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/**
* @file llmainthreadtask.h
* @author Nat Goodspeed
* @date 2019-12-04
* @brief LLMainThreadTask dispatches work to the main thread. When invoked on
* the main thread, it performs the work inline.
*
* $LicenseInfo:firstyear=2019&license=viewerlgpl$
* Copyright (c) 2019, Linden Research, Inc.
* $/LicenseInfo$
*/
#if ! defined(LL_LLMAINTHREADTASK_H)
#define LL_LLMAINTHREADTASK_H
#include "llthread.h"
#include "workqueue.h"
/**
* LLMainThreadTask provides a way to perform some task specifically on the
* main thread, waiting for it to complete. A task consists of a C++ nullary
* invocable (i.e. any callable that requires no arguments) with arbitrary
* return type.
*
* Instead of instantiating LLMainThreadTask, pass your invocable to its
* static dispatch() method. dispatch() returns the result of calling your
* task. (Or, if your task throws an exception, dispatch() throws that
* exception.)
*
* When you call dispatch() on the main thread (as determined by
* on_main_thread() in llthread.h), it simply calls your task and returns the
* result.
*
* When you call dispatch() on a secondary thread, it posts your task to
* gMainloopWork, the WorkQueue serviced by the main thread, using
* WorkQueue::waitForResult() to block the caller. Next time the main loop
* calls gMainloopWork.runFor(), your task will be run, and waitForResult()
* will return its result.
*/
class LLMainThreadTask
{
private:
// Don't instantiate this class -- use dispatch() instead.
LLMainThreadTask() {}
public:
/// dispatch() is the only way to invoke this functionality.
template <typename CALLABLE>
static auto dispatch(CALLABLE&& callable) -> decltype(callable())
{
if (on_main_thread())
{
// we're already running on the main thread, perfect
return callable();
}
else
{
auto queue{ LL::WorkQueue::getInstance("mainloop") };
// If this needs a null check and a message, please introduce a
// method in the .cpp file so consumers of this header don't drag
// in llerror.h.
// Use waitForResult_() so dispatch() can be used even from the
// calling thread's default coroutine.
return queue->waitForResult_(std::forward<CALLABLE>(callable));
}
}
};
#endif /* ! defined(LL_LLMAINTHREADTASK_H) */
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