Displaying grob ancestry

When working with grob callbacks, it can be helpful to understand a grob's ancestry. Most grobs have parents which influence the positioning of the grob. X- and Y-parents influence the horizontal and vertical positions for the grob, respectively. Additionally, each parent may have parents of its own.

Unfortunately, there are several aspects of a grob's ancestry that can lead to confusion:

• The types of parents a grob has may depend on context.
• For some grobs, the X- and Y-parents are the same.
• A particular ancestor may be related to a grob in multiple ways.
• The concept of generations is misleading.

For example, the System grob can be both parent (on the Y-side) and grandparent (twice on the X-side) to a VerticalAlignment grob.

The macro defined in this snippet prints (to the console) a textual representation of a grob's ancestry. For example, the call

{
  \once \override NoteHead.before-line-breaking = #display-ancestry
  c
}

generates the following output.

------------------------------------

NoteHead
X,Y: NoteColumn
     X: PaperColumn
        X,Y: System
     Y: VerticalAxisGroup
        X: NonMusicalPaperColumn
           X,Y: System
        Y: VerticalAlignment
           X: NonMusicalPaperColumn
              X,Y: System
           Y: System

As a consequence, you have to execute the code in this snippet by yourself, since the generated output file doesn't show the data we are interested in.

\version "2.24"

#(define (get-ancestry grob)
   (if (not (null? (ly:grob-parent grob X)))
       (list (grob::name grob)
             (get-ancestry (ly:grob-parent grob X))
             (get-ancestry (ly:grob-parent grob Y)))
       (grob::name grob)))

#(define (format-ancestry lst padding)
   (string-append
    (symbol->string (car lst)) "\n"
    (let ((X-ancestry (if (list? (cadr lst))
                          (format-ancestry (cadr lst) (+ padding 3))
                          (symbol->string (cadr lst))))
          (Y-ancestry (if (list? (caddr lst))
                          (format-ancestry (caddr lst) (+ padding 3))
                          (symbol->string (caddr lst)))))
      (if (equal? X-ancestry Y-ancestry)
          (string-append (format #f "~&")
                         (make-string padding #\space)
                         "X,Y: "
                         (if (list? (cadr lst))
                             (format-ancestry (cadr lst) (+ padding 5))
                             (symbol->string (cadr lst))))
          (string-append (format #f "~&")
                         (make-string padding #\space)
                         "X: " X-ancestry "\n"
                         (make-string padding #\space)
                         "Y: " Y-ancestry (format #f "~&"))))
    (format #f "~&")))

#(define (display-ancestry grob)
   (format (current-output-port)
           "~2&~a~2%~a~&"
           (make-string 36 #\-)
           (if (ly:grob? grob)
               (format-ancestry (get-ancestry grob) 0)
               (format #f "~a is not a grob" grob))))

\relative c' {
  \once \override NoteHead.before-line-breaking = #display-ancestry
  f4
  \once \override Accidental.before-line-breaking = #display-ancestry
  \once \override Arpeggio.before-line-breaking = #display-ancestry
  <f as c>4\arpeggio
}