Why does this code not properly center scene when doing UIPanGestureRecognizer? - objective-c

Question

I am a beginner developing a program for the AppStore using Xcode's sprite kit. I made a 'test' program so I can try out new things before adding it to my game. Right now I am fiddling around with swiping a scene - I have an SKNode (called "background") where I am adding several children as SKSpriteNodes. One sprite node is visible on the initial scene (the center, position 160,240), and two more that are not visible: to the left of the scene (position -160,240), and to the right of the scene (position 480,240).
I would like my game to be able to swipe left or right, and when it swipes left or right, the view will auto-center itself (with animation) to one of the three SKSpriteNodes. My code using the UIPanGestureRecognizer to move the background node works properly, and my code for auto-centering the view works MOSTLY (background position set to 0,0 or -320,0 or +320,0), but sometimes it has a strange offset and doesn't completely center itself (for example, the background position will be 7,0 or -34,0 when I pan right or left). What am I doing wrong?
P.S: I am using code from RayWenderlich's "iOS Games" for the SKTMoveEffect. I also want to note that if I make the function f(t)=t, there is no problem (at least in my several tests), but f(t)=t^2 or anything else seems to have an issue; if it helps to see the code for this I can post it too
#implementation LTMyScene
{
SKNode *background;
SKSpriteNode *spaceship1, *spaceship2;
}
-(id)initWithSize:(CGSize)size {
if (self = [super initWithSize:size]) {
/* Setup your scene here */
background=[SKNode node];
[self addChild:background];
self.backgroundColor = [SKColor colorWithRed:0.15 green:0.15 blue:0.3 alpha:1.0];
SKLabelNode *myLabel = [SKLabelNode labelNodeWithFontNamed:#"Chalkduster"];
myLabel.text = #"Hello, World!";
myLabel.fontSize = 30;
myLabel.position = CGPointMake(CGRectGetMidX(self.frame),
CGRectGetMidY(self.frame));
[background addChild:myLabel];
spaceship1=[SKSpriteNode spriteNodeWithImageNamed:#"Spaceship.png"];
spaceship1.position=CGPointMake(-self.size.width/2, self.size.height/2);
spaceship1.anchorPoint=CGPointMake(0.5, 0.5);
[background addChild:spaceship1];
spaceship2=[SKSpriteNode spriteNodeWithImageNamed:#"Spaceship.png"];
spaceship2.position=CGPointMake(self.size.width*3/2, self.size.height/2);
spaceship2.anchorPoint=CGPointMake(0.5, 0.5);
[background addChild:spaceship2];
}
return self;
}
- (void)didMoveToView:(SKView *)view
{
UIPanGestureRecognizer *swipe = [[UIPanGestureRecognizer alloc] initWithTarget:self action:#selector(dragPlayer:)];
[[self view] addGestureRecognizer:swipe];
}
-(void)dragPlayer: (UIPanGestureRecognizer *)gesture {
[[[self view] layer] removeAllAnimations];
CGPoint trans = [gesture translationInView:self.view];
SKAction *moveAction = [SKAction moveByX:trans.x y:0 duration:0];
[background runAction:moveAction];
[gesture setTranslation:CGPointMake(0, 0) inView:self.view];
if([gesture state] == UIGestureRecognizerStateEnded) {
CGFloat finalX=0;
if (abs(background.position.x)<self.size.width/2) {
finalX=0;
} else if (abs(background.position.x)<self.size.width*3/2) {
finalX=self.size.width*background.position.x/abs(background.position.x);
}
NSLog(#"%f",finalX);
SKTMoveEffect *upEffect =
[SKTMoveEffect effectWithNode:background duration:0.5
startPosition:background.position
endPosition:CGPointMake(finalX, 0)];
upEffect.timingFunction = ^(float t) {
// return powf(2.0f, -3.0f * t) * fabsf(cosf(t * M_PI * 1.0f)) //has bounce
// return (-1.0f*t*t+1) //no bounce ... for parabola this is only solution with (1,0) and (0,1) as intercepts and vertex at (1,0)
return (t*t)
;};
SKAction *upAction = [SKAction actionWithEffect:upEffect];
[background runAction:upAction];
}
}
-(void)update:(CFTimeInterval)currentTime {
/* Called before each frame is rendered */
NSLog(#"%f,%f",background.position.x,background.position.y);
}
#end

Answer 1

You have to remember that you are moving the larger background node with other nodes as its children. Keeping that in mind, the code you need to center on a specific node is:
_worldNode.position = CGPointMake(-(myNode.position.x-(self.size.width/2)), -(myNode.position.y-(self.size.height/2)));
The above assumes that your main background "canvas" is called _worldNode and your target node is a child of _worldNode