support for conditional parts. resolves #238.

autoload/airline/parts.vim

@@ -26,6 +26,10 @@ function! airline#parts#define_minwidth(key, width)
   call airline#parts#define(a:key, { 'minwidth': a:width })
 endfunction
 
+function! airline#parts#define_condition(key, predicate)
+  call airline#parts#define(a:key, { 'condition': a:predicate })
+endfunction
+
 function! airline#parts#define_empty(keys)
   for key in a:keys
     call airline#parts#define_raw(key, '')

autoload/airline/section.vim

@@ -36,12 +36,19 @@ function! s:create(parts, append)
     let minwidth = get(part, 'minwidth', 0)
 
     if a:append > 0 && idx != 0
-      let val .= printf('%%{airline#util#append(%s,%s)}', func, minwidth)
+      let partval = printf('%%{airline#util#append(%s,%s)}', func, minwidth)
     elseif a:append < 0 && idx != len(a:parts) - 1
-      let val .= printf('%%{airline#util#prepend(%s,%s)}', func, minwidth)
+      let partval = printf('%%{airline#util#prepend(%s,%s)}', func, minwidth)
     else
-      let val .= printf('%%{airline#util#wrap(%s,%s)}', func, minwidth)
+      let partval = printf('%%{airline#util#wrap(%s,%s)}', func, minwidth)
     endif
+
+    if exists('part.condition')
+      let partval = substitute(partval, '{', '{'.(part.condition).' ? ', '')
+      let partval = substitute(partval, '}', ' : ""}', '')
+    endif
+
+    let val .= partval
     let _ .= val
   endfor
   return _

doc/airline.txt

@@ -370,6 +370,9 @@ Here is how you would define a part that is visible only if the window width
 greater than a minimum width. >
   call airline#parts#define_minwidth('foo', 50)
 <
+Parts can be configured to be visible conditionally. >
+  call airline#parts#define_condition('foo', 'getcwd() =~ "work_dir"')
+<
 Note: Part definitions are combinative; e.g. the two examples above modify the
 same `foo` part.
 

t/parts.vim

@@ -25,5 +25,10 @@ describe 'parts'
     call airline#parts#define_minwidth('mw', 123)
     Expect airline#parts#get('mw').minwidth == 123
   end
+
+  it 'can define a condition'
+    call airline#parts#define_condition('part', '1')
+    Expect airline#parts#get('part').condition == '1'
+  end
 end
 

t/section.vim

@@ -55,5 +55,12 @@ describe 'section'
     let s = airline#section#create_right(['%t', 'asdf', '%{getcwd()}'])
     Expect s == '%t < asdf < %{getcwd()}'
   end
+
+  it 'should empty out parts that do not pass their condition'
+    call airline#parts#define_text('conditional', 'conditional')
+    call airline#parts#define_condition('conditional', '0')
+    let s = airline#section#create(['conditional'])
+    Expect s == '%{0 ? airline#util#wrap("conditional",0) : ""}'
+  end
 end