Tweaks discovered when using Compound overrides. (#4456)

- Fixes a bug where gradients wouldn't use the latest tokens after first use.
- Makes the bloom aware of colour overrides (and discovers a bug with dark mode).
- Decouples HomeScreenCell from the HomeScreenViewModel.Context (so it can be previewed with colour overrides).

ElementX/Sources/Screens/HomeScreen/View/BloomModifier.swift

@@ -17,10 +17,12 @@ extension View {
 }
 
 private struct BloomModifier: ViewModifier {
+    @Environment(\.colorScheme) private var colorScheme
+    
     @State private var standardAppearance = UINavigationBarAppearance()
     @State private var scrollEdgeAppearance = UINavigationBarAppearance()
     
-    @State private var bloomGradientImage: UIImage?
+    @State private var bloom = Bloom()
     
     func body(content: Content) -> some View {
         content
@@ -28,33 +30,39 @@ private struct BloomModifier: ViewModifier {
     }
     
     private func configureBloom(controller: UIViewController) {
-        guard controller.navigationItem.standardAppearance != standardAppearance,
-              controller.navigationItem.scrollEdgeAppearance != scrollEdgeAppearance else {
+        if controller.navigationItem.standardAppearance == standardAppearance,
+           controller.navigationItem.scrollEdgeAppearance == scrollEdgeAppearance,
+           canUse(bloom) {
             return
         }
         
-        let image = makeBloomImage()
+        let bloom = makeBloom()
         
         standardAppearance.configureWithDefaultBackground()
-        standardAppearance.backgroundImage = image
+        standardAppearance.backgroundImage = bloom.image
         standardAppearance.backgroundImageContentMode = .scaleToFill
         controller.navigationItem.standardAppearance = standardAppearance
         
         scrollEdgeAppearance.configureWithTransparentBackground()
-        scrollEdgeAppearance.backgroundImage = image
+        scrollEdgeAppearance.backgroundImage = bloom.image
         scrollEdgeAppearance.backgroundImageContentMode = .scaleToFill
         scrollEdgeAppearance.backgroundColor = .compound.bgCanvasDefault
         controller.navigationItem.scrollEdgeAppearance = scrollEdgeAppearance
     }
     
-    private func makeBloomImage() -> UIImage? {
-        if let bloomGradientImage {
-            return bloomGradientImage
+    private func makeBloom() -> Bloom {
+        if bloom.image != nil, canUse(bloom) {
+            return bloom
         }
         
-        let newImage = ImageRenderer(content: bloomGradient).uiImage
-        Task { bloomGradientImage = newImage }
-        return newImage
+        // There's a bug somewhere when rendering in dark mode (which we've mistakenly not been doing)
+        // which results in the first 5 stops not having any alpha, only the last one…
+        let newImage = ImageRenderer(content: bloomGradient /* .colorScheme(colorScheme) */ ).uiImage
+        
+        bloom.image = newImage
+        bloom.colorScheme = colorScheme
+        bloom.baseColor = .compound.gradientSubtleStop1
+        return bloom
     }
     
     private var bloomGradient: some View {
@@ -64,4 +72,18 @@ private struct BloomModifier: ViewModifier {
             .ignoresSafeArea(edges: .all)
             .frame(width: 256, height: 256)
     }
+    
+    private func canUse(_ bloom: Bloom) -> Bool {
+        // Don't check for a nil image in here, there's no point re-rendering over and over if the render fails.
+        bloom.colorScheme == colorScheme && bloom.baseColor == .compound.gradientSubtleStop1
+    }
+    
+    // This is a class to avoid a "Modifying state during view update" warning when storing
+    // the result on the same run-loop - we want to avoid dispatching that to the next loop as
+    // that can result in further (unnecessary) renders being made.
+    class Bloom {
+        var image: UIImage?
+        var colorScheme: ColorScheme?
+        var baseColor: Color?
+    }
 }