Emacs Widget Library

Question

Using this widget:

(widget-create 'editable-field :size 4 :format "text1 %v text2")
(widget-setup)

I get the following output (where the underscores appear as a fillable grey area)

text1 ____ text2

In general, when hitting Enter inside an editable field area, the effect is a jump to the next field. (widget-setup) sets the buffer read-only, therefore outside the editable area, hitting Enter gives widget-before-change: Text is read-only: "Attempt to change text outside editable field".

However, if I place the cursor immediately before text2, hitting Enter inserts an undesired newline, while that should be a read-only area.

Why is a newline added despite that is a read-only area?
How can I prevent the newline insertion?

Answer 1

Analysis of the problem

Looking at wid-edit.el, the only call to widget-field-activate happens inside widget-field-keymap:

(defvar widget-field-keymap
  (let ((map (copy-keymap widget-keymap)))
    ...
    (define-key map "\C-m" 'widget-field-activate)
    ...

I cannot find the function used elsewhere, therefore the widget-field-activate action can only be triggered when widget-field-keymap is active and when Enter is pressed. Since this keymap is associated to editable fields, it means one should be inside a field and hit Enter.

There is however a "dead" spot on the right field boundary where an anomalous behaviour happens:

1. Pressing Enter triggers widget-field-activate, which would imply you are inside the field.

2. However, using a Edebug, shows that this point is outside a field. In fact,

   (get-char-property (point) 'field)
   

   returns nil, rather than the field properties.

The function widget-field-keymap controls for this and: if Enter is pressed inside the field, activates the field; if Enter is pressed outside, executes the standard widget-global-map binding for RET, which is (newline):

(defun widget-field-activate (pos &optional event)
  "Invoke the editable field at point."
  (interactive "@d")
  (let ((field (widget-field-at pos)))    ; <- the func checks if we are in a field
    (if field
    (widget-apply-action field event)
      (call-interactively                 ; <- if not, lookup-key returns newline 
       (lookup-key widget-global-map (this-command-keys))))))

Calling the newline causes the bizarre result of visually breaking the field, spanning it over two lines. Instead, the function should do nothing if the activation cannot proceed or perhaps return the usual read-only warning. Hence, the following

General workaround

(defun widget-field-activate (pos &optional event)
(interactive "@d")
  (let ((field (widget-field-at pos)))
    (if field
    (widget-apply-action field event)
      (message "Text is read-only: \"Attempt to change text outside editable field\""))))

I don't know if this version fixes a bug, but I haven't found any side effect and that produces a more desirable behaviour.