实例成员不能用于Swift类型

Question

我在这里得到一个输入代码错误＆＃34;实例成员＆＃39;界限＆＃39;不能用于类型＆＃39; BallView＆＃39; ＆＃34; 创建目标点变量时 我该如何解决这个问题？

import UIKit
import CoreMotion

class BallView: UIView {
    var width = CGFloat(0)
    var height = CGFloat(0)
    var acceleration = CMAcceleration(x: 0, y: 0, z: 0)
    private let image = UIImage(named : "ball")!
    private let image1 = UIImage(named : "target")!
    private let image2 = UIImage(named : "target")!
    private let image3 = UIImage(named : "target")!
    private let image4 = UIImage(named : "target")!
    var currentPoint : CGPoint = CGPointZero {
        didSet {
            var newX = currentPoint.x
            var newY = currentPoint.y
            if newX < 0 {
                newX = 0
                ballXVelocity = 0
            } else if newX > bounds.size.width - image.size.width {
                newX = bounds.size.width - image.size.width
                ballXVelocity = 0
            }
            if newY < 0 {
                newY = 0
                ballYVelocity = 0
            } else if newY > bounds.size.height - image.size.height {
                newY = bounds.size.height - image.size.height
                ballYVelocity = 0
            }
            currentPoint = CGPointMake(newX, newY)

            let currentRect = CGRectMake(newX, newY,
                                         newX + image.size.width,
                                         newY + image.size.height)
            let prevRect = CGRectMake(oldValue.x, oldValue.y,
                                      oldValue.x + image.size.width,
                                      oldValue.y + image.size.height)
            setNeedsDisplayInRect(CGRectUnion(currentRect, prevRect))
        }
    }

    // set starting values for the ball's dynamic behavior:
    private var ballXVelocity = 0.0        // the ball starts standing still
    private var ballYVelocity = 0.0
    private var lastUpdateTime = NSDate()  // get the instant when we begin
    var targetPoint : CGPoint = CGPointMake(CGFloat(arc4random_uniform(UInt32(bounds.size.width))),
                                            CGFloat(arc4random_uniform(UInt32(bounds.size.height))))
    var targetPoint1 : CGPoint = CGPointMake(CGFloat(arc4random_uniform(UInt32(bounds.size.width))),
                                            CGFloat(arc4random_uniform(UInt32(bounds.size.height))))
    var targetPoint2 : CGPoint = CGPointMake(CGFloat(arc4random_uniform(UInt32(bounds.size.width))),
                                            CGFloat(arc4random_uniform(UInt32(bounds.size.height))))
    var targetPoint3 : CGPoint = CGPointMake(CGFloat(arc4random_uniform(UInt32(bounds.size.width))),
                                            CGFloat(arc4random_uniform(UInt32(bounds.size.height))))


    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        commonInit()
    }

    private func commonInit() -> Void {
        width = bounds.size.width
        height = bounds.size.height
        currentPoint = CGPointMake((bounds.size.width / 2.0) +
                                   (image.size.width / 2.0),
                                   (bounds.size.height / 2.0) +
                                   (image.size.height / 2.0))
    }

    override func drawRect(rect: CGRect) {
        // Drawing code
        image.drawAtPoint(currentPoint)
        image1.drawAtPoint(targetPoint)
        image2.drawAtPoint(targetPoint1)
        image3.drawAtPoint(targetPoint2)
        image4.drawAtPoint(targetPoint3)
    }

    func update() -> Void {
        // what time is now, i.e. the current sampling instant:
        let now = NSDate()
        // how much time passed since the last sampling instant:
        let secondsSinceLastDraw = now.timeIntervalSinceDate(lastUpdateTime)
        // compute new velocity for the ball we draw on screen:
        ballXVelocity =
                ballXVelocity + (acceleration.x * secondsSinceLastDraw)
        ballYVelocity =
                ballYVelocity - (acceleration.y * secondsSinceLastDraw)

        let xDelta = secondsSinceLastDraw * ballXVelocity * 1000
        let yDelta = secondsSinceLastDraw * ballYVelocity * 1000

        // change in position:
        currentPoint = CGPointMake(currentPoint.x + CGFloat(xDelta),
        currentPoint.y + CGFloat(yDelta))

        // make the "last sampling instant" equal to now, to be used next time through:
        lastUpdateTime = now
    }

}