Question

How can I constrain the drag-path of a button so that it can only be dragged to the centre of the screen from any point I want?

I know the current position of the UIButton and the centre coordinates.
I need the current x and y formula for the drag event.

Answer 1

This should do it. Save the start position of the drag, get the current position on each move event. Pass those to this function. It will answer a vector which can be applied to the button’s frame.

The gist of the calculation is to determine a magnitude by choosing the smallest component of the drag (x or y) and answering a vector with the same magnitude in both dimensions (preserving the signs over both axes).

// answer a vector to apply to an object's frame, constrained to a diagonal
- (CGPoint)constrainToDiagonalFrom:(CGPoint)from to:(CGPoint)to {

    CGPoint diff = CGPointMake(to.x-from.x, to.y-from.y);
    CGFloat magnitude = MIN(fabs(diff.x), fabs(diff.y));    // this is how large the drag will be
    return CGPointMake(copysignf(magnitude, diff.x), copysignf(magnitude, diff.y));
}

Call it like this:

// on touches moved
// we saved startPoint on touches began
// get location from this event's touches

CGPoint diagonal = [self constrainToDiagonalFrom:startPoint to:location];
myButton.frame = CGRectOffset(myButton.frame, diagonal.x, diagonal.y);

You can fool around with the magnitude calculation.. largest component, smallest, average, etc. As long as the result is symmetrical in both dimensions.