pass context into the builder.

autoload/airline.vim

@@ -95,7 +95,8 @@ endfunction
 function! airline#update_statusline()
   for nr in filter(range(1, winnr('$')), 'v:val != winnr()')
     call setwinvar(nr, 'airline_active', 0)
-    let builder = airline#builder#new(0, s:highlighter)
+    let context = { 'winnr': nr, 'active': 0 }
+    let builder = airline#builder#new(context, s:highlighter)
     call setwinvar(nr, '&statusline', airline#get_statusline(builder, nr, 0))
   endfor
 
@@ -107,7 +108,8 @@ function! airline#update_statusline()
     unlet! w:airline_section_{section}
   endfor
 
-  let builder = airline#builder#new(1, s:highlighter)
+  let context = { 'winnr': winnr(), 'active': 1 }
+  let builder = airline#builder#new(context, s:highlighter)
   let err = airline#util#exec_funcrefs(g:airline_statusline_funcrefs, builder)
   if err == 0
     call setwinvar(winnr(), '&statusline', airline#get_statusline(builder, winnr(), 1))

autoload/airline/builder.vim

@@ -24,16 +24,15 @@ function! s:prototype.add_raw(text)
 endfunction
 
 function! s:prototype.build()
-  if !self._active
+  if !self._context.active
     let self._line = substitute(self._line, '%#.\{-}\ze#', '\0_inactive', 'g')
   endif
   return self._line
 endfunction
 
-function! airline#builder#new(active, highlighter)
+function! airline#builder#new(context, highlighter)
   let builder = copy(s:prototype)
-  let builder._sections = []
-  let builder._active = a:active
+  let builder._context = a:context
   let builder._highlighter = a:highlighter
   let builder._side = 1
   let builder._curgroup = ''

doc/airline.txt

@@ -300,6 +300,9 @@ statuslines to your liking.  Here is an example: >
     return -1
   endfunction
 <
+The second variable is the context, which is a dictionary containing various
+values such as whether the statusline is active or not, and the window number.
+
                                              *airline-pipeline-return-codes*
 The pipeline accepts various return codes and can be used to determine the
 next action.  The following are the supported codes: >