ios - How do I move a SKSpriteNode up and down around its starting value in Swift? -

i'm working on game, there's skspritenode. want, node's location moves , down time. @ beginning, should have y value e.g. 500 . should move 200 px down (500+200=700) , should move 400 (700-400=300) creates , down, travels between 500+200 , 500-200 time. hope explained understandable. first, how effect , second, enter it? should use custom func? i'm programming swift (newbie swift)

i've written sample code can adapt purposes:

firstly, need π oscillation calculations:

// defined @ global scope. let π = cgfloat(m_pi) 

secondly, extend skaction can create actions oscillate node in question:

extension skaction {      // amplitude  - amount height vary by, set 200 in case.     // timeperiod - time takes 1 complete cycle     // midpoint   - point around oscillation occurs.      static func oscillation(amplitude a: cgfloat, timeperiod t: double, midpoint: cgpoint) -> skaction {         let action = skaction.customactionwithduration(t) { node, currenttime in             let displacement = * sin(2 * π * currenttime / cgfloat(t))             node.position.y = midpoint.y + displacement         }          return action     } } 

the displacement above comes simple harmonic motion equation displacement. see here more information (their's rearranged little, it's same equation).

thirdly, putting together, in skscene:

override func didmovetoview(view: skview) {     let node = skspritenode(color: uicolor.greencolor(),                               size: cgsize(width: 50, height: 50))     node.position = cgpoint(x: size.width / 2, y: size.height / 2)     self.addchild(node)      let oscillate = skaction.oscillation(amplitude: 200,                                          timeperiod: 1,                                           midpoint: node.position)     node.runaction(skaction.repeatactionforever(oscillate)) } 

if needed oscillate node in x-axis i'd recommend adding angle argument oscillate method , using sin , cos determine how node should oscillate in each axis.

hope helps!