text-properties: How to add a face + additional face attributes

Question

In this scenario, there exists one or more faces for a particular condition named "foo". When the condition changes to the name of "bar", the goal is to add an additional face attribute such as :strike-through t. How can this be done without explicitly defining separate faces for both conditions? E.g., eliminate the need to define my-face+strike-through.

(defface my-face
  '((t (:foreground "red")))
  "Doc-string"
  :group 'my-faces)

(defface my-face+strike-through
  '((t (:foreground "red" :strike-through t)))
  "Doc-string"
  :group 'my-faces)

(let ((txt "foo"))
  (if (equal txt "bar")
    (add-text-properties 0 (length txt) (list 'face 'my-face) txt)
    (add-text-properties 0 (length txt) (list 'face 'my-face+strike-through) txt))
  txt)

Answer 1

A face spec can be a face defined with defface, or it can be a list of face attributs and their values.

You can apply multiple face specs (multiple faces) to the same text (in a string or buffer). Those multiple faces are merged - their attributes are combined.

(defvar foo "foo")
(defvar bar "bar")

(add-text-properties 0 3 (list 'face 'my-face) foo)
(add-text-properties 0 3 (list 'face '(my-face (:strike-through t))) bar)

See the Elisp manual, nodes Defining Faces and Displaying Faces. See also node Face Remapping.