How do I constrict where SKSpriteNodes fall (in my case)

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Ok so I am making a game where things fall down the screen, its at a random where they spawn at the top on the X value randomly. sometimes when they spawn they go really close to the edge of the screen to the point where they are hard to see. So what I need is some way to make them spawn a little bit farther away from the edges. here is my code

Swift

}*/
    func spawnChlorine(){
        var Chlorine = SKSpriteNode(imageNamed: "Chlorine.png")
        
        var minValue = self.size.width / 8
        var maxValue = self.size.width - 20
        var spawnPoint = UInt32(maxValue - minValue)
        Chlorine.position = CGPoint(x: CGFloat (arc4random_uniform (spawnPoint )) ,y: self.size.height)
        let action = SKAction.moveToY(-20, duration: 5.0)
        let actionDone = SKAction.removeFromParent()
        Chlorine.runAction(SKAction.sequence([action, actionDone]))
        Chlorine.runAction(SKAction.repeatActionForever(action))
        Chlorine.physicsBody = SKPhysicsBody(rectangleOfSize: Chlorine.size)
        Chlorine.physicsBody?.categoryBitMask = PhysicsCatagory.Chlorine
        Chlorine.physicsBody?.contactTestBitMask = PhysicsCatagory.Shot
        Chlorine.physicsBody?.affectedByGravity = true
        Chlorine.physicsBody?.dynamic = true
        self.addChild(Chlorine)
    }

Posted 23-Aug-15 13:15pm

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Patrice T · Accepted Answer · 2015-08-23T15:39:00

Rather simple.

compute once the values in global variable.

Lets say:
play field width is 800
drop from edge is 100
sprite width is 30
drop width is computed
drop offset is computed

Swift

var fw= 800
var de= 100
var sw= 30

var dw= fw-2*de
var do= de-sw/2

and in your function, change to this line

Swift

Chlorine.position = CGPoint(x: CGFloat (arc4random_uniform (dw )+do) ,y: self.size.height)

variable names are up to you.

Update: I don't know swift, but this change should fit

Swift

func spawnChlorine(){
    var Chlorine = SKSpriteNode(imageNamed: "Chlorine.png")

    var fw= self.size.width
    var de= 100
    var sw= 30

    var dw= UInt32(fw-2*de) // dw is the narrow width of drop
    var ok= UInt32(de-sw/2) // ok is an offset to center the drops
    Chlorine.position = CGPoint(x: CGFloat (arc4random_uniform (dw )+ok) ,y: self.size.height)
    let action = SKAction.moveToY(-20, duration: 5.0)
    let actionDone = SKAction.removeFromParent()
    Chlorine.runAction(SKAction.sequence([action, actionDone]))
    Chlorine.runAction(SKAction.repeatActionForever(action))
    Chlorine.physicsBody = SKPhysicsBody(rectangleOfSize: Chlorine.size)
    Chlorine.physicsBody?.categoryBitMask = PhysicsCatagory.Chlorine
    Chlorine.physicsBody?.contactTestBitMask = PhysicsCatagory.Shot
    Chlorine.physicsBody?.affectedByGravity = true
    Chlorine.physicsBody?.dynamic = true
    self.addChild(Chlorine)
}