Tree view allows users to view and interact with objects arranged in hierarchical mode. Users can make single selections or multiple selections. Objects containing data are called leaf nodes and objects contianing sub-objects are called container nodes. The top-most container node is called the root node.
A tree view is an appropriate control for items that have a single, natural, hierarchical categorization that's familiar to most users with more than two levels (not including the root node). But having hierarchical data doesn't mean a tree view must be used. Very often a list view or a combination of list view and drop-down list is a simpler and a more powerful choice. Tree views can present a challenge for users when the level of complexity is not visually understood and users may take longer to familiarize themselves with the tree and each item's location. This can to confusion. Designers and developers must balance the user's ability to easily discover contact with a predictable tree view model that minimizes confusion.
Ask yourself "Is this the right control?"
- Apply a tree view to large data sets that can be categorized into two or more levels.
- A tree view should not have more than four sub-levels (not including the root node). The most commonly accessed objects should appear in the first two levels.
- Use a natural hierarchical structure that is familiar to most users. Balance discoverability with a predictable user model that minimizes confusion.
- Consider breaking down the hierarchical data. For example, into a selection by a drop-down list with an associated list view.
- Use double click to unfold items from the first item in the tree view list. Make double-click behavior redundant via button or context menu.
- Use directional arrows to provide key-based navigation. Also enable the use of Home, End, Page Up, and Page Down.
- Provide a context menu with relevant commands.
- Provide a root node only if users need the ability to perform commands on the entire tree.
- Users should always be able to expand and collapse container nodes by clicking expander buttons.
- Use headers with a meaningful caption for each column.
- Avoid using empty trees.
- If the tree has alternative access methods such as a word search or an index, optimize the tree for browsing by focusing on the most useful content.
- Multi selection:
- Use check boxes to indicate multiple selections.
- For check boxes, use the mixed state to indicate that an option is set for some, but not all, child objects. Users should not be able to set a mixed state directly (cf. check boxes).
- Clicking a mixed state check box selects all child objects and the parent check box.
- Don’t use check boxes in single selection trees.
- If high-level containers have similar contents, but have different purposes, consider using visual clues, e.g. icons to differentiate between them.
- Use persistent tree view states so that users find the list the same way they left it.
- Make controls large enough that it can show at least eight list items at a time without scrolling.
- Label the tree view with a descriptive caption to the top left (cf. alignment).
- Create a buddy relation so access keys are assigned.
- Make use of punctuation (Except for dot "." or colon ":") for a caption.
- Use sentence style capitalization for tree view items.