Question

I have a draggable view that I have set up with the following code:

#import <UIKit/UIKit.h>

@interface DraggableView : UIImageView {

    CGPoint startLocation;
}

@end

#import "DraggableView.h"

@implementation DraggableView

- (id) initWithImage: (UIImage *) anImage
{
    if (self = [super initWithImage:anImage])
        self.userInteractionEnabled = YES;
    return self;
}

- (void) touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event
{
    // Calculate and store offset, and pop view into front if needed
    CGPoint pt = [[touches anyObject] locationInView:self];
    startLocation = pt;
    [[self superview] bringSubviewToFront:self];
}

- (void) touchesMoved:(NSSet*)touches withEvent:(UIEvent*)event
{
    // Calculate offset
    CGPoint pt = [[touches anyObject] locationInView:self];
    float dx = pt.x - startLocation.x;
    float dy = pt.y - startLocation.y;
    CGPoint newcenter = CGPointMake(self.center.x + dx, self.center.y + dy);

    // Set new location
    self.center = newcenter;
}

How do I go about snapping this view to a grid? From a broad viewpoint, I understand that I could offset the new location in a touchesEnded method call. However, I am hitting a brick wall when I try to implement this.

Thanks in advance for any assistance with this issue.

Answer 1

In touchesMoved, before applying newcenter to your view, round it to your grid step size:

float step = 10.0; // Grid step size.
newcenter.x = step * floor((newcenter.x / step) + 0.5);
newcenter.y = step * floor((newcenter.y / step) + 0.5);

This will cause your view to “snap” as you drag it.