Add text line to range

A line needs to be applied over a range, not added to a single object.

You could attempt the following:
1. Select the range using the selection API
2. Call cmd('add-noteline');

It could be that the cmd() has to be wrapped within startCmd/endCmd to then correctly further process it (although I'm not sure how you can access that line object then afterwards to adjust properties of it).

That errors comes from the fact that your selection contains not only notes but also stems, ...
So the primary idea would to isolate the notes from the selection and add the bracket on the note directly.

var sel=curScore.selection.elements;

  // The way it should work
  var bracket = newElement(Element.TEXTLINE);

  for (var i=0;i<sel.length;i++) {
        var el = sel[i];
        console.debug(el.type);
        if (el.type==Element.NOTE) {
              curScore.startCmd();
              el.add(bracket); // STOP : crashes Musescore
              curScore.endCmd();
        }
  }

No error.... But a crash (this is a typical behaviour of MS). So we've got to find another solution.

The idea of jeetee is working fine. After adding the line through the cmd the line is added to the selection.
You just have to retrieve it from the selection in order to modify it:


// The way it works actually
curScore.startCmd();
cmd("add-noteline");
curScore.endCmd();

  var bracket=null;
  for (var i=0;i<sel.length;i++) {
        var el = sel[i];
        console.debug(el.type);
        if (el.type==Element.TEXTLINE) {
              bracket=el;
        }
  }

  if (el==null) {
        console.log("Couldn't find the line");
        } 
  else {
        curScore.startCmd();
        var hookHeight = 4; //desired height of line hook

        bracket.beginHookType = 1; //90 degrees
        bracket.endHookType = 1; //90 degrees
        bracket.beginHookHeight = hookHeight;
        bracket.endHookHeight = hookHeight;

        curScore.endCmd();

  }

I finally found some time to look into this and can conclude the following:

1.) There is no need to reselect something the range from within the plugin as the added line using cmd("add-noteline") is automatically included in it.

2.) The line is found as a TEXTLINE element, even if it crosses systems and thus has two underlying SLineSegments internally.

3.) Changing the position of the TEXTLINE via the plugin is not possible (and possibly a bug in the API).
You can assign offset, but it is written to the TextLine tag instead of the child Segment tag. That in itself wouldn't be an issue if it was taken into account when reading the score back in / layouting the textline. But only the offset of the underlying SLineSegment is applied.
Because of not forwarding this property to the SLineSegment assigning userOff2 entirely fails. That property doesn't exist on the TextLine itself and there's no access to the SLineSegment inside to apply it there.

Find the test plugin version attached. When testing out, best to save the example score as mscx so you can open it with a text editor and filter on <Spanner type="TextLine"> to see the effect the plugin has.