[macos] Register log handler

shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

@@ -6,6 +6,7 @@
 #import "flutter/shell/platform/darwin/macos/framework/Source/FlutterEngine_Internal.h"
 
 #include <algorithm>
+#include <iostream>
 #include <vector>
 
 #import "flutter/shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMacDelegate.h"
@@ -298,6 +299,13 @@ - (BOOL)runWithEntrypoint:(NSString*)entrypoint {
   flutterArguments.dart_entrypoint_argc = dartEntrypointArgs.size();
   flutterArguments.dart_entrypoint_argv = dartEntrypointArgs.data();
   flutterArguments.root_isolate_create_callback = _project.rootIsolateCreateCallback;
+  flutterArguments.log_message_callback = [](const char* tag, const char* message,
+                                             void* user_data) {
+    if (tag && tag[0]) {
+      std::cout << tag << ": ";
+    }
+    std::cout << message << std::endl;
+  };
 
   static size_t sTaskRunnerIdentifiers = 0;
   const FlutterTaskRunnerDescription cocoa_task_runner_description = {

shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm

@@ -58,6 +58,31 @@ @interface FlutterEngine (Test)
   EXPECT_TRUE(called);
 }
 
+TEST_F(FlutterEngineTest, CanLogToStdout) {
+  // Replace stdout stream buffer with our own.
+  std::stringstream buffer;
+  std::streambuf* old_buffer = std::cout.rdbuf();
+  std::cout.rdbuf(buffer.rdbuf());
+
+  // Launch the test entrypoint.
+  FlutterEngine* engine = GetFlutterEngine();
+  EXPECT_TRUE([engine runWithEntrypoint:@"canLogToStdout"]);
+  EXPECT_TRUE(engine.running);
+
+  // Block until completion of print statement.
+  fml::AutoResetWaitableEvent latch;
+  AddNativeCallback("SignalNativeTest",
+                    CREATE_NATIVE_ENTRY([&](Dart_NativeArguments args) { latch.Signal(); }));
+  latch.Wait();
+
+  // Restore old stdout stream buffer.
+  std::cout.rdbuf(old_buffer);
+
+  // Verify hello world was written to stdout.
+  std::string logs = buffer.str();
+  EXPECT_TRUE(logs.find("Hello logging") != std::string::npos);
+}
+
 TEST_F(FlutterEngineTest, CanToggleAccessibility) {
   FlutterEngine* engine = GetFlutterEngine();
   // Capture the update callbacks before the embedder API initializes.

shell/platform/darwin/macos/framework/Source/fixtures/flutter_desktop_test.dart

@@ -3,8 +3,17 @@
 // found in the LICENSE file.
 
 import 'dart:ui';
+
+void signalNativeTest() native 'SignalNativeTest';
+
 void main() {
+}
 
+@pragma('vm:entry-point')
+void canLogToStdout() {
+  // Emit hello world message to output then signal the test.
+  print('Hello logging');
+  signalNativeTest();
 }
 
 Picture CreateSimplePicture() {
@@ -29,8 +38,6 @@ void can_composite_platform_views() {
   PlatformDispatcher.instance.scheduleFrame();
 }
 
-void signalNativeTest() native 'SignalNativeTest';
-
 @pragma('vm:entry-point')
 void native_callback() {
   signalNativeTest();