What is the purpose of the onChange property in a widget?

The onChange property in a widget is specifically designed to execute a Gosu expression when the value of that widget changes. This functionality allows developers to create responsive, interactive user interfaces by triggering specific actions in response to user input.

For example, if a user selects a different option from a dropdown, the onChange event can be used to execute logic that updates other parts of the UI, validates input, or performs calculations based on the new selection. This enhances the user experience by providing immediate feedback and ensuring that the application reacts dynamically to user actions.

In contrast, the other options do not accurately describe the primary function of the onChange property. Although hiding the widget, blocking changes, or preventing immediate post backs can be important actions in a web application, they are not the intended purpose of the onChange event. Instead, those functionalities would be managed through other attributes or methods specific to those actions.