This event is emitted before an AJAX request is sent over the network.
The event is emitted on the layer that caused the request.
The request to be sent.
The layer this request is associated with.
If this request is intended to update an existing fragment, this is that fragment's layer.
If this request is intended to open an overlay, the associated layer is the future overlay's parent layer.
The link or form element that caused the request.
Event listeners may call this method to prevent the request from being sent.
