[kde-guidelines] Drop-down list and Combo box

[Aurélien Gâteau]

On Tuesday 11 June 2013 22:07:54 Thomas Pfeiffer wrote:
> On Friday 07 June 2013 17:28:26 Heiko Tietze wrote:
> > IMHO we should distinguish between simple, read-only drop downs and combo
> > boxes. But some of the underlying ideas, and probably the code as well,
> > are
> > very similar. So I put both into the old ComboBox article.
> > 
> > http://techbase.kde.org/Projects/Usability/HIG/Combo_Box
> 
> Agreed! The problem is that common naming and Qt naming collide here. In Qt,
> afaics, both a drop-down / select box and an actual combo box are a
> ComboBox, one with "editable" set to "false", the other with it set to
> "true". And then there is a KComboBox, which is an extended ComboBox that
> among other things has an "autocomplete" option.

> If now we refer to the same widget with an option set to one value or
> another with different names (one being the same that Qt uses) could lead
> to confusion among developers.
> On the other hand, I don't know how the respective Plasma Components (or Qt
> Desktop Components, for that matter) are or will be called. Looks like
> confusion all over the place.
> Developers, can you help us here?

Plasma components comes with a ComboBox item. It wraps a KComboBox, but does 
not expose the "editable" property, making it always read-only for QML-based 
code (C++ based code can still access and change this property).

QtQuick controls also comes with a control named "ComboBox", which is always 
read-only.

> Generally, I really like the improvements!
> For detailed feedback (and some questions for the developers among us), I'll
> copy some bullets to comment on here:
> > Use a drop-down list for single selection of one out of many items.
> > For a single selection out of a large number of items, use a list view.
> 
> What's the exact distinction here? Personally, I'd distinguish mainly based
> on available space. If there is enough vertical space for a tall list box,
> it works better for single selection because you can see more options at
> once. If you don't have the vertical space, you have to use a drop-down
> anyway. Not sure if that is a sufficient criterion for distinction, though.

+1. Additionally, when you have enough vertical space and a long list to pick 
from, you can make use of a KListWidgetSearchLine, a widget provided by 
kdelibs to easily add a filter-as-you-type lineedit on top of your list.

> > Enable vertical scrollbar for drop-down lists with more than eight items.
> 
> Devs: Does this have to be enabled explicitly or does it appear
> automatically?

The scrollbar is automatically enabled. What you can control is the number of 
visible items. See:

http://qt-project.org/doc/qt-4.8/qcombobox.html#maxVisibleItems-prop


> > Do not add controls to the drop-down (e.g. check boxes for each item).
> 
> Devs: Is this even possible technically?

It is possible, but it's not trivial: the widget which appears to show items 
when you click on the button is a QListView, the widget used to do all list 
views. QComboBox API allows you to replace this widge with your own.

> > Provide autocomplete for the whole string and not for only the last
> > letter,
> 
> i.e. ABC must not go to the first entry starting with "C".
> 
> Devs: Does the autocomplete option of KComboBox provide that automatically?

I think QComboBox behaves as one expects it to.

Given the following item lists:

- bike
- boat
- car

If one types "b", QComboBox selects "bike".
If one (rapidly) types "bo", it selects "boat".
If one types "c", it selects "car".

This behavior is independant of the "editable" property. When "editable" is 
true, the combobox lineedit marks the completed part of the item as selected, 
making it easy to change the completion. That is, if I type "b", the lineedit 
looks like this:

b*ike*

"ike" is selected.

Additionally, when "editable" is false, one can repeatedly type a letter to 
cycle through items starting with this letter. In my example, after typing 
"b", "bike" is selected. If I type "b" again, "boat" is selected. If I type 
"b" again, "bike" is selected again.

KComboBox autocompletion is a bit weird: it works on a different list than the 
combobox items.

To confuse you a bit further: kdelibs also comes with another widget: 
KHistoryComboBox :) This widget is not really meant to be used as a combo box, 
but rather as a line edit which remembers what you typed before.

Aurélien