feature: make the text input plugin use the correct view on the Windows platform (#163847)

## What's new?
- Updates the `TextInput.setClient` method to expect a view ID, which is
already being sent up by clients. This makes it so that the IME info
shows on the text input correctly across views 🎉. Also - text input
on windows works properly across views (although it was working before
too)
- Using the view ID in `TextInputPlugin::HandleMethodCall` to resolve
the view
- Update tests to no longer assume that we are using the implicit view
id
- Add two tests to ensure that the view id is set

## What's fixed?
- Partially fixes: https://github.com/flutter/flutter/issues/142845

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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[Discord]:
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[Data Driven Fixes]:
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engine/src/flutter/shell/platform/windows/keyboard_unittests.cc

@@ -2203,7 +2203,7 @@ TEST_F(KeyboardTest, TextInputSubmit) {
 
   tester.InjectPlatformMessage(
       "flutter/textinput", "TextInput.setClient",
-      R"|([108, {"inputAction": "TextInputAction.none"}])|");
+      R"|([108, {"inputAction": "TextInputAction.none", "viewId": 0}])|");
 
   // Press Enter
   tester.InjectKeyboardChanges(std::vector<KeyboardChange>{

engine/src/flutter/shell/platform/windows/testing/engine_modifier.h

@@ -43,6 +43,11 @@ class EngineModifier {
     engine_->views_[kImplicitViewId] = view;
   }
 
+  /// Associate a view with a view id.
+  void SetViewById(FlutterWindowsView* view, FlutterViewId viewId) {
+    engine_->views_[viewId] = view;
+  }
+
   /// Reset the start_time field that is used to align vsync events.
   void SetStartTime(uint64_t start_time_nanos) {
     engine_->start_time_ = std::chrono::nanoseconds(start_time_nanos);

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

@@ -38,6 +38,7 @@ static constexpr char kDeltaEndKey[] = "deltaEnd";
 static constexpr char kDeltasKey[] = "deltas";
 static constexpr char kEnableDeltaModel[] = "enableDeltaModel";
 static constexpr char kTextInputAction[] = "inputAction";
+static constexpr char kViewId[] = "viewId";
 static constexpr char kTextInputType[] = "inputType";
 static constexpr char kTextInputTypeName[] = "name";
 static constexpr char kComposingBaseKey[] = "composingBase";
@@ -216,12 +217,12 @@ void TextInputPlugin::HandleMethodCall(
   if (method.compare(kShowMethod) == 0 || method.compare(kHideMethod) == 0) {
     // These methods are no-ops.
   } else if (method.compare(kClearClientMethod) == 0) {
-    // TODO(loicsharma): Remove implicit view assumption.
+    FlutterWindowsView* view = engine_->view(view_id_);
-    // https://github.com/flutter/flutter/issues/142845
-    FlutterWindowsView* view = engine_->view(kImplicitViewId);
     if (view == nullptr) {
-      result->Error(kInternalConsistencyError,
+      std::stringstream ss;
-                    "Text input is not available in Windows headless mode");
+      ss << "Text input is not available because view with view_id=" << view_id_
+         << " cannot be found";
+      result->Error(kInternalConsistencyError, ss.str());
       return;
     }
     if (active_model_ != nullptr && active_model_->composing()) {
@@ -255,6 +256,15 @@ void TextInputPlugin::HandleMethodCall(
         enable_delta_model_json->value.IsBool()) {
       enable_delta_model = enable_delta_model_json->value.GetBool();
     }
+    auto view_id_json = client_config.FindMember(kViewId);
+    if (view_id_json != client_config.MemberEnd() &&
+        view_id_json->value.IsInt()) {
+      view_id_ = view_id_json->value.GetInt();
+    } else {
+      result->Error(kBadArgumentError,
+                    "Could not set client, view ID is null.");
+      return;
+    }
     input_action_ = "";
     auto input_action_json = client_config.FindMember(kTextInputAction);
     if (input_action_json != client_config.MemberEnd() &&
@@ -328,12 +338,12 @@ void TextInputPlugin::HandleMethodCall(
           TextRange(composing_base, composing_extent), cursor_offset);
     }
   } else if (method.compare(kSetMarkedTextRect) == 0) {
-    // TODO(loicsharma): Remove implicit view assumption.
+    FlutterWindowsView* view = engine_->view(view_id_);
-    // https://github.com/flutter/flutter/issues/142845
-    FlutterWindowsView* view = engine_->view(kImplicitViewId);
     if (view == nullptr) {
-      result->Error(kInternalConsistencyError,
+      std::stringstream ss;
-                    "Text input is not available in Windows headless mode");
+      ss << "Text input is not available because view with view_id=" << view_id_
+         << " cannot be found";
+      result->Error(kInternalConsistencyError, ss.str());
       return;
     }
     if (!method_call.arguments() || method_call.arguments()->IsNull()) {
@@ -359,12 +369,12 @@ void TextInputPlugin::HandleMethodCall(
     Rect transformed_rect = GetCursorRect();
     view->OnCursorRectUpdated(transformed_rect);
   } else if (method.compare(kSetEditableSizeAndTransform) == 0) {
-    // TODO(loicsharma): Remove implicit view assumption.
+    FlutterWindowsView* view = engine_->view(view_id_);
-    // https://github.com/flutter/flutter/issues/142845
-    FlutterWindowsView* view = engine_->view(kImplicitViewId);
     if (view == nullptr) {
-      result->Error(kInternalConsistencyError,
+      std::stringstream ss;
-                    "Text input is not available in Windows headless mode");
+      ss << "Text input is not available because view with view_id=" << view_id_
+         << " cannot be found";
+      result->Error(kInternalConsistencyError, ss.str());
       return;
     }
     if (!method_call.arguments() || method_call.arguments()->IsNull()) {

engine/src/flutter/shell/platform/windows/text_input_plugin.h

@@ -16,6 +16,7 @@
 #include "flutter/shell/platform/common/json_method_codec.h"
 #include "flutter/shell/platform/common/text_editing_delta.h"
 #include "flutter/shell/platform/common/text_input_model.h"
+#include "flutter/shell/platform/embedder/embedder.h"
 #include "flutter/shell/platform/windows/keyboard_handler_base.h"
 
 namespace flutter {
@@ -70,6 +71,9 @@ class TextInputPlugin {
   virtual void ComposeChangeHook(const std::u16string& text, int cursor_pos);
 
  private:
+  // Allows modifying the TextInputPlugin in tests.
+  friend class TextInputPluginModifier;
+
   // Sends the current state of the given model to the Flutter engine.
   void SendStateUpdate(const TextInputModel& model);
 
@@ -98,6 +102,9 @@ class TextInputPlugin {
   // The active client id.
   int client_id_;
 
+  // The active view id.
+  FlutterViewId view_id_ = 0;
+
   // The active model. nullptr if not set.
   std::unique_ptr<TextInputModel> active_model_;
 

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

@@ -20,6 +20,22 @@
 #include "gtest/gtest.h"
 
 namespace flutter {
+
+class TextInputPluginModifier {
+ public:
+  explicit TextInputPluginModifier(TextInputPlugin* text_input_plugin)
+      : text_input_plugin(text_input_plugin) {}
+
+  void SetViewId(FlutterViewId view_id) {
+    text_input_plugin->view_id_ = view_id;
+  }
+
+ private:
+  TextInputPlugin* text_input_plugin;
+
+  FML_DISALLOW_COPY_AND_ASSIGN(TextInputPluginModifier);
+};
+
 namespace testing {
 
 namespace {
@@ -33,6 +49,7 @@ static constexpr int kDefaultClientId = 42;
 // Should be identical to constants in text_input_plugin.cc.
 static constexpr char kChannelName[] = "flutter/textinput";
 static constexpr char kEnableDeltaModel[] = "enableDeltaModel";
+static constexpr char kViewId[] = "viewId";
 static constexpr char kSetClientMethod[] = "TextInput.setClient";
 static constexpr char kAffinityDownstream[] = "TextAffinity.downstream";
 static constexpr char kTextKey[] = "text";
@@ -63,6 +80,7 @@ static std::unique_ptr<rapidjson::Document> EncodedClientConfig(
   rapidjson::Value config(rapidjson::kObjectType);
   config.AddMember("inputAction", input_action, allocator);
   config.AddMember(kEnableDeltaModel, false, allocator);
+  config.AddMember(kViewId, 456, allocator);
   rapidjson::Value type_info(rapidjson::kObjectType);
   type_info.AddMember("name", type_name, allocator);
   config.AddMember("inputType", type_info, allocator);
@@ -149,7 +167,20 @@ class TextInputPluginTest : public WindowsTest {
                                                      std::move(window));
 
     EngineModifier modifier{engine_.get()};
-    modifier.SetImplicitView(view_.get());
+    modifier.SetViewById(view_.get(), 456);
+  }
+
+  std::unique_ptr<MockFlutterWindowsView> AddViewWithId(int view_id) {
+    EXPECT_NE(engine_, nullptr);
+    auto window = std::make_unique<MockWindowBindingHandler>();
+    EXPECT_CALL(*window, SetView).Times(1);
+    EXPECT_CALL(*window, GetWindowHandle).WillRepeatedly(Return(nullptr));
+    auto view = std::make_unique<MockFlutterWindowsView>(engine_.get(),
+                                                         std::move(window));
+
+    EngineModifier modifier{engine_.get()};
+    modifier.SetViewById(view_.get(), view_id);
+    return view;
   }
 
  private:
@@ -194,6 +225,8 @@ TEST_F(TextInputPluginTest, ClearClientResetsComposing) {
   BinaryReply reply_handler = [](const uint8_t* reply, size_t reply_size) {};
 
   TextInputPlugin handler(&messenger, engine());
+  TextInputPluginModifier modifier(&handler);
+  modifier.SetViewId(456);
 
   EXPECT_CALL(*view(), OnResetImeComposing());
 
@@ -224,9 +257,10 @@ TEST_F(TextInputPluginTest, ClearClientRequiresView) {
   messenger.SimulateEngineMessage(kChannelName, message->data(),
                                   message->size(), reply_handler);
 
-  EXPECT_EQ(reply,
+  EXPECT_EQ(
-            "[\"Internal Consistency Error\",\"Text input is not available in "
+      reply,
-            "Windows headless mode\",null]");
+      "[\"Internal Consistency Error\",\"Text input is not available because "
+      "view with view_id=0 cannot be found\",null]");
 }
 
 // Verify that the embedder sends state update messages to the framework during
@@ -253,6 +287,7 @@ TEST_F(TextInputPluginTest, VerifyComposingSendStateUpdate) {
   config.AddMember("inputAction", "done", allocator);
   config.AddMember("inputType", "text", allocator);
   config.AddMember(kEnableDeltaModel, false, allocator);
+  config.AddMember(kViewId, 456, allocator);
   arguments->PushBack(config, allocator);
   auto message =
       codec.EncodeMethodCall({"TextInput.setClient", std::move(arguments)});
@@ -392,6 +427,71 @@ TEST_F(TextInputPluginTest, VerifyInputActionSendDoesNotInsertNewLine) {
                          messages.front().begin()));
 }
 
+TEST_F(TextInputPluginTest, SetClientRequiresViewId) {
+  UseEngineWithView();
+
+  TestBinaryMessenger messenger([](const std::string& channel,
+                                   const uint8_t* message, size_t message_size,
+                                   BinaryReply reply) {});
+
+  TextInputPlugin handler(&messenger, engine());
+
+  auto args = std::make_unique<rapidjson::Document>(rapidjson::kArrayType);
+  auto& allocator = args->GetAllocator();
+  args->PushBack(123, allocator);  // client_id
+
+  rapidjson::Value client_config(rapidjson::kObjectType);
+
+  args->PushBack(client_config, allocator);
+  auto encoded = JsonMethodCodec::GetInstance().EncodeMethodCall(
+      MethodCall<rapidjson::Document>(kSetClientMethod, std::move(args)));
+
+  std::string reply;
+  BinaryReply reply_handler = [&reply](const uint8_t* reply_bytes,
+                                       size_t reply_size) {
+    reply = std::string(reinterpret_cast<const char*>(reply_bytes), reply_size);
+  };
+
+  EXPECT_TRUE(messenger.SimulateEngineMessage(kChannelName, encoded->data(),
+                                              encoded->size(), reply_handler));
+  EXPECT_EQ(
+      reply,
+      "[\"Bad Arguments\",\"Could not set client, view ID is null.\",null]");
+}
+
+TEST_F(TextInputPluginTest, SetClientRequiresViewIdToBeInteger) {
+  UseEngineWithView();
+
+  TestBinaryMessenger messenger([](const std::string& channel,
+                                   const uint8_t* message, size_t message_size,
+                                   BinaryReply reply) {});
+
+  TextInputPlugin handler(&messenger, engine());
+
+  auto args = std::make_unique<rapidjson::Document>(rapidjson::kArrayType);
+  auto& allocator = args->GetAllocator();
+  args->PushBack(123, allocator);  // client_id
+
+  rapidjson::Value client_config(rapidjson::kObjectType);
+  client_config.AddMember(kViewId, "Not an integer", allocator);  // view_id
+
+  args->PushBack(client_config, allocator);
+  auto encoded = JsonMethodCodec::GetInstance().EncodeMethodCall(
+      MethodCall<rapidjson::Document>(kSetClientMethod, std::move(args)));
+
+  std::string reply;
+  BinaryReply reply_handler = [&reply](const uint8_t* reply_bytes,
+                                       size_t reply_size) {
+    reply = std::string(reinterpret_cast<const char*>(reply_bytes), reply_size);
+  };
+
+  EXPECT_TRUE(messenger.SimulateEngineMessage(kChannelName, encoded->data(),
+                                              encoded->size(), reply_handler));
+  EXPECT_EQ(
+      reply,
+      "[\"Bad Arguments\",\"Could not set client, view ID is null.\",null]");
+}
+
 TEST_F(TextInputPluginTest, TextEditingWorksWithDeltaModel) {
   UseEngineWithView();
 
@@ -413,6 +513,7 @@ TEST_F(TextInputPluginTest, TextEditingWorksWithDeltaModel) {
 
   rapidjson::Value client_config(rapidjson::kObjectType);
   client_config.AddMember(kEnableDeltaModel, true, allocator);
+  client_config.AddMember(kViewId, 456, allocator);
 
   args->PushBack(client_config, allocator);
   auto encoded = JsonMethodCodec::GetInstance().EncodeMethodCall(
@@ -479,6 +580,7 @@ TEST_F(TextInputPluginTest, CompositionCursorPos) {
   auto& allocator = args->GetAllocator();
   args->PushBack(123, allocator);  // client_id
   rapidjson::Value client_config(rapidjson::kObjectType);
+  client_config.AddMember(kViewId, 456, allocator);
   args->PushBack(client_config, allocator);
   auto encoded = JsonMethodCodec::GetInstance().EncodeMethodCall(
       MethodCall<rapidjson::Document>(kSetClientMethod, std::move(args)));
@@ -535,6 +637,8 @@ TEST_F(TextInputPluginTest, TransformCursorRect) {
   BinaryReply reply_handler = [](const uint8_t* reply, size_t reply_size) {};
 
   TextInputPlugin handler(&messenger, engine());
+  TextInputPluginModifier modifier(&handler);
+  modifier.SetViewId(456);
 
   auto& codec = JsonMethodCodec::GetInstance();
 
@@ -611,9 +715,62 @@ TEST_F(TextInputPluginTest, SetMarkedTextRectRequiresView) {
   messenger.SimulateEngineMessage(kChannelName, message->data(),
                                   message->size(), reply_handler);
 
-  EXPECT_EQ(reply,
+  EXPECT_EQ(
-            "[\"Internal Consistency Error\",\"Text input is not available in "
+      reply,
-            "Windows headless mode\",null]");
+      "[\"Internal Consistency Error\",\"Text input is not available because "
+      "view with view_id=0 cannot be found\",null]");
+}
+
+TEST_F(TextInputPluginTest, SetAndUseMultipleClients) {
+  UseEngineWithView();  // Creates the default view
+  AddViewWithId(789);   // Creates the next view
+
+  bool sent_message = false;
+  TestBinaryMessenger messenger(
+      [&sent_message](const std::string& channel, const uint8_t* message,
+                      size_t message_size,
+                      BinaryReply reply) { sent_message = true; });
+
+  TextInputPlugin handler(&messenger, engine());
+
+  auto const set_client_and_send_message = [&](int client_id, int view_id) {
+    auto args = std::make_unique<rapidjson::Document>(rapidjson::kArrayType);
+    auto& allocator = args->GetAllocator();
+    args->PushBack(client_id, allocator);  // client_id
+
+    rapidjson::Value client_config(rapidjson::kObjectType);
+    client_config.AddMember(kViewId, view_id, allocator);  // view_id
+
+    args->PushBack(client_config, allocator);
+    auto encoded = JsonMethodCodec::GetInstance().EncodeMethodCall(
+        MethodCall<rapidjson::Document>(kSetClientMethod, std::move(args)));
+
+    std::string reply;
+    BinaryReply reply_handler = [&reply](const uint8_t* reply_bytes,
+                                         size_t reply_size) {
+      reply =
+          std::string(reinterpret_cast<const char*>(reply_bytes), reply_size);
+    };
+
+    EXPECT_TRUE(messenger.SimulateEngineMessage(
+        kChannelName, encoded->data(), encoded->size(), reply_handler));
+
+    sent_message = false;
+    handler.ComposeBeginHook();
+    EXPECT_TRUE(sent_message);
+    sent_message = false;
+    handler.ComposeChangeHook(u"4", 1);
+    EXPECT_TRUE(sent_message);
+    sent_message = false;
+    handler.ComposeCommitHook();
+    EXPECT_FALSE(sent_message);
+    sent_message = false;
+    handler.ComposeEndHook();
+    EXPECT_TRUE(sent_message);
+  };
+
+  set_client_and_send_message(123, 456);  // Set and send for the first view
+  set_client_and_send_message(123, 789);  // Set and send for the next view
 }
 
 }  // namespace testing