Add dry run option to CLI

Sources/swift-openapi-generator/GenerateCommand.swift

@@ -48,10 +48,17 @@ struct _GenerateCommand: AsyncParsableCommand {
     )
     var isPluginInvocation: Bool = false
 
+    @Flag(
+        help:
+            "Simulate the command and print the operations, without actually affecting the file system."
+    )
+    var isDryRun: Bool = false
+
     func run() async throws {
         try generate.runGenerator(
             outputDirectory: outputDirectory,
-            isPluginInvocation: isPluginInvocation
+            isPluginInvocation: isPluginInvocation,
+            isDryRun: isDryRun
         )
     }
 }

Sources/swift-openapi-generator/GenerateOptions+runGenerator.swift

@@ -27,9 +27,13 @@ extension _GenerateOptions {
     ///   Server.swift) regardless of which generator mode was requested, with
     ///   the caveat that the not requested files are empty. This is due to
     ///   a limitation of the build system used by SwiftPM under the hood.
+    ///   - isDryRun: A Boolean value that indicates whether this invocation should
+    ///   be run in a testing mode to preview all the operations being carried out without
+    ///   making any actual changes.
     func runGenerator(
         outputDirectory: URL,
-        isPluginInvocation: Bool
+        isPluginInvocation: Bool,
+        isDryRun: Bool
     ) throws {
         let config = try loadedConfig()
         let sortedModes = try resolvedModes(config)
@@ -63,6 +67,7 @@ extension _GenerateOptions {
             - Diagnostics output path: \(diagnosticsOutputPath?.path ?? "<none - logs to stderr>")
             - Current directory: \(FileManager.default.currentDirectoryPath)
             - Is plugin invocation: \(isPluginInvocation)
+            - Is dry run: \(isDryRun)
             - Additional imports: \(resolvedAdditionalImports.isEmpty ? "<none>" : resolvedAdditionalImports.joined(separator: ", "))
             """
         )
@@ -72,6 +77,7 @@ extension _GenerateOptions {
                 configs: configs,
                 isPluginInvocation: isPluginInvocation,
                 outputDirectory: outputDirectory,
+                isDryRun: isDryRun,
                 diagnostics: diagnostics
             )
         } catch let error as Diagnostic {

Sources/swift-openapi-generator/runGenerator.swift

@@ -16,7 +16,6 @@ import ArgumentParser
 import _OpenAPIGeneratorCore
 
 extension _Tool {
-
     /// Runs the generator with the specified configuration values.
     /// - Parameters:
     ///   - doc: A path to the OpenAPI document.
@@ -25,12 +24,15 @@ extension _Tool {
     ///   generator invocation is coming from a SwiftPM plugin.
     ///   - outputDirectory: The directory to which the generator writes
     ///   the generated Swift files.
+    ///   - isDryRun: A Boolean value that indicates whether this invocation should
+    ///   be a dry run.
     ///   - diagnostics: A collector for diagnostics emitted by the generator.
     static func runGenerator(
         doc: URL,
         configs: [Config],
         isPluginInvocation: Bool,
         outputDirectory: URL,
+        isDryRun: Bool,
         diagnostics: any DiagnosticCollector
     ) throws {
         let docData: Data
@@ -48,14 +50,15 @@ extension _Tool {
                 docData: docData,
                 config: config,
                 outputFilePath: filePathForMode(config.mode),
+                isDryRun: isDryRun,
                 diagnostics: diagnostics
             )
         }
         if isPluginInvocation {
             let nonGeneratedModes = Set(GeneratorMode.allCases).subtracting(configs.map(\.mode))
             for mode in nonGeneratedModes.sorted() {
                 let path = filePathForMode(mode)
-                try replaceFileContents(at: path, with: { Data() })
+                try replaceFileContents(at: path, with: { Data() }, isDryRun: isDryRun)
             }
         }
     }
@@ -67,45 +70,72 @@ extension _Tool {
     ///   - config: A set of configuration values for the generator.
     ///   - outputFilePath: The directory to which the generator writes
     ///   the generated Swift files.
+    ///   - isDryRun: A Boolean value that indicates whether this invocation should
+    ///   be a dry run.
     ///   - diagnostics: A collector for diagnostics emitted by the generator.
     static func runGenerator(
         doc: URL,
         docData: Data,
         config: Config,
         outputFilePath: URL,
+        isDryRun: Bool,
         diagnostics: any DiagnosticCollector
     ) throws {
-        let didChange = try replaceFileContents(at: outputFilePath) {
-            let output = try _OpenAPIGeneratorCore.runGenerator(
-                input: .init(absolutePath: doc, contents: docData),
-                config: config,
-                diagnostics: diagnostics
-            )
-            return output.contents
+        let result = try replaceFileContents(
+            at: outputFilePath,
+            with: {
+                let output = try _OpenAPIGeneratorCore.runGenerator(
+                    input: .init(absolutePath: doc, contents: docData),
+                    config: config,
+                    diagnostics: diagnostics
+                )
+                return output.contents
+            },
+            isDryRun: isDryRun
+        )
+        if result.didCreate && isDryRun {
+            print("File \(outputFilePath.lastPathComponent) does not exist.\nCreating new file...")
+        }
+        if result.didChange {
+            print("File \(outputFilePath.lastPathComponent) will be overwritten.")
+        } else {
+            print("File \(outputFilePath.lastPathComponent) will remain unchanged.")
         }
-        print("File \(outputFilePath.lastPathComponent): \(didChange ? "changed" : "unchanged")")
     }
 
     /// Evaluates a closure to generate file data and writes the data to disk
-    /// if the data is different than the current file contents.
+    /// if the data is different than the current file contents. Will write to disk
+    /// only if `isDryRun` is set as `false`.
     /// - Parameters:
     ///   - path: A path to the file.
     ///   - contents: A closure evaluated to produce the file contents data.
+    ///   - isDryRun: A Boolean value that indicates whether this invocation should
+    ///   be a dry run. File system changes will not be written to disk in this mode.
     /// - Throws: When writing to disk fails.
-    /// - Returns: `true` if the generated contents changed, otherwise `false`.
+    /// - Returns: A tuple of two booleans didChange and didCreate.
+    ///   The former will be `true` if the generated contents changed, otherwise `false` and
+    ///   the latter will be `true` if the file did not exist before, otherwise `false`.
     @discardableResult
-    static func replaceFileContents(at path: URL, with contents: () throws -> Data) throws -> Bool {
+    static func replaceFileContents(
+        at path: URL,
+        with contents: () throws -> Data,
+        isDryRun: Bool
+    ) throws -> (didChange: Bool, didCreate: Bool) {
         let data = try contents()
         let didChange: Bool
+        var didCreate = false
         if FileManager.default.fileExists(atPath: path.path) {
             let existingData = try? Data(contentsOf: path)
             didChange = existingData != data
         } else {
             didChange = true
+            didCreate = true
         }
         if didChange {
-            try data.write(to: path)
+            if !isDryRun {
+                try data.write(to: path)
+            }
         }
-        return didChange
+        return (didChange, didCreate)
     }
 }