Observe touches on multiple views in one motion - objective-c

Question

I have a parent view with 3 separate child views. The child views are spread out within the parent with no overlap (and with some space in between). As a user moves her finger around the screen (without lifting it), I'd like to track touches as they enter and exit each of the child views.
Example: If the user begins touching somewhere on the screen outside of the child views, then swipes her finger over child 1, off of child 1, over child 2, and then lets go, I would expect these events to be triggered:
Touch began
Touch entered child 1
Touch exited child 1
Touch entered child 2
Touch ended
It seems as if touchesBegan:withEvent: and touchesEnded:withEvent: methods would be helpful in this case, but when I define them on the child view controllers, they don't do exactly what I want -- if the user begins touching outside the child view, then swipes over the child view, no touch events are triggered on the child itself.
Current Solution: I'm currently using a solution that feels really hacky to accomplish this. I'm observing touchesBegan:withEvent:, touchesEnded:withEvent:, and touchesMoved:withEvent: on the parent, grabbing the coordinates of each event, and determining if they lie within the bounds of a child. If they do, I trigger the appropriate events as described above.
This method mostly works, but feels very inefficient. It feels like the framework should handle this work for me. My state management code also sometimes misses an "enter" or "exit" trigger and I suspect it's because touch events were either dropped or came to me in an unexpected order. Am I missing a better method here?

Answer 1

The simplest solution would be something like:
- (void)viewDidLoad
{
[super viewDidLoad];
UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:#selector(pan:)];
[self.view addGestureRecognizer:pan];
// Do any additional setup after loading the view.
}
- (void)pan:(UIPanGestureRecognizer *)sender
{
static NSInteger startViewIndex;
static NSInteger endViewIndex;
CGPoint location = [sender locationInView:self.view];
if (sender.state == UIGestureRecognizerStateBegan)
{
if (CGRectContainsPoint(self.view0.frame, location))
startViewIndex = 0;
else if (CGRectContainsPoint(self.view1.frame, location))
startViewIndex = 1;
else if (CGRectContainsPoint(self.view2.frame, location))
startViewIndex = 2;
else
startViewIndex = -1;
}
else if (sender.state == UIGestureRecognizerStateEnded)
{
if (CGRectContainsPoint(self.view0.frame, location))
endViewIndex = 0;
else if (CGRectContainsPoint(self.view1.frame, location))
endViewIndex = 1;
else if (CGRectContainsPoint(self.view2.frame, location))
endViewIndex = 2;
else
endViewIndex = -1;
if (startViewIndex != -1 && endViewIndex != -1 && startViewIndex != endViewIndex)
{
// successfully moved between subviews!
NSLog(#"Moved from %1d to %1d", startViewIndex, endViewIndex);
}
}
}
Perhaps a little more elegant would be to define your own custom gesture recognizer (that way if you aren't dragging from one of your subviews, it will fail which will allow other gesture recognizers you might have going on elsewhwere to work ... probably not an issue unless you're use multiple gesture recognizers; it also isolates the gory details of the gesture logic from the rest of your view controller):
#interface PanBetweenSubviewsGestureRecognizer : UIPanGestureRecognizer
{
NSMutableArray *_arrayOfFrames;
}
#property NSInteger startingIndex;
#property NSInteger endingIndex;
#end
#implementation PanBetweenSubviewsGestureRecognizer
#synthesize startingIndex = _startingIndex;
#synthesize endingIndex = _endingIndex;
- (void)dealloc
{
_arrayOfFrames = nil;
}
- (id)initWithTarget:(id)target action:(SEL)action
{
self = [super initWithTarget:target action:action];
if (self)
{
_arrayOfFrames = [[NSMutableArray alloc] init];
}
return self;
}
- (void)addSubviewToArrayOfFrames:(UIView *)view
{
[_arrayOfFrames addObject:[NSValue valueWithCGRect:view.frame]];
}
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
[super touchesBegan:touches withEvent:event];
UITouch *touch = [touches anyObject];
CGPoint location = [touch locationInView:self.view];
for (NSInteger i = 0; i < [_arrayOfFrames count]; i++)
{
if (CGRectContainsPoint([[_arrayOfFrames objectAtIndex:i] CGRectValue], location))
{
self.startingIndex = i;
return;
}
}
self.startingIndex = -1;
self.endingIndex = -1;
self.state = UIGestureRecognizerStateCancelled;
}
- (void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
{
[super touchesEnded:touches withEvent:event];
UITouch *touch = [touches anyObject];
CGPoint location = [touch locationInView:self.view];
for (NSInteger i = 0; i < [_arrayOfFrames count]; i++)
{
if (CGRectContainsPoint([[_arrayOfFrames objectAtIndex:i] CGRectValue], location))
{
self.endingIndex = i;
return;
}
}
self.endingIndex = -1;
self.state = UIGestureRecognizerStateCancelled;
}
#end
Which you could then use as follows:
- (void)viewDidLoad
{
[super viewDidLoad];
PanBetweenSubviewsGestureRecognizer *pan = [[PanBetweenSubviewsGestureRecognizer alloc] initWithTarget:self action:#selector(pan:)];
[pan addSubviewToArrayOfFrames:self.view0];
[pan addSubviewToArrayOfFrames:self.view1];
[pan addSubviewToArrayOfFrames:self.view2];
[self.view addGestureRecognizer:pan];
// Do any additional setup after loading the view.
}
- (void)pan:(PanBetweenSubviewsGestureRecognizer *)sender
{
if (sender.state == UIGestureRecognizerStateEnded && sender.startingIndex >= 0 && sender.endingIndex >= 0 && sender.startingIndex != sender.endingIndex)
{
// successfully moved between subviews!
NSLog(#"Moved from %1d to %1d", sender.startingIndex, sender.endingIndex);
}
}