typedef void __fastcall (__closure *TWndMethod)(Winapi::Messages::TMessage &Message);
TWndMethod is the type for Windows message procedures.
Each Window has a message procedure that responds to calls from Windows. These calls can originate from Windows (such as paint messages), from user actions (such as mouse messages), or from your messages that you send in your own code.
TWndMethod is the type for VCL-based message procedures. Message procedures of this type can be assigned to controls using the WindowProc method. In addition, a window that implements this procedure but which is not associated with a control can be created using the AllocateHWnd function.
The Message parameter encapsulates the message to which the procedure responds. It contains fields for the incoming message parameters and a result field that holds the return value.