Tech News Back Issues Issue: 042707

The Tree List Component: An Introduction

By David Swain
Polymath Business Systems

One of the common mistakes made by Omnis Studio programmers is to confuse a List Display Field (List Box, Headed List Box, Data Grid, etc.) with the List Variable it represents. This leads to all sorts of programming errors, causing great amounts of frustration and wasted time. It is a difficult habit to break, but eventually most Omnis Studio programmers figure out that the display characteristics and formatting provided by the field have no effect on the contents of the associated list variable.

But once the Omnis Studio programmer learns this important lesson, there is then the Tree List component to master - and the whole learning process has to be done in reverse! There is no Omnis variable type designed to contain the hierarchical content represented by a Tree List field. Yes, we can use a list variable to hold an expanded version of that content and we can also use a list variable as a transfer mechanism for adding children to a specific Tree List node. But the Tree List field itself contains the real hierarchical list value - and it stands on its own. That field has no dataname, which should be a strong signal of this fact.

So let's begin at the beginning and see what this Tree List component is all about...

The Static Tree List

To remove the use of any variables from our early discussion of the Tree List component, let's first examine the use of a Tree List field that contains only default lines. That is, we will add our content to the Tree List hierarchy directly using the treedefaultlines property in the Property Manager rather than using any "fancier" technique at this point. We can easily set this field up to perform some simple operations and help us to become familiar with the fields many features.

So to begin our exploration, let's create a new Window class and place a Tree List field onto it from the Standard Fields section of the Component Store window. This field looks like any other field we might place on a window, with the exception of the sample tree list image it displays. This image is useful here to discuss some basic hierarchical tree list terminology.

First, a tree list is used to contain a hierarchical collection of data elements in a way that allows the user to hide or expose sections of the hierarchy as needed. Each element in this hierarchy is called a node. Each node has certain properties (many detailed below), some of which relate to the content of the node and some of which govern the nodes display features.

Node images in default tree list field

Elements within the hierarchy are associated through parent-child relationships, which is one characteristic that distinguishes a tree list from a simple list. Each node can potentially have multiple "children" and most nodes have a "parent". Those nodes that do not have parents are called root nodes. These form the "base" for the tree (to use the "tree" analogy) and their names are aligned furthest to the left in our Tree List display field (at least for left-to-right writing systems and the non-Unicode version of Omnis Studio). Child nodes are displayed indented below their respective parent nodes. The amount of indentation from the left edge of the field is determined by the level within the hierarchy at which a child node resides.

Hierarchical level indentations

Nodes that do not have any children are sometimes called leaf nodes. In this article we will treat such nodes as the "outer edge" of the hierarchy, but we will see in a subsequent article how child nodes can be added dynamically to such nodes if our application requires it.

The triangles on the left of this image are disclosure or expand-collapse icons. Different icons are standard on different platforms and for different purposes, but their function is the same. These are embedded controls that allow the user to expand or collapse the display of a branch of the hierarchy. This feature is, perhaps, the greatest convenience provided by the Tree List field.

Disclosure icons

We can explore a little more about Tree List fields by examining some of its many properties.

Basic Tree List Properties

A Tree List field contains the normal properties for an Omnis Studio Window class field. Height and width, horizontal and vertical position, name, visible, active, enabled, tooltip, userinfo, text properties, basic appearance properties, etc. Nothing new here.

A Tree List field also contains some of the basic properties for a list display field. These include horizontal and vertical scroll bars, number of design columns, an optional header (with optional bold text), column names (we will discuss multiple column tree lists in another article, but a single column name can be used to put text into the list header for a basic tree list), multiple line selection, etc. So the "list" nature of a Tree List field is well supported.

But there are also special properties that are specifically intended for a Tree List field. These are primarily found under the Appearance tab in the Property Manager. For example, there are a number of choices for the disclosure icons we can use. This is controlled using the $expandcollapseicon property. We simply supply this property with the number of a multistate icon set. Here are some examples applied to the default Tree List field image display:

Disclosure icon options

In addition to (or instead of) the disclosure icons, we can specify node icons. These can vary from one node to another to indicate specific information about each node. We set this using the $iconid property of an individual node. We can also specify a default node icon (to be used when we do not assign one to a given node) by setting the $defaultnodeicon property of the Tree List field. Node icons (or the space they would occupy if none is assigned) can be hidden or shown by appropriately setting the $shownodeicons property of the field. If this is set to kFalse, there is less of a gap between the node name and its disclosure icon than we saw in the examples above:

Node icon options

Notice that those node icons look a bit cramped. We can give them more vertical breathing room by using the $treelinehtextra property and more horizontal space by increasing the value of the $treeindentlevel property. And if the position of the disclosure icons seems a bit off, we can also adjust the value of the $treeleftmargin property. The exact settings we ultimately choose for these properties will depend on the font size we use for the field as well as the size of the icons.

Node icons with additional spacing

While this may appear to be simple window dressing to some developers, these icons are intended to be used to impart information to the user. But there are more fundamental things for us to understand before having that discussion...

Adding Default Tree List Lines

So far we have only changed the display of a default design image of our Tree List field. If we instantiate our window without further work, we will see absolutely nothing within the fields borders. So we need to add some content to explore further. Since we intend to work with a static tree list for now, this means working with the treedefaultlines property of our Tree List field, found under the General tab of the Property Manager. Clicking in the property value box for treedefaultlines shows it to be a dropdown list. But if we click the dropdown disclosure arrow, we are presented with the following interface (which I display here with some content already in place):

treedefaultlines interface

This dialog offers us a number of tools for managing the default lines of our Tree List field. The second through the fourth pushbuttons assume that we have clicked on (and, therefore, highlighted) a node in the list on the left. The action of each of these buttons is performed relative to the selected node. The first pushbutton adds a new root node at the bottom of the list, no matter which node is currently selected. Child and sibling (same level) nodes are also added to the bottom of their respective groups. It does not appear that a means of dragging nodes to different positions has been provided with this interface, so a little planning goes a long way in arranging the default hierarchy we desire.

When we add a new node to the hierarchy, we are prompted to type in a label for that node. This becomes the value of the name property for that node. If we need to rename one of our default nodes in this design mode, we simply click the node once to select it and then click again (two separate clicks, not a double-click) to edit the text. But there are other node properties we can set while we're here. If we context-click on a node, a context menu appears with even more options:

treedefaultlines node context menu

Let's look at some of these options in detail...

Basic Node Properties

Each node in an Omnis Studio Tree List has a variety of properties. We will examine the most basic of these here, but will cover additional node properties in a later article (as we discuss techniques that require them).

The most basic property that we assign to a Tree List node is its name. This is the actual text that is displayed for the node. When we want to access this property using Omnis Notation, we use the $name property. The data type for the value of this property is a character string.

As with other notational items in Omnis Studio, each node also has a $ident property. But unlike most Omnis items, we have the option - and the responsibility - to assign this property its value for each node. Omnis Studio does not do this for us. This property requires a long integer value. Character strings and decimal values will not work here, but negative values are allowed. The context menu shown above offers us a Node Ident... option for setting this value for our default nodes. It also displays the current $ident value for the selected node with the Clear Ident option. Here I have set this value to 110 for the selected node.

But Omnis Studio also maintains a unique sequential numeric ID value for each node as well. This is called the $seedid property. It is assigned to each node as it is added to a Tree List instance. It also is a long integer value. Default lines are numbered from the top during instantiation in the order they would appear in a fully expanded tree. Numbering begins with 1 and continues in increments of 1. Each new node subsequently added to the tree is given the next sequential value - no matter where in the tree it is added. So except for static nodes, there is no guarantee that a given node will have the same $seedid value from one instance to the next. For this reason, the $seedid property is generally for internal use only.

By default, the text for each node is given the text color assigned to the field. However, each node has its own $textcolor property so that we can assign different colors to different nodes if we wish. This property is stored as a long integer and takes on a value of kColorDefault (-2147483599) if a specific text color value has not been assigned to the node. The context menu shown above gives us the opportunity to set a Node Color... and displays the current setting for the selected node.

We can also view and set the value of the $nodeicon property for the selected node using this context menu. By default, the value of this property is 0 - even if the field itself has been assigned a non-zero $defaultnodeicon value.

And the node context menu allows us to set the $enterable and $showexpandalways property values for the selected node using the top two options. We will discuss these in more detail in a subsequent article. For now, these are both Boolean properties and they each default to kFalse. Note that any node that contains children automatically shows the disclosure icon, so it is not necessary to set $showexpandalways to kTrue for such nodes.

Each node knows its parent. It has a $nodeparent property that contains a reference to the node containing it as a child. If we test the value of this property and find that it is NULL, then we know we are dealing with a root node. The value of this property is automatically set based on the location of the node within the tree.

Another way to test whether a node is a root node is to examine its $level property value. Root nodes have a $level property value of 1. The value of this property increases the more a node is nested within the Tree List. This property can be useful for making sense of the $ident property value when we build Tree List contents dynamically and where different levels contain data derived from different tables in the database or have some other meaning that is tied to that level. We will discuss this in detail in a subsequent article.

Basic Tree List Events

As with basic properties, a Tree List field also inherits the basic events for a field. In fact, it also inherits the basic events for a list display field as well. But there is a bit of a difference in how we use those events and their parameters. To finish this first Tree List article, let's examine how we might use the simple evClick and evDoubleClick events for a Tree List field.

The static Tree List content I built for this demo might be used as a navigation tree through a simple application. There are root nodes for Customers, Products and Sales, with child node hierarchies that point to various windows (perhaps managed using a single Subwindow field) from the library. We might use this to construct an interface similar to the one used in iTunes, for example. All we have to do is to detect which line of our list was clicked or double-clicked.

But this is a bit different than for other kinds of list display fields. Sure, the click and double-click events return even parameters pLineNumber and pRow, but neither of these is populated for clicks on a Tree List field. And no, we can't access $ident, $name or even $seedid of the clicked line using pRow. There is no line number or row reference to tap into because a Tree List just isn't organized that way - but it still inherits these vestigial event parameters from its list display field cousins.

Instead, we need to use the $currentnode() method of the field itself, understanding that the node most recently clicked upon becomes the current node. Note that this is not a property, but a method that returns a reference to the current node of our Tree List field. Since this method returns a notational reference to the current node when it is executed, we can follow the closing parenthesis that resolves the method with the notation for a node property to yield the value of that property for the current node. For example, if we want to know the $ident value of the current node, we could use:


In all of the examples in this and any subsequent article on this subject, treelistRef is an Item reference variable that points to the Tree List field on our window.

Suppose we want to open a specific window in a Subwindow field if the user double-clicks on a node. But some nodes are simply parents and do not point to a specific window. In those cases, double-clicking on the node acts as an alternate to clicking on the disclosure icon, automatically expanding or collapsing that branch of the hierarchical display. If the user double-clicks such a node, we do not want that node to remain the current one, while we don't mind if a node that switches windows remains highlighted.

To indicate that a node contains children, I gave a $ident value that is a multiple of 100 to each such node. This makes such a node easy to distinguish during event processing. Here is some code we might put into the $event method of the Tree List field to handle this situation:

On evDoubleClick
  Process event and continue
  If mod($cobj.$currentnode().$ident,100)=0
    Do $cobj.$setcurrentnode()
    Do $cobj.$redraw()
    ; set window class for subwindow field
  End If

There is a lot more we can do here, but it will have to wait until next time...

Next Time

I hope this has been a helpful introduction to the standard Tree List field. In the next issue of Omnis Tech News, we will dig a little deeper into this subject and explore how we can transfer node information to and from a Tree List using a list variable. There are also more properties, methods and events to discuss.

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