You can use the function
format-mode-line to compute the text
that would appear in a mode line or header line based on a certain
mode line construct.
This function formats a line of text according to format as if it were generating the mode line for window, but it also returns the text as a string. The argument window defaults to the selected window. If buffer is non-
nil, all the information used is taken from buffer; by default, it comes from window's buffer.
The value string normally has text properties that correspond to the faces, keymaps, etc., that the mode line would have. Any character for which no
faceproperty is specified by format gets a default value determined by face. If face is
t, that stands for either
mode-lineif window is selected, otherwise
mode-line-inactive. If face is
nilor omitted, that stands for the default face. If face is an integer, the value returned by this function will have no text properties.
You can also specify other valid faces as the value of face. If specified, that face provides the
faceproperty for characters whose face is not specified by format.
Note that using
header-lineas face will actually redisplay the mode line or the header line, respectively, using the current definitions of the corresponding face, in addition to returning the formatted string. (Other faces do not cause redisplay.)
(format-mode-line header-line-format)returns the text that would appear in the selected window's header line (
""if it has no header line).
(format-mode-line header-line-format 'header-line)returns the same text, with each character carrying the face that it will have in the header line itself, and also redraws the header line.