* Markup programmer interface::
* Contexts for programmers::
* Scheme procedures as properties::
+* TODO moved into scheme::
@end menu
The inner @code{ly:make-simple-closure} supplies the grob as argument
to @var{proc}, the outer ensures that result of the function is
returned, rather than the @code{simple-closure} object.
+
+
+@node TODO moved into scheme
+@section TODO moved into scheme
+
+@menu
+* Using Scheme code instead of \tweak::
+* Difficult tweaks::
+@end menu
+
+@node Using Scheme code instead of \tweak
+@subsection Using Scheme code instead of @code{\tweak}
+
+The main disadvantage of @code{\tweak} is its syntactical
+inflexibility. For example, the following produces a syntax error.
+
+@example
+F = \tweak #'font-size #-3 -\flageolet
+
+\relative c'' @{
+ c4^\F c4_\F
+@}
+@end example
+
+@noindent
+With other words, @code{\tweak} doesn't behave like an articulation
+regarding the syntax; in particular, it can't be attached with
+@code{^} and @code{_}.
+
+Using Scheme, this problem can be circumvented. The route to the
+result is given in @ref{Adding articulation to notes (example)},
+especially how to use @code{\displayMusic} as a helping guide.
+
+@example
+F = #(let ((m (make-music 'ArticulationEvent
+ 'articulation-type "flageolet")))
+ (set! (ly:music-property m 'tweaks)
+ (acons 'font-size -3
+ (ly:music-property m 'tweaks)))
+ m)
+
+\relative c'' @{
+ c4^\F c4_\F
+@}
+@end example
+
+@noindent
+Here, the @code{tweaks} properties of the flageolet object
+@code{m} (created with @code{make-music}) are extracted with
+@code{ly:music-property}, a new key-value pair to change the
+font size is prepended to the property list with the
+@code{acons} Scheme function, and the result is finally
+written back with @code{set!}. The last element of the
+@code{let} block is the return value, @code{m} itself.
+
+
+
+@node Difficult tweaks
+@subsection Difficult tweaks
+
+There are a few classes of difficult adjustments.
+
+@itemize
+
+
+@item
+One type of difficult adjustment is the appearance of spanner objects,
+such as slur and tie. Initially, only one of these objects is created,
+and they can be adjusted with the normal mechanism. However, in some
+cases the spanners cross line breaks. If this happens, these objects
+are cloned. A separate object is created for every system that it is
+in. These are clones of the original object and inherit all
+properties, including @code{\override}s.
+
+
+In other words, an @code{\override} always affects all pieces of a
+broken spanner. To change only one part of a spanner at a line break,
+it is necessary to hook into the formatting process. The
+@code{after-line-breaking} callback contains the Scheme procedure that
+is called after the line breaks have been determined, and layout
+objects have been split over different systems.
+
+In the following example, we define a procedure
+@code{my-callback}. This procedure
+
+@itemize
+@item
+determines if we have been split across line breaks
+@item
+if yes, retrieves all the split objects
+@item
+checks if we are the last of the split objects
+@item
+if yes, it sets @code{extra-offset}.
+@end itemize
+
+This procedure is installed into @rinternals{Tie}, so the last part
+of the broken tie is translated up.
+
+@lilypond[quote,verbatim,ragged-right]
+#(define (my-callback grob)
+ (let* (
+ ; have we been split?
+ (orig (ly:grob-original grob))
+
+ ; if yes, get the split pieces (our siblings)
+ (siblings (if (ly:grob? orig)
+ (ly:spanner-broken-into orig) '() )))
+
+ (if (and (>= (length siblings) 2)
+ (eq? (car (last-pair siblings)) grob))
+ (ly:grob-set-property! grob 'extra-offset '(-2 . 5)))))
+
+\relative c'' {
+ \override Tie #'after-line-breaking =
+ #my-callback
+ c1 ~ \break c2 ~ c
+}
+@end lilypond
+
+@noindent
+When applying this trick, the new @code{after-line-breaking} callback
+should also call the old one @code{after-line-breaking}, if there is
+one. For example, if using this with @code{Hairpin},
+@code{ly:hairpin::after-line-breaking} should also be called.
+
+
+@item Some objects cannot be changed with @code{\override} for
+technical reasons. Examples of those are @code{NonMusicalPaperColumn}
+and @code{PaperColumn}. They can be changed with the
+@code{\overrideProperty} function, which works similar to @code{\once
+\override}, but uses a different syntax.
+
+@example
+\overrideProperty
+#"Score.NonMusicalPaperColumn" % Grob name
+#'line-break-system-details % Property name
+#'((next-padding . 20)) % Value
+@end example
+
+Note, however, that @code{\override}, applied to
+@code{NoteMusicalPaperColumn} and @code{PaperColumn}, still works as
+expected within @code{\context} blocks.
+
+@end itemize
+
+
+
+
+
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