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Build a tree with lazy children

Select nodes, insert a child and complete a lazy expansion request.

1 Form / control designs1 complete BASIC filesGuided sample project
Download complete projectComplete event code

TreeNodes.Item uses one-based collection positions. Lazy expansion has an identity: copy RequestedId to CompletionId before supplying LazyChildren. Using only a row index would risk filling a different request after the visible tree changes.

Step 1Prepare a safe practice project#

Extract the archive into a new writable folder, separate from your own application. Open TreeView.mcbproj in McBasic™. Keep every declared source, layout, resource and migration file together.

To reconstruct it yourself, create a DesktopApp project and add the files declared below. In the complete project, follow these steps as a guided rebuild: replace each named source file once and Check after all its procedures are present.

Exact project configuration
project.mcproj
{
  "schemaVersion": 1,
  "projectId": "f0447b52-5144-4a86-bec2-54ebd79fb826",
  "name": "TreeViewSample",
  "kind": "DesktopApp",
  "ai": {
    "storeChatsInProject": false
  },
  "language": {
    "dialect": "McBasic1",
    "requireExplicit": true,
    "compare": "Binary",
    "defaultArrayBase": 0
  },
  "application": {
    "bundleIdentifier": "com.mcbasic.samples.treeview",
    "version": "0.1.0",
    "buildNumber": "1",
    "startupFormId": "form-main",
    "shutdownMode": "OnStartupFormClosed"
  },
  "forms": [
    {
      "id": "form-main",
      "typeName": "frmMain",
      "layout": "Forms/frmMain.form.json",
      "code": "Forms/frmMain.bas"
    }
  ],
  "modules": [],
  "classes": [],
  "tests": [],
  "references": [],
  "resources": [],
  "targets": [
    "osx-arm64"
  ],
  "capabilities": []
}

Step 2Build the Form and set its properties#

frmMain

Select the Form first and set its properties. Add containers before their children. These are initial Designer values, not statements to paste into a procedure.

Form propertyValue
Caption"TreeView — McBasic"
Width600
Height360
MinWidth480
MinHeight280
TypeNameParent
TreeViewtvwTreeForm
LabellblStatusForm
CommandButtoncmdCloseForm
CommandButtoncmdInsertChildForm
tvwTree · exact Inspector values
PropertyValue
Left48
Top64
Width300
Height210
Items"Root\n Child 1\n Child 2\nOther"
SelectedIndex1
ExpandAllTrue
TabIndex0
Anchor edgesLeft, Top
lblStatus · exact Inspector values
PropertyValue
Left48
Top288
Width480
Height30
Caption"Selected node: 1"
Anchor edgesLeft, Top
cmdClose · exact Inspector values
PropertyValue
Left388
Top64
Width140
Height36
Caption"Close"
TabIndex1
Anchor edgesLeft, Top
cmdInsertChild · exact Inspector values
PropertyValue
Left388
Top120
Width140
Height36
Caption"Insert child"
TabIndex2
Anchor edgesLeft, Top

Step 3Connect the events explicitly#

frmMain

ObjectEventProcedureParameters
tvwTreeChangetvwTree_ChangeChange()
cmdCloseClickcmdClose_ClickClick()
cmdInsertChildClickcmdInsertChild_ClickClick()
FormLoadForm_Load()

Step 4Enter the complete behavior#

Open the named source file. Enter declarations first, then every procedure. Expand a procedure for its full body, or copy the complete file. Helpers are required: do not stop after entering the first Click handler.

Forms/frmMain.bas

Enter these declarations at file scope, before the procedures.

Forms/frmMain.bas · declarations
Option Explicit

Private mInserted As Boolean
Private Sub Form_Load()

Initialize state before enabling dependent actions. Keep the shared declarations and startup dependencies with this procedure.

Forms/frmMain.bas · Form_Load
Private Sub Form_Load()
    Dim root As TreeNode
    tvwTree.ExpandAll = False
    Set root = tvwTree.Nodes.Item(1)
    Set tvwTree.SelectedNode = root.Nodes.Item(1)
    lblStatus.Caption = "Selected node: " & tvwTree.SelectedNode.Text
    tvwTree.LazyIndex = 3
End Sub
Private Sub cmdInsertChild_Click()

Enter the complete action, including its input checks and calls to helper procedures.

Forms/frmMain.bas · cmdInsertChild_Click
Private Sub cmdInsertChild_Click()
    Dim root As TreeNode
    Dim added As TreeNode
    If mInserted Then Exit Sub
    Set root = tvwTree.Nodes.Item(1)
    Set added = root.Nodes.Add("New child")
    added.Tag = "inserted"
    added.Expanded = False
    mInserted = True
    tvwTree.LazyIndex = 4
    cmdInsertChild.Enabled = False
    lblStatus.Caption = "Child inserted: " & added.Text & " (" & CStr(root.Nodes.Count) & " children)"
End Sub
Private Sub tvwTree_ExpandRequested()

Enter this helper in the named source file, outside every other procedure. Its callers are included in the same project.

Forms/frmMain.bas · tvwTree_ExpandRequested
Private Sub tvwTree_ExpandRequested()
    If tvwTree.RequestedIndex < 0 Then Exit Sub
    tvwTree.CompletionId = tvwTree.RequestedId
    tvwTree.LazyChildren = "Loaded child A" & Chr(10) & "Loaded child B"
End Sub
Private Sub tvwTree_Change()

Read the current value or selection and update its dependent interface. The binding above determines which object calls this handler.

Forms/frmMain.bas · tvwTree_Change
Private Sub tvwTree_Change()
    If tvwTree.SelectedNode Is Nothing Then
        lblStatus.Caption = "No selected node"
    Else
        lblStatus.Caption = "Selected node: " & tvwTree.SelectedNode.Text
    End If
End Sub
Private Sub cmdClose_Click()

Enter the complete action, including its input checks and calls to helper procedures.

Forms/frmMain.bas · cmdClose_Click
Private Sub cmdClose_Click()
    Unload Me
End Sub
Copy the complete Forms/frmMain.bas file

Replace the file once; do not append duplicate procedures.

Forms/frmMain.bas
Option Explicit

Private mInserted As Boolean

Private Sub Form_Load()
    Dim root As TreeNode
    tvwTree.ExpandAll = False
    Set root = tvwTree.Nodes.Item(1)
    Set tvwTree.SelectedNode = root.Nodes.Item(1)
    lblStatus.Caption = "Selected node: " & tvwTree.SelectedNode.Text
    tvwTree.LazyIndex = 3
End Sub

Private Sub cmdInsertChild_Click()
    Dim root As TreeNode
    Dim added As TreeNode
    If mInserted Then Exit Sub
    Set root = tvwTree.Nodes.Item(1)
    Set added = root.Nodes.Add("New child")
    added.Tag = "inserted"
    added.Expanded = False
    mInserted = True
    tvwTree.LazyIndex = 4
    cmdInsertChild.Enabled = False
    lblStatus.Caption = "Child inserted: " & added.Text & " (" & CStr(root.Nodes.Count) & " children)"
End Sub

Private Sub tvwTree_ExpandRequested()
    If tvwTree.RequestedIndex < 0 Then Exit Sub
    tvwTree.CompletionId = tvwTree.RequestedId
    tvwTree.LazyChildren = "Loaded child A" & Chr(10) & "Loaded child B"
End Sub

Private Sub tvwTree_Change()
    If tvwTree.SelectedNode Is Nothing Then
        lblStatus.Caption = "No selected node"
    Else
        lblStatus.Caption = "Selected node: " & tvwTree.SelectedNode.Text
    End If
End Sub

Private Sub cmdClose_Click()
    Unload Me
End Sub

Step 5Keep resources and storage reproducible#

No external resource or migration files are declared. Any inline migration, seed data or local JSON is present in the code above. Runtime application data and private preferences are not included in the archive.

Step 6Check, run and test the result#

Save all files and run Check. Resolve diagnostics before Run. Developer CLI users can work from the extracted folder:

Developer CLI
mcbasic check TreeView.mcbproj
mcbasic run TreeView.mcbproj

Acceptance checklist

  • Inspect the initial selected node.
  • Insert the new child once; the command becomes disabled.
  • Expand the lazy node and verify the two loaded children.
  • Select a different node and verify the caption.

Step 7Make one change and repeat the tests#

Add a second lazy branch and preserve the request identity when completing it.

Save a working Git checkpoint first. Change one behavior, Check again and repeat the acceptance checklist. Compare with the downloaded solution if a regression appears.

Source baseline 20f4bfed6b · Build manifest