Full Code with Explanation Below

import SwiftUI

struct LayerPreviewer: View {
    var body: some View {
        LayerDrawer<CAGradientLayer> { layer in
            layer.colors = [
                UIColor.red.cgColor,
                UIColor.blue.cgColor
            ]
        }
    }
}

struct LayerPreviewer_Previews: PreviewProvider {
    static var previews: some View {
        LayerPreviewer()
    }
}

struct LayerDrawer<Layer: CALayer>: UIViewRepresentable {
    let callback: (Layer) -> Void
    
    init(_ callback: @escaping (Layer) -> Void) {
        self.callback = callback
    }
    
    func makeUIView(context: Context) -> DrawerView<Layer> {
        let view = DrawerView<Layer>(frame: .zero)
        view.setContentCompressionResistancePriority(.defaultLow, for: .horizontal)
        view.setContentHuggingPriority(.defaultLow, for: .vertical)
        return view
    }

    func updateUIView(_ uiView: DrawerView<Layer>, context: Context) {
        callback(uiView.layer as! Layer)
    }
}

final class DrawerView<Layer: CALayer>: UIView {
    override class var layerClass: AnyClass { Layer.self }
    var actualLayer: Layer { layer as! Layer }
}

Explain Please

1. LayerPreviewer is a SwiftUI View that you can use to preview a CALayer. It uses the LayerDrawer view to configure and draw the layer.

struct LayerPreviewer: View {
    var body: some View {
        LayerDrawer<CAGradientLayer> { layer in
            layer.colors = [
                UIColor.red.cgColor,
                UIColor.blue.cgColor
            ]
        }
    }
}

2. In the body of LayerPreviewer, it uses LayerDrawer<CAGradientLayer> to create a CAGradientLayer. Inside the LayerDrawer initializer, you pass a closure that configures the properties of the CAGradientLayer, setting its colors property to a gradient from red to blue.

LayerDrawer<CAGradientLayer> { layer in
    layer.colors = [
        UIColor.red.cgColor,
        UIColor.blue.cgColor
    ]
}

3. LayerDrawer is a SwiftUI UIViewRepresentable view that bridges between SwiftUI and UIKit. It's generic over a Layer type that should be a subclass of CALayer. It takes a callback closure that allows you to configure the layer.

struct LayerDrawer<Layer: CALayer>: UIViewRepresentable {
    let callback: (Layer) -> Void
    
    init(_ callback: @escaping (Layer) -> Void) {
        self.callback = callback
    }
    
    func makeUIView(context: Context) -> DrawerView<Layer> {
        let view = DrawerView<Layer>(frame: .zero)
        view.setContentCompressionResistancePriority(.defaultLow, for: .horizontal)
        view.setContentHuggingPriority(.defaultLow, for: .vertical)
        return view
    }

    func updateUIView(_ uiView: DrawerView<Layer>, context: Context) {
        callback(uiView.layer as! Layer)
    }
}

4. In makeUIView(context:), it creates an instance of DrawerView<Layer>, a UIKit UIView subclass that acts as a container for your custom layer. It sets the compression resistance and content hugging priorities for this view.

5. In updateUIView(_:context:), it calls the callback closure with the actual layer (uiView.layer as! Layer) to configure it based on your specifications.

6. DrawerView is a UIKit UIView subclass that provides a layer-based view. It overrides the layerClass property to specify the type of layer it should use, which is defined by the generic Layer type.

final class DrawerView<Layer: CALayer>: UIView {
    override class var layerClass: AnyClass { Layer.self }
    var actualLayer: Layer { layer as! Layer }
}

7. Finally, there's a LayerPreviewer_Previews struct that allows you to preview the LayerPreviewer in Xcode's SwiftUI canvas.