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Build 881
Hands-on tutorial · Input & events

Respond to text as it changes

Update an output label immediately while respecting the current option state.

1 Form / control designs1 complete BASIC filesGuided sample project

A Change handler is invoked after a value changes. Keep this handler short: normalize or display the input, but do not start an expensive operation for every keystroke. When a checkbox changes how text is displayed, update the output from both the checkbox and text events.

Step 1Prepare a safe practice project#

Extract the archive into a new writable folder, separate from your own application. Open LiveText.mcproj 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
LiveText.mcproj
{
  "schemaVersion": 1,
  "projectId": "d60b0640-478b-4800-9521-5810e93f3190",
  "name": "LiveText",
  "kind": "DesktopApp",
  "language": {
    "dialect": "McBasic1",
    "requireExplicit": true,
    "compare": "Binary",
    "defaultArrayBase": 0
  },
  "application": {
    "bundleIdentifier": "com.mcbasic.livetext",
    "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"Live Text — BASIC events"
Height400
Width640
TypeNameParent
TextBoxtxtMessageForm
LabellblLabel1Form
CommandButtoncmdButton1Form
CheckBoxchkCheckBox1Form
txtMessage · exact Inspector values
PropertyValue
AccessibleName"Message text"
EnabledTrue
Height104
Left24
MultiLineTrue
ReadOnlyFalse
TabIndex0
Text"Hello, McBasic!\nType on a second line."
Top24
VisibleTrue
Width520
WordWrapTrue
Anchor edgesLeft, Top
lblLabel1 · exact Inspector values
PropertyValue
Caption"Edit the text above to update this preview."
EnabledTrue
Height96
Left24
TabIndex1
Top152
VisibleTrue
Width520
Anchor edgesLeft, Top
cmdButton1 · exact Inspector values
PropertyValue
Caption"Assign text with BASIC"
EnabledTrue
Height32
Left24
TabIndex2
Top280
VisibleTrue
Width220
Anchor edgesLeft, Top
chkCheckBox1 · exact Inspector values
PropertyValue
Caption"Read-only input"
CheckedFalse
EnabledTrue
Height32
Left24
TabIndex3
Top328
VisibleTrue
Width220
Anchor edgesLeft, Top

Step 3Connect the events explicitly#

frmMain

ObjectEventProcedureParameters
txtMessageChangetxtMessage_ChangeChange()
cmdButton1ClickcmdButton1_ClickClick()
chkCheckBox1ChangechkCheckBox1_ChangeChange()

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 Sub chkCheckBox1_Change()

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

Forms/frmMain.bas · chkCheckBox1_Change
Private Sub chkCheckBox1_Change()
    txtMessage.ReadOnly = chkCheckBox1.Checked
End Sub
Private Sub cmdButton1_Click()

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

Forms/frmMain.bas · cmdButton1_Click
Private Sub cmdButton1_Click()
    txtMessage.Text = "Updated by BASIC"
    txtMessage.AccessibleName = "Message updated by BASIC"
End Sub
Private Sub txtMessage_Change()

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

Forms/frmMain.bas · txtMessage_Change
Private Sub txtMessage_Change()
    lblLabel1.Caption = txtMessage.Text
End Sub
Copy the complete Forms/frmMain.bas file

Replace the file once; do not append duplicate procedures.

Forms/frmMain.bas
Option Explicit

Private Sub chkCheckBox1_Change()
    txtMessage.ReadOnly = chkCheckBox1.Checked
End Sub

Private Sub cmdButton1_Click()
    txtMessage.Text = "Updated by BASIC"
    txtMessage.AccessibleName = "Message updated by BASIC"
End Sub

Private Sub txtMessage_Change()
    lblLabel1.Caption = txtMessage.Text
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 LiveText.mcproj
mcbasic run LiveText.mcproj

Acceptance checklist

  • Type a short message and verify the output updates.
  • Toggle the checkbox after typing; the output should reflect the new option.
  • Clear the text and test the command button.

Step 7Make one change and repeat the tests#

Move the shared display calculation into a private procedure and call it from both Change handlers.

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