline_section_y = airline#section#create(['ffenc'])
+    let g:airline_section_z = airline#section#create(['%3p%% %{g:airline_symbols.linenr} %3l:%3c '])
+    let g:airline_section_warning = airline#section#create(['syntastic', 'whitespace'])
 
     let s:airline_theme_defined = exists('g:airline_theme')
     if s:airline_theme_defined || !airline#switch_matching_theme()