Projects/Usability/HIG/Combo Box

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Purpose

A combo box is a combination of a drop-down list and an edit control, thus allowing users to enter a value that isn't in the list. It behaves like a drop-down list and allows the user to choose from a list of existing items but adds the option to type a value directly into the control. Newly typed items are usually added to the list and can be selected next time. Combo boxes are typically applied to provide auto-complete or auto-type functionality in a convenient way to the user.

The list provides auto-complete feature for the whole string, independently of the "editable" property. Given the items of "bike", "boat", and "car":

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

The input field of the combo box ("editable" is true) marks the completed part of the item as selected, making it easy to change the completion.

Guidelines

Is this the right control

  • Use a combo box for single selection of one out of many items of lists that can be extended by the user. Prefer a simple drop-down list in case of read-only interaction.
  • Consider to replace the combo box by a list view with a connected edit control.

Behavior

  • Show a maximum of eight items at once (maxVisibleItems=8).
  • When possible apply changes immediately but do not initiate an action (like print, send, delete) when the user selects an item from the list.
  • Do not add controls to the drop-down (e.g. check boxes for each item).
  • Place options that represent general options (e.g. all, none) at the beginning of the list.
  • Sort list items in a logical order. Make sure sorting fits translation.
  • Make sure the items are easily accessible via keyboard by moving distinctive letters to the beginning of each option. For example, in a list of countries on continents, write "Germany (Europe)" instead of "Europe/Germany".
  • Do not have blank list items; use meta-options, e.g. (None) instead

Appearance

  • Combo boxes are distinguished visually from drop-down lists (normally by the raised or lowered bevel). Do not override the common processing, e.g. by using a combo box and making it read only in order to simulate a simple drop-down list.
  • Label the combo box with a descriptive label to the left of the combo box (cf. Alignment).
  • Create a buddy relation so access keys are assigned.
  • If activating a choice affects the appearance or the enabled state of other controls, place them next to the combo box or below the combo box with a space indentation.
  • Use sentence style capitalization for the label and the options.

Implementation

KComboBox


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