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Cursor Control 2
Volume Number:6
Issue Number:9
Column Tag:C Workshop

Related Info: Quickdraw

Cursor Control

By Robert S. T. Gibson, Ontario, Canada

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.

[Robert S. T. Gibson is a Senior Software Engineer at Atryx Software Design in Canada]

Although it is not suggested by anyone who follows the Macintosh user interface, it is occasionally necessary for a program to change or restrict the cursor position. Some programs move the cursor to lock it into a grid, some restrict the cursor to a specific region, and others (usually games) allow it to wrap around the screen. Games sometimes read the cursor position, compare it to the middle of the screen, and then move the cursor to the screen’s center to keep it from reaching the edge.

Cursor tracking is maintained by many system globals. The three we are interested in, however, are MTemp, RawMouse, Mouse, and CrsrNew (see Figure 1).

It is often dangerous to change the System’s low-memory globals if you don’t know their exact effects on your system.

As always, there is a right way and a wrong way to do this. I’ll skip the wrong way and get right to what I believe to be the correct way. The C techniques used here can easily be adapted into other programming languages.

It is a relatively simple task to set the mouse position, but it requires a bit of insight into how the System keeps control.

Figure 1. Mouse Globals

System Control

Operation of the mouse depends on two routines that are called during interrupts. The first routine simply sets the global variable CrsrNew to true if the mouse position has changed. The second routine is a vertical retrace (VBL) task, which checks the value of CrsrNew and draws the cursor in the new position if the value is true.

Providing CrsrNew is set, each time the VBL task is executed, it draws the cursor and copies the value of MTemp, the new location of the mouse, into the variable RawMouse, the saved (old) location. This allows the next execution to examine the two variables. If MTemp is a valid position, RawMouse is set to the new position and the cycle continues. The mouse location is copied in to the global variable Mouse for use by routines such as GetMouse().

Program Control

Only a few simple steps are required to set the mouse position properly and safely:


1: set MTemp to the new position

2: set RawMouse to new position

3: set CrsrNew to true

First, we should define our variables:

int             *MTemp;
char            *CrsrNew;

MTemp   = (int *)  0x828;
CrsrNew = (char *) 0x8CE;

Notice that MTemp was declared as an integer, and RawMouse and Mouse were not declared at all. Since the three points are in a row, it is much easier to simply declare the first variable as an integer and increment the pointer to each position to be set. NewPos is passed to our routine, containing the point to which the mouse should be moved in the following code excerpt. Remember that points are stored internally as (v,h) or (row,column), instead of (h,v).


odeexamplestart/* 1 */

for (i=0;i<3;i++) {
 *(MTemp+2*i) = newPos.v;
 *(MTemp+2*i+1) = newPos.h;

Once the three points are set, the only task which remains to be done is the setting of the CrsrNew variable. To tell the System that the mouse has moved, the boolean must be set with a non-zero (true) value. Since it is set to -1 by the System, it will be so set here. Any true value should work, however.

/* 2 */

*CrsrNew = -1;

The System routine will realize that the global CrsrNew has been changed, will find the new position in MTemp and RawMouse and will draw the cursor in the new position.

Curse Control

It’s unwise to have your program confuse the user by moving the cursor too dramatically from where he would expect it to appear. If you are in a programming situation which is leading you to move the cursor to where you want it, rather than where the user expects it, you should think twice -- and then think again.

Listing:  Control.c

 * HandleMouse()
 * Checks the cursor position and calls MousePos if necessary
Rect  *boundsRect;
Point   mousePoint;
Point   newPoint;

 newPoint.h = newPoint.v = -1;
 if (mousePoint.h <= boundsRect->left)
 newPoint.h = boundsRect->right - 2;
 if (mousePoint.h >= boundsRect->right - 1)
 newPoint.h = boundsRect->left + 1;
 if (mousePoint.v <= boundsRect->top)
 newPoint.v = boundsRect->bottom - 2;
 if (mousePoint.v >= boundsRect->bottom - 1)
 newPoint.v = boundsRect->top + 1;
 if ( (newPoint.h + 1) || (newPoint.v + 1) )
 newPoint.h = (newPoint.h != -1) ? newPoint.h : mousePoint.h;
 newPoint.v = (newPoint.v != -1) ? newPoint.v : mousePoint.v;
Listing:  ControlMain.C

/* CursorControl */ /* by Rob Gibson */ /* August 22, 1989. */
/* MacHeaders Included */
/*********  Project file: Control.c  ControlMain.c             Functions.c 
 MacTraps MousePos.c
 Type: APPL Creator: CCTL **********/

#define ControlDialogID 1000
#define nil 0L
/* important dialog items */
enum{   quitItem = 1,   setItem,   topPosItem,     leftPosItem, 
 bottomPosItem,  rightPosItem,   boxItem     };

  /* Our global variables */
DialogPtr ControlDialog;
Rect    boundsRect;

/**InitMacintosh()  Initialize all the managers & memory ***/
{  MaxApplZone();
 FlushEvents(everyEvent, 0);
} /* end InitMacintosh */

/**GetBounds()  *  * Get the rect specified in dialog  ***/
DialogPtr theDialog;
{  Str255 str;   longdummy; = GetETNum(theDialog, topPosItem);
 boundsRect.left = GetETNum(theDialog, leftPosItem);
 boundsRect.bottom = GetETNum(theDialog, bottomPosItem);
 boundsRect.right = GetETNum(theDialog, rightPosItem);
} /* end GetBounds */

/*TrackRect() Frame old, new bound rects in current GrafPort**/
TrackRect(oldRect, r)
Rect  *oldRect;
Rect  *r;
{  FrameRect(oldRect);
} /* end TrackRect */

/**DisplayBounds() Display a rect in the dialog*/
DisplayBounds(theRect, theDialog) 
Rect    *theRect;
DialogPtr theDialog;
{  SetETNum(theDialog, topPosItem, (long)theRect->top);
 SetETNum(theDialog, leftPosItem, (long)theRect->left);
 SetETNum(theDialog, bottomPosItem, (long)theRect->bottom);
 SetETNum(theDialog, rightPosItem, (long)theRect->right);
 SelIText(theDialog, topPosItem, 0, 32767);
} /* end DisplayBounds */

/**SetBoundsLoop() User drags to specify new rect**/
DialogPtr theDialog;
{  Rect oldRect;
 Rect   newRect;
 Point  firstPoint;
 Point  secondPoint;
 Point  lastSecondPoint;

 OpenPort(deskPort = (GrafPtr)NewPtr(sizeof(GrafPort)));
 PenSize(2, 2);
 GetMouse(&firstPoint); = lastSecondPoint.v = firstPoint.v;
 newRect.left = lastSecondPoint.h = firstPoint.h;
 newRect.bottom = newRect.right = 0;
 while(Button()) {
 oldRect = newRect;
 /* If the mouse location has changed then track mouse */
 if (secondPoint.v != lastSecondPoint.v || secondPoint.h != lastSecondPoint.h) 
 /* Create a new Rect making sure it is not an empty Rect */
 if (secondPoint.v > firstPoint.v) { = firstPoint.v;
 newRect.bottom = secondPoint.v;
 else { = secondPoint.v;
 newRect.bottom = firstPoint.v;
 if (secondPoint.h > firstPoint.h) {
 newRect.left = firstPoint.h;
 newRect.right = secondPoint.h;
 else {
 newRect.left = secondPoint.h;
 newRect.right = firstPoint.h;
 lastSecondPoint = secondPoint;

 TrackRect(&oldRect, &newRect);
 DisplayBounds(&newRect, theDialog);
 boundsRect = newRect;
 } /* end SetBoundsLoop */

/*HandleControlDialog()  Main event loop  *  ****/
DialogPtr theDialog;
{  EventRecord   event; /*  Filled by GetNextEvent */
 Booleanfinished = false; /*  Are we done? */
 int    chosen;
 char   theChar;

 while (!finished) /*  do this until we selected quit */
 { /* continue with the normal get next event stuff... */
 if (GetNextEvent(everyEvent, &event))
 /*  if there was an event... then  */
 if (event.what == keyDown || event.what == autoKey)
 theChar = (char) (event.message & charCodeMask);
 switch(theChar) { 
 case ‘Q’:
 case ‘q’:
 case ‘.’:
 chosen = quitItem;
 event.what = 0; /* remove event */
 finished = true;
 ClickButton(theDialog, quitItem, 2);
   case ‘\t’:
 case ‘\b’: 
   case ‘\r’:
 case ‘\003’:
 case ‘S’:
 case ‘s’:
 chosen = setItem;
 event.what = 0; /* remove event */
 ClickButton(theDialog, setItem, true);
 ClickButton(theDialog, setItem, false);
 if (theChar < ‘0’ || theChar > ‘9’)
 event.what = 0;
 } else if (event.what == updateEvt)
 FrameItem(theDialog, boxItem);
 DrawDefaultBtn (theDialog, setItem);

 if (!finished) {
 if   (IsDialogEvent(&event))
 if (DialogSelect(&event, &theDialog, &chosen)) {
 switch (chosen) {
 case 1:
 finished = true;
 case 2:
 ClickButton(theDialog, setItem, true);
 ClickButton(theDialog, setItem, false);
 } /*  end of if switch  */
 } /*  end of if (!finished) */
 } /*  of event loop  */

} /* end HandleControlDialog */

/*****  * SetUpDialog()  *  * Set up dialog stuff  *  *****/
SetUpDialog() {
 int    itemType;
 Handle Hdl;

 ControlDialog = GetNewDialog(ControlDialogID, nil, -1L);
 CenterWindow(ControlDialog, &screenBits.bounds);
 DisplayBounds(&screenBits.bounds, ControlDialog);
 boundsRect = screenBits.bounds;
} /* end SetUpDialog */

/*****  * main()  *  * Call the main procedures  *  *****/

{  InitMacintosh();
} /* end main */
Listings:  Functions.C

 * GetEText()
 * Get text of an ETItem
GetEText (theDialog, theItem, s)
DialogPtr theDialog;
char    *s;
 int     theType;
 Handle Hdl;
 Rect box;

 GetDItem (theDialog, theItem, &theType, &Hdl, &box);
 GetIText (Hdl, s);
} /* end GetEText */

 * GetETNum()
 * Get number from an ETItem
GetETNum(theDialog, theItem)
DialogPtr theDialog;
 Str255 s;
 long theNum;
 GetEText(theDialog, theItem, &s);
 StringToNum(s, &theNum);
} /* end GetETNum */

 * SetEText()
 * Set text of an ETItem
SetEText (theDialog, theItem, s)
DialogPtr theDialog;
Str255  s;
 int     theType;
 Handle Hdl;
 Rect box;

 GetDItem (theDialog, theItem, &theType, &Hdl, &box);
 SetIText (Hdl, s);
} /* end SetEText */

 * SetETNum()
 * Set number in an ETItem
SetETNum(theDialog, theItem, theNum)
DialogPtr theDialog;
long    theNum;
 Str255 s;
 NumToString(theNum, s);
 SetEText(theDialog, theItem, &s);
} /* end GetETNum */

/***** LToGRect()  Convert a local rect to global *****/
Rect  *r;
 Point  pt1,

 pt1 = topLeft(*r);
 pt2 = botRight(*r);
 Pt2Rect(pt1, pt2, r);
} /* end LToGRect */

 * CenterWindowPoint()
 * Calculates the topleft co-ords of a window,
 * taking screen size into account
Point CenterWindowPoint (theRect)
Rect  *theRect;
 int    theInd = (screenBits.bounds.bottom<350) ? 3:4;
 Point  thePt;
 int    int1, int2;

/ 2);
/ theInd);
 SetPt(&thePt, int1, int2);
} /* end CenterWindowPoint */

 * CenterWindow()
 * Centers a dialog or window
void CenterWindow(theDialog)
DialogPtr theDialog;
/* Center window - center slightly higher for large screens */
 Point  thePt;
 Rect newBounds;

 newBounds = *&theDialog->portRect;
 thePt = CenterWindowPoint(&newBounds);
 MoveWindow(theDialog, thePt.h, thePt.v, 0);
} /* end CenterWindow */

/****ClickButton() Simulate a click in a button ****/
ClickButton(theDialog, ID, method) 
/* 0 is off, 1 is on, 2 simulates a click */
DialogPtr theDialog;
 int    itemType;
 Handle item;
 Rect box;
 long ticks;

 GetDItem(theDialog, ID, &itemType, &item, &box);
 HiliteControl((ControlHandle) item, (method >= 1));
 if (method >= 2)
 Delay(8L, &ticks);
 HiliteControl((ControlHandle) item, 0);
} /* end ClickButton */

 * FrameItem()
 * Frame a dialog item in current pen modes
void FrameItem (theDialog, item)
DialogPtr theDialog;
 int    optType;
 Handle btnHdl;
 Rect   optBox;

 GetDItem(theDialog, item, &optType, &btnHdl, &optBox);
} /* end FrameItem */

/**** DrawDefaultBtn() Outline the default button ****/
void DrawDefaultBtn (theDialog, item)
DialogPtr theDialog;
 int    optType;
 Handle btnHdl;
 Rect   optBox;

 GetDItem(theDialog, item, &optType, &btnHdl, &optBox);
 PenSize(3, 3);
 InsetRect(&optBox, -4, -4);
 FrameRoundRect(&optBox, 16, 16);
} /* end DrawDefaultBtn */
Listing:  MousePos.C

* MousePos()
* Set the mouse position, August 22, 1989
* by Robert S. T. Gibson for MacTutor
void MousePos(newPos)
Point newPos;
 int    *MTemp;
 char *CrsrNew;
 int    i;

 MTemp = (int *)  0x828;  /* Set up our globals... */
 CrsrNew = (char *) 0x8CE;

/* Points are stored as (row, column) or (v, h) in memory...*/
 /* Set our globals... */
 for (i=0;i<3;i++) {
 *(MTemp+2*i) = newPos.v;
 *(MTemp+2*i+1) = newPos.h;
 *CrsrNew = -1;  /* There’s a new position */

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