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

i beginner developing program appstore using xcode's sprite kit. made 'test' program can try out new things before adding game. right fiddling around swiping scene - have sknode (called "background") adding several children skspritenodes. 1 sprite node visible on initial scene (the center, position 160,240), , 2 more not visible: left of scene (position -160,240), , right of scene (position 480,240).

i game able swipe left or right, , when swipes left or right, view auto-center (with animation) 1 of 3 skspritenodes. code using uipangesturerecognizer move background node works properly, , code auto-centering view works (background position set 0,0 or -320,0 or +320,0), has strange offset , doesn't center (for example, background position 7,0 or -34,0 when pan right or left). doing wrong?

p.s: using code raywenderlich's "ios games" sktmoveeffect. want note if make function f(t)=t, there no problem (at least in several tests), f(t)=t^2 or else seems have issue; if helps see code can post too

@implementation ltmyscene {     sknode *background;     skspritenode *spaceship1, *spaceship2; } -(id)initwithsize:(cgsize)size {         if (self = [super initwithsize:size]) {         /* setup 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 ... parabola solution (1,0) , (0,1) intercepts , vertex @ (1,0)             return (t*t)             ;};          skaction *upaction = [skaction actionwitheffect:upeffect];         [background runaction:upaction];     } } -(void)update:(cftimeinterval)currenttime {     /* called before each frame rendered */     nslog(@"%f,%f",background.position.x,background.position.y); }  @end 

you have remember moving larger background node other nodes children. keeping in mind, code need center on specific node is:

_worldnode.position = cgpointmake(-(mynode.position.x-(self.size.width/2)), -(mynode.position.y-(self.size.height/2))); 

the above assumes main background "canvas" called _worldnode , target node child of _worldnode