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MouseUp and MouseDown in VB6

The MouseDown event fires when the user presses a mouse button over a control or form. Similarly the MouseUp event occurs when the user releases the mouse button over a control or form. Note that if the user moves the mouse between the time the button was pressed and released, the same control (i.e., the control that originally received the MouseDown) will receive the MouseUp event.

During the MouseDown and MouseUp event procedures, you might want to know whether the left or right mouse button was pressed. You might also want to know whether or not one of the auxiliary keys (Shift, Alt, or Ctrl) also was depressed during the mouse event.

Finally, it might be nice to know the relative position of the mouse pointer within the form or control receiving the event.

All of the foregoing information is available in parameters that the MouseUp or MouseDown event procedure receives from the system. The four parameters are:

  • Button As Integer. This is a value representing which mouse button fired the event. The value of this parameter is either vbLeftButton, vbRightButton, or vbMiddleButton. Again these terms are from the point of view of a right-handed mouse. vbLeftButton always refers to the primary button, regardless of whether it's physically the left or right button.
  • Shift As Integer. This parameter represents an integer that indicates whether an auxiliary key is pressed during the Mouse event. It contains a value of 0 (none), 1 (Shift), 2 (Ctrl), 4 (Alt), or the sum of any combination of those keys. For example, if both the Ctrl and Alt key were pressed, the value of the Shift parameter is 6. You can check for the state of any one of the auxiliary keys with one of the VB constants vbAltMask, vbCtrlMask, or vbShiftMask. The following code illustrates how you could store the state of each auxiliary key in a Boolean variable within the MouseDown or MouseUp event procedure. The bit-wise representation of 1, 2, or 4 in the Shift parameter is 000000001, 000000010, 00000100. By doing a logical AND between the Shift parameter and one of the VB Shift-key constants, you can pick out whether each of the three Shift keys is currently pressed, as illustrated in Listing 3.1.
  • X As Single. This is the horizontal position of the mouse pointer from the internal left edge of the control or form receiving the event.
  • Y As Single. This is the vertical position of the mouse pointer from the internal top edge of the control or form receiving the event.

LISTING 3.1
TESTING THE SHIFT MASK IN A MOUSEDOWN OR MOUSEUP EVENT PROCEDURE

Dim blnIsAlt As Boolean
Dim blnIsCtrl As Boolean
Dim blnIsShift As Boolean
blnIsAlt = Shift And vbAltMask
blnIsCtrl = Shift And vbCtrlMask
blnIsShift = Shift And vbShiftMask

"Of course," you may be thinking, "isn't a Click event simply the combination of a MouseUp and a MouseDown?" How does the system handle this fact when the user clicks or double-clicks the mouse? In the next section we discuss how a VB program handles the combination of these various events.


  

 

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