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逐步解說：建立資料流程代理程式

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本檔示範如何建立以資料流程為基礎的代理程式型應用程式，而不是控制流程。

控制流程 是指程式中作業的執行順序。 控制流程是使用條件陳述式、迴圈等控制結構進行管制。 或者，資料流程 是指程式設計模型， 只有在所有必要資料都可用時，才會進行計算。 資料流程程式設計模型與訊息傳遞的概念有關，其中程式的獨立元件會藉由傳送訊息彼此通訊。

非同步代理程式同時支援控制流程和資料流程程式設計模型。 雖然在許多情況下，控制流程模型是適當的，但資料流程模型適用于其他模型，例如，當代理程式接收資料並執行以該資料承載為基礎的動作時。

必要條件

開始本逐步解說之前，請先閱讀下列檔：

區段

本逐步解說包含下列各節：

建立基本控制流程代理程式

請考慮下列定義 類別的 control_flow_agent 範例。 類別 control_flow_agent 在三個訊息緩衝區上作用：一個輸入緩衝區和兩個輸出緩衝區。 方法 run 會從迴圈中的來源訊息緩衝區讀取，並使用條件陳述式來引導程式執行的流程。 代理程式會針對非零、負值遞增一個計數器，並為非零、正值遞增另一個計數器。 代理程式收到零的 sentinel 值之後，它會將計數器的值傳送至輸出訊息緩衝區。 和 negatives positives 方法可讓應用程式從代理程式讀取負值和正值計數。

// 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;
};

雖然此範例會基本使用代理程式中的控制流程，但它示範以控制流程為基礎的程式設計序列本質。 即使輸入訊息緩衝區中可能有多個訊息，每個訊息都必須循序處理。 資料流程模型可讓條件陳述式的兩個分支同時評估。 資料流程模型也可讓您建立更複雜的傳訊網路，以在資料可供使用時採取行動。

[靠上]

建立基本資料流程代理程式

本節說明如何將 類別轉換成 control_flow_agent 使用資料流程模型來執行相同的工作。

資料流程代理程式的運作方式是建立訊息緩衝區網路，每個訊息緩衝區都有特定用途。 某些訊息區塊會使用篩選函式，根據其承載來接受或拒絕訊息。 篩選函式可確保消息塊只接收特定值。

將控制流程代理程式轉換為數據流代理程式

  1. 將 類別的 control_flow_agent 主體複製到另一個類別，例如 dataflow_agent 。 或者，您可以重新命名 類別 control_flow_agent 。

  2. 移除從 run 方法呼叫 receive 的迴圈主體。

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 方法中 run ，在變數 negative_count 和 positive_count 初始化之後，新增 countdown_event 物件來追蹤使用中作業計數。
// Tracks the count of active operations.
countdown_event active;
// An event that is set by the sentinel.
event received_sentinel;

本主題稍後會顯示 類別 countdown_event 。

  1. 建立將參與資料流程網路的訊息緩衝區物件。
 //
 // 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. 連線訊息緩衝區以形成網路。
//
// 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. 等候 event 設定 和 countdown event 物件。 這些事件表示代理程式已收到 sentinel 值，且所有作業都已完成。
// Wait for the sentinel event and for all operations to finish.
received_sentinel.wait();
active.wait();

下圖顯示 類別的完整資料流程網路 dataflow_agent ：

The dataflow network.

下表描述網路的成員。

member 描述
increment_active 並行：：transformer 物件，會遞增使用中事件計數器，並將輸入值傳遞至網路的其餘部分。
negatives, positives concurrency：：call 物件，以遞增數位計數並遞減使用中事件計數器。 每個物件都會使用篩選準則來接受負數或正數。
sentinel concurrency：：call 物件，只接受零的 sentinel 值，並遞減使用中事件計數器。
connector 將 來源訊息緩衝區連線到內部網路的並行：：unbounded_buffer 物件。

run因為方法是在個別執行緒上呼叫，其他執行緒可以在網路完全連線之前，將訊息傳送至網路。 資料 _source 成員是物件 unbounded_buffer ，會緩衝從應用程式傳送至代理程式的所有輸入。 為了確保網路會處理所有輸入訊息，代理程式會先連結網路的內部節點，然後將該網路的 connector 開頭連結至 _source 資料成員。 這可確保訊息不會在形成網路時進行處理。

由於此範例中的網路是以資料流程為基礎，而不是以控制流程為基礎，因此網路必須與代理程式通訊，該代理程式已完成處理每個輸入值，而且 sentinel 節點已接收其值。 這個範例會使用 countdown_event 物件來表示已處理所有輸入值，以及 concurrency：：event 物件，表示 sentinel 節點已收到其值。 類別 countdown_event 會使用 event 物件，在計數器值達到零時發出訊號。 資料流程網路的前端會在每次收到值時遞增計數器。 網路的每個終端節點會在處理輸入值之後遞減計數器。 代理程式形成資料流程網路之後，它會等候 sentinel 節點設定 event 物件，並讓 countdown_event 物件發出其計數器已達到零的訊號。

下列範例顯示 control_flow_agent 、 dataflow_agent 和 countdown_event 類別。 函 wmain 式會 control_flow_agent 建立 和 dataflow_agent 物件，並使用 函 send_values 式將一系列隨機值傳送給代理程式。

// 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;
}

此範例會產生下列範例輸出：

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.

編譯程式碼

複製範例程式碼，並將其貼到 Visual Studio 專案中，或貼到名為 dataflow-agent.cpp 的檔案中，然後在 Visual Studio 命令提示字元視窗中執行下列命令。

cl.exe /EHsc dataflow-agent.cpp

[靠上]

建立訊息記錄代理程式

下列範例顯示 log_agent 類別，類似于 dataflow_agent 類別。 類別 log_agent 會實作非同步記錄代理程式，以將記錄訊息寫入檔案和主控台。 類別 log_agent 可讓應用程式將訊息分類為參考、警告或錯誤。 它也可讓應用程式指定每個記錄類別是否寫入檔案、主控台或兩者。 此範例會將所有記錄訊息寫入檔案，並將錯誤訊息只寫入主控台。

// 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);
}

此範例會將下列輸出寫入主控台。

error: This is a sample error message.

此範例也會產生 log.txt 檔案，其中包含下列文字。

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

編譯程式碼

複製範例程式碼，並將其貼到 Visual Studio 專案中，或貼到名為 log-filter.cpp 的檔案中，然後在 Visual Studio 命令提示字元視窗中執行下列命令。

cl.exe /EHsc log-filter.cpp

[靠上]

另請參閱

並行執行階段逐步解說