Using UIPinchGestureRecognizer to scale uiviews in single direction

Question

I would like to know how we can use UIPinchGestureRecognizer to scale UIView in single (x or y) directions alone. Say, if the user moves his two fi

Answer 1

I created a custom version of a UIPinchGestureRecognizer to do exactly what you're looking for. It uses the slope of line between the two fingers to determine the direction of the scale. It does 3 types: Vertical; Horizontal; and Combined(diagonal). Please see my notes at the bottom.

-(void) scaleTheView:(UIPinchGestureRecognizer *)pinchRecognizer
    {
    if ([pinchRecognizer state] == UIGestureRecognizerStateBegan || [pinchRecognizer state] == UIGestureRecognizerStateChanged) {

    if ([pinchRecognizer numberOfTouches] > 1) {

        UIView *theView = [pinchRecognizer view];

        CGPoint locationOne = [pinchRecognizer locationOfTouch:0 inView:theView];
        CGPoint locationTwo = [pinchRecognizer locationOfTouch:1 inView:theView];
            NSLog(@"touch ONE  = %f, %f", locationOne.x, locationOne.y);
            NSLog(@"touch TWO  = %f, %f", locationTwo.x, locationTwo.y);
        [scalableView setBackgroundColor:[UIColor redColor]];

        if (locationOne.x == locationTwo.x) {
                // perfect vertical line
                // not likely, but to avoid dividing by 0 in the slope equation
            theSlope = 1000.0;
        }else if (locationOne.y == locationTwo.y) {
                // perfect horz line
                // not likely, but to avoid any problems in the slope equation
            theSlope = 0.0;
        }else {
            theSlope = (locationTwo.y - locationOne.y)/(locationTwo.x - locationOne.x);
        }

        double abSlope = ABS(theSlope);

        if (abSlope < 0.5) {
                    //  Horizontal pinch - scale in the X
            [arrows setImage:[UIImage imageNamed:@"HorzArrows.png"]];
            arrows.hidden = FALSE;
                    // tranform.a  = X-axis
                NSLog(@"transform.A = %f", scalableView.transform.a);
                    // tranform.d  = Y-axis
                NSLog(@"transform.D = %f", scalableView.transform.d);

                    //  if hit scale limit along X-axis then stop scale and show Blocked image
            if (((pinchRecognizer.scale > 1.0) && (scalableView.transform.a >= 2.0)) || ((pinchRecognizer.scale < 1.0) && (scalableView.transform.a <= 0.1))) {
                blocked.hidden = FALSE;
                arrows.hidden = TRUE;
            } else {
                        // scale along X-axis
                scalableView.transform = CGAffineTransformScale(scalableView.transform, pinchRecognizer.scale, 1.0);
                pinchRecognizer.scale = 1.0;
                blocked.hidden = TRUE;
                arrows.hidden = FALSE;
            }
        }else if (abSlope > 1.7) {
                    // Vertical pinch - scale in the Y
            [arrows setImage:[UIImage imageNamed:@"VerticalArrows.png"]];
            arrows.hidden = FALSE;
                NSLog(@"transform.A = %f", scalableView.transform.a);
                NSLog(@"transform.D = %f", scalableView.transform.d);

                    //  if hit scale limit along Y-axis then don't scale and show Blocked image
            if (((pinchRecognizer.scale > 1.0) && (scalableView.transform.d >= 2.0)) || ((pinchRecognizer.scale < 1.0) && (scalableView.transform.d <= 0.1))) {
                blocked.hidden = FALSE;
                arrows.hidden = TRUE;
            } else {
                        // scale along Y-axis
                scalableView.transform = CGAffineTransformScale(scalableView.transform, 1.0, pinchRecognizer.scale);
                pinchRecognizer.scale = 1.0;
                blocked.hidden = TRUE;
                arrows.hidden = FALSE;
            }
        } else {
                    // Diagonal pinch - scale in both directions
            [arrows setImage:[UIImage imageNamed:@"CrossArrows.png"]];
            blocked.hidden = TRUE;
            arrows.hidden = FALSE;

                NSLog(@"transform.A = %f", scalableView.transform.a);
                NSLog(@"transform.D = %f", scalableView.transform.d);

                    // if we have hit any limit don't allow scaling
            if ((((pinchRecognizer.scale > 1.0) && (scalableView.transform.a >= 2.0)) || ((pinchRecognizer.scale < 1.0) && (scalableView.transform.a <= 0.1))) || (((pinchRecognizer.scale > 1.0) && (scalableView.transform.d >= 2.0)) || ((pinchRecognizer.scale < 1.0) && (scalableView.transform.d <= 0.1)))) {
                blocked.hidden = FALSE;
                arrows.hidden = TRUE;
            } else {
                        // scale in both directions
                scalableView.transform = CGAffineTransformScale(scalableView.transform, pinchRecognizer.scale, pinchRecognizer.scale);
                pinchRecognizer.scale = 1.0;
                blocked.hidden = TRUE;
                arrows.hidden = FALSE;
            }
        }  // else for diagonal pinch
    }  // if numberOfTouches
}  // StateBegan if

if ([pinchRecognizer state] == UIGestureRecognizerStateEnded || [pinchRecognizer state] == UIGestureRecognizerStateCancelled) {
    NSLog(@"StateEnded StateCancelled");
    [scalableView setBackgroundColor:[UIColor whiteColor]];
    arrows.hidden = TRUE;
    blocked.hidden = TRUE;
    }
}

Remember to add the protocol to the view controller header file:

@interface WhiteViewController : UIViewController 
{
IBOutlet UIView *scalableView;
IBOutlet UIView *mainView;
IBOutlet UIImageView *arrows;
IBOutlet UIImageView *blocked;
}
@property (strong, nonatomic) IBOutlet UIView *scalableView;
@property (strong, nonatomic) IBOutlet UIView *mainView;
@property (strong, nonatomic)IBOutlet UIImageView *arrows;
@property (strong, nonatomic)IBOutlet UIImageView *blocked;

-(void) scaleTheView:(UIPinchGestureRecognizer *)pinchRecognizer;
@end

And add the recognizer in the viewDidLoad:

- (void)viewDidLoad
{ UIPinchGestureRecognizer *pinchGesture = [[UIPinchGestureRecognizer alloc] initWithTarget:self action:@selector(scaleTheView:)];
[pinchGesture setDelegate:self];
[mainView addGestureRecognizer:pinchGesture];

arrows.hidden = TRUE;
blocked.hidden = TRUE;
[scalableView setBackgroundColor:[UIColor whiteColor]];}

This is set up to use the main view to capture the pinch; and manipulate a second view. This way you can still scale it as the view gets small. You can change it to react directly to the scalable view.

LIMITS: I arbitrarily chose the starting size of my view so a scale limit of 2.0 would equal full screen. My lower scale is set at 0.1.

USER INTERACTION: I mess around with a lot of user interaction things like changing the view's background color and adding/changing arrows over the view to show direction. It's important to give them feedback during the scaling process, especially when changing directions like this codes allows.

BUG: There is a bug in Apple's UIPinchGestureRecognizer. It registers UIGestureRecognizerStateBegan with the touch of 2 fingers as you would expect. But once it is in StateBegan or StateChanged you can lift one finger and the state remains. It doesn't move to StateEnded or StateCancelled until BOTH fingers are lifted. This created a bug in my code and many headaches! The if numberOfTouches > 1 fixes it.

FUTURE: You can change the slope settings to scale in just one direction, or just 2. If you add the arrows images, you can see them change as you rotate your fingers.