Question

I am using a uiscrollview to display an UIImageview that is larger then screen size (just like photo viewing in the Photo Library). next i needed to add a point (i am using a UIImageview to show a dot) to the screen when the user taps. got that working.

now what i am trying to do is have that point to stay in the same location relative to the image. UIScrollview has some notifications for when dragging starts and ends, but nothing that gives a constant update of its contentoffset. say i place a dot at X location. i then scroll down 50 pixels, the point needs to move up 50 pixels smoothly (so the point always stays at the same location relative to the image).

does anybody have any ideas?

EDIT: adding code

- (void)viewDidLoad
{
    [super viewDidLoad];
    //other code...
    scrollView.delegate = self;
    scrollView.clipsToBounds = YES;
    scrollView.decelerationRate = UIScrollViewDecelerationRateFast;
    scrollView.minimumZoomScale = (scrollView.frame.size.width  / imageView.frame.size.width);
    scrollView.maximumZoomScale = 10.0;    
    //other code...
}

adding a point

-(void) foundTap:(UITapGestureRecognizer *) recognizer
{
    CGPoint pixelPos = [recognizer locationInView:self.view];

    UIImageView *testPoint = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"inner-circle.png"]];
    testPoint.frame = CGRectMake(0, 0, 20, 20);
    testPoint.center = CGPointMake(pixelPos.x, pixelPos.y);
    [self.scrollView addSubview:testPoint];

    NSMutableArray *tempTestPointArray = [[NSMutableArray alloc] initWithArray:testPointArray];
    [tempTestPointArray addObject:testPoint];
    testPointArray = tempTestPointArray;

    NSLog(@"recorded point %f,%f",pixelPos.x,pixelPos.y);
}

getting pixel points

-(void)scrollViewDidZoom:(UIScrollView *)scrollView
{
    NSLog(@"zoom factor: %f", scrollView.zoomScale);
    UIPinchGestureRecognizer *test = scrollView.pinchGestureRecognizer;

    UIView *piece = test.view;
    CGPoint locationInView = [test locationInView:piece];
    CGPoint locationInSuperview = [test locationInView:piece.superview];

    NSLog(@"locationInView: %@ locationInSuperView: %@", NSStringFromCGPoint(locationInView), NSStringFromCGPoint(locationInSuperview));
}

Answer 1

You can implement the UIScrollViewDelegate method scrollViewDidScroll: to get a continuous callback whenever the content offset changes.