Why does mouseDragged position lag behind actual mouse? - objective-c

Question

I'm trying to make a SpriteKit game where the player can drag groups of sprites around, but I can't figure out how to get the sprite to follow the mouse cursor. Using the SpriteKit boilerplate, I get this:
Here is the relevant logic for how I move the "Hello, world!" sprite in the SKNode babies
SKNode *babies;
-(void)mouseDown:(NSEvent *)theEvent {
dragStart = [theEvent locationInWindow];
babiesStart = babies.position;
}
-(void)mouseDragged:(NSEvent *)theEvent {
CGPoint translation = CGPointMake([theEvent locationInWindow].x - dragStart.x,
[theEvent locationInWindow].y - dragStart.y);
float adjust = 1.0;
babies.position = CGPointMake(babiesStart.x + translation.x * adjust,
babiesStart.y + translation.y * adjust);
}
I've tried a number of different methods, such as deltaX and Y on theEvent but I get the same result. The only solution I've found is to play with the adjust variable, but that's clearly a hack.

Answer 1

NSEvent has another method in SpriteKit, - (CGPoint)locationInNode:(SKNode *)node. By using this I was able to get correct offset values for move the SKNode along with the mouse.

Answer 2

please try this [theEvent locationInNode:self];

Answer 3

if you don't accumulate delta in mouseDragged, the loss is inevitable.
In my case the following works quite ok.
mouseDown()
{
previous = event.location(in: self)
}
mouseDragged()
{
current = event.location(in: self)
...
delta += (current - previous)
previous = current
...
}
update()
{
...
use up your delta
delta = 0
}
cheers

Answer 4

My guess is that the issue is with coordinate spaces. You're performing calculations based on -[NSEvent locationInWindow] which is, of course, in the window coordinate system. In what coordinate system is babies.position? It's at least in a view's coordinate system, although maybe SprikeKit also imposes another coordinate space.
To convert the point to the view's coordinate space, you will want to use NSPoint point = [theView convertPoint:[NSEvent locationInWindow] fromView:nil];. To convert the point from the view's coordinate space to the scene's, you'd use CGPoint cgpoint = [theScene convertPointFromView:NSPointToCGPoint(point)];. If babies is not the scene object, then to convert to the coordinate system used by babies.position, you'd do cgpoint = [babies.parent convertPoint:cgpoint fromNode:scene];. You'd then compute translation by taking the difference between babiesStart and cgpoint.
Update: actually, you wouldn't compare the result with babiesStart as such. You'd compare it with the result of the same coordinate transformation done on the original cursor location. So, you'd compute dragStart similar to how you'd compute cgpoint. Later, you'd take the difference between those.

Answer 5

This is normal behavior.
When you take the mouse position as reported by the event, some time passes before the Sprite Kit view is redrawn, at which point the cursor has already moved to a new position.
There isn't much you can do about it, except maybe predict the position for very fast movement by factoring in the previous mouse events distances and thus predicting where the next position is likely going to be, and then take the actual mouse position and adjust it a little according to the most recent general movement direction.
Usually this is overkill though.