Using DataWindow information to populate a TreeView

A useful implementation of the TreeView control is to populate it with information that you retrieve from a DataWindow. To do this your application must:

  • Declare and instantiate a DataStore and assign a DataWindow object

  • Retrieve information as needed

  • Use the retrieved information to populate the TreeView

  • Destroy the DataStore instance when you have finished

Because a TreeView can display different types of information at different levels, you will probably define additional DataWindows, one for each level. Those DataWindows usually refer to different but related tables. When an item is expanded, the item becomes a retrieval argument for getting child items.

Populating the first level

This example populates a TreeView with a list of composers. The second level of the TreeView displays music by each composer. In the database there are two tables: composer names and music titles (with composer name as a foreign key).

This example declares two DataStore instance variables for the window containing the TreeView control:

datastore ids_data, ids_info

This example uses the TreeView control's Constructor event to:

  • Instantiate the DataStore

  • Associate it with a DataWindow and retrieve information

  • Use the retrieved data to populate the root level of the TreeView:

    //Constructor event for tv_1
    treeviewitem tvi1, tvi2
    long ll_lev1, ll_lev2, ll_rowcount, ll_row
    //Create instance variable datastore
    ids_data = CREATE datastore
    ids_data.DataObject = "d_composers"
    ll_rowcount = ids_data.Retrieve()
    //Create the first level of the TreeView
    tvi1.PictureIndex = 1
    tvi1.Children = TRUE
    //Populate the TreeView with
    //data retrieved from the datastore
    FOR ll_row = 1 to ll_rowcount
          tvi1.Label = ids_data.GetItemString(ll_row, &
          This.InsertItemLast(0, tvi1)

Populating the second level

When the user expands a root level item, the ItemPopulate event occurs. This script for the event:

  • Instantiates a second DataStore

    Its DataWindow uses the composer name as a retrieval argument for the music titles table.

  • Inserts music titles as child items for the selected composer

    The handle argument of ItemPopulate will be the parent of the new items:

    //ItemPopulate event for tv_1
    TreeViewItem tvi1, tvi2
    long ll_row, ll_rowcount
    //Create instance variable datastore
    ids_info = CREATE datastore
    ids_info.DataObject = "d_music"
    //Use the label of the item being populated 
    // as the retrieval argument
    This.GetItem(handle, tvi1)
    ll_rowcount = ids_info.Retrieve(tvi1.Label)
    //Use information retrieved from the database
    //to populate the expanded item
    FOR ll_row = 1 to ll_rowcount
          This.InsertItemLast(handle, &
          ids_info.GetItemString(ll_row, &
          music_title'), 2)

Destroying DataStore instances

When the window containing the TreeView control closes, this example destroys the DataStore instances:

//Close event for w_treeview
DESTROY ids_data
DESTROY ids_info