Add AsyncPreview package

.gitignore

@@ -0,0 +1,8 @@
+.DS_Store
+/.build
+/Packages
+xcuserdata/
+DerivedData/
+.swiftpm/configuration/registries.json
+.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
+.netrc

Package.swift

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+// swift-tools-version: 5.9
+// The swift-tools-version declares the minimum version of Swift required to build this package.
+
+import PackageDescription
+
+let package = Package(
+    name: "AsyncPreview",
+    platforms: [
+        .macOS(.v14), .iOS(.v13), .tvOS(.v13), .visionOS(.v1)
+    ],
+    products: [
+        // Products define the executables and libraries a package produces, making them visible to other packages.
+        .library(
+            name: "AsyncPreview",
+            targets: ["AsyncPreview"]),
+    ],
+    targets: [
+        // Targets are the basic building blocks of a package, defining a module or a test suite.
+        // Targets can depend on other targets in this package and products from dependencies.
+        .target(
+            name: "AsyncPreview",
+            exclude: ["README.md", "LICENSE"]),
+    ]
+)

README.md

@@ -1,2 +1,15 @@
 # AsyncPreview
-Preview asynchronous views on Xcode canvas.
+
+Preview views with asynchronous dependencies, on Xcode canvas.
+
+## Usage
+
+```
+#Preview {
+    AsyncModel { model in
+        SomeView().environment(model)
+    } model: {
+        try await SomeAsynchronousModel()
+    }
+}
+```

Sources/AsyncPreview/AsyncPreview.swift

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+//
+//  AsyncPreview.swift
+//  Circuit
+//
+//  Created by Alt on 2024-03-09.
+//
+
+import Foundation
+import SwiftUI
+
+struct AsyncPreview<V: View, M>: View {
+
+    var view: (M) -> V
+    var model: () async throws -> M?
+
+    @State internal var internalModel: M?
+    @State internal var error: Error?
+
+    var body: some View {
+        internalBody
+        .task {
+            do {
+                self.internalModel = try await model()
+            } catch {
+                self.error = error
+                print(error)
+            }
+        }
+    }
+
+    @ViewBuilder
+    internal var internalBody: some View {
+        if let internalModel {
+            view(internalModel)
+        }
+        else if let error { 
+            Label(error.localizedDescription, systemImage: "exclamationmark.triangle.fill")
+        }
+        else {
+            // Note: We need an actual view to trigger the `task` in `body`
+            ProgressView().progressViewStyle(.circular)
+        }
+    }
+}