Extend lead/trail edge support for throttled_func (#19210)

You can now create throttled functions which trigger both on the leading
and trailing edge. This was then also ported to `ThrottledFunc` for
`DispatcherQueue`s and used for title/taskbar updates.

Closes #19188

## Validation Steps Performed
* In CMD run:
  ```batch
  FOR /L %N IN () DO @echo %time%
  ```
* Doesn't hang the UI ✅

src/cascadia/TerminalApp/AppLogic.cpp

@@ -135,12 +135,19 @@ namespace winrt::TerminalApp::implementation
         _isElevated = ::Microsoft::Console::Utils::IsRunningElevated();
         _canDragDrop = ::Microsoft::Console::Utils::CanUwpDragDrop();
 
-        _reloadSettings = std::make_shared<ThrottledFuncTrailing<>>(winrt::Windows::System::DispatcherQueue::GetForCurrentThread(), std::chrono::milliseconds(100), [weakSelf = get_weak()]() {
-            if (auto self{ weakSelf.get() })
-            {
-                self->ReloadSettings();
-            }
-        });
+        _reloadSettings = std::make_shared<ThrottledFunc<>>(
+            DispatcherQueue::GetForCurrentThread(),
+            til::throttled_func_options{
+                .delay = std::chrono::milliseconds{ 100 },
+                .debounce = true,
+                .trailing = true,
+            },
+            [weakSelf = get_weak()]() {
+                if (auto self{ weakSelf.get() })
+                {
+                    self->ReloadSettings();
+                }
+            });
 
         _languageProfileNotifier = winrt::make_self<LanguageProfileNotifier>([this]() {
             _reloadSettings->Run();

src/cascadia/TerminalApp/AppLogic.h

@@ -64,7 +64,7 @@ namespace winrt::TerminalApp::implementation
         bool _hasSettingsStartupActions{ false };
         ::TerminalApp::AppCommandlineArgs _settingsAppArgs;
 
-        std::shared_ptr<ThrottledFuncTrailing<>> _reloadSettings;
+        std::shared_ptr<ThrottledFunc<>> _reloadSettings;
 
         std::vector<Microsoft::Terminal::Settings::Model::SettingsLoadWarnings> _warnings{};
 

src/cascadia/TerminalApp/MinMaxCloseControl.cpp

@@ -35,17 +35,23 @@ namespace winrt::TerminalApp::implementation
         // (which should be the default, see:
         // https://docs.microsoft.com/en-us/windows/win32/api/winuser/ns-winuser-trackmouseevent#remarks)
         unsigned int hoverTimeoutMillis{ 400 };
-        LOG_IF_WIN32_BOOL_FALSE(SystemParametersInfoW(SPI_GETMOUSEHOVERTIME, 0, &hoverTimeoutMillis, 0));
-        const auto toolTipInterval = std::chrono::milliseconds(hoverTimeoutMillis);
+        if (FAILED(SystemParametersInfoW(SPI_GETMOUSEHOVERTIME, 0, &hoverTimeoutMillis, 0)))
+        {
+            hoverTimeoutMillis = 400;
+        }
 
         // Create a ThrottledFunc for opening the tooltip after the hover
         // timeout. If we hover another button, we should make sure to call
         // Run() with the new button. Calling `_displayToolTip.Run(nullptr)`,
         // which will cause us to not display a tooltip, which is used when we
         // leave the control entirely.
-        _displayToolTip = std::make_shared<ThrottledFuncTrailing<Controls::Button>>(
+        _displayToolTip = std::make_shared<ThrottledFunc<Controls::Button>>(
             dispatcher,
-            toolTipInterval,
+            til::throttled_func_options{
+                .delay = std::chrono::milliseconds{ hoverTimeoutMillis },
+                .debounce = true,
+                .trailing = true,
+            },
             [weakThis = get_weak()](Controls::Button button) {
                 // If we provide a button, then open the tooltip on that button.
                 // We can "dismiss" this throttled func by calling it with null,

src/cascadia/TerminalApp/MinMaxCloseControl.h

@@ -32,7 +32,7 @@ namespace winrt::TerminalApp::implementation
         til::typed_event<TerminalApp::MinMaxCloseControl, winrt::Windows::UI::Xaml::RoutedEventArgs> MaximizeClick;
         til::typed_event<TerminalApp::MinMaxCloseControl, winrt::Windows::UI::Xaml::RoutedEventArgs> CloseClick;
 
-        std::shared_ptr<ThrottledFuncTrailing<winrt::Windows::UI::Xaml::Controls::Button>> _displayToolTip{ nullptr };
+        std::shared_ptr<ThrottledFunc<winrt::Windows::UI::Xaml::Controls::Button>> _displayToolTip{ nullptr };
         std::optional<CaptionButton> _lastPressedButton{ std::nullopt };
     };
 }

src/cascadia/TerminalApp/Tab.cpp

@@ -1035,27 +1035,49 @@ namespace winrt::TerminalApp::implementation
     void Tab::_AttachEventHandlersToContent(const uint32_t paneId, const TerminalApp::IPaneContent& content)
     {
         auto weakThis{ get_weak() };
-        auto dispatcher = TabViewItem().Dispatcher();
+        auto dispatcher = DispatcherQueue::GetForCurrentThread();
         ContentEventTokens events{};
 
+        auto throttledTitleChanged = std::make_shared<ThrottledFunc<>>(
+            dispatcher,
+            til::throttled_func_options{
+                .delay = std::chrono::milliseconds{ 200 },
+                .leading = true,
+                .trailing = true,
+            },
+            [weakThis]() {
+                if (const auto tab = weakThis.get())
+                {
+                    tab->UpdateTitle();
+                }
+            });
+
         events.TitleChanged = content.TitleChanged(
             winrt::auto_revoke,
-            [dispatcher, weakThis](auto&&, auto&&) -> safe_void_coroutine {
-                // The lambda lives in the `std::function`-style container owned by `control`. That is, when the
-                // `control` gets destroyed the lambda struct also gets destroyed. In other words, we need to
-                // copy `weakThis` onto the stack, because that's the only thing that gets captured in coroutines.
-                // See: https://devblogs.microsoft.com/oldnewthing/20211103-00/?p=105870
-                const auto weakThisCopy = weakThis;
-                co_await wil::resume_foreground(dispatcher);
-                // Check if Tab's lifetime has expired
-                if (auto tab{ weakThisCopy.get() })
+            [func = std::move(throttledTitleChanged)](auto&&, auto&&) {
+                func->Run();
+            });
+
+        auto throttledTaskbarProgressChanged = std::make_shared<ThrottledFunc<>>(
+            dispatcher,
+            til::throttled_func_options{
+                .delay = std::chrono::milliseconds{ 200 },
+                .leading = true,
+                .trailing = true,
+            },
+            [weakThis]() {
+                if (const auto tab = weakThis.get())
                 {
-                    // The title of the control changed, but not necessarily the title of the tab.
-                    // Set the tab's text to the active panes' text.
-                    tab->UpdateTitle();
+                    tab->_UpdateProgressState();
                 }
             });
 
+        events.TaskbarProgressChanged = content.TaskbarProgressChanged(
+            winrt::auto_revoke,
+            [func = std::move(throttledTaskbarProgressChanged)](auto&&, auto&&) {
+                func->Run();
+            });
+
         events.TabColorChanged = content.TabColorChanged(
             winrt::auto_revoke,
             [dispatcher, weakThis](auto&&, auto&&) -> safe_void_coroutine {
@@ -1071,18 +1093,6 @@ namespace winrt::TerminalApp::implementation
                 }
             });
 
-        events.TaskbarProgressChanged = content.TaskbarProgressChanged(
-            winrt::auto_revoke,
-            [dispatcher, weakThis](auto&&, auto&&) -> safe_void_coroutine {
-                const auto weakThisCopy = weakThis;
-                co_await wil::resume_foreground(dispatcher);
-                // Check if Tab's lifetime has expired
-                if (auto tab{ weakThisCopy.get() })
-                {
-                    tab->_UpdateProgressState();
-                }
-            });
-
         events.ConnectionStateChanged = content.ConnectionStateChanged(
             winrt::auto_revoke,
             [dispatcher, weakThis](auto&&, auto&&) -> safe_void_coroutine {

src/cascadia/TerminalApp/TerminalPage.cpp

@@ -4876,8 +4876,6 @@ namespace winrt::TerminalApp::implementation
     safe_void_coroutine TerminalPage::_ControlCompletionsChangedHandler(const IInspectable sender,
                                                                         const CompletionsChangedEventArgs args)
     {
-        // This will come in on a background (not-UI, not output) thread.
-
         // This won't even get hit if the velocity flag is disabled - we gate
         // registering for the event based off of
         // Feature_ShellCompletions::IsEnabled back in _RegisterTerminalEvents

src/cascadia/TerminalControl/ControlCore.cpp

@@ -174,8 +174,12 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         //
         // NOTE: Calling UpdatePatternLocations from a background
         // thread is a workaround for us to hit GH#12607 less often.
-        shared->outputIdle = std::make_unique<til::debounced_func_trailing<>>(
-            std::chrono::milliseconds{ 100 },
+        shared->outputIdle = std::make_unique<til::throttled_func<>>(
+            til::throttled_func_options{
+                .delay = std::chrono::milliseconds{ 100 },
+                .debounce = true,
+                .trailing = true,
+            },
             [this, weakThis = get_weak(), dispatcher = _dispatcher]() {
                 dispatcher.TryEnqueue(DispatcherQueuePriority::Normal, [weakThis]() {
                     if (const auto self = weakThis.get(); self && !self->_IsClosing())
@@ -195,8 +199,12 @@ namespace winrt::Microsoft::Terminal::Control::implementation
 
         // If you rapidly show/hide Windows Terminal, something about GotFocus()/LostFocus() gets broken.
         // We'll then receive easily 10+ such calls from WinUI the next time the application is shown.
-        shared->focusChanged = std::make_unique<til::debounced_func_trailing<bool>>(
-            std::chrono::milliseconds{ 25 },
+        shared->focusChanged = std::make_unique<til::throttled_func<bool>>(
+            til::throttled_func_options{
+                .delay = std::chrono::milliseconds{ 25 },
+                .debounce = true,
+                .trailing = true,
+            },
             [this](const bool focused) {
                 // Theoretically `debounced_func_trailing` should call `WaitForThreadpoolTimerCallbacks()`
                 // with cancel=true on destruction, which should ensure that our use of `this` here is safe.
@@ -204,9 +212,12 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             });
 
         // Scrollbar updates are also expensive (XAML), so we'll throttle them as well.
-        shared->updateScrollBar = std::make_shared<ThrottledFuncTrailing<Control::ScrollPositionChangedArgs>>(
+        shared->updateScrollBar = std::make_shared<ThrottledFunc<Control::ScrollPositionChangedArgs>>(
             _dispatcher,
-            std::chrono::milliseconds{ 8 },
+            til::throttled_func_options{
+                .delay = std::chrono::milliseconds{ 8 },
+                .trailing = true,
+            },
             [weakThis = get_weak()](const auto& update) {
                 if (auto core{ weakThis.get() }; core && !core->_IsClosing())
                 {
@@ -2757,15 +2768,9 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         }
     }
 
-    safe_void_coroutine ControlCore::_terminalCompletionsChanged(std::wstring_view menuJson,
-                                                                 unsigned int replaceLength)
+    void ControlCore::_terminalCompletionsChanged(std::wstring_view menuJson, unsigned int replaceLength)
     {
-        auto args = winrt::make_self<CompletionsChangedEventArgs>(winrt::hstring{ menuJson },
-                                                                  replaceLength);
-
-        co_await winrt::resume_background();
-
-        CompletionsChanged.raise(*this, *args);
+        CompletionsChanged.raise(*this, winrt::make<CompletionsChangedEventArgs>(winrt::hstring{ menuJson }, replaceLength));
     }
 
     // Select the region of text between [s.start, s.end), in buffer space

src/cascadia/TerminalControl/ControlCore.h

@@ -307,9 +307,9 @@ namespace winrt::Microsoft::Terminal::Control::implementation
     private:
         struct SharedState
         {
-            std::unique_ptr<til::debounced_func_trailing<>> outputIdle;
-            std::unique_ptr<til::debounced_func_trailing<bool>> focusChanged;
-            std::shared_ptr<ThrottledFuncTrailing<Control::ScrollPositionChangedArgs>> updateScrollBar;
+            std::unique_ptr<til::throttled_func<>> outputIdle;
+            std::unique_ptr<til::throttled_func<bool>> focusChanged;
+            std::shared_ptr<ThrottledFunc<Control::ScrollPositionChangedArgs>> updateScrollBar;
         };
 
         void _setupDispatcherAndCallbacks();
@@ -339,7 +339,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         void _terminalSearchMissingCommand(std::wstring_view missingCommand, const til::CoordType& bufferRow);
         void _terminalWindowSizeChanged(int32_t width, int32_t height);
 
-        safe_void_coroutine _terminalCompletionsChanged(std::wstring_view menuJson, unsigned int replaceLength);
+        void _terminalCompletionsChanged(std::wstring_view menuJson, unsigned int replaceLength);
 #pragma endregion
 
 #pragma region RendererCallbacks

src/cascadia/TerminalControl/TermControl.cpp

@@ -358,9 +358,12 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         // These three throttled functions are triggered by terminal output and interact with the UI.
         // Since Close() is the point after which we are removed from the UI, but before the
         // destructor has run, we MUST check control->_IsClosing() before actually doing anything.
-        _playWarningBell = std::make_shared<ThrottledFuncLeading>(
+        _playWarningBell = std::make_shared<ThrottledFunc<>>(
             dispatcher,
-            TerminalWarningBellInterval,
+            til::throttled_func_options{
+                .delay = TerminalWarningBellInterval,
+                .leading = true,
+            },
             [weakThis = get_weak()]() {
                 if (auto control{ weakThis.get() }; control && !control->_IsClosing())
                 {
@@ -368,9 +371,12 @@ namespace winrt::Microsoft::Terminal::Control::implementation
                 }
             });
 
-        _updateScrollBar = std::make_shared<ThrottledFuncTrailing<ScrollBarUpdate>>(
+        _updateScrollBar = std::make_shared<ThrottledFunc<ScrollBarUpdate>>(
             dispatcher,
-            ScrollBarUpdateInterval,
+            til::throttled_func_options{
+                .delay = ScrollBarUpdateInterval,
+                .trailing = true,
+            },
             [weakThis = get_weak()](const auto& update) {
                 if (auto control{ weakThis.get() }; control && !control->_IsClosing())
                 {

src/cascadia/TerminalControl/TermControl.h

@@ -284,7 +284,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
         bool _quickFixesAvailable{ false };
         til::CoordType _quickFixBufferPos{};
 
-        std::shared_ptr<ThrottledFuncLeading> _playWarningBell;
+        std::shared_ptr<ThrottledFunc<>> _playWarningBell;
 
         struct ScrollBarUpdate
         {
@@ -294,7 +294,7 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             double newViewportSize;
         };
 
-        std::shared_ptr<ThrottledFuncTrailing<ScrollBarUpdate>> _updateScrollBar;
+        std::shared_ptr<ThrottledFunc<ScrollBarUpdate>> _updateScrollBar;
 
         bool _isInternalScrollBarUpdate;
 

src/cascadia/TerminalSettingsEditor/Profiles_Appearance.cpp

@@ -66,9 +66,13 @@ namespace winrt::Microsoft::Terminal::Settings::Editor::implementation
     {
         if (!_updatePreviewControl)
         {
-            _updatePreviewControl = std::make_shared<ThrottledFuncTrailing<>>(
+            _updatePreviewControl = std::make_shared<ThrottledFunc<>>(
                 winrt::Windows::System::DispatcherQueue::GetForCurrentThread(),
-                std::chrono::milliseconds{ 100 },
+                til::throttled_func_options{
+                    .delay = std::chrono::milliseconds{ 100 },
+                    .debounce = true,
+                    .trailing = true,
+                },
                 [this]() {
                     const auto settings = _Profile.TermSettings();
                     _previewConnection->DisplayPowerlineGlyphs(_Profile.DefaultAppearance().HasPowerlineCharacters());

src/cascadia/TerminalSettingsEditor/Profiles_Appearance.h

@@ -33,7 +33,7 @@ namespace winrt::Microsoft::Terminal::Settings::Editor::implementation
 
         winrt::com_ptr<PreviewConnection> _previewConnection{ nullptr };
         Microsoft::Terminal::Control::TermControl _previewControl{ nullptr };
-        std::shared_ptr<ThrottledFuncTrailing<>> _updatePreviewControl;
+        std::shared_ptr<ThrottledFunc<>> _updatePreviewControl;
         Windows::UI::Xaml::Data::INotifyPropertyChanged::PropertyChanged_revoker _ViewModelChangedRevoker;
         Windows::UI::Xaml::Data::INotifyPropertyChanged::PropertyChanged_revoker _AppearanceViewModelChangedRevoker;
         Editor::IHostedInWindow _windowRoot;

src/cascadia/TerminalSettingsModel/ApplicationState.cpp

@@ -96,7 +96,14 @@ namespace winrt::Microsoft::Terminal::Settings::Model::implementation
     ApplicationState::ApplicationState(const std::filesystem::path& stateRoot) noexcept :
         _sharedPath{ stateRoot / stateFileName },
         _elevatedPath{ stateRoot / elevatedStateFileName },
-        _throttler{ std::chrono::seconds(1), [this]() { _write(); } }
+        _throttler{
+            til::throttled_func_options{
+                .delay = std::chrono::seconds{ 1 },
+                .debounce = true,
+                .trailing = true,
+            },
+            [this]() { _write(); }
+        }
     {
         _read();
     }

src/cascadia/TerminalSettingsModel/ApplicationState.h

@@ -91,7 +91,7 @@ namespace winrt::Microsoft::Terminal::Settings::Model::implementation
         til::shared_mutex<state_t> _state;
         std::filesystem::path _sharedPath;
         std::filesystem::path _elevatedPath;
-        til::throttled_func_trailing<> _throttler;
+        til::throttled_func<> _throttler;
 
         void _write() const noexcept;
         void _read() const noexcept;

src/cascadia/WinRTUtils/inc/ThrottledFunc.h

@@ -3,40 +3,44 @@
 
 #pragma once
 
-#include "til/throttled_func.h"
+#include <til/throttled_func.h>
 
 // ThrottledFunc is a copy of til::throttled_func,
 // specialized for the use with a WinRT Dispatcher.
-template<bool leading, typename... Args>
-class ThrottledFunc : public std::enable_shared_from_this<ThrottledFunc<leading, Args...>>
+template<typename... Args>
+class ThrottledFunc : public std::enable_shared_from_this<ThrottledFunc<Args...>>
 {
 public:
-    using filetime_duration = std::chrono::duration<int64_t, std::ratio<1, 10000000>>;
+    using filetime_duration = til::throttled_func_options::filetime_duration;
     using function = std::function<void(Args...)>;
 
-    // Throttles invocations to the given `func` to not occur more often than `delay`.
+    // Throttles invocations to the given `func` to not occur more often than specified in options.
     //
-    // If this is a:
-    // * ThrottledFuncLeading: `func` will be invoked immediately and
-    //   further invocations prevented until `delay` time has passed.
-    // * ThrottledFuncTrailing: On the first invocation a timer of `delay` time will
-    //   be started. After the timer has expired `func` will be invoked just once.
+    // Options:
+    // * delay: The minimum time between invocations
+    // * leading: If true, `func` will be invoked immediately on first call
+    // * trailing: If true, `func` will be invoked after the delay
+    // * debounce: If true, resets the timer on each call
     //
-    // After `func` was invoked the state is reset and this cycle is repeated again.
-    ThrottledFunc(
-        winrt::Windows::System::DispatcherQueue dispatcher,
-        filetime_duration delay,
-        function func) :
+    // At least one of leading or trailing must be true.
+    ThrottledFunc(winrt::Windows::System::DispatcherQueue dispatcher, til::throttled_func_options opts, function func) :
         _dispatcher{ std::move(dispatcher) },
         _func{ std::move(func) },
-        _timer{ _create_timer() }
+        _timer{ _create_timer() },
+        _debounce{ opts.debounce },
+        _leading{ opts.leading },
+        _trailing{ opts.trailing }
     {
-        const auto d = -delay.count();
+        if (!_leading && !_trailing)
+        {
+            throw std::invalid_argument("neither leading nor trailing");
+        }
+
+        const auto d = -opts.delay.count();
         if (d >= 0)
         {
             throw std::invalid_argument("non-positive delay specified");
         }
-
         memcpy(&_delay, &d, sizeof(d));
     }
 
@@ -54,88 +58,119 @@ public:
     template<typename... MakeArgs>
     void Run(MakeArgs&&... args)
     {
-        if (!_storage.emplace(std::forward<MakeArgs>(args)...))
-        {
-            _leading_edge();
-        }
+        _lead(std::make_tuple(std::forward<MakeArgs>(args)...));
     }
 
-    // Modifies the pending arguments for the next function
-    // invocation, if there is one pending currently.
-    //
-    // `func` will be invoked as func(Args...). Make sure to bind any
-    // arguments in `func` by reference if you'd like to modify them.
     template<typename F>
     void ModifyPending(F func)
     {
-        _storage.modify_pending(func);
+        const auto guard = _lock.lock_exclusive();
+        if (_pendingRunArgs)
+        {
+            std::apply(func, *_pendingRunArgs);
+        }
     }
 
 private:
     static void __stdcall _timer_callback(PTP_CALLBACK_INSTANCE /*instance*/, PVOID context, PTP_TIMER /*timer*/) noexcept
+    try
     {
-        static_cast<ThrottledFunc*>(context)->_trailing_edge();
+        static_cast<ThrottledFunc*>(context)->_trail();
     }
+    CATCH_LOG()
 
-    void _leading_edge()
-    {
-        if constexpr (leading)
-        {
-            _dispatcher.TryEnqueue(winrt::Windows::System::DispatcherQueuePriority::Normal, [weakSelf = this->weak_from_this()]() {
-                if (auto self{ weakSelf.lock() })
-                {
-                    try
-                    {
-                        self->_func();
-                    }
-                    CATCH_LOG();
-
-                    SetThreadpoolTimerEx(self->_timer.get(), &self->_delay, 0, 0);
-                }
-            });
-        }
-        else
-        {
-            SetThreadpoolTimerEx(_timer.get(), &_delay, 0, 0);
-        }
-    }
-
-    void _trailing_edge()
-    {
-        if constexpr (leading)
-        {
-            _storage.reset();
-        }
-        else
-        {
-            _dispatcher.TryEnqueue(winrt::Windows::System::DispatcherQueuePriority::Normal, [weakSelf = this->weak_from_this()]() {
-                if (auto self{ weakSelf.lock() })
-                {
-                    try
-                    {
-                        self->_storage.apply(self->_func);
-                    }
-                    CATCH_LOG();
-                }
-            });
-        }
-    }
-
-    inline wil::unique_threadpool_timer _create_timer()
+    wil::unique_threadpool_timer _create_timer()
     {
         wil::unique_threadpool_timer timer{ CreateThreadpoolTimer(&_timer_callback, this, nullptr) };
         THROW_LAST_ERROR_IF(!timer);
         return timer;
     }
 
-    FILETIME _delay;
+    void _lead(std::tuple<Args...> args)
+    {
+        bool timerRunning = false;
+
+        {
+            const auto guard = _lock.lock_exclusive();
+
+            timerRunning = _timerRunning;
+            _timerRunning = true;
+
+            if (!timerRunning && _leading)
+            {
+                // Call the function immediately on the leading edge.
+                // See below (out of lock).
+            }
+            else if (_trailing)
+            {
+                _pendingRunArgs.emplace(std::move(args));
+            }
+        }
+
+        const auto execute = !timerRunning && _leading;
+        const auto schedule = !timerRunning || _debounce;
+
+        if (execute)
+        {
+            _execute(std::move(args), schedule);
+        }
+        else if (schedule)
+        {
+            SetThreadpoolTimerEx(_timer.get(), &_delay, 0, 0);
+        }
+    }
+
+    void _trail()
+    {
+        decltype(_pendingRunArgs) args;
+
+        {
+            const auto guard = _lock.lock_exclusive();
+
+            _timerRunning = false;
+            args = std::exchange(_pendingRunArgs, std::nullopt);
+        }
+
+        if (args)
+        {
+            _execute(std::move(*args), false);
+        }
+    }
+
+    void _execute(std::tuple<Args...> args, bool schedule)
+    {
+        _dispatcher.TryEnqueue(
+            winrt::Windows::System::DispatcherQueuePriority::Normal,
+            [weakSelf = this->weak_from_this(), args = std::move(args), schedule]() mutable {
+                if (auto self{ weakSelf.lock() })
+                {
+                    try
+                    {
+                        std::apply(self->_func, std::move(args));
+                    }
+                    CATCH_LOG();
+
+                    if (schedule)
+                    {
+                        SetThreadpoolTimerEx(self->_timer.get(), &self->_delay, 0, 0);
+                    }
+                }
+            });
+    }
+
     winrt::Windows::System::DispatcherQueue _dispatcher;
+
+    // Everything below this point is just like til::throttled_func.
+
     function _func;
-
     wil::unique_threadpool_timer _timer;
-    til::details::throttled_func_storage<Args...> _storage;
-};
+    FILETIME _delay;
 
-template<typename... Args>
-using ThrottledFuncTrailing = ThrottledFunc<false, Args...>;
-using ThrottledFuncLeading = ThrottledFunc<true>;
+    wil::srwlock _lock;
+    std::optional<std::tuple<Args...>> _pendingRunArgs;
+    bool _timerRunning = false;
+
+    bool _debounce;
+    bool _leading;
+    bool _trailing;
+};

src/cascadia/WindowsTerminal/AppHost.cpp

@@ -288,9 +288,12 @@ void AppHost::Initialize()
     // the PTY requesting a change to the window state and the Terminal
     // realizing it, but should mitigate issues where the Terminal and PTY get
     // de-sync'd.
-    _showHideWindowThrottler = std::make_shared<ThrottledFuncTrailing<bool>>(
+    _showHideWindowThrottler = std::make_shared<ThrottledFunc<bool>>(
         winrt::Windows::System::DispatcherQueue::GetForCurrentThread(),
-        std::chrono::milliseconds(200),
+        til::throttled_func_options{
+            .delay = std::chrono::milliseconds{ 200 },
+            .trailing = true,
+        },
         [this](const bool show) {
             _window->ShowWindowChanged(show);
         });

src/cascadia/WindowsTerminal/AppHost.h

@@ -39,7 +39,7 @@ private:
     std::unique_ptr<IslandWindow> _window;
     winrt::TerminalApp::AppLogic _appLogic{ nullptr };
     winrt::TerminalApp::TerminalWindow _windowLogic{ nullptr };
-    std::shared_ptr<ThrottledFuncTrailing<bool>> _showHideWindowThrottler;
+    std::shared_ptr<ThrottledFunc<bool>> _showHideWindowThrottler;
     SafeDispatcherTimer _frameTimer;
     LARGE_INTEGER _lastActivatedTime{};
     winrt::guid _virtualDesktopId{};

src/inc/til/throttled_func.h

@@ -5,120 +5,49 @@
 
 namespace til
 {
-    namespace details
+    struct throttled_func_options
     {
-        template<typename... Args>
-        class throttled_func_storage
-        {
-        public:
-            template<typename... MakeArgs>
-            bool emplace(MakeArgs&&... args)
-            {
-                std::unique_lock guard{ _lock };
-                const bool hadValue = _pendingRunArgs.has_value();
-                _pendingRunArgs.emplace(std::forward<MakeArgs>(args)...);
-                return hadValue;
-            }
+        using filetime_duration = std::chrono::duration<int64_t, std::ratio<1, 10000000>>;
 
-            template<typename F>
-            void modify_pending(F f)
-            {
-                std::unique_lock guard{ _lock };
-                if (_pendingRunArgs)
-                {
-                    std::apply(f, *_pendingRunArgs);
-                }
-            }
+        filetime_duration delay{};
+        bool debounce = false;
+        bool leading = false;
+        bool trailing = false;
+    };
 
-            void apply(const auto& func)
-            {
-                decltype(_pendingRunArgs) args;
-                {
-                    std::unique_lock guard{ _lock };
-                    args = std::exchange(_pendingRunArgs, std::nullopt);
-                }
-                // Theoretically it should always have a value, because the throttled_func
-                // should not call the callback without there being a reason.
-                // But in practice a failure here was observed at least once.
-                // It's unknown to me what caused it, so the best we can do is avoid a crash.
-                assert(args.has_value());
-                if (args)
-                {
-                    std::apply(func, *args);
-                }
-            }
-
-            explicit operator bool() const
-            {
-                std::shared_lock guard{ _lock };
-                return _pendingRunArgs.has_value();
-            }
-
-        private:
-            // std::mutex uses imperfect Critical Sections on Windows.
-            // --> std::shared_mutex uses SRW locks that are small and fast.
-            mutable std::shared_mutex _lock;
-            std::optional<std::tuple<Args...>> _pendingRunArgs;
-        };
-
-        template<>
-        class throttled_func_storage<>
-        {
-        public:
-            bool emplace()
-            {
-                return _isPending.exchange(true, std::memory_order_relaxed);
-            }
-
-            void apply(const auto& func)
-            {
-                if (_isPending.exchange(false, std::memory_order_relaxed))
-                {
-                    func();
-                }
-            }
-
-            void reset()
-            {
-                _isPending.store(false, std::memory_order_relaxed);
-            }
-
-            explicit operator bool() const
-            {
-                return _isPending.load(std::memory_order_relaxed);
-            }
-
-        private:
-            std::atomic<bool> _isPending;
-        };
-    } // namespace details
-
-    template<bool Debounce, bool Leading, typename... Args>
+    template<typename... Args>
     class throttled_func
     {
     public:
-        using filetime_duration = std::chrono::duration<int64_t, std::ratio<1, 10000000>>;
+        using filetime_duration = throttled_func_options::filetime_duration;
         using function = std::function<void(Args...)>;
 
-        // Throttles invocations to the given `func` to not occur more often than `delay`.
+        // Throttles invocations to the given `func` to not occur more often than specified in options.
         //
-        // If this is a:
-        // * throttled_func_leading: `func` will be invoked immediately and
-        //   further invocations prevented until `delay` time has passed.
-        // * throttled_func_trailing: On the first invocation a timer of `delay` time will
-        //   be started. After the timer has expired `func` will be invoked just once.
+        // Options:
+        // * delay: The minimum time between invocations
+        // * debounce: If true, resets the timer on each call
+        // * leading: If true, `func` will be invoked immediately on first call
+        // * trailing: If true, `func` will be invoked after the delay
         //
-        // After `func` was invoked the state is reset and this cycle is repeated again.
-        throttled_func(filetime_duration delay, function func) :
+        // At least one of leading or trailing must be true.
+        throttled_func(throttled_func_options opts, function func) :
             _func{ std::move(func) },
-            _timer{ _createTimer() }
+            _timer{ _create_timer() },
+            _debounce{ opts.debounce },
+            _leading{ opts.leading },
+            _trailing{ opts.trailing }
         {
-            const auto d = -delay.count();
+            if (!_leading && !_trailing)
+            {
+                throw std::invalid_argument("neither leading nor trailing");
+            }
+
+            const auto d = -opts.delay.count();
             if (d >= 0)
             {
                 throw std::invalid_argument("non-positive delay specified");
             }
-
             memcpy(&_delay, &d, sizeof(d));
         }
 
@@ -139,27 +68,7 @@ namespace til
         template<typename... MakeArgs>
         void operator()(MakeArgs&&... args)
         {
-            const auto hadValue = _storage.emplace(std::forward<MakeArgs>(args)...);
-
-            if constexpr (Debounce)
-            {
-                SetThreadpoolTimerEx(_timer.get(), &_delay, 0, 0);
-            }
-            else
-            {
-                if (!hadValue)
-                {
-                    SetThreadpoolTimerEx(_timer.get(), &_delay, 0, 0);
-                }
-            }
-
-            if constexpr (Leading)
-            {
-                if (!hadValue)
-                {
-                    _func();
-                }
-            }
+            _lead(std::make_tuple(std::forward<MakeArgs>(args)...));
         }
 
         // Modifies the pending arguments for the next function
@@ -170,7 +79,11 @@ namespace til
         template<typename F>
         void modify_pending(F func)
         {
-            _storage.modify_pending(func);
+            const auto guard = _lock.lock_exclusive();
+            if (_pendingRunArgs)
+            {
+                std::apply(func, *_pendingRunArgs);
+            }
         }
 
         // Makes sure that the currently pending timer is executed
@@ -190,36 +103,76 @@ namespace til
         static void __stdcall _timer_callback(PTP_CALLBACK_INSTANCE /*instance*/, PVOID context, PTP_TIMER /*timer*/) noexcept
         try
         {
-            const auto self = static_cast<throttled_func*>(context);
-            if constexpr (Leading)
-            {
-                self->_storage.reset();
-            }
-            else
-            {
-                self->_storage.apply(self->_func);
-            }
+            static_cast<throttled_func*>(context)->_trail();
         }
         CATCH_LOG()
 
-        wil::unique_threadpool_timer _createTimer()
+        wil::unique_threadpool_timer _create_timer()
         {
             wil::unique_threadpool_timer timer{ CreateThreadpoolTimer(&_timer_callback, this, nullptr) };
             THROW_LAST_ERROR_IF(!timer);
             return timer;
         }
 
-        FILETIME _delay;
+        void _lead(std::tuple<Args...> args)
+        {
+            bool timerRunning = false;
+
+            {
+                const auto guard = _lock.lock_exclusive();
+
+                timerRunning = _timerRunning;
+                _timerRunning = true;
+
+                if (!timerRunning && _leading)
+                {
+                    // Call the function immediately on the leading edge.
+                    // See below (out of lock).
+                }
+                else if (_trailing)
+                {
+                    _pendingRunArgs.emplace(std::move(args));
+                }
+            }
+
+            if (!timerRunning && _leading)
+            {
+                std::apply(_func, std::move(args));
+            }
+
+            if (!timerRunning || _debounce)
+            {
+                SetThreadpoolTimerEx(_timer.get(), &_delay, 0, 0);
+            }
+        }
+
+        void _trail()
+        {
+            decltype(_pendingRunArgs) args;
+
+            {
+                const auto guard = _lock.lock_exclusive();
+
+                _timerRunning = false;
+                args = std::exchange(_pendingRunArgs, std::nullopt);
+            }
+
+            if (args)
+            {
+                std::apply(_func, *std::move(args));
+            }
+        }
+
         function _func;
         wil::unique_threadpool_timer _timer;
-        details::throttled_func_storage<Args...> _storage;
+        FILETIME _delay;
+
+        wil::srwlock _lock;
+        std::optional<std::tuple<Args...>> _pendingRunArgs;
+        bool _timerRunning = false;
+
+        bool _debounce;
+        bool _leading;
+        bool _trailing;
     };
-
-    template<typename... Args>
-    using throttled_func_trailing = throttled_func<false, false, Args...>;
-    using throttled_func_leading = throttled_func<false, true>;
-
-    template<typename... Args>
-    using debounced_func_trailing = throttled_func<true, false, Args...>;
-    using debounced_func_leading = throttled_func<true, true>;
 } // namespace til

src/til/ut_til/throttled_func.cpp

@@ -19,21 +19,25 @@ class ThrottledFuncTests
     TEST_METHOD(Basic)
     {
         using namespace std::chrono_literals;
-        using throttled_func = til::throttled_func_trailing<bool>;
+        using throttled_func = til::throttled_func<bool>;
 
         til::latch latch{ 2 };
 
         std::unique_ptr<throttled_func> tf;
-        tf = std::make_unique<throttled_func>(10ms, [&](bool reschedule) {
-            latch.count_down();
+        tf = std::make_unique<throttled_func>(
+            til::throttled_func_options{
+                .delay = 10ms,
+                .trailing = true },
+            [&](bool reschedule) {
+                latch.count_down();
 
-            // This will ensure that the callback is called even if we
-            // invoke the throttled_func from inside the callback itself.
-            if (reschedule)
-            {
-                tf->operator()(false);
-            }
-        });
+                // This will ensure that the callback is called even if we
+                // invoke the throttled_func from inside the callback itself.
+                if (reschedule)
+                {
+                    tf->operator()(false);
+                }
+            });
         // This will ensure that the throttled_func invokes the callback in general.
         tf->operator()(true);