重要的 API
了解如何通过向线程池提交工作项，在单独的线程中完成工作。 使用此函数可以保持响应式 UI，同时仍在完成需要明显时间的工作，并使用它并行完成多个任务。
创建和提交工作项
通过调用 RunAsync 创建工作项。 提供委派来完成工作（你可使用一个 lambda 或 delegate 函数）。 请注意， RunAsync 返回 IAsyncAction 对象;存储此对象以供下一步使用。
可以使用三个版本的 RunAsync，以便可以选择指定工作项的优先级，并控制它是否与其他工作项并发运行。
以下示例创建一个工作项并提供 lambda 来执行该工作：
// The nth prime number to find.
const uint n = 9999;
// Receives the result.
ulong nthPrime = 0;
// Simulates work by searching for the nth prime number. Uses a
// naive algorithm and counts 2 as the first prime number.
// Capture the DispatcherQueue before entering the background lambda.
var dispatcherQueue = Microsoft.UI.Dispatching.DispatcherQueue.GetForCurrentThread();
IAsyncAction asyncAction = Windows.System.Threading.ThreadPool.RunAsync(
(workItem) =>
{
uint progress = 0; // For progress reporting.
uint primes = 0; // Number of primes found so far.
ulong i = 2; // Number iterator.
if ((n >= 0) && (n <= 2))
{
nthPrime = n;
return;
}
while (primes < (n - 1))
{
if (workItem.Status == AsyncStatus.Canceled)
{
break;
}
// Go to the next number.
i++;
// Check for prime.
bool prime = true;
for (uint j = 2; j < i; ++j)
{
if ((i % j) == 0)
{
prime = false;
break;
}
};
if (prime)
{
// Found another prime number.
primes++;
// Report progress at every 10 percent.
uint temp = progress;
progress = (uint)(10.0*primes/n);
if (progress != temp)
{
String updateString;
updateString = "Progress to " + n + "th prime: "
+ (10 * progress) + "%\n";
// Update the UI thread with the DispatcherQueue.
dispatcherQueue.TryEnqueue(
Microsoft.UI.Dispatching.DispatcherQueuePriority.High,
() => UpdateUI(updateString));
}
}
}
// Return the nth prime number.
nthPrime = i;
});
// A reference to the work item is cached so that we can trigger a
// cancellation when the user presses the Cancel button.
m_workItem = asyncAction;
// The nth prime number to find.
const unsigned int n{ 9999 };
// A shared pointer to the result.
// We use a shared pointer to keep the result alive until the
// work is done.
std::shared_ptr<unsigned long> nthPrime = std::make_shared<unsigned long>(0);
// Simulates work by searching for the nth prime number. Uses a
// naive algorithm and counts 2 as the first prime number.
// Capture the DispatcherQueue before entering the background lambda.
auto dispatcherQueue{ Microsoft::UI::Dispatching::DispatcherQueue::GetForCurrentThread() };
// A reference to the work item is cached so that we can trigger a
// cancellation when the user presses the Cancel button.
m_workItem = Windows::System::Threading::ThreadPool::RunAsync(
[=, strongThis = get_strong()](Windows::Foundation::IAsyncAction const& workItem)
{
unsigned int progress = 0; // For progress reporting.
unsigned int primes = 0; // Number of primes found so far.
unsigned long int i = 2; // Number iterator.
if ((n >= 0) && (n <= 2))
{
*nthPrime = n;
return;
}
while (primes < (n - 1))
{
if (workItem.Status() == Windows::Foundation::AsyncStatus::Canceled)
{
break;
}
// Go to the next number.
i++;
// Check for prime.
bool prime = true;
for (unsigned int j = 2; j < i; ++j)
{
if ((i % j) == 0)
{
prime = false;
break;
}
};
if (prime)
{
// Found another prime number.
primes++;
// Report progress at every 10 percent.
unsigned int temp = progress;
progress = static_cast<unsigned int>(10.f*primes / n);
if (progress != temp)
{
std::wstringstream updateStream;
updateStream << L"Progress to " << n << L"th prime: " << (10 * progress) << std::endl;
std::wstring updateString = updateStream.str();
// Update the UI thread with the DispatcherQueue.
dispatcherQueue.TryEnqueue(
Microsoft::UI::Dispatching::DispatcherQueuePriority::High,
[strongThis, updateString]()
{
strongThis->UpdateUI(updateString);
});
}
}
}
// Return the nth prime number.
*nthPrime = i;
});
// The nth prime number to find.
const unsigned int n = 9999;
// A shared pointer to the result.
// We use a shared pointer to keep the result alive until the
// work is done.
std::shared_ptr<unsigned long> nthPrime = std::make_shared<unsigned long>(0);
// Simulates work by searching for the nth prime number. Uses a
// naive algorithm and counts 2 as the first prime number.
auto workItem = ref new Windows::System::Threading::WorkItemHandler(
[this, n, nthPrime](IAsyncAction^ workItem)
{
unsigned int progress = 0; // For progress reporting.
unsigned int primes = 0; // Number of primes found so far.
unsigned long int i = 2; // Number iterator.
if ((n >= 0) && (n <= 2))
{
*nthPrime = n;
return;
}
while (primes < (n - 1))
{
if (workItem->Status == AsyncStatus::Canceled)
{
break;
}
// Go to the next number.
i++;
// Check for prime.
bool prime = true;
for (unsigned int j = 2; j < i; ++j)
{
if ((i % j) == 0)
{
prime = false;
break;
}
};
if (prime)
{
// Found another prime number.
primes++;
// Report progress at every 10 percent.
unsigned int temp = progress;
progress = static_cast<unsigned int>(10.f*primes / n);
if (progress != temp)
{
String^ updateString;
updateString = "Progress to " + n + "th prime: "
+ (10 * progress).ToString() + "%\n";
// Update the UI thread with the CoreDispatcher.
CoreApplication::MainView->CoreWindow->Dispatcher->RunAsync(
CoreDispatcherPriority::High,
ref new DispatchedHandler([this, updateString]()
{
UpdateUI(updateString);
}));
}
}
}
// Return the nth prime number.
*nthPrime = i;
});
auto asyncAction = ThreadPool::RunAsync(workItem);
// A reference to the work item is cached so that we can trigger a
// cancellation when the user presses the Cancel button.
m_workItem = asyncAction;
调用 RunAsync 后，工作项由线程池排队，并在线程可用时运行。 线程池工作项以异步方式运行，并且可以按任意顺序运行，因此请确保工作项独立运行。
请注意，工作项检查 IAsyncInfo.Status 属性，如果取消工作项，则退出。
处理工作项完成
通过设置工作项的 IAsyncAction.Completed 属性来提供完成处理程序。 提供委派（可使用 lambda 或 delegate 函数）来处理工作项的完成。 例如，使用 DispatcherQueue.TryEnqueue 访问 UI 线程并显示结果。
以下示例使用在步骤 1 中提交的工作项的结果来更新 UI：
asyncAction.Completed = new AsyncActionCompletedHandler(
(IAsyncAction asyncInfo, AsyncStatus asyncStatus) =>
{
if (asyncStatus == AsyncStatus.Canceled)
{
return;
}
String updateString;
updateString = "\n" + "The " + n + "th prime number is "
+ nthPrime + ".\n";
// Update the UI thread with the DispatcherQueue.
dispatcherQueue.TryEnqueue(
Microsoft.UI.Dispatching.DispatcherQueuePriority.High,
() => UpdateUI(updateString));
});
m_workItem.Completed(
[=, strongThis = get_strong()](Windows::Foundation::IAsyncAction const& asyncInfo, Windows::Foundation::AsyncStatus const& asyncStatus)
{
if (asyncStatus == Windows::Foundation::AsyncStatus::Canceled)
{
return;
}
std::wstringstream updateStream;
updateStream << std::endl << L"The " << n << L"th prime number is " << *nthPrime << std::endl;
std::wstring updateString = updateStream.str();
// Update the UI thread with the DispatcherQueue.
dispatcherQueue.TryEnqueue(
Microsoft::UI::Dispatching::DispatcherQueuePriority::High,
[strongThis, updateString]()
{
strongThis->UpdateUI(updateString);
});
});
请注意，完成处理程序在调度 UI 更新之前检查工作项是否已取消。
摘要和后续步骤
可以从GitHub上的Windows 应用 SDK线程处理示例了解详细信息。
