use statusline function for better control.

autoload/airline.vim

@@ -43,7 +43,6 @@ function! airline#switch_theme(name)
   let w:airline_lastmode = ''
   call airline#update_statusline()
   call airline#load_theme()
-  call airline#check_mode()
 endfunction
 
 function! airline#switch_matching_theme()
@@ -82,6 +81,7 @@ function! airline#update_statusline()
   call s:invoke_funcrefs(context, g:airline_statusline_funcrefs)
 endfunction
 
+let s:contexts = {}
 let s:core_funcrefs = [
       \ function('airline#extensions#apply'),
       \ function('airline#extensions#default#apply') ]
@@ -89,11 +89,13 @@ function! s:invoke_funcrefs(context, funcrefs)
   let builder = airline#builder#new(a:context)
   let err = airline#util#exec_funcrefs(a:funcrefs + s:core_funcrefs, builder, a:context)
   if err == 1
-    call setwinvar(a:context.winnr, '&statusline', builder.build())
+    let a:context.line = builder.build()
+    let s:contexts[a:context.winnr] = a:context
+    call setwinvar(a:context.winnr, '&statusline', '%!airline#statusline('.a:context.winnr.')')
   endif
 endfunction
 
-function! airline#check_mode()
+function! airline#statusline(winnr)
   if get(w:, 'airline_active', 1)
     let l:m = mode()
     if l:m ==# "i"
@@ -123,6 +125,7 @@ function! airline#check_mode()
     call airline#highlighter#highlight(l:mode)
     let w:airline_lastmode = mode_string
   endif
-  return ''
+
+  return s:contexts[a:winnr].line
 endfunction
 

autoload/airline/builder.vim

@@ -39,7 +39,7 @@ function! airline#builder#new(context)
   let builder._context = a:context
   let builder._side = 1
   let builder._curgroup = ''
-  let builder._line = '%{airline#check_mode()}'
+  let builder._line = ''
   return builder
 endfunction
 

t/airline.vim

@@ -1,3 +1,4 @@
+let g:airline_theme = 'dark'
 call airline#init#bootstrap()
 call airline#init#sections()
 source plugin/airline.vim
@@ -28,7 +29,7 @@ describe 'airline'
     call airline#add_statusline_func('MyFuncref')
     let &statusline = ''
     call airline#update_statusline()
-    Expect &statusline =~ 'hello world'
+    Expect airline#statusline(1) =~ 'hello world'
   end
 
   it 'should not change the statusline with -1'
@@ -42,13 +43,13 @@ describe 'airline'
     call airline#add_statusline_func('MyAppend1')
     call airline#add_statusline_func('MyAppend2')
     call airline#update_statusline()
-    Expect &statusline =~ 'helloworld'
+    Expect airline#statusline(1) =~ 'helloworld'
   end
 
   it 'should allow users to redefine sections'
     let g:airline_section_a = airline#section#create(['mode', 'mode'])
     call airline#update_statusline()
-    Expect &statusline =~ '%{airline#util#wrap(airline#parts#mode(),0)}%{airline#util#wrap(airline#parts#mode(),0)}'
+    Expect airline#statusline(1) =~ '%{airline#util#wrap(airline#parts#mode(),0)}%{airline#util#wrap(airline#parts#mode(),0)}'
   end
 
   it 'should remove funcrefs properly'
@@ -60,8 +61,8 @@ describe 'airline'
 
   it 'should overwrite the statusline with active and inactive splits'
     wincmd s
-    Expect getwinvar(1, '&statusline') !~ 'inactive'
+    Expect airline#statusline(1) !~ 'inactive'
-    Expect getwinvar(2, '&statusline') =~ 'inactive'
+    Expect airline#statusline(2) =~ 'inactive'
     wincmd c
   end
 

t/builder.vim

@@ -5,9 +5,9 @@ describe 'active builder'
     let s:builder = airline#builder#new({'active': 1})
   end
 
-  it 'should have a call to check mode'
+  it 'should start with an empty statusline'
     let stl = s:builder.build()
-    Expect stl =~ '%{airline#check_mode()}'
+    Expect stl == ''
   end
 
   it 'should transition colors from one to the next'

t/extensions_default.vim

@@ -23,8 +23,8 @@ describe 'default'
 
   it 'should only render warning section in active splits'
     wincmd s
-    Expect getwinvar(1, '&statusline') =~ 'whitespace'
+    Expect airline#statusline(1) =~ 'whitespace'
-    Expect getwinvar(2, '&statusline') !~ 'whitespace'
+    Expect airline#statusline(2) !~ 'whitespace'
     wincmd c
   end
 end