What's in a Mouse Event

A mouse event occurs when a user moves the mouse in the user interface of an application. The VCL has three mouse events.

Mouse events

Event	Description
OnMouseDown event	Occurs when the user presses a mouse button with the mouse pointer over a control.
OnMouseMove event	Occurs when the user moves the mouse while the mouse pointer is over a control.
OnMouseUp event	Occurs when the user releases a mouse button that was pressed with the mouse pointer over a component.

When an application detects a mouse action, it calls whatever event handler you've defined for the corresponding event, passing five parameters. Use the information in those parameters to customize your responses to the events. The five parameters are as follows:

Mouse-event parameters

Parameter	Meaning
Sender	The object that detected the mouse action
Button	Indicates which mouse button was involved: mbLeft, mbMiddle, or mbRight
Shift	Indicates the state of the Alt, Ctrl, and Shift keys at the time of the mouse action
X, Y	The coordinates where the event occurred

Most of the time, you need the coordinates returned in a mouse-event handler, but sometimes you also need to check Button to determine which mouse button caused the event.

Note: Delphi uses the same criteria as Microsoft Windows in determining which mouse button has been pressed. Thus, if you have switched the default "primary" and "secondary" mouse buttons (so that the right mouse button is now the primary button), clicking the primary (right) button will record mbLeft as the value of the Button parameter.