教學課程：在驅動程式之間傳遞中繼資料

本指南說明如何使用中繼資料程式庫，將中繼資料從某個驅動程式傳送至另一個驅動程式。中繼資料是指驅動程式繫結至節點時所建立的任意資料，且該節點應可供繫結至其子節點的驅動程式存取。通常，驅動程式可以建立自己的 FIDL 通訊協定來傳遞中繼資料，但 metadata 程式庫提供在程式碼較少的驅動程式之間傳遞中繼資料的功能。

如需駕駛人傳送、擷取及轉寄中繼資料的範例，請參閱這篇文章。

中繼資料定義

首先，中繼資料類型必須定義為 FIDL 類型：

library fuchsia.examples.metadata;

// Type of the metadata to be passed.
type Metadata = table {
    1: test_property string:MAX;
};

這項中繼資料會透過 fuchsia.driver.metadata/Service FIDL 服務，在驅動程式的傳出命名空間中提供。不過，外送命名空間中的服務名稱不會是 fuchsia.driver.metadata.Service。而是我們提供的關聯中繼資料類型自訂字串。在本教學課程中，我們會在上述相同的 FIDL 程式庫中定義該字串：

library fuchsia.examples.metadata;

const SERVICE string = "fuchsia.examples.metadata.Metadata";

FIDL 程式庫的建構目標將定義如下：

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

中繼資料資料庫

為了使用中繼資料程式庫傳送及接收中繼資料，我們需要將 fdf_metadata::ObjectDetails 範本類別專門化，如下所示：

// Defines `fdf_metadata::ObjectDetails`.
#include <lib/driver/metadata/cpp/metadata.h>

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

// `ObjectDetails` must be specialized within the `fdf_metadata` namespace.
namespace fdf_metadata {

template <>
// Pass the metadata type as the template argument.
struct ObjectDetails<fuchsia_examples_metadata::Metadata> {

  // Specify the name of the service used to serve the metadata.
  inline static const char* Name = fuchsia_examples_metadata::kService;
};

}  // namespace fdf_metadata

由於傳送和接收驅動程式都需要執行這項操作，我們可以將這段程式碼插入兩個驅動程式都會納入的新程式庫：

source_set("examples_metadata") {
  # This header file should contain the above code.
  sources = [ "examples_metadata.h" ]

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

傳送中繼資料

初始設定

假設我們有一個驅動程式，想要將中繼資料傳送至子項：

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

class Sender : public fdf::DriverBase {
 public:
  Sender(fdf::DriverStartArgs start_args, fdf::UnownedSynchronizedDispatcher driver_dispatcher)
      : DriverBase("parent", std::move(start_args), std::move(driver_dispatcher)) {}
};

FUCHSIA_DRIVER_EXPORT(Sender);

元件資訊清單如下所示：

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

其建構目標的定義如下：

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

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

傳送程序

為了讓這個驅動程式將中繼資料傳送至子驅動程式，它需要 fdf_metadata::MetadataServer 類別的例項，透過呼叫其 fdf_metadata::MetadataServer::SetMetadata() 方法設定中繼資料，然後透過呼叫其 fdf_metadata::MetadataServer::Serve() 方法，將中繼資料提供至驅動程式的傳出目錄：


// Make sure to include the metadata library that specializes the
// `fdf_metadata::ObjectDetails` class. It is needed by
// `fdf_metadata::MetadataServer`.
#include "examples_metadata.h"

#include <lib/driver/metadata/cpp/metadata_server.h>

class Sender : public fdf::DriverBase {
 public:
  zx::result<> Start() override {
    // Set the metadata to be served.
    fuchsia_examples_metadata::Metadata metadata{{.test_property = "test value"}};
    ZX_ASSERT(metadata_server_.SetMetadata(std::move(metadata)) == ZX_OK);

    // Serve the metadata to the driver's outgoing directory.
    ZX_ASSERT(metadata_server_.Serve(*outgoing(), dispatcher()) == ZX_OK);

    return zx::ok();
  }

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

您可在 fdf_metadata::MetadataServer::Serve() 之前或之後多次呼叫 fdf_metadata::MetadataServer::SetMetadata()，中繼資料伺服器將提供最新的中繼資料執行個體。如果在嘗試擷取中繼資料之前未呼叫 fdf_metadata::MetadataServer::SetMetadata()，驅動程式就無法擷取中繼資料。

需要更新 driver 建構目標：fuchsia_cc_driver("driver") { testonly = true output_name = "parent" sources = [ "parent.cc", ] deps = [ "//src/devices/lib/driver:driver_runtime", ":examples_metadata", ] }

公開中繼資料服務

最後，驅動程式庫必須在元件資訊清單中宣告並公開 fuchsia.examples.metadata.Metadata FIDL 服務：

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

從技術層面來說，並沒有 fuchsia.examples.metadata.Metadata FIDL 服務。實際上，fdf_metadata::MetadataServer 會提供 fuchsia.driver.metadata/Service Fidl 服務，以便傳送中繼資料。不過，fdf_metadata::MetadataServer 不會像一般 FIDL 服務一樣，以 fuchsia.driver.metadata.Service 的名稱提供這項服務。而是以 fuchsia.examples.metadata.Metadata 名稱提供服務。這可讓接收驅動程式識別要傳遞的中繼資料類型。這表示即使 fuchsia.examples.metadata.Metadata 服務不存在，我們也需要宣告並公開該服務。

擷取中繼資料

初始設定

假設某個驅動程式庫想接收來自父項驅動程式庫的中繼資料：

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

class Retriever : public fdf::DriverBase {
 public:
  Retriever(fdf::DriverStartArgs start_args, fdf::UnownedSynchronizedDispatcher driver_dispatcher)
      : DriverBase("child", std::move(start_args), std::move(driver_dispatcher)) {}
};

FUCHSIA_DRIVER_EXPORT(Retriever);

元件資訊清單如下所示：

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

其建構目標的定義如下：

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

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

擷取程序

為從驅動程式庫的父項驅動程式庫擷取中繼資料，驅動程式庫會呼叫 fdf_metadata::GetMetadata()：

#include <lib/driver/metadata/cpp/metadata.h>

// Make sure to include the metadata library that specializes the
// `fdf_metadata::ObjectDetails` class. It is needed by
// `fdf_metadata::GetMetadata()`.
#include "examples_metadata.h"

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

    return zx::ok();
  }
};

需要更新 driver 建構目標：

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

使用中繼資料服務

最後，驅動程式需要在元件資訊清單中宣告 fuchsia.examples.metadata.Metadata FIDL 服務的用法：

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

轉送中繼資料

初始設定

假設有個驅動程式想要從父項驅動程式擷取中繼資料，並將該中繼資料轉送至子項驅動程式：

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

class Forwarder : public fdf::DriverBase {
 public:
  Forwarder(fdf::DriverStartArgs start_args, fdf::UnownedSynchronizedDispatcher driver_dispatcher)
      : DriverBase("forward", std::move(start_args), std::move(driver_dispatcher)) {}
};

FUCHSIA_DRIVER_EXPORT(Forwarder);

元件資訊清單如下所示：

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

其建構目標的定義如下：

fuchsia_cc_driver("driver") {
  testonly = true
  output_name = "forward_driver"
  sources = [
    "forward_driver.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.
}

轉送程序

為了讓驅動程式將中繼資料從父項驅動程式轉送至子項驅動程式，驅動程式需要 fdf_metadata::MetadataServer 類別的例項，透過呼叫其 fdf_metadata::MetadataServer::ForwardMetadata() 方法設定中繼資料，然後透過呼叫其 fdf_metadata::MetadataServer::Serve() 方法，將中繼資料提供至驅動程式的傳出目錄：

// Make sure to include the metadata library that specializes the
// `fdf_metadata::ObjectDetails` class. It is needed by
// `fdf_metadata::MetadataServer`.
#include "examples_metadata.h"

#include <lib/driver/metadata/cpp/metadata_server.h>

class Forwarder : public fdf::DriverBase {
 public:
  zx::result<> Start() override {
    // Set metadata using the driver's parent driver metadata.
    ZX_ASSERT(metadata_server_.ForwardMetadata(incoming()) == ZX_OK);

    // Serve the metadata to the driver's outgoing directory.
    ZX_ASSERT(metadata_server_.Serve(*outgoing(), dispatcher()) == ZX_OK);

    return zx::ok();
  }

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

請注意，fdf_metadata::MetadataServer::ForwardMetadata() 不會在 fdf_metadata::MetadataServer::ForwardMetadata() 呼叫後，檢查父項驅動程式是否變更所提供的中繼資料。驅動程式必須再次呼叫 fdf_metadata::MetadataServer::ForwardMetadata()，才能納入變更。

您需要更新 driver 建構目標：

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

公開及使用中繼資料服務

最後，驅動程式需要在其元件資訊清單中宣告、使用及公開 fuchsia.examples.metadata.Metadata FIDL 服務：

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