[testing] Extract StreamCapture test utility (flutter/engine#47774)

Factors out an RAII-based class that can be used to capture std::cout, std::cerr, or technically any other std::ostream, though that's unlikely to be useful.

This makes the logic reusable but more importantly, ensures the capture is cleaned up at the end of the test.

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm

@@ -25,6 +25,7 @@
 #include "flutter/shell/platform/embedder/embedder.h"
 #include "flutter/shell/platform/embedder/embedder_engine.h"
 #include "flutter/shell/platform/embedder/test_utils/proc_table_replacement.h"
+#include "flutter/testing/stream_capture.h"
 #include "flutter/testing/test_dart_native_resolver.h"
 #include "gtest/gtest.h"
 
@@ -189,9 +190,7 @@ TEST_F(FlutterEngineTest, CanLogToStdout) {
                     CREATE_NATIVE_ENTRY([&](Dart_NativeArguments args) { latch.Signal(); }));
 
   // Replace stdout stream buffer with our own.
-  std::stringstream buffer;
-  std::streambuf* old_buffer = std::cout.rdbuf();
-  std::cout.rdbuf(buffer.rdbuf());
+  StreamCapture stdout_capture(&std::cout);
 
   // Launch the test entrypoint.
   FlutterEngine* engine = GetFlutterEngine();
@@ -200,12 +199,10 @@ TEST_F(FlutterEngineTest, CanLogToStdout) {
 
   latch.Wait();
 
-  // Restore old stdout stream buffer.
-  std::cout.rdbuf(old_buffer);
+  stdout_capture.Stop();
 
   // Verify hello world was written to stdout.
-  std::string logs = buffer.str();
-  EXPECT_TRUE(logs.find("Hello logging") != std::string::npos);
+  EXPECT_TRUE(stdout_capture.GetOutput().find("Hello logging") != std::string::npos);
 }
 
 // TODO(cbracken): Needs deflaking. https://github.com/flutter/flutter/issues/124677

engine/src/flutter/shell/platform/windows/flutter_windows_unittests.cc

@@ -15,6 +15,7 @@
 #include "flutter/shell/platform/windows/testing/windows_test.h"
 #include "flutter/shell/platform/windows/testing/windows_test_config_builder.h"
 #include "flutter/shell/platform/windows/testing/windows_test_context.h"
+#include "flutter/testing/stream_capture.h"
 #include "gtest/gtest.h"
 #include "third_party/tonic/converter/dart_converter.h"
 
@@ -45,27 +46,20 @@ TEST_F(WindowsTest, LaunchMain) {
 // Verify there is no unexpected output from launching main.
 TEST_F(WindowsTest, LaunchMainHasNoOutput) {
   // Replace stdout & stderr stream buffers with our own.
-  std::stringstream cout_buffer;
-  std::stringstream cerr_buffer;
-  std::streambuf* old_cout_buffer = std::cout.rdbuf();
-  std::streambuf* old_cerr_buffer = std::cerr.rdbuf();
-  std::cout.rdbuf(cout_buffer.rdbuf());
-  std::cerr.rdbuf(cerr_buffer.rdbuf());
+  StreamCapture stdout_capture(&std::cout);
+  StreamCapture stderr_capture(&std::cerr);
 
   auto& context = GetContext();
   WindowsConfigBuilder builder(context);
   ViewControllerPtr controller{builder.Run()};
   ASSERT_NE(controller, nullptr);
 
-  // Restore original stdout & stderr stream buffer.
-  std::cout.rdbuf(old_cout_buffer);
-  std::cerr.rdbuf(old_cerr_buffer);
+  stdout_capture.Stop();
+  stderr_capture.Stop();
 
   // Verify stdout & stderr have no output.
-  std::string cout = cout_buffer.str();
-  std::string cerr = cerr_buffer.str();
-  EXPECT_TRUE(cout.empty());
-  EXPECT_TRUE(cerr.empty());
+  EXPECT_TRUE(stdout_capture.GetOutput().empty());
+  EXPECT_TRUE(stderr_capture.GetOutput().empty());
 }
 
 // Verify we can successfully launch a custom entry point.

engine/src/flutter/testing/BUILD.gn

@@ -25,6 +25,8 @@ source_set("testing_lib") {
     "mock_canvas.h",
     "post_task_sync.cc",
     "post_task_sync.h",
+    "stream_capture.cc",
+    "stream_capture.h",
     "test_args.cc",
     "test_args.h",
     "test_timeout_listener.cc",

engine/src/flutter/testing/stream_capture.cc

@@ -0,0 +1,31 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "flutter/testing/stream_capture.h"
+
+namespace flutter {
+namespace testing {
+
+StreamCapture::StreamCapture(std::ostream* ostream)
+    : ostream_(ostream), old_buffer_(ostream_->rdbuf()) {
+  ostream_->rdbuf(buffer_.rdbuf());
+}
+
+StreamCapture::~StreamCapture() {
+  Stop();
+}
+
+void StreamCapture::Stop() {
+  if (old_buffer_) {
+    ostream_->rdbuf(old_buffer_);
+    old_buffer_ = nullptr;
+  }
+}
+
+std::string StreamCapture::GetOutput() const {
+  return buffer_.str();
+}
+
+}  // namespace testing
+}  // namespace flutter

engine/src/flutter/testing/stream_capture.h

@@ -0,0 +1,47 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef FLUTTER_TESTING_STREAM_CAPTURE_H_
+#define FLUTTER_TESTING_STREAM_CAPTURE_H_
+
+#include <ostream>
+#include <sstream>
+#include <string>
+
+namespace flutter {
+namespace testing {
+
+// Temporarily replaces the specified stream's output buffer to capture output.
+//
+// Example:
+// StreamCapture captured_stdout(&std::cout);
+// ... code that writest to std::cout ...
+// std::string output = captured_stdout.GetCapturedOutput();
+class StreamCapture {
+ public:
+  // Begins capturing output to the specified stream.
+  StreamCapture(std::ostream* ostream);
+
+  // Stops capturing output to the specified stream, and restores the original
+  // output buffer, if |Stop| has not already been called.
+  ~StreamCapture();
+
+  // Stops capturing output to the specified stream, and restores the original
+  // output buffer.
+  void Stop();
+
+  // Returns any output written to the captured stream between construction and
+  // the first call to |Stop|, if any, or now.
+  std::string GetOutput() const;
+
+ private:
+  std::ostream* ostream_;
+  std::stringstream buffer_;
+  std::streambuf* old_buffer_;
+};
+
+}  // namespace testing
+}  // namespace flutter
+
+#endif  // FLUTTER_TESTING_STREAM_CAPTURE_H_