Adds an event handler that runs when the element receives focus.
An element receives focus through user actions, such as clicking and tabbing, or programmatically, using the focus( ) function.
You can also define an event handler using the Properties and Events panel.
Note: onFocus()
has no effect when applied to RadioButtonGroup elements.
handler(event: Event): void The name of the function or the function expression to run when the element receives focus.