How do I create a UIView with a transparent circle

问题:

I want to create a view that looks like this:

I figure what I need is a uiview with some sort of mask, I can make a mask in the shape of a circle using a UIBezierpath, however I cannot invert this makes so that it masks everything but the circle. I need this to be a mask of a view and not a fill layer because the view that I intend to mask has a UIBlurEffect on it. The end goal is to animate this UIView overtop of my existing views to provide instruction.

Please note that I am using swift. Is there away to do this? If so, how?

回答1:

You can use this function to create what you need.

func createOverlay(frame : CGRect)
{
    let overlayView = UIView(frame: frame)
    overlayView.alpha = 0.6
    overlayView.backgroundColor = UIColor.blackColor()
    self.view.addSubview(overlayView)

    let maskLayer = CAShapeLayer()

    // Create a path with the rectangle in it.
    var path = CGPathCreateMutable()

    let radius : CGFloat = 50.0
    let xOffset : CGFloat = 10
    let yOffset : CGFloat = 10

    CGPathAddArc(path, nil, overlayView.frame.width - radius/2 - xOffset, yOffset, radius, 0.0, 2 * 3.14, false)
    CGPathAddRect(path, nil, CGRectMake(0, 0, overlayView.frame.width, overlayView.frame.height))


    maskLayer.backgroundColor = UIColor.blackColor().CGColor

    maskLayer.path = path;
    maskLayer.fillRule = kCAFillRuleEvenOdd

    // Release the path since it's not covered by ARC.
    overlayView.layer.mask = maskLayer
    overlayView.clipsToBounds = true
}

Adjust the radius and xOffset and yOffset to change the radius and position of the circle.

回答2:

For Swift 3, here is rakeshbs' answer formatted so it returns the UIView needed:

func createOverlay(frame : CGRect, xOffset: CGFloat, yOffset: CGFloat, radius: CGFloat) -> UIView
{
    let overlayView = UIView(frame: frame)
    overlayView.alpha = 0.6
    overlayView.backgroundColor = UIColor.black

    // Create a path with the rectangle in it.
    let path = CGMutablePath()
    path.addArc(center: CGPoint(x: xOffset, y: yOffset), radius: radius, startAngle: 0.0, endAngle: 2 * 3.14, clockwise: false)
    path.addRect(CGRect(x: 0, y: 0, width: overlayView.frame.width, height: overlayView.frame.height))

    let maskLayer = CAShapeLayer()
    maskLayer.backgroundColor = UIColor.black.cgColor
    maskLayer.path = path;
    maskLayer.fillRule = kCAFillRuleEvenOdd

    // Release the path since it's not covered by ARC.
    overlayView.layer.mask = maskLayer
    overlayView.clipsToBounds = true

    return overlayView
}

回答3:

Updated again for Swift 4 & removed a few items to make the code tighter.

Please note that maskLayer.fillRule is set differently between Swift 4 and Swift 4.2.

func createOverlay(frame: CGRect,
                   xOffset: CGFloat,
                   yOffset: CGFloat,
                   radius: CGFloat) -> UIView {
    // Step 1
    let overlayView = UIView(frame: frame)
    overlayView.backgroundColor = UIColor.black.withAlphaComponent(0.6)
    // Step 2
    let path = CGMutablePath()
    path.addArc(center: CGPoint(x: xOffset, y: yOffset),
                radius: radius,
                startAngle: 0.0,
                endAngle: 2.0 * .pi,
                clockwise: false)
    path.addRect(CGRect(origin: .zero, size: overlayView.frame.size))
    // Step 3
    let maskLayer = CAShapeLayer()
    maskLayer.backgroundColor = UIColor.black.cgColor
    maskLayer.path = path
    // For Swift 4.0
    maskLayer.fillRule = kCAFillRuleEvenOdd
    // For Swift 4.2
    maskLayer.fillRule = .evenOdd
    // Step 4
    overlayView.layer.mask = maskLayer
    overlayView.clipsToBounds = true

    return overlayView
}

A rough breakdown on what is happening:

1. Create a view sized to the specified frame, with a black background set to 60% opacity
2. Create the path for drawing the circle using the provided starting point and radius
3. Create the mask for the area to remove
4. Apply the mask & clip to bounds

The following code snippet will call this and place a circle in the middle of the screen with radius of 50:

let overlay = createOverlay(frame: view.frame,
                            xOffset: view.frame.midX,
                            yOffset: view.frame.midY,
                            radius: 50.0)
view.addSubview(overlay)

Which looks like this: