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# 7.2 Mouse Events

When a mouse event occurs, this is communicated to GAP via a function call which in turn triggers a callback. As described in GraphicSheet to CtrlRightPBDown the following callback keys are predefined as reactions on mouse events: `LeftPBDown`, `RightPBDown`, `ShiftLeftPBDown`, `ShiftRightPBDown`, `CtrlLeftPBDown`, `CtrlRightPBDown`.

Note that when your function gets called, the mouse button will still be pressed. So it can react and for example wait for the release. There is an easy way to find out about the state of the mouse buttons after the event:

- `WcQueryPointer(` id`) F`

  id must be a `WindowId` of an XGAP sheet. This function returns a vector of four integers. The first two are the coordinates of the mouse pointer relative to the XGAP sheet. Values outside the window are represented by -1. The third element is a number where the pressed buttons are coded. If no mouse button is pressed, the value is zero. `BUTTONS.left` is added to the value, if the left mouse button is pressed and `BUTTONS.right` is added, if the right mouse button is pressed. The fourth value codes the state of the shift and control. Here the values `BUTTONS.shift` and `BUTTONS.ctrl` are used.

  This function is used in

- `Drag(` sheet`,` x`,` y`,` bt`,` func`) O`

  Call this function when a button event has occurred, so the button bt is still pressed. It waits until the user releases the mouse button and calls func for every change of the mouse position with the new x and y position as two integer parameters. You can implement a dragging procedure in this way as in the following example: (we assume that a `LeftPBDown` event just occurred and x and y contain the current mouse pointer position):

        storex := x;
        storey := y;
        box := Rectangle(sheet,x,y,0,0);
        if Drag(sheet,x,y,BUTTONS.left,
                function(x,y)
                  local bx,by,bw,bh;
                  if x < storex then
                    bx := x;
                    bw := storex - x;
                  else
                    bx := storex;
                    bw := x - storex;
                  fi;
                  if y < storey then
                    by := y;
                    bh := storey - y;
                  else
                    by := storey;
                    bh := y - storey;
                  fi;
                  if bx <> box!.x or by <> box!.y then
                    Move(box,bx,by);
                  fi;
                  if bw <> box!.w or bh <> box!.h then
                    Reshape(box,bw,bh);
                  fi;
                end) then
          the box had at one time at least a certain size
          ... work with box ...
        else
          the box was never big enough, we do nothing
        fi;
        Delete(box);

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