Snap to grid effect with Quartz 2d? iphone dev

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;

Answer 2

You'll need some kind of data structure that represents the grid in your view controller. Maybe it'll contain an array of coordinates, each representing where the vertical and horizontal lines cross.

Then when you touch the view, you compare the y value with all the y values in your grid structure, and choose the one that is closest. Same for the x value. That should give the idea of a snap to grid.