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futures.h
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/**********************************************************************
* LeechCraft - modular cross-platform feature rich internet client.
* Copyright (C) 2006-2014 Georg Rudoy
*
* Distributed under the Boost Software License, Version 1.0.
* (See accompanying file LICENSE or copy at https://www.boost.org/LICENSE_1_0.txt)
**********************************************************************/
#pragma once
#include <type_traits>
#include <functional>
#include <memory>
#include <optional>
#include <QFutureInterface>
#include <QFutureWatcher>
#include <util/sll/slotclosure.h>
#include <util/sll/detector.h>
#include "threadsconfig.h"
#include "concurrentexception.h"
namespace LC::Util
{
template<typename R, typename F, typename... Args>
void ReportFutureResult (QFutureInterface<R>& iface, F&& f, Args&&... args)
{
try
{
constexpr bool isVoid = std::is_same_v<R, void>;
if constexpr (!isVoid && !std::is_invocable_v<std::decay_t<F>, Args...>)
{
static_assert (std::is_constructible_v<R, F>);
static_assert (sizeof... (Args) == 0,
"Extra args when a value is passed. Perhaps you wanted to pass in a function?");
const R result { std::forward<F> (f) };
iface.reportResult (result);
}
else if constexpr (!isVoid)
{
const auto result = std::invoke (std::forward<F> (f), std::forward<Args> (args)...);
iface.reportResult (result);
}
else
std::invoke (std::forward<F> (f), std::forward<Args> (args)...);
}
catch (const QtException_t& e)
{
iface.reportException (e);
}
catch (const std::exception& e)
{
iface.reportException (ConcurrentStdException { e });
}
iface.reportFinished ();
}
namespace detail
{
template<typename T>
struct UnwrapFutureTypeBase {};
template<typename T>
struct UnwrapFutureTypeBase<QFuture<T>>
{
using type = T;
};
template<typename T>
struct UnwrapFutureType : UnwrapFutureTypeBase<std::decay_t<T>>
{
};
}
template<typename T>
using UnwrapFutureType_t = typename detail::UnwrapFutureType<T>::type;
namespace detail
{
/** @brief Incapsulates the sequencing logic of asynchronous
* actions.
*
* The objects of this class are expected to be created on heap.
* They will delete themselves automatically after the chain is
* walked (or an exception is thrown).
*
* @tparam Future The type of the initial future.
*/
template<typename Future>
class Sequencer final : public QObject
{
public:
/** @brief The type instantinating the QFuture returned by the
* \em Executor.
*/
using RetType_t = UnwrapFutureType_t<Future>;
private:
Future Future_;
QFutureWatcher<RetType_t> BaseWatcher_;
QFutureWatcherBase *LastWatcher_ = &BaseWatcher_;
public:
/** @brief Constructs the sequencer.
*
* @param[in] future The initial future in the chain.
* @param[in] parent The parent object for the sequencer.
*/
Sequencer (const Future& future, QObject *parent)
: QObject { parent }
, Future_ { future }
, BaseWatcher_ { this }
{
}
/** @brief Starts the first action in the chain.
*
* All the actions should be chained before calling this
* method to avoid a race condition.
*/
void Start ()
{
connect (LastWatcher_,
&QFutureWatcherBase::finished,
this,
&QObject::deleteLater);
BaseWatcher_.setFuture (Future_);
}
/** @brief Chains the given asynchronous action.
*
* The \em action is a functor callable with a single
* parameter of type \em ArgT and returning a value of type
* <code>QFuture<RetT></code> for some \em RetT.
*
* The parameter type \em ArgT should match exactly the
* "unwrapped" \em RetT for the previous call of Then() (or
* RetType_t if this is the second action in the asynchronous
* chain). Otherwise, an exception will be thrown at runtime.
*
* @note The SequenceProxy class takes care of compile-time
* type-checking of arguments and return types.
*
* @param[in] action The action to add to the sequence chain.
* @tparam RetT The type instantiating the return type
* <code>QFuture<RetT></code> of the \em action.
* @tparam ArgT The type of the argument passed to the
* \em action.
*/
template<typename RetT, typename ArgT>
void Then (const std::function<QFuture<RetT> (ArgT)>& action)
{
const auto last = dynamic_cast<QFutureWatcher<ArgT>*> (LastWatcher_);
if (!last)
{
deleteLater ();
throw std::runtime_error { std::string { "invalid type in " } + Q_FUNC_INFO };
}
const auto watcher = new QFutureWatcher<RetT> { this };
LastWatcher_ = watcher;
new SlotClosure<DeleteLaterPolicy>
{
[this, last, watcher, action]
{
if (static_cast<QObject*> (last) != &BaseWatcher_)
last->deleteLater ();
watcher->setFuture (action (last->result ()));
},
last,
SIGNAL (finished ()),
last
};
}
/** @brief Chains the given asynchronous action and closes the
* chain.
*
* The \em action is a functor callable with a single
* parameter of type \em ArgT and returning <code>void</code>.
*
* No more functors may be chained after adding a
* <code>void</code>-returning functor.
*
* The parameter type \em ArgT should match exactly the
* "unwrapped" \em RetT for the previous call of Then() (or
* RetType_t if this is the second action in the asynchronous
* chain). Otherwise, an exception will be thrown at runtime.
*
* @note The SequenceProxy class takes care of compile-time
* type-checking of arguments and return types.
*
* @tparam ArgT The type of the argument passed to the
* \em action.
*/
template<typename ArgT>
void Then (const std::function<void (ArgT)>& action)
{
const auto last = dynamic_cast<QFutureWatcher<ArgT>*> (LastWatcher_);
if (!last)
{
deleteLater ();
throw std::runtime_error { std::string { "invalid type in " } + Q_FUNC_INFO };
}
new SlotClosure<DeleteLaterPolicy>
{
[last, action]
{
action (last->result ());
},
LastWatcher_,
SIGNAL (finished ()),
LastWatcher_
};
}
void Then (const std::function<void ()>& action)
{
const auto last = dynamic_cast<QFutureWatcher<void>*> (LastWatcher_);
if (!last)
{
deleteLater ();
throw std::runtime_error { std::string { "invalid type in " } + Q_FUNC_INFO };
}
new SlotClosure<DeleteLaterPolicy>
{
action,
LastWatcher_,
SIGNAL (finished ()),
LastWatcher_
};
}
template<typename Handler>
void MultipleResults (const Handler& handler,
const std::function<void ()>& finishHandler = {},
const std::function<void ()>& startHandler = {})
{
if (LastWatcher_ != &BaseWatcher_)
{
qWarning () << Q_FUNC_INFO
<< "multiple results handler should be chained directly to the source";
throw std::runtime_error { "invalid multiple results handler chaining" };
}
connect (&BaseWatcher_,
&QFutureWatcherBase::resultReadyAt,
&BaseWatcher_,
[handler, this] (int index) { handler (BaseWatcher_.resultAt (index)); });
if (finishHandler)
new Util::SlotClosure<Util::DeleteLaterPolicy>
{
finishHandler,
&BaseWatcher_,
SIGNAL (finished ()),
&BaseWatcher_
};
if (startHandler)
new Util::SlotClosure<Util::DeleteLaterPolicy>
{
startHandler,
&BaseWatcher_,
SIGNAL (started ()),
&BaseWatcher_
};
connect (&BaseWatcher_,
SIGNAL (finished ()),
this,
SLOT (deleteLater ()));
}
};
template<typename T>
using SequencerRetType_t = typename Sequencer<T>::RetType_t;
struct EmptyDestructionTag;
/** @brief A proxy object allowing type-checked sequencing of
* actions and responsible for starting the initial action.
*
* SequenceProxy manages a Sequencer object, which itself is
* directly responsible for walking the chain of sequenced
* actions.
*
* Internally, objects of this class are reference-counted. As
* soon as the last instance is destroyed, the initial action is
* started.
*
* @tparam Ret The type \em T that <code>QFuture<T></code>
* returned by the last chained executor is specialized with.
* @tparam E0 The type of the first executor.
* @tparam A0 The types of the arguments to the executor \em E0.
*/
template<typename Ret, typename Future, typename DestructionTag>
class SequenceProxy
{
template<typename, typename, typename>
friend class SequenceProxy;
std::shared_ptr<void> ExecuteGuard_;
Sequencer<Future> * const Seq_;
std::optional<QFuture<Ret>> ThisFuture_;
std::function<DestructionTag ()> DestrHandler_;
SequenceProxy (const std::shared_ptr<void>& guard, Sequencer<Future> *seq,
const std::function<DestructionTag ()>& destrHandler)
: ExecuteGuard_ { guard }
, Seq_ { seq }
, DestrHandler_ { destrHandler }
{
}
template<typename F1, typename Ret1>
using ReturnsFutureDetector_t = UnwrapFutureType_t<std::result_of_t<F1 (Ret1)>>;
template<typename F, typename... Args>
using ReturnsVoidDetector_t = std::result_of_t<F (Args...)>;
public:
using Ret_t = Ret;
/** @brief Constructs a sequencer proxy managing the given
* \em sequencer.
*
* @param[in] sequencer The sequencer to manage.
*/
SequenceProxy (Sequencer<Future> *sequencer)
: ExecuteGuard_ { nullptr, [sequencer] (void*) { sequencer->Start (); } }
, Seq_ { sequencer }
{
}
/** @brief Copy-constructs from \em proxy.
*
* @param[in] proxy The proxy object to share the managed
* sequencer with.
*/
SequenceProxy (const SequenceProxy& proxy) = delete;
/** @brief Move-constructs from \em proxy.
*
* @param[in] proxy The proxy object from which the sequencer
* should be borrowed.
*/
SequenceProxy (SequenceProxy&& proxy) = default;
/** @brief Adds the functor \em f to the chain of actions.
*
* @param[in] f The functor to add to the chain.
* @return A new SequenceProxy if the chain can be continued further on, \code void otherwise..
* @tparam F The type of the functor to chain.
*/
template<typename F>
auto Then (F&& f)
{
if (ThisFuture_)
throw std::runtime_error { "SequenceProxy::Then(): cannot chain more after being converted to a QFuture" };
if constexpr (IsDetected_v<ReturnsFutureDetector_t, F, Ret>)
{
using Next_t = UnwrapFutureType_t<decltype (f (std::declval<Ret> ()))>;
Seq_->template Then<Next_t, Ret> (f);
return SequenceProxy<Next_t, Future, DestructionTag> { ExecuteGuard_, Seq_, DestrHandler_ };
}
else if constexpr (std::is_same<IsDetected_t<struct Dummy, ReturnsVoidDetector_t, F, Ret>, void> {})
Seq_->template Then<Ret> (f);
else if constexpr (std::is_same<void, Ret>::value &&
std::is_same<IsDetected_t<struct Dummy, ReturnsVoidDetector_t, F>, void> {})
Seq_->Then (std::function<void ()> { f });
else
static_assert (std::is_same<F, struct Dummy> {}, "Invalid functor passed to SequenceProxy::Then()");
}
template<typename F>
auto operator>> (F&& f) -> decltype (this->Then (std::forward<F> (f)))
{
return Then (std::forward<F> (f));
}
template<typename F>
SequenceProxy<Ret, Future, std::result_of_t<F ()>> DestructionValue (F&& f)
{
static_assert (std::is_same<DestructionTag, EmptyDestructionTag>::value,
"Destruction handling function has been already set.");
return { ExecuteGuard_, Seq_, std::forward<F> (f) };
}
template<typename F>
void MultipleResults (F&& f)
{
Seq_->MultipleResults (std::forward<F> (f));
}
template<typename F, typename Finish>
void MultipleResults (F&& f, Finish&& finish)
{
Seq_->MultipleResults (std::forward<F> (f),
std::forward<Finish> (finish));
}
template<typename F, typename Finish, typename Start>
void MultipleResults (F&& f, Finish&& finish, Start&& start)
{
Seq_->MultipleResults (std::forward<F> (f),
std::forward<Finish> (finish),
std::forward<Start> (start));
}
operator QFuture<Ret> ()
{
constexpr bool isEmptyDestr = std::is_same<DestructionTag, EmptyDestructionTag>::value;
static_assert (std::is_same<DestructionTag, Ret>::value || isEmptyDestr,
"Destruction handler's return type doesn't match expected future type.");
if (ThisFuture_)
return *ThisFuture_;
QFutureInterface<Ret> iface;
iface.reportStarted ();
SlotClosure<DeleteLaterPolicy> *deleteGuard = nullptr;
if constexpr (!isEmptyDestr)
{
deleteGuard = new SlotClosure<DeleteLaterPolicy>
{
[destrHandler = DestrHandler_, iface] () mutable
{
if (iface.isFinished ())
return;
const auto res = destrHandler ();
iface.reportFinished (&res);
},
Seq_->parent (),
SIGNAL (destroyed ()),
Seq_
};
}
Then ([deleteGuard, iface] (const Ret& ret) mutable
{
iface.reportFinished (&ret);
delete deleteGuard;
});
const auto& future = iface.future ();
ThisFuture_ = future;
return future;
}
};
}
/** @brief Creates a sequencer that allows chaining multiple futures.
*
* This function creates a sequencer object that starts with the
* passed \em future, and, after this future being completed, passes
* its result to the next function in chain, and so on, until either
* there are no more functions in the chain or a function returns
* something different from <code>QFuture<T></code>
* and so on.
*
* Each function in the chain may return a <code>QFuture<T></code> for
* some <code>T != void</code>, in which case it will be unwrapped and
* passed along to the next function in the chain.
*
* The functors may also return <code>QFuture<void></code>, meaning
* that the next function in the chain will be invoked without
* arguments when this future is completed.
*
* If a functor returns <code>void</code>, no further chaining is
* possible.
*
* The functions are chained via the detail::SequenceProxy::Then()
* method or via the <code>operator>>()</code> (leading to a nice
* somewhat monadic-like syntax).
*
* The \em parent QObject controls the lifetime of the sequencer: as
* soon as it is destroyed, the sequencer is destroyed as well, and
* all pending actions are cancelled (the currently executing action
* will still continue to execute, though). This parameter is optional
* and may be <code>nullptr</code>.
*
* A sample usage may look like:
* \code
Util::Sequence (this,
QtConcurrent::run ([this, &]
{
const auto& contents = file->readAll ();
file->close ();
file->remove ();
return DoSomethingWith (contents);
})) >>
[this, url, script] (const QString& contents)
{
const auto& result = Parse (contents);
if (result.isEmpty ())
{
qWarning () << Q_FUNC_INFO
<< "empty result for"
<< url;
return;
}
const auto id = DoSomethingSynchronouslyWith (result);
emit gotResult (id);
};
\endcode
*
* @param[in] parent The parent object of the sequencer (may be
* <code>nullptr</code>.
* @param[in] future The future to pass to the sequencer.
* @return The sequencer object.
* @tparam T The underlying type of the passed future (the async
* computation result type).
*
* @sa detail::SequenceProxy
*/
template<typename T>
detail::SequenceProxy<
detail::SequencerRetType_t<QFuture<T>>,
QFuture<T>,
detail::EmptyDestructionTag
>
Sequence (QObject *parent, const QFuture<T>& future)
{
return { new detail::Sequencer<QFuture<T>> { future, parent } };
}
/** @brief Creates a ready future holding the given value.
*
* This function creates a ready future containing the value \em t.
* That is, calling <code>QFuture<T>::get()</code> on the returned
* future will not block.
*
* @param[in] t The value to keep in the future.
* @return The ready future with the value \em t.
*
* @tparam T The type of the value in the future.
*/
template<typename T>
QFuture<T> MakeReadyFuture (const T& t)
{
QFutureInterface<T> iface;
iface.reportStarted ();
iface.reportFinished (&t);
return iface.future ();
}
}