教程：在驱动程序之间传递元数据

本指南介绍了如何使用 元数据库将元数据从一个驱动程序传递给另一个驱动程序。元数据是驱动程序创建的任何任意数据，应可供绑定到其子节点的驱动程序访问。通常，驱动程序可以创建自己的 FIDL 协议来传递 元数据，但 元数据 库提供了一些 功能，可用于在驱动程序之间传递元数据，而无需编写太多代码。

您可以在此处找到驱动程序发送、检索和转发元数据的示例 here。

元数据定义

首先，必须将元数据的类型定义为 FIDL 类型，并使用 @serializable 进行注释：

library fuchsia.examples.metadata;

// Type of the metadata to be passed.
// Make sure to annotate it with `@serializable`.
@serializable
type Metadata = table {
    1: test_property string:MAX;
};

此元数据将使用 fuchsia.driver.metadata/Service FIDL 服务在驱动程序的传出命名空间中提供。驱动程序的传出命名空间中的 FIDL 服务的名称不会是 fuchsia.driver.metadata.Service。相反，它将是 FIDL 类型的可序列化名称。只有在 FIDL 类型使用 @serializable 进行注释时，才会创建可序列化名称。

FIDL 库的 build 目标将定义如下：

fidl("fuchsia.examples.metadata") {
  sources = [ "fuchsia.examples.metadata.fidl" ]
}

发送元数据

初始设置

假设我们有一个驱动程序想要向其子驱动程序发送元数据：

#include <lib/driver/component/cpp/driver_export2.h>

class Sender : public fdf::DriverBase2 {
 public:
  Sender() : fdf::DriverBase2("sender") {}
};

FUCHSIA_DRIVER_EXPORT2(Sender);

其组件清单如下：

{
    include: [
        "inspect/client.shard.cml",
        "syslog/client.shard.cml",
    ],
    program: {
        runner: "driver",
        binary: "driver/sender.so",
        bind: "meta/bind/sender.bindbc",
    },
}

其 build 目标定义如下：

fuchsia_cc_driver("driver") {
  testonly = true
  output_name = "sender"
  sources = [
    "sender.cc",
  ]
  deps = [
    "//src/devices/lib/driver:driver_runtime",
  ]
}

fuchsia_driver_component("component") {
  testonly = true
  component_name = "sender"
  manifest = "meta/sender.cml"
  deps = [
    ":driver",
    ":bind", # Bind rules not specified in this tutorial.
  ]
  info = "sender.json" # Info not specified in this tutorial.
}

发送过程

为了让此驱动程序向其子驱动程序发送元数据，它需要 创建 fdf_metadata::MetadataServer 类的一个实例，调用 fdf_metadata::MetadataServer::Serve() 以提供 元数据，并将 fdf_metadata::MetadataServer::CreateOffer()创建的元数据服务器的 offer 传递给子节点：


// Defines the fuchsia.examples.metadata types in C++.
#include <fidl/fuchsia.examples.metadata/cpp/fidl.h>

// Defines fdf::MetadataServer.
#include <lib/driver/metadata/cpp/metadata_server.h>

class Sender : public fdf::DriverBase2 {
 public:
  zx::result<> Start(fdf::DriverContext context) override {
    // Serve the metadata to the driver's outgoing directory.
    fuchsia_examples_metadata::Metadata metadata({.test_property = "test value"});
    ZX_ASSERT(metadata_server_.Serve(*outgoing(), dispatcher(), metadata).is_ok());

    std::vector<fuchsia_driver_framework::Offer> offers;

    std::optional<fuchsia_driver_framework::Offer> metadata_offer =
      metadata_server_.CreateOffer();

    // The metadata server should create an offer because it is serving
    // metadata.
    ZX_ASSERT(metadata_offer.has_value());

    // The metadata server's offer must be provided to the child node.
    offers.push_back(*metadata_offer);

    zx::result child = AddChild("child", {}, offers);
    if (child.is_error()) {
      return child.take_error();
    }

    return zx::ok();
  }

 private:
  // Responsible for serving metadata.
  fdf_metadata::MetadataServer<fuchsia_examples_metadata::Metadata> metadata_server_;
};

可以多次调用 fdf_metadata::MetadataServer::Serve()，元数据服务器将提供最新的元数据实例。

需要更新 driver build 目标，以包含元数据库和定义元数据类型的 FIDL 库：

fuchsia_cc_driver("driver") {
  testonly = true
  output_name = "sender"
  sources = [
    "sender.cc",
  ]
  deps = [
    "//src/devices/lib/driver:driver_runtime",

    # This should be the fuchsia.examples.metadata FIDL library's C++ target.
    ":fuchsia.examples.metadata_cpp",

    # This library contains `fdf::MetadataServer`.
    "//sdk/lib/driver/metadata/cpp",
  ]
}

公开元数据服务

最后，驱动程序需要在其组件清单中声明并公开 fuchsia.examples.metadata.Metadata FIDL 服务：

{
    include: [
        "inspect/client.shard.cml",
        "syslog/client.shard.cml",
    ],
    program: {
        runner: "driver",
        binary: "driver/sender.so",
        bind: "meta/bind/sender.bindbc",
    },
    capabilities: [
        { service: "fuchsia.examples.metadata.Metadata" },
    ],
    expose: [
        {
            service: "fuchsia.examples.metadata.Metadata",
            from: "self",
        },
    ],
}

从技术上讲，没有 fuchsia.examples.metadata.Metadata FIDL 服务。 在底层， fdf_metadata::MetadataServer 提供 fuchsia.driver.metadata/Service FIDL 服务以发送元数据。但是， fdf_metadata::MetadataServer 不会像普通的 FIDL 服务那样以名称 fuchsia.driver.metadata.Service 提供此服务。相反，该服务以元数据类型的可序列化名称提供。在本例中，该名称为 fuchsia.examples.metadata.Metadata。您可以在 C++ 字段 fuchsia_examples_metadata::Metadata::kSerializableName 中找到此名称。这样，接收驱动程序就可以识别正在传递的元数据类型。这意味着，即使 fuchsia.examples.metadata.Metadata FIDL 服务不存在，驱动程序也需要声明并公开该服务。

检索元数据

初始设置

假设我们有一个驱动程序想要从其父驱动程序检索元数据：

#include <lib/driver/component/cpp/driver_export2.h>

class Retriever : public fdf::DriverBase2 {
 public:
  Retriever() : fdf::DriverBase2("retriever") {}
};

FUCHSIA_DRIVER_EXPORT2(Retriever);

其组件清单如下：

{
    include: [
        "inspect/client.shard.cml",
        "syslog/client.shard.cml",
    ],
    program: {
        runner: "driver",
        binary: "driver/retriever.so",
        bind: "meta/bind/retriever.bindbc",
    },
}

其 build 目标定义如下：

fuchsia_cc_driver("driver") {
  testonly = true
  output_name = "retriever"
  sources = [
    "retriever.cc",
  ]
  deps = [
    "//src/devices/lib/driver:driver_runtime",
  ]
}

fuchsia_driver_component("component") {
  testonly = true
  component_name = "retriever"
  manifest = "meta/retriever.cml"
  deps = [
    ":driver",
    ":bind", # Bind rules not specified in this tutorial.
  ]
  info = "retriever.json" # Info not specified in this tutorial.
}

检索过程

为了让驱动程序从其父驱动程序检索元数据，驱动程序需要调用 fdf_metadata::GetMetadata()：

// Defines the fuchsia.examples.metadata types in C++.
#include <fidl/fuchsia.examples.metadata/cpp/fidl.h>

// Defines fdf::GetMetadata().
#include <lib/driver/metadata/cpp/metadata.h>

class Retriever : public fdf::DriverBase2 {
 public:
  zx::result<> Start(fdf::DriverContext context) override {
    zx::result<fuchsia_examples_metadata::Metadata> metadata =
      fdf_metadata::GetMetadata<fuchsia_examples_metadata::Metadata>(context.incoming());
    ZX_ASSERT(!metadata.is_error());

    return zx::ok();
  }
};

需要更新 driver build 目标，以依赖于元数据库和定义元数据类型的 FIDL 库：

fuchsia_cc_driver("driver") {
  testonly = true
  output_name = "retriever"
  sources = [
    "retriever.cc",
  ]
  deps = [
    "//src/devices/lib/driver:driver_runtime",

    # This should be the fuchsia.examples.metadata FIDL library's C++ target.
    ":fuchsia.examples.metadata_cpp",

    # This library contains `fdf::GetMetadata()`.
    "//sdk/lib/driver/metadata/cpp",
  ]
}

使用元数据服务

最后，驱动程序需要在其组件清单中声明其对 fuchsia.examples.metadata.Metadata FIDL 服务的用法：

{
    include: [
        "inspect/client.shard.cml",
        "syslog/client.shard.cml",
    ],
    program: {
        runner: "driver",
        binary: "driver/retriever.so",
        bind: "meta/bind/retriever.bindbc",
    },
    use: [
        { service: "fuchsia.examples.metadata.Metadata" },
    ],
}

转发元数据

初始设置

假设我们有一个驱动程序想要从其父驱动程序检索元数据，并将该元数据转发给其子驱动程序：

#include <lib/driver/component/cpp/driver_export2.h>

class Forwarder : public fdf::DriverBase2 {
 public:
  Forwarder() : fdf::DriverBase2("forwarder") {}
};

FUCHSIA_DRIVER_EXPORT2(Forwarder);

其组件清单如下：

{
    include: [
        "inspect/client.shard.cml",
        "syslog/client.shard.cml",
    ],
    program: {
        runner: "driver",
        binary: "driver/forwarder.so",
        bind: "meta/bind/forwarder.bindbc",
    },
}

其 build 目标定义如下：

fuchsia_cc_driver("driver") {
  testonly = true
  output_name = "forwarder"
  sources = [
    "forwarder.cc",
  ]
  deps = [
    "//src/devices/lib/driver:driver_runtime",
  ]
}

fuchsia_driver_component("component") {
  testonly = true
  component_name = "forward"
  manifest = "meta/forward.cml"
  deps = [
    ":driver",
    ":bind", # Bind rules not specified in this tutorial.
  ]
  info = "forward.json" # Info not specified in this tutorial.
}

转发过程

为了让驱动程序将其父驱动程序的元数据转发给其子 驱动程序，它需要创建 fdf_metadata::MetadataServer 类的一个实例，调用 fdf_metadata::MetadataServer::ForwardAndServe() 以从其父驱动程序检索元数据并将其提供给驱动程序的传出 目录，并将 fdf_metadata::MetadataServer::CreateOffer()创建的元数据服务器的 offer 传递给子节点。

如果 fdf_metadata::MetadataServer::ForwardAndServe() 无法从其父驱动程序检索元数据，则不会返回错误。相反，它会返回 zx::ok(false)，并且不会提供任何内容。 ```cpp // Defines the fuchsia.examples.metadata types in C++.

include

// Defines fdf::MetadataServer.

include

class Forwarder : public fdf::DriverBase2 { public: zx::result<> Start(fdf::DriverContext context) override { // Retrieve the metadata from the driver's parent and serve it to the // driver's outgoing directory. zx::result is_serving = metadataserver.ForwardAndServe( *outgoing(), dispatcher(), context.incoming()).is_ok(); ZX_ASSERT(is_serving.is_ok());

std::vector<fuchsia_driver_framework::Offer> offers;
std::optional<fuchsia_driver_framework::Offer> metadata_offer =
  metadata_server_.CreateOffer();

// The metadata server may not be serving metadata if the driver failed to
// retrieve it from the driver's parent. In that case, the metadata server
// will not have an offer.
if (metadata_offer.has_value()) {
  // The metadata server's offer must be provided to the child node.
  offers.push_back(metadata_offer.value());
}

zx::result child = AddChild("child", {}, offers);
if (child.is_error()) {
  return child.take_error();
}

return zx::ok();

}

private: // Responsible for serving metadata. fdf_metadata::MetadataServer metadataserver; }; ```

请注意，fdf_metadata::MetadataServer::ForwardAndServe() 不会检查父驱动程序在调用 fdf_metadata::MetadataServer::ForwardAndServe() 后是否更改了其提供的元数据。驱动程序必须再次调用 fdf_metadata::MetadataServer::ForwardAndServe() 才能纳入更改。

需要更新 driver build 目标，以依赖于元数据库和定义元数据类型的 FIDL 库：

fuchsia_cc_driver("driver") {
  testonly = true
  output_name = "forwarder"
  sources = [
    "forwarder.cc",
  ]
  deps = [
    "//src/devices/lib/driver:driver_runtime",

    # This should be the fuchsia.examples.metadata FIDL library's C++ target.
    ":fuchsia.examples.metadata_cpp",

    # This library contains `fdf::MetadataServer`.
    "//sdk/lib/driver/metadata/cpp",
  ]
}

公开和使用元数据服务

最后，驱动程序需要在其组件清单中声明、使用和公开 fuchsia.examples.metadata.Metadata FIDL 服务：

{
    include: [
        "inspect/client.shard.cml",
        "syslog/client.shard.cml",
    ],
    program: {
        runner: "driver",
        binary: "driver/forwarder.so",
        bind: "meta/bind/forwarder.bindbc",
    },
    capabilities: [
        { service: "fuchsia.examples.metadata.Metadata" },
    ],
    expose: [
        {
            service: "fuchsia.examples.metadata.Metadata",
            from: "self",
        },
    ],
    use: [
        { service: "fuchsia.examples.metadata.Metadata" },
    ],
}