Make CartonFrontend dev's file-watching opt-in to easily use it as a standalone CLI

Sources/CartonCLI/Commands/Dev.swift

@@ -18,6 +18,22 @@ import CartonKit
 import Foundation
 
 struct Dev: AsyncParsableCommand {
+  enum Error: Swift.Error & CustomStringConvertible {
+    case noBuildRequestOption
+    case noBuildResponseOption
+    case failedToOpenBuildRequestPipe
+    case failedToOpenBuildResponsePipe
+
+    var description: String {
+      switch self {
+      case .noBuildRequestOption: "--build-request option is necessary if you want to watch, but has not been specified."
+      case .noBuildResponseOption: "--build-response option is necessary if you want to watch, but has not been specified."
+      case .failedToOpenBuildRequestPipe: "failed to open build request pipe"
+      case .failedToOpenBuildResponsePipe: "failed to open build response pipe"
+      }
+    }
+  }
+
   static let entrypoint = Entrypoint(fileName: "dev.js", content: StaticResource.dev)
 
   @Option(help: "Specify name of an executable product in development.")
@@ -68,15 +84,15 @@ struct Dev: AsyncParsableCommand {
       visibility: .private
     )
   )
-  var buildRequest: String
+  var buildRequest: String?
 
   @Option(
     help: ArgumentHelp(
       "Internal: Path to the named pipe used to receive build responses from the SwiftPM Plugin process.",
       visibility: .private
     )
   )
-  var buildResponse: String
+  var buildResponse: String?
 
   @Option(
     help: ArgumentHelp(
@@ -90,27 +106,46 @@ struct Dev: AsyncParsableCommand {
     abstract: "Watch the current directory, host the app, rebuild on change."
   )
 
-  func run() async throws {
-    let terminal = InteractiveWriter.stdout
+  private func makeBuilderIfNeed() throws -> SwiftPMPluginBuilder? {
+    guard !watchPaths.isEmpty else {
+      return nil
+    }
+
+    guard let buildRequest else {
+      throw Error.noBuildRequestOption
+    }
+    guard let buildResponse else {
+      throw Error.noBuildResponseOption
+    }
 
-    let paths = try watchPaths.map {
+    let pathsToWatch = try watchPaths.map {
       try AbsolutePath(validating: $0, relativeTo: localFileSystem.currentWorkingDirectory!)
     }
 
+    guard let buildRequest = FileHandle(forWritingAtPath: buildRequest) else {
+      throw Error.failedToOpenBuildRequestPipe
+    }
+    guard let buildResponse = FileHandle(forReadingAtPath: buildResponse) else {
+      throw Error.failedToOpenBuildResponsePipe
+    }
+
+    return SwiftPMPluginBuilder(
+      pathsToWatch: pathsToWatch,
+      buildRequest: buildRequest,
+      buildResponse: buildResponse
+    )
+  }
+
+  func run() async throws {
+    let terminal = InteractiveWriter.stdout
+
     if !verbose {
       terminal.revertCursorAndClear()
     }
-    terminal.write("\nWatching these directories for changes:\n", inColor: .green)
-    paths.forEach { terminal.logLookup("", $0) }
-    terminal.write("\n")
 
     let server = try await Server(
       .init(
-        builder: SwiftPMPluginBuilder(
-          pathsToWatch: paths,
-          buildRequest: FileHandle(forWritingAtPath: buildRequest)!,
-          buildResponse: FileHandle(forReadingAtPath: buildResponse)!
-        ),
+        builder: try makeBuilderIfNeed(),
         mainWasmPath: AbsolutePath(
           validating: mainWasmPath, relativeTo: localFileSystem.currentWorkingDirectory!),
         verbose: verbose,

Sources/CartonKit/Server/Server.swift

@@ -165,6 +165,12 @@ public actor Server {
     }
 
     if !builder.pathsToWatch.isEmpty {
+      let terminal = configuration.terminal
+
+      terminal.write("\nWatching these directories for changes:\n", inColor: .green)
+      builder.pathsToWatch.forEach { terminal.logLookup("", $0) }
+      terminal.write("\n")
+
       watcher = FSWatch(paths: builder.pathsToWatch, latency: 0.1) { [weak self] changes in
         guard let self = self, !changes.isEmpty else { return }
         Task { try await self.onChange(changes, configuration) }