windows C++-创建数据流代理(二)

 完整的数据流演示 

下图显示了 dataflow_agent 类的完整数据流网络：

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;
}

输出如下:

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