check if an instance of a spritenode is at a certain posion on the screen - sprite-kit

Question

Im trying to make it so that the program will print something to the console when an instance of the sprite is in the center of the screen from top to bottom
var sprite = SKSpriteNode()
override func didMoveToView(view: SKView) {
//spawns an instance of the sprite
NSTimer.scheduledTimerWithTimeInterval(0.3, target: self, selector: Selector("spawnObject"), userInfo: nil, repeats: true)
//call the check middle function
checkMiddle()
}
//spawns a sprite
func spawnObject() {
let size = CGSizeMake(self.frame.size.width/3, self.frame.size.width3)
self.physicsWorld.gravity = CGVectorMake(0, -CGFloat(objectSpeed))
sprite = SKSpriteNode(imageNamed: "spriteImage")
sprite.position = CGPointMake(CGFloat(100), self.frame.height)
sprite.size = size
sprite.physicsBody = SKPhysicsBody(circleOfRadius: self.frame.size.width)
sprite.physicsBody?.affectedByGravity = true
addChild(self.sprite)
}
//PRINT WHEN SPRITE IS IN THE CENTER OF SCREEN FROM TOP TO BOTTOM
func checkMiddle() {
if sprite.position.y == self.frame.size.height/2 {
print("center")
}
}

Answer 1

if this is for debugging purposes you can use :
override func update(currentTime: NSTimeInterval)
{
checkMiddle()
}
The update() function is called before every frame so you need to be careful what you put in there
If you don't need it to be checked as frequently as that (60 x per second), you could also use an action:
let checkCenterAction = SKAction.runBlock(){ self.checkMiddle() }
let waitAction = SKAction.waitForDuration( 0.1 ) // every 1/10th of a second
let checkAndWaitAction = SKAction.sequence([checkCenterAction, waitAction])
sprite.runAction(SKAction.repeatForever(checkAndWaitAction))
And yet another option is to use the SKPhysicsBody to detect collisions