如何：创建在延迟一段时间后完成的任务
此示例演示如何使用、、、、concurrency::timerconcurrency::taskconcurrency::cancellation_token_sourceconcurrency::cancellation_tokenconcurrency::task_completion_eventconcurrency::call 和类在创建延迟后完成的任务。 你可使用此方法来生成偶尔轮询数据的循环，引入超时，将对用户输入的处理延迟预定的一段时间等。
示例
下面的示例显示 complete_after 和 cancel_after_timeout 函数。 complete_after 函数创建完成，然后指定的延迟后的 task 对象。 它使用 timer 对象和 call 对象来设置 task_completion_event 对象，在指定后延迟。 通过使用 task_completion_event 类，您可以定义执行的任务，在线程或其他表示的任务之后值可用为止。 设置事件后，侦听器任务完成，它们的继续按计划运行。
提示
有关 timer 和 call 类的详细信息，该异步代理库的一部分，请参见 异步消息块。
该任务不在特定超时前，在 complete_after 的 cancel_after_timeout 函数生成函数取消任务。 cancel_after_timeout 函数创建两个任务。 在提供的任务完成后，第一个任务并指示成功完成；第二个任务指示失败且在指定的超时后完成。 cancel_after_timeout 函数创建运行的任务，在成功或失败的任务完成。 如果未先完成任务，则延续将取消标记源取消整个任务。
// Creates a task that completes after the specified delay.
task<void> complete_after(unsigned int timeout)
{
// A task completion event that is set when a timer fires.
task_completion_event<void> tce;
// Create a non-repeating timer.
auto fire_once = new timer<int>(timeout, 0, nullptr, false);
// Create a call object that sets the completion event after the timer fires.
auto callback = new call<int>([tce](int)
{
tce.set();
});
// Connect the timer to the callback and start the timer.
fire_once->link_target(callback);
fire_once->start();
// Create a task that completes after the completion event is set.
task<void> event_set(tce);
// Create a continuation task that cleans up resources and
// and return that continuation task.
return event_set.then([callback, fire_once]()
{
delete callback;
delete fire_once;
});
}
// Cancels the provided task after the specifed delay, if the task
// did not complete.
template<typename T>
task<T> cancel_after_timeout(task<T> t, cancellation_token_source cts, unsigned int timeout)
{
// Create a task that returns true after the specified task completes.
task<bool> success_task = t.then([](T)
{
return true;
});
// Create a task that returns false after the specified timeout.
task<bool> failure_task = complete_after(timeout).then([]
{
return false;
});
// Create a continuation task that cancels the overall task
// if the timeout task finishes first.
return (failure_task || success_task).then([t, cts](bool success)
{
if(!success)
{
// Set the cancellation token. The task that is passed as the
// t parameter should respond to the cancellation and stop
// as soon as it can.
cts.cancel();
}
// Return the original task.
return t;
});
}
下面的示例分范围[0, 100000]多次计算一个数组中质数的计数。 如果在特定时间限制，无法完成操作失败。 count_primes 函数演示如何使用 cancel_after_timeout 函数。 *如果任务提供的时间，而在完成给定范围计数数字填装和失败。 wmain 函数调用 count_primes 函数多次。 每次，该二进制时间限制。 在操作中当前时间期限后，无法完成程序完成。
// Determines whether the input value is prime.
bool is_prime(int n)
{
if (n < 2)
return false;
for (int i = 2; i < n; ++i)
{
if ((n % i) == 0)
return false;
}
return true;
}
// Counts the number of primes in the range [0, max_value].
// The operation fails if it exceeds the specified timeout.
bool count_primes(unsigned int max_value, unsigned int timeout)
{
cancellation_token_source cts;
// Create a task that computes the count of prime numbers.
// The task is canceled after the specified timeout.
auto t = cancel_after_timeout(task<size_t>([max_value, timeout]
{
combinable<size_t> counts;
parallel_for<unsigned int>(0, max_value + 1, [&counts](unsigned int n)
{
// Respond if the overall task is cancelled by canceling
// the current task.
if (is_task_cancellation_requested())
{
cancel_current_task();
}
// NOTE: You can replace the calls to is_task_cancellation_requested
// and cancel_current_task with a call to interruption_point.
// interruption_point();
// Increment the local counter if the value is prime.
if (is_prime(n))
{
counts.local()++;
}
});
// Return the sum of counts across all threads.
return counts.combine(plus<size_t>());
}, cts.get_token()), cts, timeout);
// Print the result.
try
{
auto primes = t.get();
wcout << L"Found " << primes << L" prime numbers within "
<< timeout << L" ms." << endl;
return true;
}
catch (const task_canceled& e)
{
wcout << L"The task timed out." << endl;
return false;
}
}
int wmain()
{
// Compute the count of prime numbers in the range [0, 100000]
// until the operation fails.
// Each time the test succeeds, the time limit is halved.
unsigned int max = 100000;
unsigned int timeout = 5000;
bool success = true;
do
{
success = count_primes(max, timeout);
timeout /= 2;
} while (success);
}
/* Sample output:
Found 9592 prime numbers within 5000 ms.
Found 9592 prime numbers within 2500 ms.
Found 9592 prime numbers within 1250 ms.
Found 9592 prime numbers within 625 ms.
The task timed out.
*/
当您使用此技术来取消任务，然后推迟，任何 unstarted 任务不会启动之后，整个任务也将被取消后。 但是，对于所有长期运行的任务很重要及时响应取消。 在此示例中，count_primes 方法调用响应的 concurrency::is_task_cancellation_requested 和 cancel_current_task 函数对取消。(或者，您可能调用 concurrency::interruption_point 函数)。 有关任务取消的更多信息，请参见 PPL 中的取消操作。
这是此示例的完整代码：
// task-delay.cpp
// compile with: /EHsc
#include <ppltasks.h>
#include <agents.h>
#include <iostream>
using namespace concurrency;
using namespace std;
// Creates a task that completes after the specified delay.
task<void> complete_after(unsigned int timeout)
{
// A task completion event that is set when a timer fires.
task_completion_event<void> tce;
// Create a non-repeating timer.
auto fire_once = new timer<int>(timeout, 0, nullptr, false);
// Create a call object that sets the completion event after the timer fires.
auto callback = new call<int>([tce](int)
{
tce.set();
});
// Connect the timer to the callback and start the timer.
fire_once->link_target(callback);
fire_once->start();
// Create a task that completes after the completion event is set.
task<void> event_set(tce);
// Create a continuation task that cleans up resources and
// and return that continuation task.
return event_set.then([callback, fire_once]()
{
delete callback;
delete fire_once;
});
}
// Cancels the provided task after the specifed delay, if the task
// did not complete.
template<typename T>
task<T> cancel_after_timeout(task<T> t, cancellation_token_source cts, unsigned int timeout)
{
// Create a task that returns true after the specified task completes.
task<bool> success_task = t.then([](T)
{
return true;
});
// Create a task that returns false after the specified timeout.
task<bool> failure_task = complete_after(timeout).then([]
{
return false;
});
// Create a continuation task that cancels the overall task
// if the timeout task finishes first.
return (failure_task || success_task).then([t, cts](bool success)
{
if(!success)
{
// Set the cancellation token. The task that is passed as the
// t parameter should respond to the cancellation and stop
// as soon as it can.
cts.cancel();
}
// Return the original task.
return t;
});
}
// Determines whether the input value is prime.
bool is_prime(int n)
{
if (n < 2)
return false;
for (int i = 2; i < n; ++i)
{
if ((n % i) == 0)
return false;
}
return true;
}
// Counts the number of primes in the range [0, max_value].
// The operation fails if it exceeds the specified timeout.
bool count_primes(unsigned int max_value, unsigned int timeout)
{
cancellation_token_source cts;
// Create a task that computes the count of prime numbers.
// The task is canceled after the specified timeout.
auto t = cancel_after_timeout(task<size_t>([max_value, timeout]
{
combinable<size_t> counts;
parallel_for<unsigned int>(0, max_value + 1, [&counts](unsigned int n)
{
// Respond if the overall task is cancelled by canceling
// the current task.
if (is_task_cancellation_requested())
{
cancel_current_task();
}
// NOTE: You can replace the calls to is_task_cancellation_requested
// and cancel_current_task with a call to interruption_point.
// interruption_point();
// Increment the local counter if the value is prime.
if (is_prime(n))
{
counts.local()++;
}
});
// Return the sum of counts across all threads.
return counts.combine(plus<size_t>());
}, cts.get_token()), cts, timeout);
// Print the result.
try
{
auto primes = t.get();
wcout << L"Found " << primes << L" prime numbers within "
<< timeout << L" ms." << endl;
return true;
}
catch (const task_canceled& e)
{
wcout << L"The task timed out." << endl;
return false;
}
}
int wmain()
{
// Compute the count of prime numbers in the range [0, 100000]
// until the operation fails.
// Each time the test succeeds, the time limit is halved.
unsigned int max = 100000;
unsigned int timeout = 5000;
bool success = true;
do
{
success = count_primes(max, timeout);
timeout /= 2;
} while (success);
}
/* Sample output:
Found 9592 prime numbers within 5000 ms.
Found 9592 prime numbers within 2500 ms.
Found 9592 prime numbers within 1250 ms.
Found 9592 prime numbers within 625 ms.
The task timed out.
*/
编译代码
若要编译代码，请复制代码并将其粘贴到Visual Studio项目中或一个名为task-delay.cpp的文件中，然后在Visual Studio命令提示符窗口中运行以下命令。
cl.exe /EHsc task-delay.cpp
请参见
参考
is_task_cancellation_requested 函数