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演练：创建数据流代理

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本文档演示如何创建基于数据流，而不是基于控制流的基于代理的应用程序。

控制流指的是程序中操作的执行顺序。 控制流是通过使用条件语句、循环等控制结构调节的。 此外，数据流指的是一种编程模型，在这种模型中，只有当所有需要的数据都可用时，才会进行计算。 数据流编程模型与消息传递这一概念相关，其中程序的独立组件通过发送消息相互通信。

异步代理同时支持控制流和数据流编程模型。 尽管控制流模型适用于许多情况，但数据流模型也适用于其他一些情况，例如，当代理接收数据并执行基于该数据的有效负载的操作时。

先决条件

在开始操作本演练之前，请阅读以下文档：

部分

本演练包含以下各节：

创建基本控制流代理

请看下面的示例，该示例定义了 control_flow_agent 类。 control_flow_agent 类对三个消息缓冲区（一个输入缓冲区和两个输出缓冲区）执行操作。 run 方法在一个循环中从源消息缓冲区读取，并使用条件语句来引导程序执行流。 代理针对非零负值递增一个计数器值，并为非零正值递增另一个计数器值。 代理收到为零的 Sentinel 值后，它会将计数器的值发送到输出消息缓冲区。 通过 negatives 和 positives 方法，应用程序可以从代理读取负值和正值的计数。

// A basic agent that uses control-flow to regulate the order of program 
// execution. This agent reads numbers from a message buffer and counts the 
// number of positive and negative values.
class control_flow_agent : public agent
{
public:
   explicit control_flow_agent(ISource<int>& source)
      : _source(source)
   {
   }

   // Retrieves the count of negative numbers that the agent received.
   size_t negatives() 
   {
      return receive(_negatives);
   }

   // Retrieves the count of positive numbers that the agent received.
   size_t positives()
   {
      return receive(_positives);
   }

protected:
   void run()
   {
      // Counts the number of negative and positive values that
      // the agent receives.
      size_t negative_count = 0;
      size_t positive_count = 0;

      // Read from the source buffer until we receive
      // the sentinel value of 0.
      int value = 0;      
      while ((value = receive(_source)) != 0)
      {
         // Send negative values to the first target and
         // non-negative values to the second target.
         if (value < 0)
            ++negative_count;
         else
            ++positive_count;
      }

      // Write the counts to the message buffers.
      send(_negatives, negative_count);
      send(_positives, positive_count);

      // Set the agent to the completed state.
      done();
   }
private:
   // Source message buffer to read from.
   ISource<int>& _source;

   // Holds the number of negative and positive numbers that the agent receives.
   single_assignment<size_t> _negatives;
   single_assignment<size_t> _positives;
};

虽然在此示例中看到的是代理中的控制流的基本用法，但它展示了基于控制流的编程的连续性。 必须按顺序处理每个消息，即使在输入消息缓冲区中可能有多个消息。 数据流模型使条件语句的两个分支能同时求值。 数据流模型还使你能够创建更复杂的消息传送网络，在数据可用时对其执行操作。

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创建基本数据流代理

本部分介绍如何转换 control_flow_agent 类以使用数据流模型执行相同的任务。

数据流代理的工作方式是创建一个消息缓冲区网络，每个缓冲区都有特定用途。 某些消息块使用筛选功能，根据消息的有效负载接受或拒绝消息。 筛选器函数可确保消息块仅接收特定值。

将控制流代理转换为数据流代理

  1. 将 control_flow_agent 类的主体复制到另一个类，例如 dataflow_agent。 或者，可以重命名 control_flow_agent 类。

  2. 从 run 方法移除用来调用 receive 的循环主体。

void run()
{
   // Counts the number of negative and positive values that
   // the agent receives.
   size_t negative_count = 0;
   size_t positive_count = 0;

   // Write the counts to the message buffers.
   send(_negatives, negative_count);
   send(_positives, positive_count);

   // Set the agent to the completed state.
   done();
}
  1. 在 run 方法中，在变量 negative_count 和 positive_count 初始化后，添加 countdown_event 对象，用于跟踪活动操作的计数。
// Tracks the count of active operations.
countdown_event active;
// An event that is set by the sentinel.
event received_sentinel;

本主题稍后会对 countdown_event 类进行介绍。

  1. 创建将参与数据流网络的消息缓冲区对象。
 //
 // Create the members of the dataflow network.
 //

 // Increments the active counter.
 transformer<int, int> increment_active(
    [&active](int value) -> int {
       active.add_count();
       return value;
    });

 // Increments the count of negative values.
 call<int> negatives(
    [&](int value) {
       ++negative_count;
       // Decrement the active counter.
       active.signal();
    },
    [](int value) -> bool {
       return value < 0;
    });

 // Increments the count of positive values.
 call<int> positives(
    [&](int value) {
       ++positive_count;
       // Decrement the active counter.
       active.signal();
    },
    [](int value) -> bool {
       return value > 0;
    });

 // Receives only the sentinel value of 0.
 call<int> sentinel(
    [&](int value) {            
       // Decrement the active counter.
       active.signal();
       // Set the sentinel event.
       received_sentinel.set();
    },
    [](int value) -> bool { 
       return value == 0; 
    });

 // Connects the _source message buffer to the rest of the network.
 unbounded_buffer<int> connector;
  1. 连接消息缓冲区以构成网络。
//
// Connect the network.
//

// Connect the internal nodes of the network.
connector.link_target(&negatives);
connector.link_target(&positives);
connector.link_target(&sentinel);
increment_active.link_target(&connector);

// Connect the _source buffer to the internal network to 
// begin data flow.
_source.link_target(&increment_active);
  1. 等待 event 和 countdown event 对象设置完成。 这些事件表明代理已收到 Sentinel 值，并且所有操作都已完成。
// Wait for the sentinel event and for all operations to finish.
received_sentinel.wait();
active.wait();

下图显示了 dataflow_agent 类的完整数据流网络：

The dataflow network.

下表描述了网络的成员。

成员 说明
increment_active 一个 concurrency::transformer 对象，用于递增活动事件计数器值，并将输入值传递到网络的其余部分。
negatives，positives concurrency::call 对象，用于递增数字计数值并递减活动事件计数器值。 每个对象都使用筛选器来接受负数或正数。
sentinel 一个 concurrency::call 对象，用于仅接受为零的 Sentinel 值并递减活动事件计数器值。
connector 一个 concurrency::unbounded_buffer 对象，用于将源消息缓冲区连接到内部网络。

由于 run 方法是在一个单独的线程上调用的，因此在完全连接网络之前，其他线程可以将消息发送到网络。 _source 数据成员是一个 unbounded_buffer 对象，用于缓冲从应用程序发送到代理的所有输入。 为了确保网络能够处理所有输入消息，代理会首先链接网络的内部节点，然后将该网络的起点 connector 链接到 _source 数据成员。 这可以保证在形成网络的过程中不会处理消息。

由于此示例中的网络是基于数据流，而不是基于控制流的，网络必须向代理传达它已经完成了对每个输入值的处理，并且 Sentinel 节点也已接收到它的值。 此示例使用 countdown_event 对象来指示所有输入值均已经过处理，使用 concurrency::event 对象指示 Sentinel 节点已接收到它的值。 countdown_event 类使用 event 对象指示计数器值达到零。 每当数据流网络的头收到一个值时，都会递增计数器值。 在处理输入值后，网络的每个终端节点都会递减计数器值。 代理形成数据流网络后，它会等待 Sentinel 节点设置 event 对象，还会等待 countdown_event 对象指示其计数器值已达到零。

下面的示例展示了 control_flow_agent、dataflow_agent 和 countdown_event 类。 wmain 函数创建了 control_flow_agent 和 dataflow_agent 对象，并使用 send_values 函数将一系列随机值发送到代理。

// dataflow-agent.cpp
// compile with: /EHsc 
#include <windows.h>
#include <agents.h>
#include <iostream>
#include <random>

using namespace concurrency;
using namespace std;

// A basic agent that uses control-flow to regulate the order of program 
// execution. This agent reads numbers from a message buffer and counts the 
// number of positive and negative values.
class control_flow_agent : public agent
{
public:
   explicit control_flow_agent(ISource<int>& source)
      : _source(source)
   {
   }

   // Retrieves the count of negative numbers that the agent received.
   size_t negatives() 
   {
      return receive(_negatives);
   }

   // Retrieves the count of positive numbers that the agent received.
   size_t positives()
   {
      return receive(_positives);
   }

protected:
   void run()
   {
      // Counts the number of negative and positive values that
      // the agent receives.
      size_t negative_count = 0;
      size_t positive_count = 0;

      // Read from the source buffer until we receive
      // the sentinel value of 0.
      int value = 0;      
      while ((value = receive(_source)) != 0)
      {
         // Send negative values to the first target and
         // non-negative values to the second target.
         if (value < 0)
            ++negative_count;
         else
            ++positive_count;
      }

      // Write the counts to the message buffers.
      send(_negatives, negative_count);
      send(_positives, positive_count);

      // Set the agent to the completed state.
      done();
   }
private:
   // Source message buffer to read from.
   ISource<int>& _source;

   // Holds the number of negative and positive numbers that the agent receives.
   single_assignment<size_t> _negatives;
   single_assignment<size_t> _positives;
};

// A synchronization primitive that is signaled when its 
// count reaches zero.
class countdown_event
{
public:
   countdown_event(unsigned int count = 0L)
      : _current(static_cast<long>(count)) 
   {
      // Set the event if the initial count is zero.
      if (_current == 0L)
         _event.set();
   }
     
   // Decrements the event counter.
   void signal() {
      if(InterlockedDecrement(&_current) == 0L) {
         _event.set();
      }
   }

   // Increments the event counter.
   void add_count() {
      if(InterlockedIncrement(&_current) == 1L) {
         _event.reset();
      }
   }
   
   // Blocks the current context until the event is set.
   void wait() {
      _event.wait();
   }
 
private:
   // The current count.
   volatile long _current;
   // The event that is set when the counter reaches zero.
   event _event;

   // Disable copy constructor.
   countdown_event(const countdown_event&);
   // Disable assignment.
   countdown_event const & operator=(countdown_event const&);
};

// A basic agent that resembles control_flow_agent, but uses uses dataflow to 
// perform computations when data becomes available.
class dataflow_agent : public agent
{
public:
   dataflow_agent(ISource<int>& source)
      : _source(source)
   {
   }

   // Retrieves the count of negative numbers that the agent received.
   size_t negatives() 
   {
      return receive(_negatives);
   }

   // Retrieves the count of positive numbers that the agent received.
   size_t positives()
   {
      return receive(_positives);
   }

protected:
   void run()
   {
      // Counts the number of negative and positive values that
      // the agent receives.
      size_t negative_count = 0;
      size_t positive_count = 0;

      // Tracks the count of active operations.
      countdown_event active;
      // An event that is set by the sentinel.
      event received_sentinel;
      
      //
      // Create the members of the dataflow network.
      //
     
      // Increments the active counter.
      transformer<int, int> increment_active(
         [&active](int value) -> int {
            active.add_count();
            return value;
         });

      // Increments the count of negative values.
      call<int> negatives(
         [&](int value) {
            ++negative_count;
            // Decrement the active counter.
            active.signal();
         },
         [](int value) -> bool {
            return value < 0;
         });

      // Increments the count of positive values.
      call<int> positives(
         [&](int value) {
            ++positive_count;
            // Decrement the active counter.
            active.signal();
         },
         [](int value) -> bool {
            return value > 0;
         });

      // Receives only the sentinel value of 0.
      call<int> sentinel(
         [&](int value) {            
            // Decrement the active counter.
            active.signal();
            // Set the sentinel event.
            received_sentinel.set();
         },
         [](int value) -> bool { 
            return value == 0; 
         });

      // Connects the _source message buffer to the rest of the network.
      unbounded_buffer<int> connector;
       
      //
      // Connect the network.
      //

      // Connect the internal nodes of the network.
      connector.link_target(&negatives);
      connector.link_target(&positives);
      connector.link_target(&sentinel);
      increment_active.link_target(&connector);

      // Connect the _source buffer to the internal network to 
      // begin data flow.
      _source.link_target(&increment_active);

      // Wait for the sentinel event and for all operations to finish.
      received_sentinel.wait();
      active.wait();
           
      // Write the counts to the message buffers.
      send(_negatives, negative_count);
      send(_positives, positive_count);

      // Set the agent to the completed state.
      done();
   }

private:
   // Source message buffer to read from.
   ISource<int>& _source;
   
   // Holds the number of negative and positive numbers that the agent receives.
   single_assignment<size_t> _negatives;
   single_assignment<size_t> _positives;
};

// Sends a number of random values to the provided message buffer.
void send_values(ITarget<int>& source, int sentinel, size_t count)
{
   // Send a series of random numbers to the source buffer.
   mt19937 rnd(42);
   for (size_t i = 0; i < count; ++i)
   {
      // Generate a random number that is not equal to the sentinel value.
      int n;
      while ((n = rnd()) == sentinel);

      send(source, n);      
   }
   // Send the sentinel value.
   send(source, sentinel);   
}

int wmain()
{
   // Signals to the agent that there are no more values to process.
   const int sentinel = 0;
   // The number of samples to send to each agent.
   const size_t count = 1000000;

   // The source buffer that the application writes numbers to and 
   // the agents read numbers from.
   unbounded_buffer<int> source;

   //
   // Use a control-flow agent to process a series of random numbers.
   //
   wcout << L"Control-flow agent:" << endl;

   // Create and start the agent.
   control_flow_agent cf_agent(source);
   cf_agent.start();
   
   // Send values to the agent.
   send_values(source, sentinel, count);
   
   // Wait for the agent to finish.
   agent::wait(&cf_agent);
   
   // Print the count of negative and positive numbers.
   wcout << L"There are " << cf_agent.negatives() 
         << L" negative numbers."<< endl;
   wcout << L"There are " << cf_agent.positives() 
         << L" positive numbers."<< endl;  

   //
   // Perform the same task, but this time with a dataflow agent.
   //
   wcout << L"Dataflow agent:" << endl;

   // Create and start the agent.
   dataflow_agent df_agent(source);
   df_agent.start();
   
   // Send values to the agent.
   send_values(source, sentinel, count);
   
   // Wait for the agent to finish.
   agent::wait(&df_agent);
   
   // Print the count of negative and positive numbers.
   wcout << L"There are " << df_agent.negatives() 
         << L" negative numbers."<< endl;
   wcout << L"There are " << df_agent.positives() 
         << L" positive numbers."<< endl;
}

此示例产生以下示例输出：

Control-flow agent:
There are 500523 negative numbers.
There are 499477 positive numbers.
Dataflow agent:
There are 500523 negative numbers.
There are 499477 positive numbers.

编译代码

复制示例代码，并将它粘贴到 Visual Studio 项目中，或粘贴到名为 dataflow-agent.cpp 的文件中，再在 Visual Studio 命令提示符窗口中运行以下命令。

cl.exe /EHsc dataflow-agent.cpp

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创建消息记录代理

以下示例展示了 log_agent 类，它类似于 dataflow_agent 类。 log_agent 类实现异步记录代理，用于将日志消息写入文件和控制台。 log_agent 类使应用程序能够将消息分类为信息性、警告或错误消息。 它还使应用程序能够指定每个日志类别是写入文件、控制台还是同时写入这两者。 本示例将所有日志消息写入文件，并仅将错误消息写入控制台。

// log-filter.cpp
// compile with: /EHsc 
#include <windows.h>
#include <agents.h>
#include <sstream>
#include <fstream>
#include <iostream>

using namespace concurrency;
using namespace std;

// A synchronization primitive that is signaled when its 
// count reaches zero.
class countdown_event
{
public:
    countdown_event(unsigned int count = 0L)
        : _current(static_cast<long>(count)) 
    {
        // Set the event if the initial count is zero.
        if (_current == 0L)
        {
            _event.set();
        }
    }

    // Decrements the event counter.
    void signal()
    {
        if(InterlockedDecrement(&_current) == 0L)
        {
            _event.set();
        }
    }

    // Increments the event counter.
    void add_count()
    {
        if(InterlockedIncrement(&_current) == 1L)
        {
            _event.reset();
        }
    }

    // Blocks the current context until the event is set.
    void wait()
    {
        _event.wait();
    }

private:
    // The current count.
    volatile long _current;
    // The event that is set when the counter reaches zero.
    event _event;

    // Disable copy constructor.
    countdown_event(const countdown_event&);
    // Disable assignment.
    countdown_event const & operator=(countdown_event const&);
};

// Defines message types for the logger.
enum log_message_type
{
    log_info    = 0x1,
    log_warning = 0x2,
    log_error   = 0x4,
};

// An asynchronous logging agent that writes log messages to 
// file and to the console.
class log_agent : public agent
{
    // Holds a message string and its logging type.
    struct log_message
    {
        wstring message;
        log_message_type type;
    };

public:
    log_agent(const wstring& file_path, log_message_type file_messages, log_message_type console_messages)
        : _file(file_path)
        , _file_messages(file_messages)
        , _console_messages(console_messages)
        , _active(0)
    {
        if (_file.bad())
        {
            throw invalid_argument("Unable to open log file.");
        }
    }

    // Writes the provided message to the log.
    void log(const wstring& message, log_message_type type)
    {  
        // Increment the active message count.
        _active.add_count();

        // Send the message to the network.
        log_message msg = { message, type };
        send(_log_buffer, msg);
    }

    void close()
    {
        // Signal that the agent is now closed.
        _closed.set();
    }

protected:

    void run()
    {
        //
        // Create the dataflow network.
        //

        // Writes a log message to file.
        call<log_message> writer([this](log_message msg)
        {
            if ((msg.type & _file_messages) != 0)
            {
                // Write the message to the file.
                write_to_stream(msg, _file);
            }
            if ((msg.type & _console_messages) != 0)
            {
                // Write the message to the console.
                write_to_stream(msg, wcout);
            }
            // Decrement the active counter.
            _active.signal();
        });

        // Connect _log_buffer to the internal network to begin data flow.
        _log_buffer.link_target(&writer);

        // Wait for the closed event to be signaled.
        _closed.wait();

        // Wait for all messages to be processed.
        _active.wait();

        // Close the log file and flush the console.
        _file.close();
        wcout.flush();

        // Set the agent to the completed state.
        done();
    }

private:
    // Writes a logging message to the specified output stream.
    void write_to_stream(const log_message& msg, wostream& stream)
    {
        // Write the message to the stream.
        wstringstream ss;

        switch (msg.type)
        {
        case log_info:
            ss << L"info: ";
            break;
        case log_warning:
            ss << L"warning: ";
            break;
        case log_error:
            ss << L"error: ";
        }

        ss << msg.message << endl;
        stream << ss.str();
    }

private:   
    // The file stream to write messages to.
    wofstream _file;

    // The log message types that are written to file.
    log_message_type _file_messages;

    // The log message types that are written to the console.
    log_message_type _console_messages;

    // The head of the network. Propagates logging messages
    // to the rest of the network.
    unbounded_buffer<log_message> _log_buffer;

    // Counts the number of active messages in the network.
    countdown_event _active;

    // Signals that the agent has been closed.
    event _closed;
};

int wmain()
{
    // Union of all log message types.
    log_message_type log_all = log_message_type(log_info | log_warning  | log_error);

    // Create a logging agent that writes all log messages to file and error 
    // messages to the console.
    log_agent logger(L"log.txt", log_all, log_error);

    // Start the agent.
    logger.start();

    // Log a few messages.

    logger.log(L"===Logging started.===", log_info);

    logger.log(L"This is a sample warning message.", log_warning);
    logger.log(L"This is a sample error message.", log_error);

    logger.log(L"===Logging finished.===", log_info);

    // Close the logger and wait for the agent to finish.
    logger.close();
    agent::wait(&logger);
}

本示例将以下输出写入控制台。

error: This is a sample error message.

此示例还生成 log.txt 文件，其中包含以下文本。

info: ===Logging started.===
warning: This is a sample warning message.
error: This is a sample error message.
info: ===Logging finished.===

编译代码

复制示例代码，并将它粘贴到 Visual Studio 项目中，或粘贴到名为 log-filter.cpp 的文件中，再在 Visual Studio 命令提示符窗口中运行以下命令。

cl.exe /EHsc log-filter.cpp

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另请参阅

并发运行时演练