实现 C++ FIDL 服务器

前提条件

本教程基于网域对象教程。对于 完整的 FIDL 教程，请参阅概览。

概览

本教程介绍了如何针对 FIDL 协议实现服务器 (fuchsia.examples/Echo) 并在 Fuchsia 上运行它。此协议有一种方法 以下各项：单向方法、双向方法和事件：

@discoverable
closed protocol Echo {
    strict EchoString(struct {
        value string:MAX_STRING_LENGTH;
    }) -> (struct {
        response string:MAX_STRING_LENGTH;
    });
    strict SendString(struct {
        value string:MAX_STRING_LENGTH;
    });
    strict -> OnString(struct {
        response string:MAX_STRING_LENGTH;
    });
};

如需详细了解 FIDL 方法和消息传递模型，请参阅 FIDL 概念页面。

本文档介绍了如何完成以下任务：

• 实现 FIDL 协议。
• 发布协议实现。
• 运行服务器。

服务器结构示例

本教程附带的示例代码位于 Fuchsia 结账中 //examples/fidl/cpp/server。它由一个服务器 组件及其包含的软件包。想要详细了解 请参阅构建组件。

要启动并运行服务器组件，需要设置 在 //examples/fidl/cpp/server/BUILD.gn 中定义：

1. 服务器的原始可执行文件。这会生成一个包含 可以在 Fuchsia 上运行的指定输出名称：

   executable("bin") {
     output_name = "fidl_echo_cpp_server"
     sources = [ "main.cc" ]
     deps = [
       "//examples/fidl/fuchsia.examples:fuchsia.examples_cpp",
   
       # This library is used to log messages.
       "//sdk/lib/syslog/cpp",
   
       # This library is used to publish capabilities, e.g. protocols,
       # to the component's outgoing directory.
       "//sdk/lib/component/outgoing/cpp",
   
       # This library provides an the asynchronous event loop implementation.
       "//zircon/system/ulib/async-loop:async-loop-cpp",
     ]
   }
   
   

2. 一种设置为运行服务器可执行文件的组件。 组件是 Fuchsia 上软件执行的单位。组件是 由其清单文件描述在本例中为 meta/server.cml 将 echo-server 配置为可执行 “:bin”中有 fidl_echo_cpp_server。

   fuchsia_component("echo-server") {
     component_name = "echo_server"
     manifest = "meta/server.cml"
     deps = [ ":bin" ]
   }
   
   

   服务器组件清单位于 //examples/fidl/cpp/server/meta/server.cml。此处的二进制文件名称 清单必须与在以下文件中定义的 executable 的输出名称一致： BUILD.gn。

   {
       include: [ "syslog/client.shard.cml" ],
   
       // Information about the program to run.
       program: {
           // Use the built-in ELF runner.
           runner: "elf",
   
           // The binary to run for this component.
           binary: "bin/fidl_echo_cpp_server",
       },
   
       // Capabilities provided by this component.
       capabilities: [
           { protocol: "fuchsia.examples.Echo" },
       ],
       expose: [
           {
               protocol: "fuchsia.examples.Echo",
               from: "self",
           },
       ],
   }
   
   

3. 然后，该组件被放入软件包中，软件包是软件单元 在 Fuchsia 上的应用分发。在本例中， 包只包含 单个组件。

   fuchsia_package("echo-cpp-server") {
     package_name = "echo-cpp-server"
     deps = [ ":echo-server" ]
   }
   
   

构建服务器

您可以通过以下方式构建服务器软件包：

1. 将该服务器添加到您的构建配置中。此操作只需执行一次：

   fx set core.x64 --with //examples/fidl/cpp/server
   

2. 构建服务器软件包：

   fx build examples/fidl/cpp/server
   

。

实现 FIDL 协议

EchoImpl 会为 fuchsia.examples/Echo 实现服务器请求处理程序 协议。为此，EchoImpl 会继承生成的纯虚拟服务器。 接口 fidl::Server<fuchsia_examples::Echo>，并替换其纯虚拟 与单向和双向调用相对应：

class EchoImpl : public fidl::Server<fuchsia_examples::Echo> {
 public:
  // The handler for `fuchsia.examples/Echo.EchoString`.
  //
  // For two-way methods (those with a response) like this one, the completer is
  // used complete the call: either to send the reply via |completer.Reply|, or
  // close the channel via |completer.Close|.
  //
  // |EchoStringRequest| exposes the same API as the request struct domain
  // object, that is |fuchsia_examples::EchoEchoStringRequest|.
  void EchoString(EchoStringRequest& request, EchoStringCompleter::Sync& completer) override {
    // Call |Reply| to reply synchronously with the request value.
    completer.Reply({{.response = request.value()}});
  }

  // The handler for `fuchsia.examples/Echo.SendString`.
  //
  // For fire-and-forget methods like this one, the completer is normally not
  // used, but its |Close(zx_status_t)| method can be used to close the channel,
  // either when the protocol has reached its intended terminal state or the
  // server encountered an unrecoverable error.
  //
  // |SendStringRequest| exposes the same API as the request struct domain
  // object, that is |fuchsia_examples::EchoSendStringRequest|.
  void SendString(SendStringRequest& request, SendStringCompleter::Sync& completer) override {
    ZX_ASSERT(binding_ref_.has_value());

    // Handle a SendString request by sending an |OnString| event (an
    // unsolicited server-to-client message) back to the client.
    fit::result result = fidl::SendEvent(*binding_ref_)->OnString({request.value()});
    if (!result.is_ok()) {
      FX_LOGS(ERROR) << "Error sending event: " << result.error_value();
    }
  }

  // ... other methods from examples/fidl/cpp/server/main.cc omitted, to be covered later.

 private:
  // `ServerBindingRef` can be used to:
  // - Control the binding, such as to unbind the server from the channel or
  //   close the channel.
  // - Send events back to the client.
  // See the documentation comments on |fidl::ServerBindingRef|.
  std::optional<fidl::ServerBindingRef<fuchsia_examples::Echo>> binding_ref_;
};

将实现绑定到服务器端点

实现请求处理程序只是成功的一半。服务器需要 能够监控发送到 服务器端点。为此，EchoImpl 定义了另外两个 方法：一个 BindSelfManagedServer 静态工厂函数，用于创建 新的 EchoImpl 实例，用于处理新的服务器端点上的请求 fidl::ServerEnd<fuchsia_examples::Echo> 和 OnUnbound 方法， 在连接中断时调用：

/* Inside `class EchoImpl {`... */

  // Bind a new implementation of |EchoImpl| to handle requests coming from
  // the server endpoint |server_end|.
  static void BindSelfManagedServer(async_dispatcher_t* dispatcher,
                                    fidl::ServerEnd<fuchsia_examples::Echo> server_end) {
    // Create a new instance of |EchoImpl|.
    std::unique_ptr impl = std::make_unique<EchoImpl>();
    EchoImpl* impl_ptr = impl.get();

    // |fidl::BindServer| takes a FIDL protocol server implementation and a
    // channel. It asynchronously reads requests off the channel, decodes them
    // and dispatches them to the correct handler on the server implementation.
    //
    // The FIDL protocol server implementation can be passed as a
    // |std::shared_ptr|, |std::unique_ptr|, or raw pointer. For shared and
    // unique pointers, the binding will manage the lifetime of the
    // implementation object. For raw pointers, it's up to the caller to ensure
    // that the implementation object outlives the binding but does not leak.
    //
    // See the documentation comment of |fidl::BindServer|.
    fidl::ServerBindingRef binding_ref = fidl::BindServer(
        dispatcher, std::move(server_end), std::move(impl), std::mem_fn(&EchoImpl::OnUnbound));
    // Put the returned |binding_ref| into the |EchoImpl| object.
    impl_ptr->binding_ref_.emplace(std::move(binding_ref));
  }

  // This method is passed to the |BindServer| call as the last argument,
  // which means it will be called when the connection is torn down.
  // In this example we use it to log some connection lifecycle information.
  // Production code could do more things such as resource cleanup.
  void OnUnbound(fidl::UnbindInfo info, fidl::ServerEnd<fuchsia_examples::Echo> server_end) {
    // |is_user_initiated| returns true if the server code called |Close| on a
    // completer, or |Unbind| / |Close| on the |binding_ref_|, to proactively
    // teardown the connection. These cases are usually part of normal server
    // shutdown, so logging is unnecessary.
    if (info.is_user_initiated()) {
      return;
    }
    if (info.is_peer_closed()) {
      // If the peer (the client) closed their endpoint, log that as INFO.
      FX_LOGS(INFO) << "Client disconnected";
    } else {
      // Treat other unbind causes as errors.
      FX_LOGS(ERROR) << "Server error: " << info;
    }
  }

发布协议实现

实现 FIDL 协议的组件可公开该 FIDL 连接到其他组件只需发布协议 添加到该组件的 传出目录 ，了解所有最新动态。整个流程 开放协议的生命周期中对此有进一步的描述。 我们可以使用 C++ 组件运行时库中的 component::OutgoingDirectory 来完成繁杂的工作

如需依赖于组件运行时库，请执行以下操作：

executable("bin") {
  output_name = "fidl_echo_cpp_server"
  sources = [ "main.cc" ]
  deps = [
    "//examples/fidl/fuchsia.examples:fuchsia.examples_cpp",

    # This library is used to log messages.
    "//sdk/lib/syslog/cpp",

    # This library is used to publish capabilities, e.g. protocols,
    # to the component's outgoing directory.
    "//sdk/lib/component/outgoing/cpp",

    # This library provides an the asynchronous event loop implementation.
    "//zircon/system/ulib/async-loop:async-loop-cpp",
  ]
}

在 examples/fidl/cpp/server/main.cc 的顶部导入库：

#include <fidl/fuchsia.examples/cpp/fidl.h>
#include <lib/async-loop/cpp/loop.h>
#include <lib/component/outgoing/cpp/outgoing_directory.h>
#include <lib/syslog/cpp/log_settings.h>
#include <lib/syslog/cpp/macros.h>

提供组件的传出目录：

int main(int argc, const char** argv) {
  // The event loop is used to asynchronously listen for incoming connections
  // and requests from the client. The following initializes the loop, and
  // obtains the dispatcher, which will be used when binding the server
  // implementation to a channel.
  async::Loop loop(&kAsyncLoopConfigNeverAttachToThread);
  async_dispatcher_t* dispatcher = loop.dispatcher();
  fuchsia_logging::LogSettingsBuilder builder;
  builder.WithDispatcher(dispatcher).BuildAndInitialize();

  // Create an |OutgoingDirectory| instance.
  //
  // The |component::OutgoingDirectory| class serves the outgoing directory for
  // our component. This directory is where the outgoing FIDL protocols are
  // installed so that they can be provided to other components.
  component::OutgoingDirectory outgoing = component::OutgoingDirectory(dispatcher);

  // The `ServeFromStartupInfo()` function sets up the outgoing directory with
  // the startup handle. The startup handle is a handle provided to every
  // component by the system, so that they can serve capabilities (e.g. FIDL
  // protocols) to other components.
  zx::result result = outgoing.ServeFromStartupInfo();
  if (result.is_error()) {
    FX_LOGS(ERROR) << "Failed to serve outgoing directory: " << result.status_string();
    return -1;
  }

  // ...

提供协议

然后，服务器会使用 outgoing.AddProtocol 注册 Echo 协议。

int main(int argc, const char** argv) {
  // The event loop is used to asynchronously listen for incoming connections
  // and requests from the client. The following initializes the loop, and
  // obtains the dispatcher, which will be used when binding the server
  // implementation to a channel.
  async::Loop loop(&kAsyncLoopConfigNeverAttachToThread);
  async_dispatcher_t* dispatcher = loop.dispatcher();
  fuchsia_logging::LogSettingsBuilder builder;
  builder.WithDispatcher(dispatcher).BuildAndInitialize();

  // Create an |OutgoingDirectory| instance.
  //
  // The |component::OutgoingDirectory| class serves the outgoing directory for
  // our component. This directory is where the outgoing FIDL protocols are
  // installed so that they can be provided to other components.
  component::OutgoingDirectory outgoing = component::OutgoingDirectory(dispatcher);

  // The `ServeFromStartupInfo()` function sets up the outgoing directory with
  // the startup handle. The startup handle is a handle provided to every
  // component by the system, so that they can serve capabilities (e.g. FIDL
  // protocols) to other components.
  zx::result result = outgoing.ServeFromStartupInfo();
  if (result.is_error()) {
    FX_LOGS(ERROR) << "Failed to serve outgoing directory: " << result.status_string();
    return -1;
  }

  // Register a handler for components trying to connect to fuchsia.examples.Echo.
  result = outgoing.AddUnmanagedProtocol<fuchsia_examples::Echo>(
      [dispatcher](fidl::ServerEnd<fuchsia_examples::Echo> server_end) {
        FX_LOGS(INFO) << "Incoming connection for "
                      << fidl::DiscoverableProtocolName<fuchsia_examples::Echo>;
        EchoImpl::BindSelfManagedServer(dispatcher, std::move(server_end));
      });
  if (result.is_error()) {
    FX_LOGS(ERROR) << "Failed to add Echo protocol: " << result.status_string();
    return -1;
  }

  FX_LOGS(INFO) << "Running C++ echo server with natural types";

  // This runs the event loop and blocks until the loop is quit or shutdown.
  // See documentation comments on |async::Loop|.
  loop.Run();
  return 0;
}

调用 AddProtocol 会以 FIDL 协议的名称安装处理程序 （fidl::DiscoverableProtocolName<fuchsia_examples::Echo>，它是 "fuchsia.examples.Echo"）。当客户端组件连接到 fuchsia.examples.Echo，outgoing 会调用 lambda 函数， 使用与 此 lambda 函数将调用 EchoImpl::BindSelfManagedServer 将服务器端点绑定到 EchoImpl 的新实例。

我们的主方法将持续监听 异步循环。

测试服务器

构建服务器后，您可以在正在运行的 Fuchsia 模拟器实例

ffx component run /core/ffx-laboratory:echo-server fuchsia-pkg://fuchsia.com/echo-cpp-server#meta/echo_server.cm

您应该会在设备日志 (ffx log) 中看到类似于以下内容的输出：

[ffx-laboratory:echo_server][][I] Running C++ echo server with natural types

服务器现在正在运行并等待传入请求。 下一步是编写一个发送 Echo 协议请求的客户端。 目前，您可以直接终止服务器组件：

ffx component destroy /core/ffx-laboratory:echo_server

。

使用传输网域对象处理请求

上面的教程使用 自然网域对象：服务器接收请求 以自然域对象的形式表示，并发送由自然域对象编码的回复， 域对象。在针对性能和堆分配进行优化时， 实现一个使用“有线网域对象”（即有线）的服务器 服务器。以下是重新编写的 EchoImpl，以使用传输网域对象：

class EchoImpl final : public fidl::WireServer<fuchsia_examples::Echo> {
 public:
  // The handler for `fuchsia.examples/Echo.EchoString`.
  //
  // For two-way methods (those with a response) like this one, the completer is
  // used complete the call: either to send the reply via |completer.Reply|, or
  // close the channel via |completer.Close|.
  //
  // |EchoStringRequestView| exposes the same API as a pointer to the request
  // struct domain object, that is
  // |fuchsia_examples::wire::EchoEchoStringRequest*|.
  void EchoString(EchoStringRequestView request, EchoStringCompleter::Sync& completer) override {
    // Call |Reply| to reply synchronously with the request value.
    completer.Reply(request->value);
  }

  // The handler for `fuchsia.examples/Echo.SendString`.
  //
  // For fire-and-forget methods like this one, the completer is normally not
  // used, but its |Close(zx_status_t)| method can be used to close the channel,
  // either when the protocol has reached its intended terminal state or the
  // server encountered an unrecoverable error.
  //
  // |SendStringRequestView| exposes the same API as a pointer to the request
  // struct domain object, that is
  // |fuchsia_examples::wire::EchoSendStringRequest*|.
  void SendString(SendStringRequestView request, SendStringCompleter::Sync& completer) override {
    // Handle a SendString request by sending an |OnString| event (an
    // unsolicited server-to-client message) back to the client.
    fidl::Status status = fidl::WireSendEvent(binding_)->OnString(request->value);
    if (!status.ok()) {
      FX_LOGS(ERROR) << "Error sending event: " << status.error();
    }
  }

  // ... |BindSelfManagedServer| etc omitted. Those stay the same.
};

电线服务器中使用的相关类和函数具有与 那些在自然服务器中使用的调用其他类或函数时 对应的线缆通常带有 Wire 前缀。还有 指针、引用和参数结构上的细微差别：

• 由自然服务器实现的服务器接口 fidl::Server<fuchsia_examples::Echo>。由 有线服务器为 fidl::WireServer<fuchsia_examples::Echo>。

• 自然服务器中的处理程序函数采用对请求的引用 消息。Reply 方法接受一个参数，即响应 载荷网域对象：

  void EchoString(EchoStringRequest& request, EchoStringCompleter::Sync& completer) override {
    // Call |Reply| to reply synchronously with the request value.
    completer.Reply({{.response = request.value()}});
  }
  

  而有线服务器中的处理程序函数可获取视图（类似于指针） 。当响应载荷是结构体时，Reply 方法将响应载荷中的结构体字段列表扁平化为 单独的参数（此处为一个 fidl::StringView 参数）：

  void EchoString(EchoStringRequestView request, EchoStringCompleter::Sync& completer) override {
    // Call |Reply| to reply synchronously with the request value.
    completer.Reply(request->value);
  }
  

• 用于发送自然类型的事件的函数是 fidl::SendEvent。通过 使用传输类型发送事件的函数为 fidl::WireSendEvent。结构体 字段也会在发送事件时展平为单独的参数。

可以使用同一个 fidl::BindServer 函数来绑定自然服务器 或有线服务器。

线服务器的完整示例代码位于 Fuchsia 结账部分 //examples/fidl/cpp/server/wire。