ios - How can I make my bombs either fall faster or spawn faster the longer the game lasts? -

how can make bombs either fall faster or spawn faster longer game lasts? trying make game harder longer stay alive. harder every 10 bombs. have been researching days , have tried multiple answers similar questions. believe way had make bombs making harder me this. in advance!

var bombspawnspeed : nstimeinterval = 0.40 var bombcounter : int = 0 

this in update func. understanding regular update func of whole scene.

  override func update(currenttime: cftimeinterval) {     /* called before each frame rendered */    if bombcounter >= 10 {         if bombspawnspeed > 0.15 {             bombspawnspeed -= 0.05             bombcounter = 0          }         else {             bombspawnspeed = 0.1          }      } 

here bombs

func enemies() {      let bomb1 = skspritenode(imagenamed: "atomicbomb")     let bomb2 = skspritenode(imagenamed: "emp")     let bomb3 = skspritenode(imagenamed: "biobomb")     let bomb4 = skspritenode(imagenamed: "chembomb")      let enemy = [bomb1, bomb2, bomb3, bomb4]       // enemy physics     bomb in enemy {         bomb1.size = cgsize(width: 55, height: 55)         bomb2.size = cgsize(width: 55, height: 70)         bomb3.size = cgsize(width: 30, height: 30)         bomb4.size = cgsize(width: 45, height: 70)           bomb.physicsbody = skphysicsbody(circleofradius: bomb.size.width / 4)         bomb.physicsbody?.categorybitmask = physicscategory.enemy         bomb.physicsbody?.collisionbitmask = physicscategory.missile | physicscategory.airdefense         bomb.physicsbody?.contacttestbitmask = physicscategory.missile | physicscategory.airdefense         bomb.physicsbody?.affectedbygravity = false          bomb.runaction(skaction.speedby(10, duration: 30))         bomb.physicsbody?.dynamic = true         bomb1.name = "enemy1"         bomb2.name = "enemy2"         bomb3.name = "enemy3"         bomb4.name = "enemy4"          print("bomb")      }      let deployatrandomposition = arc4random() % 4      switch deployatrandomposition {     case 0:         bomb1.position.y = frame.size.height          let positionx = arc4random_uniform(uint32(frame.size.width))          bomb1.position.x = cgfloat(positionx)          self.addchild(bomb1)          break      case 1:         bomb2.position.y = frame.size.height          let positionx = arc4random_uniform(uint32(frame.size.width))          bomb2.position.x = cgfloat(positionx)          self.addchild(bomb2)          break      case 2:         bomb3.position.y = frame.size.height          let positionx = arc4random_uniform(uint32(frame.size.width))          bomb3.position.x = cgfloat(positionx)          self.addchild(bomb3)          break      case 3:         bomb4.position.y = frame.size.height          let positionx = arc4random_uniform(uint32(frame.size.width))          bomb4.position.x = cgfloat(positionx)          self.addchild(bomb4)          break      default:          break      }     bomb1.runaction(skaction.moveto(airdefense.position, duration: 5))    bomb2.runaction(skaction.moveto(airdefense.position, duration: 5))    bomb3.runaction(skaction.moveto(airdefense.position, duration: 5))    bomb4.runaction(skaction.moveto(airdefense.position, duration: 5))  } 

this calling in touches began create bombs

enemytimer = nstimer.scheduledtimerwithtimeinterval(0.4, target: self, selector: selector("enemies"), userinfo: nil, repeats: true) 

i think better approach is

1. add new var called lasttimeaddedbomb

2. decide on interval tracking how bombs released

3. check if need add new bomb in update method

like this:

var lasttimeaddedbomb:nsdate var numberofbombsdropped:int = 0 var difficultylevel:float = 1  override func update(currenttime: cftimeinterval) {      if (numberofbombsdropped % 10 == 0) {         difficultylevel-= 0.1;     }      let intervalforbombs = difficultylevel      let timefromlastbomb = nsdate().timeintervalsincedate(lasttimeaddedbomb)     if (timefromlastbomb >= intervalforbombs) {         //call bomb         //save last bomb date         numberofbombsdropped++         lasttimeaddedbomb = nsdate()     }  }