Improve startup UX with placeholders and prefetch

Sources/Extensions/View+Shimmer.swift

@@ -0,0 +1,51 @@
+import SwiftUI
+
+struct ShimmerModifier: ViewModifier {
+    var active: Bool
+    @State private var phase: CGFloat = -1
+
+    func body(content: Content) -> some View {
+        content
+            .overlay(
+                shimmer
+                    .mask(content)
+                    .opacity(active ? 1 : 0)
+            )
+            .onAppear {
+                guard active else { return }
+                animate()
+            }
+            .onChange(of: active) { isActive in
+                if isActive {
+                    phase = -1
+                    animate()
+                }
+            }
+    }
+
+    private var shimmer: some View {
+        LinearGradient(
+            gradient: Gradient(colors: [
+                .clear,
+                Color.white.opacity(0.6),
+                .clear
+            ]),
+            startPoint: .top,
+            endPoint: .bottom
+        )
+        .rotationEffect(.degrees(70))
+        .offset(x: phase * 250)
+    }
+
+    private func animate() {
+        withAnimation(.linear(duration: 1.2).repeatForever(autoreverses: false)) {
+            phase = 1
+        }
+    }
+}
+
+extension View {
+    func shimmering(active: Bool) -> some View {
+        modifier(ShimmerModifier(active: active))
+    }
+}