Question

Still an iphone dev starter but trying.

I would like to have the user touch the screen at which point a pin (dot) will be placed on the view.

I have accomplished this (yey) but i would like to have snap to grip effect, and can not come up with a solution.
This is how i am placing that dot:

CGPoint drawPoint = CGPointMake(lastTouch.x – horizontalOffset, lastTouch.y – >verticalOffset);
[drawImage drawAtPoint:drawPoint]; //Tell the image to draw itself

I have a grid as a background on a Viewcontroller, overlapping that grid i have a UIView with custom drawing that displays a pin every time the user touches the screen.

(and for some reason the dot already appears before the user touches… all though that is another query that i am looking into at the moment)

So snap to grid effect. Any ideas?

Answer 1

Assuming that your approach is to let the user tap, and then put the pin at the closest grid intersection, this is basically just a rounding problem.

// assume these constants
// kVGridOffset, kHGridOffset: the difference between the origin of the view
//   and the origin of the grid
// kVGridSpacing, kHGridSpacing: the size of the grid itself

// set initial coordinates from touch
CGPoint drawPoint = CGPointMake(lastTouch.x - horizontalOffset, lastTouch.y - verticalOffset);

// remove the offset, round to nearest increment of spacing, and return offset
drawPoint.x = floor((drawPoint.x - kHGridOffset) / kHGridSpacing + 0.5) * kHGridSpacing + kHGridOffset;
drawPoint.y = floor((drawPoint.y - kVGridOffset) / kVGridSpacing + 0.5) * kVGridSpacing + kVGridOffset;

// draw the image
[drawImage drawAtPoint:drawPoint];

EDIT: with the availability of nearbyint() for rounding, things can be a little neater:

drawPoint.x = nearbyint((drawPoint.x - kHGridOffset) / kHGridSpacing) * kHGridSpacing + kHGridOffset;
drawPoint.y = nearbyint((drawPoint.y - kVGridOffset) / kVGridSpacing) * kVGridSpacing + kVGridOffset;