Components often need to respond to notifications from the underlying operating system. The operating system informs the application of occurrences such as what the user does with the mouse and keyboard. Some controls also generate notifications, such as the results from user actions such as selecting an item in a list box. The component library handles most of the common notifications already. It is possible, however, that you will need to write your own code for handling such notifications. 

For VCL applications, notifications arrive in the form of messages. These messages can come from any source, including Windows, VCL components, and components you have defined. There are three aspects to working with messages:

• Understanding the message-handling system.
• Changing message handling.
• Creating new message handlers.