property OnBeforeCommandHandler: TIdCmdTCPClientBeforeCommandHandlerEvent;

OnBeforeCommandHandler is a TIdCmdTCPClientBeforeCommandHandlerEvent event handler property used to respond to the notification ocurring prior to execution of a TIdCommand instance the CommandHandlers collection. 

Procedures assigned to the OnBeforeCommandHandler and OnAfterCommandHandler event handlers are in turn executed using the corresponding methods in the CommandHandlers collection. 

Arguments provided to the event handler identify the TIdCmdTCPClient client, the TIdContext for the event notification, and the data read from the peer connection and passed to the event handler.