A dependent collection object can only exist in the context of a parent COM object.
Often we find those collections as part of an OCX control. COM-controls that support dependent collections include the TreeView Ocx, ListView Ocx, ListBox OCX, Toolbar Ocx, etc. These Ocx-es provide run-time access to their functionality through Dependent Collection Objects. For example, the TreeView Ocx control has a Nodes collection of Node objects that represent the items in the hierarchy the Tree-View control displays. Tree-View Node items are appended or inserted by using the Add method of the Nodes collection. For a List-View control you’ll use the ListItems to add ListItem objects. A dependent collection doesn’t exist until an Ocx is created and are – in COM terms - Classes That Are Not Creatable.
These collections are also provided as a means to iterate over the items of a control. This could concern a collection of all items or a collection of selected items. The collection helps in quickly locating a specific item using a key or an index. Therefor each collection object comes with properties and methods you can use to insert, delete, and retrieve the items in the collection.
|Property or method||Description|
|Add method||Add items to the collection.|
|Count property||Return the number of items in the collection. Read-only.|
|Item method||Return an item, by index or by key.|
|Remove method||Delete an item from the collection, by index or by key.|
I won’t go into the details about the basic services of adding, deleting, and retrieving from a collection. Suffice to say, that the methods and properties depend on keys (type String) and/or indexes. An index is a Long between one (1) and the number of items in the collection (Count). The Add method determines whether or not you associate a key with an item, otherwise the collection items are only accessible through their index. A typical statement to add a Node object, representing the first position, to a TreeView Ocx is:
Form frm1 Ocx TreeView tv1 = "", 10, 10, 230, 200 Dim nodes As Nodes, node As Node ' Add an item through Nodes Collection tv1.Nodes.Add , , , "Item #1" ' Use Dependent Collection Set nodes = tv1.Nodes nodes.Add , , , "Item #2" ' These are exactly the same Set node = nodes.Item(1) Set node = nodes(1) // .Item is hidden 'Using With when adding multiple nodes: With nodes .Add 1, tvwChild , , "SubItem1/1" .Add 1, tvwChild , , "SubItem1/2" ' ... etc. .Item(1).Expanded = True EndWith ' Additional GB32 - optimizing ways tv1.AddItem 2, tvwChild, , "SubItem2/1" tv1.Add 2, tvwChild , , "SubItem2/2" ' GB32 Shortcut Property tv1(2).Expanded = True For Each node In tv1 // rather than tv1.Nodes Debug.Print node.Text Next Do Sleep Until IsNothing(Me)
In fact, these intermediate dependent collections insert additional code and slow down execution speed. These collections are introduced by VB to provide a systematic way of managing items of an OCX. In the end they only consume code and time. To overcome these disadvantages, GFA-BASIC provides collection properties and methods to the OCX control directly. This concept makes the use of the dependent collections redundant. Even the iteration properties used with For Each is simplified, see listing above.
The following collection properties and methods have been added to the OCX-es:
|Property or method||Description of shortcuts|
|Ocx.AddItem or Ocx.Add method||Add items to the collection of the Ocx.|
|Ocx.Count property||Return the number of items in the collection. Read-only.|
|Ocx.Item method||Return an item, by index or by key.|
|Ocx.Remove method||Delete an item from the collection, by index or by key.|
|Ocx.Clear method||Clears the entire collection|
Using these shortcuts makes the intermediate collections redundant. The OCX-es that benefit are:
|OCX||Available Dependent Collections||Shortcut To|
|ListView||ListItems, ColumnHeaders, SelectedItems, CheckedItems, Icons, SmallIcons||ListItems|
Whenever you operate on an OCX-item, first check if the collection-properties are implemented onto to OCX directly.
There is one more dependent collection; the MenuItems collection of the Form Object. In the next post I’ll show you how to handle MenuItems.
Post a Comment