onCancel property
[final Function(String) onCancel;[ is declaring an instance variable [onCancel[ of type [Function[ that takes a single argument of type [String[. This variable is passed as a required parameter to the [InAppWebView[ constructor and is used to handle the cancellation of the WebView. When the cancel button is pressed, the [_handleNavigation[ function is called with an empty string as the argument, which in turn calls the [onCancel[ function passed to the constructor with the argument ['Success'[. This allows the parent widget to handle the cancellation event and perform any necessary actions.
Implementation
final Function(String) onCancel;