mouseExited isn't called when mouse leaves trackingArea while scrolling

Question

Why mouseExited/mouseEntered isn't called when mouse exits from NStrackingArea by scrolling or doing animation?

I create code like this:

Mouse entered and exited:

-(void)mouseEntered:(NSEvent *)theEvent {
    NSLog(@"Mouse entered");
}

-(void)mouseExited:(NSEvent *)theEvent
{
    NSLog(@"Mouse exited");
}

Tracking area:

-(void)updateTrackingAreas
{ 
    if(trackingArea != nil) {
        [self removeTrackingArea:trackingArea];
        [trackingArea release];
    }

    int opts = (NSTrackingMouseEnteredAndExited | NSTrackingActiveAlways);
    trackingArea = [ [NSTrackingArea alloc] initWithRect:[self bounds]
                                             options:opts
                                               owner:self
                                            userInfo:nil];
    [self addTrackingArea:trackingArea];
}

More details:

I have added NSViews as subviews in NSScrollView's view. Each NSView have his own tracking area and when I scroll my scrollView and leave tracking area "mouseExited" isn't called but without scrolling everything works fine. Problem is that when I scroll "updateTrackingAreas" is called and I think this makes problems.

* Same problem with just NSView without adding it as subview so that's not a problem.

There is few things to take into consideration. What is the superclass ? Do you overwrite any superclass method without sending super ? Then, here is options I always pass to the trackingArea to be sure that the mouse is really always tracked: NSTrackingMouseEnteredAndExited|NSTrackingMouseMoved|NSTrackingActiveInKeyWindow — Dimillian
@Dimillian77 I changed to "NSTrackingMouseEnteredAndExited|NSTrackingMouseMoved|NSTrackingActiveInKeyWindow" but that did'nt help.. same problem. And without "NSTrackingActiveAlways" it didn't work at all.. I updated my question that it be more clearer. — Justin Boo
you need to call [super updateTrackingAreas]. And this code is inside the NSViews subclasses or the NSScrollView? — Marcelo Alves
@MarceloAlves I didn't call it, it is automatically called when I do scroll. — Justin Boo

Michael Buckley Michael Buckley · Accepted Answer · 2012-02-02T04:35:34

As you noted in the title of the question, mouseEntered and mouseExited are only called when the mouse moves. To see why this is the case, let's first look at the process of adding NSTrackingAreas for the first time.

As a simple example, let's create a view that normally draws a white background, but if the user hovers over the view, it draws a red background. This example uses ARC.

@interface ExampleView

- (void) createTrackingArea

@property (nonatomic, retain) backgroundColor;
@property (nonatomic, retain) trackingArea;

@end

@implementation ExampleView

@synthesize backgroundColor;
@synthesize trackingArea

- (id) awakeFromNib
{
    [self setBackgroundColor: [NSColor whiteColor]];
    [self createTrackingArea];
}

- (void) createTrackingArea
{
    int opts = (NSTrackingMouseEnteredAndExited | NSTrackingActiveAlways);
    trackingArea = [ [NSTrackingArea alloc] initWithRect:[self bounds]
                                             options:opts
                                               owner:self
                                            userInfo:nil];
    [self addTrackingArea:trackingArea];
}

- (void) drawRect: (NSRect) rect
{
    [[self backgroundColor] set];
    NSRectFill(rect);
}

- (void) mouseEntered: (NSEvent*) theEvent
{
    [self setBackgroundColor: [NSColor redColor]];
}

- (void) mouseEntered: (NSEvent*) theEvent
{
    [self setBackgroundColor: [NSColor whiteColor]];
}

@end

There are two problems with this code. First, when -awakeFromNib is called, if the mouse is already inside the view, -mouseEntered is not called. This means that the background will still be white, even though the mouse is over the view. This is actually mentioned in the NSView documentation for the assumeInside parameter of -addTrackingRect:owner:userData:assumeInside:

If YES, the first event will be generated when the cursor leaves aRect, regardless if the cursor is inside aRect when the tracking rectangle is added. If NO the first event will be generated when the cursor leaves aRect if the cursor is initially inside aRect, or when the cursor enters aRect if the cursor is initially outside aRect.

In both cases, if the mouse is inside the tracking area, no events will be generated until the mouse leaves the tracking area.

So to fix this, when we add the tracking area, we need to find out if the cursor is within in the tracking area. Our -createTrackingArea method thus becomes

- (void) createTrackingArea
{
    int opts = (NSTrackingMouseEnteredAndExited | NSTrackingActiveAlways);
    trackingArea = [ [NSTrackingArea alloc] initWithRect:[self bounds]
                                             options:opts
                                               owner:self
                                            userInfo:nil];
    [self addTrackingArea:trackingArea];

    NSPoint mouseLocation = [[self window] mouseLocationOutsideOfEventStream];
    mouseLocation = [self convertPoint: mouseLocation
                              fromView: nil];

    if (NSPointInRect(mouseLocation, [self bounds]))
    {
        [self mouseEntered: nil];
    }
    else
    {
        [self mouseExited: nil];
    }
}

The second problem is scrolling. When scrolling or moving a view, we need to recalculate the NSTrackingAreas in that view. This is done by removing the tracking areas and then adding them back in. As you noted, -updateTrackingAreas is called when you scroll the view. This is the place to remove and re-add the area.

- (void) updateTrackingAreas
{
    [self removeTrackingArea:trackingArea];
    [self createTrackingArea];
    [super updateTrackingAreas]; // Needed, according to the NSView documentation
}

And that should take care of your problem. Admittedly, needing to find the mouse location and then convert it to view coordinates every time you add a tracking area is something that gets old quickly, so I would recommend creating a category on NSView that handles this automatically. You won't always be able to call [self mouseEntered: nil] or [self mouseExited: nil], so you might want to make the category accept a couple blocks. One to run if the mouse is in the NSTrackingArea, and one to run if it is not.

mouseExited isn't called when mouse leaves trackingArea while scrolling

2 Answers