Use Swift C++ interop to more closely integrate with Flutter engine

Author: lhoward

Examples/counter/macos/Flutter/GeneratedPluginRegistrant.swift

@@ -5,6 +5,4 @@
 import FlutterMacOS
 import Foundation
 
-
-func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {
-}
+func RegisterGeneratedPlugins(registry: FlutterPluginRegistry) {}

Examples/counter/swift/runner.swift

@@ -144,7 +144,11 @@ enum Counter {
       title: "Counter",
       appId: "com.example.counter"
     )
-    let window = FlutterWindow(properties: viewProperties, project: dartProject)
+    let window = FlutterWindow(
+      properties: viewProperties,
+      project: dartProject,
+      enableImpeller: true
+    )
     guard let window else {
       exit(2)
     }

Package.swift

@@ -13,7 +13,7 @@ var targetPluginUsages = [Target.PluginUsage]()
 var platformCxxSettings: [CXXSetting] = []
 var platformSwiftSettings: [SwiftSetting] = [.swiftLanguageMode(.v5)]
 
-func tryGuessSwiftLibRoot() -> String {
+func tryGuessSwiftRoot() -> String {
   let task = Process()
   task.executableURL = URL(fileURLWithPath: "/bin/sh")
   task.arguments = ["-c", "which swift"]
@@ -22,13 +22,14 @@ func tryGuessSwiftLibRoot() -> String {
     try task.run()
     let outputData = (task.standardOutput as! Pipe).fileHandleForReading.readDataToEndOfFile()
     let path = URL(fileURLWithPath: String(decoding: outputData, as: UTF8.self))
-    return path.deletingLastPathComponent().path + "/../lib/swift"
+    return path.deletingLastPathComponent().deletingLastPathComponent().deletingLastPathComponent()
+      .path
   } catch {
-    return "/usr/lib/swift"
+    return ""
   }
 }
 
-let SwiftLibRoot = tryGuessSwiftLibRoot()
+let SwiftRoot = tryGuessSwiftRoot()
 var FlutterPlatform: String
 var FlutterUnsafeLinkerFlags: [String] = []
 
@@ -235,10 +236,19 @@ packageDependencies += [
 
 let FlutterELinuxBackend = FlutterELinuxBackendType.defaultBackend
 
+let CxxIncludeDirs: [String] = [
+  "\(SwiftRoot)/usr/include",
+  "\(SwiftRoot)/usr/lib/swift",
+  "/usr/include/drm",
+]
+
+let CxxIncludeFlags = CxxIncludeDirs.flatMap { ["-I", $0] }
+
 platformSwiftSettings += [
   .define("DISPLAY_BACKEND_TYPE_\(FlutterELinuxBackend.displayBackendType)"),
   .define("FLUTTER_TARGET_BACKEND_\(FlutterELinuxBackend.flutterTargetBackend)"),
   .interoperabilityMode(.Cxx),
+  .unsafeFlags(CxxIncludeFlags),
 ]
 
 targets += [
@@ -404,7 +414,7 @@ targets += [
       .headerSearchPath("flutter-embedded-linux/src/third_party/rapidjson/include"),
       // FIXME: .cxxLanguageStandard breaks Foundation compile
       // FIXME: include path for swift/bridging.h
-      .unsafeFlags(["-pthread", "-I", SwiftLibRoot, "-I", "/usr/include/drm", "-std=c++17"]),
+      .unsafeFlags(["-pthread", "-std=c++17"] + CxxIncludeFlags),
     ],
     linkerSettings: [
       // .unsafeFlags(["-pthread"]),
@@ -446,6 +456,8 @@ platformCxxSettings += [
   .headerSearchPath(
     "../CxxFlutterSwift/flutter-embedded-linux/src/flutter/shell/platform/common/client_wrapper/include"
   ),
+  .headerSearchPath("../CxxFlutterSwift/flutter-embedded-linux/src/third_party/rapidjson/include"),
+  .unsafeFlags(CxxIncludeFlags),
 ]
 
 #else

Sources/CxxFlutterSwift/include/CxxFlutterSwift.h

@@ -1,5 +1,5 @@
 //
-// Copyright (c) 2023-2024 PADL Software Pty Ltd
+// Copyright (c) 2023-2025 PADL Software Pty Ltd
 //
 // Licensed under the Apache License, Version 2.0 (the License);
 // you may not use this file except in compliance with the License.
@@ -21,11 +21,18 @@
 
 #include <flutter_messenger.h>
 #include <flutter_elinux.h>
+#include <flutter_elinux_engine.h>
+#include <flutter_elinux_state.h>
+#include <flutter_elinux_view.h>
 #include <flutter_plugin_registrar.h>
-// #include <flutter_elinux_state.h>
 #include <flutter_platform_views.h>
 
 #ifdef __cplusplus
+#include <vector>
+#include <string>
+
+using CxxVectorOfString = std::vector<std::string, std::allocator<std::string>>;
+
 extern "C" {
 #endif
 

Sources/CxxFlutterSwift/include/module.modulemap

@@ -0,0 +1,3 @@
+module CxxFlutterSwift {
+    header "CxxFlutterSwift.h"
+}

Sources/FlutterSwift/Base/StringHelpers.swift

@@ -10,7 +10,12 @@
 //
 //===----------------------------------------------------------------------===//
 
-#if os(Linux)
+#if os(Linux) && canImport(Glibc)
+
+@_implementationOnly
+import CxxFlutterSwift
+import CxxStdlib
+
 /// Compute the prefix sum of `seq`.
 func scan<
   S: Sequence, U
@@ -60,4 +65,16 @@ extension String {
     return u32.withUnsafeBufferPointer { body($0.baseAddress!) }
   }
 }
+
+extension Array where Element == String {
+  var cxxVector: CxxVectorOfString {
+    var tmp = CxxVectorOfString()
+
+    for element in self {
+      tmp.push_back(std.string(element))
+    }
+
+    return tmp
+  }
+}
 #endif

Sources/FlutterSwift/Client/FlutterEngine.swift

@@ -1,5 +1,5 @@
 //
-// Copyright (c) 2023-2024 PADL Software Pty Ltd
+// Copyright (c) 2023-2025 PADL Software Pty Ltd
 //
 // Licensed under the Apache License, Version 2.0 (the License);
 // you may not use this file except in compliance with the License.
@@ -17,17 +17,18 @@
 #if os(Linux) && canImport(Glibc)
 @_implementationOnly
 import CxxFlutterSwift
+import CxxStdlib
 
 public final class FlutterEngine: FlutterPluginRegistry, @unchecked Sendable {
-  var engine: FlutterDesktopEngineRef! // strong or weak ref
   var pluginPublications = [String: Any]()
   let project: DartProject
   weak var viewController: FlutterViewController?
+  private var engine: flutter.FlutterELinuxEngine! // strong or weak ref
   private var _binaryMessenger: FlutterDesktopMessenger!
   private var ownsEngine = true
   private var hasBeenRun = false
 
-  public init?(project: DartProject) {
+  public init?(project: DartProject, switches: [String: Any] = [:]) {
     var properties = FlutterDesktopEngineProperties()
 
     debugPrint("Initializing Flutter engine with \(project)")
@@ -48,7 +49,9 @@ public final class FlutterEngine: FlutterPluginRegistry, @unchecked Sendable {
               )
             cStrings.withUnsafeMutableBufferPointer { pointer in
               properties.dart_entrypoint_argv = pointer.baseAddress
-              self.engine = FlutterDesktopEngineCreate(&properties)
+              let engine = FlutterDesktopEngineCreate(&properties)
+              self.engine = unsafeBitCast(engine, to: flutter.FlutterELinuxEngine.self)
+              setSwitches(switches.map { key, value in "--\(key)=\(String(describing: value))" })
               self._binaryMessenger = FlutterDesktopMessenger(engine: self.engine)
             }
           }
@@ -61,6 +64,10 @@ public final class FlutterEngine: FlutterPluginRegistry, @unchecked Sendable {
     shutDown()
   }
 
+  private var _handle: FlutterDesktopEngineRef {
+    unsafeBitCast(engine, to: FlutterDesktopEngineRef.self)
+  }
+
   // note we can't use public private(set) because we need the type to be FlutterDesktopMessenger!
   // in order for callbacks to work (otherwise self must be first initialized). But we want to
   // present a non-optional type to callers.
@@ -69,39 +76,31 @@ public final class FlutterEngine: FlutterPluginRegistry, @unchecked Sendable {
   }
 
   public func run(entryPoint: String? = nil) -> Bool {
-    if hasBeenRun {
-      debugPrint("Cannot run an engine more than once.")
-      return false
-    }
-    let runSucceeded = FlutterDesktopEngineRun(engine, entryPoint)
-    if !runSucceeded {
-      debugPrint("Failed to start engine.")
-    }
-    hasBeenRun = true
-    return runSucceeded
+    guard !hasBeenRun else { return false }
+    hasBeenRun = engine.RunWithEntrypoint(entryPoint)
+    return hasBeenRun
   }
 
   public func shutDown() {
     pluginPublications.removeAll()
-    if let engine, ownsEngine {
-      FlutterDesktopEngineDestroy(engine)
+    if engine != nil, ownsEngine {
+      FlutterDesktopEngineDestroy(_handle)
     }
     engine = nil
   }
 
   public func processMessages() -> UInt64 {
     precondition(engine != nil)
-    return FlutterDesktopEngineProcessMessages(engine)
+    return FlutterDesktopEngineProcessMessages(_handle)
   }
 
   public func reloadSystemFonts() {
-    precondition(engine != nil)
-    FlutterDesktopEngineReloadSystemFonts(engine)
+    engine.ReloadSystemFonts()
   }
 
   func relinquishEngine() -> FlutterDesktopEngineRef {
     ownsEngine = false
-    return engine
+    return _handle
   }
 
   public func registrar(for pluginKey: String) -> FlutterPluginRegistrar? {
@@ -116,5 +115,38 @@ public final class FlutterEngine: FlutterPluginRegistry, @unchecked Sendable {
   public func valuePublished(by pluginKey: String) -> Any? {
     pluginPublications[pluginKey]
   }
+
+  public var isRunning: Bool {
+    engine.running()
+  }
+
+  func onVsync(lastFrameTimeNS: UInt64, vsyncIntervalTimeNS: UInt64) {
+    engine.OnVsync(lastFrameTimeNS, vsyncIntervalTimeNS)
+  }
+
+  public var isImpellerEnabled: Bool {
+    engine.IsImpellerEnabled()
+  }
+
+  public func setSystemSettings(textScalingFactor: Float, enableHighContrast: Bool) {
+    engine.SetSystemSettings(textScalingFactor, enableHighContrast)
+  }
+
+  public func setView(_ view: FlutterView) {
+    engine.SetView(view.view)
+  }
+
+  public func setSwitches(_ switches: [String]) {
+    engine.SetSwitches(switches.cxxVector)
+  }
+
+  func getRegistrar(pluginName: String) -> FlutterDesktopPluginRegistrarRef? {
+    FlutterDesktopEngineGetPluginRegistrar(_handle, pluginName)
+  }
+
+  var textureRegistrar: FlutterDesktopTextureRegistrarRef {
+    FlutterDesktopEngineGetTextureRegistrar(_handle)
+  }
 }
+
 #endif

Sources/FlutterSwift/Client/FlutterPlugin.swift

@@ -116,7 +116,7 @@ public final class FlutterDesktopPluginRegistrar: FlutterPluginRegistrar {
   ) {
     self.engine = engine
     pluginKey = pluginName
-    registrar = FlutterDesktopEngineGetPluginRegistrar(engine.engine, pluginName)
+    registrar = engine.getRegistrar(pluginName: pluginName)
     FlutterDesktopPluginRegistrarSetDestructionHandlerBlock(registrar!) { _ in
       self.registrar = nil
     }

Sources/FlutterSwift/Client/FlutterTexture.swift

@@ -141,7 +141,7 @@ public struct FlutterDesktopTextureRegistrar {
   private let registrar: FlutterDesktopTextureRegistrarRef
 
   public init(engine: FlutterEngine) {
-    registrar = FlutterDesktopEngineGetTextureRegistrar(engine.engine)
+    registrar = engine.textureRegistrar
   }
 
   init?(plugin: FlutterDesktopPluginRegistrar) {

Sources/FlutterSwift/Client/FlutterView.swift

@@ -1,5 +1,5 @@
 //
-// Copyright (c) 2023-2024 PADL Software Pty Ltd
+// Copyright (c) 2023-2025 PADL Software Pty Ltd
 //
 // Licensed under the Apache License, Version 2.0 (the License);
 // you may not use this file except in compliance with the License.
@@ -21,7 +21,7 @@ import CxxFlutterSwift
 let kChannelName = "flutter/platform_views"
 
 public struct FlutterView {
-  let view: FlutterDesktopViewRef
+  let view: flutter.FlutterELinuxView
   var platformViewsPluginRegistrar: FlutterPluginRegistrar?
   var platformViewsHandler: FlutterPlatformViewsPlugin?
   var viewController: FlutterViewController? {
@@ -38,16 +38,20 @@ public struct FlutterView {
     }
   }
 
-  init(_ view: FlutterDesktopViewRef) {
+  init(_ view: flutter.FlutterELinuxView) {
     self.view = view
   }
 
+  init(_ view: FlutterDesktopViewRef) {
+    self.init(unsafeBitCast(view, to: flutter.FlutterELinuxView.self))
+  }
+
   public func dispatchEvent() -> Bool {
-    FlutterDesktopViewDispatchEvent(view)
+    view.DispatchEvent()
   }
 
   public var frameRate: Int32 {
-    FlutterDesktopViewGetFrameRate(view)
+    view.GetFrameRate()
   }
 }
 #endif

Sources/FlutterSwift/Client/FlutterViewController.swift

@@ -91,11 +91,16 @@ public final class FlutterViewController {
     }
   }
 
-  public init?(properties viewProperties: ViewProperties, project: DartProject) {
+  public init?(
+    properties viewProperties: ViewProperties,
+    project: DartProject,
+    switches: [String: Any] = [:]
+  ) {
     var cViewProperties = FlutterDesktopViewProperties()
     var controller: FlutterDesktopViewControllerRef?
 
-    guard let engine = FlutterEngine(project: project) else { return nil }
+    guard let engine = FlutterEngine(project: project, switches: switches)
+    else { return nil }
     self.engine = engine
 
     cViewProperties.width = viewProperties.width
@@ -145,7 +150,7 @@ public final class FlutterViewController {
 
   public var view: FlutterView {
     didSet {
-      FlutterDesktopEngineSetView(engine.engine, view.view)
+      engine.setView(view)
     }
   }
 

Sources/FlutterSwift/Client/FlutterWindow.swift

@@ -27,11 +27,17 @@ public struct FlutterWindow {
 
   public init?(
     properties viewProperties: FlutterViewController.ViewProperties,
-    project: DartProject
+    project: DartProject,
+    enableImpeller: Bool = false
   ) {
+    var switches = [String: Any]()
+    if enableImpeller {
+      switches["enable-impeller"] = true
+    }
     guard let viewController = FlutterViewController(
       properties: viewProperties,
-      project: project
+      project: project,
+      switches: switches
     ) else {
       return nil
     }