@IBDesignableと@IBInspectableを使ってグラデーション可能なカスタムViewを作ってみた ref: http://qiita.com/mo_to_44/items/762f42c22ae70c689cb9

file0.swift

@IBDesignable class GradationView: UIView {

    @IBInspectable var gradationStartColor : UIColor = UIColor.whiteColor()
    @IBInspectable var gradationEndColor : UIColor = UIColor.blackColor()

    @IBInspectable var gradationXPosition : CGFloat = 0.5
    @IBInspectable var gradationYPosition : CGFloat = 0.5
    @IBInspectable var circleSize : CGFloat = 1.0

    override func drawRect(rect: CGRect) {

        var startRed: CGFloat = 0
        var startGreen: CGFloat = 0
        var startBlue: CGFloat = 0
        var startAlpha: CGFloat = 0
        self.gradationStartColor.getRed(&startRed, green: &startGreen, blue: &startBlue, alpha: &startAlpha)

        var endRed: CGFloat = 0
        var endGreen: CGFloat = 0
        var endBlue: CGFloat = 0
        var endAlpha: CGFloat = 0
        self.gradationEndColor.getRed(&endRed, green: &endGreen, blue: &endBlue, alpha: &endAlpha)

        var context = UIGraphicsGetCurrentContext()
        CGContextSaveGState(context)

        CGContextAddEllipseInRect(context, self.frame)

        var colorSpaceRef:CGColorSpaceRef = CGColorSpaceCreateDeviceRGB()
        var components:[CGFloat] = [
            startRed, startGreen, startBlue, startAlpha,
            endRed, endGreen, endBlue, endAlpha,
        ]

        var locations:[CGFloat] = [ 0.0, 1.0 ]
        let locationCount = UInt(locations.count)
        var gradientRef: CGGradientRef = CGGradientCreateWithColorComponents(colorSpaceRef, components, locations, locationCount)

        var frame:CGRect = self.bounds
        var radius:CGFloat = CGRectGetWidth(frame)

        var startCenter: CGPoint = frame.origin
        startCenter.x += frame.size.width  * self.gradationXPosition
        startCenter.y += frame.size.height * self.gradationYPosition

        var endCenter = startCenter

        var startRadius: CGFloat = 0.0
        var endRadius  : CGFloat = CGRectGetWidth(self.bounds) * self.circleSize

        CGContextDrawRadialGradient(context, gradientRef, startCenter, startRadius, endCenter, endRadius, CGGradientDrawingOptions(kCGGradientDrawsAfterEndLocation))

        CGContextRestoreGState(context)
    }
}

file1.swift

func prepareForInterfaceBuilder()

file2.swift

override func drawRect(rect: CGRect) {
    #if TARGET_INTERFACE_BUILDER
       // IBで作業時に描画させたいもの
    #endif
}