如何从已设置遮罩的视图中提取CGPathRef？

Got it, let's break this down. When you've set a mask on a view, pulling out the underlying CGPathRef (or CGPath in Swift) depends entirely on what type of mask you're using—most often, it's a CAShapeLayer since that's how custom shape masks are typically created. Here's how to do it correctly, covering common scenarios:

Case 1: Your mask is a CAShapeLayer #

This is the straightforward scenario. If you used a CAShapeLayer directly (either as the view's mask or its layer's mask), you can access the path directly, then adjust for coordinate system differences:

// Assume your masked view is named `maskedView`
if let maskLayer = maskedView.layer.mask as? CAShapeLayer {
    guard let maskPath = maskLayer.path else {
        print("No path found on the mask layer")
        return
    }
    
    // Important: The path is relative to the mask layer's coordinate system
    // Convert it to the masked view's coordinate system if needed
    let pathInViewCoords = maskedView.layer.convert(maskPath, from: maskLayer)
    
    // Now you can use pathInViewCoords as your extracted CGPath
}

Key Note: Coordinate Conversion #

The mask layer's path is tied to its own bounds, not the parent view's. The convert(_:from:) method ensures the path aligns with the masked view's layout.

Case 2: Your mask is a UIView #

If you used another UIView as the mask, things are a bit trickier—UIViews use their layer's alpha channel for masking, so there's no direct CGPath property. You have two options here:

Option A: Recreate the path from the mask view's properties #

If the mask view has simple shapes (like rounded corners), you can rebuild the path manually:

if let maskView = maskedView.mask {
    let cornerRadius = maskView.layer.cornerRadius
    let maskPath = UIBezierPath(roundedRect: maskView.bounds, cornerRadius: cornerRadius).cgPath
    
    // Convert to the masked view's coordinate system
    let convertedPath = maskedView.convert(maskPath, from: maskView)
}

Option B: Expose the path from a custom mask view #

If the mask view uses custom drawing, modify the mask view class to store its drawing path:

class CustomMaskView: UIView {
    // Store the path used for drawing
    var drawingPath: CGPath?
    
    override func draw(_ rect: CGRect) {
        super.draw(rect)
        // Your custom drawing code
        let customPath = UIBezierPath(ovalIn: rect)
        drawingPath = customPath.cgPath
        customPath.fill()
    }
}

// Extract the path later:
if let customMaskView = maskedView.mask as? CustomMaskView {
    guard let maskPath = customMaskView.drawingPath else { return }
    let convertedPath = maskedView.convert(maskPath, from: customMaskView)
}

Common Pitfalls to Avoid #

• Don't mix up UIView.mask and CALayer.mask: The view's mask property is a UIView?, while the layer's mask is a CALayer?. If you set a UIView as the mask, maskedView.layer.mask will point to that view's underlying layer.
• Always validate the mask type: Make sure to check if the mask is actually a CAShapeLayer (or your custom view type) before casting, to avoid runtime crashes.

内容的提问来源于stack exchange，提问作者Sudhanshu Monga