Adds .previewGuide() view modifier to add a guide line that is visibl… (

#50)

* Adds .previewGuide() view modifier to add a guide line that is visible only in the preview

* Preview guides: code review feedback

Sources/UIExtensions/SwiftUIPreviews/PreviewGuide.swift

@@ -0,0 +1,110 @@
+// Copyright © 2023 Lautsprecher Teufel GmbH. All rights reserved.
+
+import Foundation
+import SwiftUI
+
+#if DEBUG
+public struct PreviewGuide: ViewModifier {
+    private let edge: Edge
+    private let color: Color
+    private let distance: CGFloat
+
+    private var isRunningInAPreviewCanvas: Bool {
+        // From https://stackoverflow.com/a/61741858 - env is "1" when running in a canvas.
+        ProcessInfo.processInfo.environment["XCODE_RUNNING_FOR_PREVIEWS"] == "1"
+    }
+
+    public init(edge: Edge, color: Color, distance: CGFloat) {
+        self.edge = edge
+        self.color = color
+        self.distance = distance
+    }
+
+    public func body(content: Content) -> some View {
+        if #available(iOS 15.0, *), isRunningInAPreviewCanvas {
+            content
+                .background {
+                    guide()
+                    .ignoresSafeArea()
+                }
+        } else {
+            content
+        }
+    }
+
+    @ViewBuilder
+    private func guide() -> some View {
+        switch edge {
+        case .top:
+            fromTop()
+        case .bottom:
+            fromBottom()
+        case .leading:
+            fromLeading()
+        case .trailing:
+            fromTrailing()
+        }
+    }
+
+    private func fromBottom() -> some View {
+        VStack {
+            Spacer()
+            color
+                .frame(height: 1.0)
+            Color.clear
+                .frame(height: distance)
+        }
+    }
+
+    private func fromTop() -> some View {
+        VStack {
+            Color.clear
+                .frame(height: distance)
+            color
+                .frame(height: 1.0)
+            Spacer()
+        }
+    }
+
+    private func fromLeading() -> some View {
+        HStack {
+            Color.clear
+                .frame(width: distance)
+            color
+                .frame(width: 1.0)
+            Spacer()
+        }
+    }
+
+    private func fromTrailing() -> some View {
+        HStack {
+            Spacer()
+            color
+                .frame(width: 1.0)
+            Color.clear
+                .frame(width: distance)
+        }
+    }
+}
+
+extension View {
+    /// Adds a guide line to the view. The guide line is only visible in the preview, neither in snapshot tests nor in production.
+    ///
+    /// Example usage:
+    /// ```
+    /// VStack {
+    ///     Spacer()
+    ///     Text("OHAI")
+    /// }
+    /// .previewGuide(.bottom, 48)
+    /// ```
+    ///
+    /// - Parameters:
+    ///   - edge: reference Edge, from where the distance should be used
+    ///   - distance: distance from the given edge of the view
+    public func previewGuide(_ edge: Edge, color: Color, distance: CGFloat) -> some View {
+        self.modifier(PreviewGuide(edge: edge, color: color, distance: distance))
+    }
+}
+
+#endif