Qt: Fix 100% CPU usage while downloading files

The wonders of having fast internet, you never realize when this happens
because it completes too quickly...

src/duckstation-qt/autoupdaterdialog.cpp

@@ -544,11 +544,13 @@ void AutoUpdaterDialog::downloadUpdateClicked()
   m_http_poll_timer->stop();
 
   // Block until completion.
-  while (m_http->HasAnyRequests())
-  {
-    QApplication::processEvents(QEventLoop::AllEvents, HTTP_POLL_INTERVAL);
-    m_http->PollRequests();
-  }
+  QtUtils::ProcessEventsWithSleep(
+    QEventLoop::AllEvents,
+    [this]() {
+      m_http->PollRequests();
+      return m_http->HasAnyRequests();
+    },
+    HTTP_POLL_INTERVAL);
 
   if (download_result.value_or(false))
   {

src/duckstation-qt/mainwindow.cpp

@@ -281,7 +281,7 @@ std::optional<WindowInfo> MainWindow::acquireRenderWindow(bool recreate_window,
     m_display_widget->setFocus();
     updateWindowState();
 
-    QCoreApplication::processEvents(QEventLoop::ExcludeUserInputEvents);
+    QApplication::processEvents(QEventLoop::ExcludeUserInputEvents);
     return m_display_widget->getWindowInfo();
   }
 
@@ -710,8 +710,7 @@ void MainWindow::quit()
   if (s_system_valid)
   {
     g_emu_thread->shutdownSystem(false, true);
-    while (s_system_valid)
-      QApplication::processEvents(QEventLoop::ExcludeUserInputEvents, 1);
+    QtUtils::ProcessEventsWithSleep(QEventLoop::ExcludeUserInputEvents, []() { return s_system_valid; });
   }
 
   // Big picture might still be active.
@@ -749,9 +748,8 @@ void MainWindow::recreate()
   if (was_display_created)
   {
     g_emu_thread->setSurfaceless(true);
-    while (m_display_widget || !g_emu_thread->isSurfaceless())
-      QApplication::processEvents(QEventLoop::ExcludeUserInputEvents, 1);
-
+    QtUtils::ProcessEventsWithSleep(QEventLoop::ExcludeUserInputEvents,
+                                    [this]() { return (m_display_widget || !g_emu_thread->isSurfaceless()); });
     m_display_created = false;
   }
 
@@ -2005,8 +2003,8 @@ void MainWindow::switchToGameListView()
 
       // switch to surfaceless. we have to wait until the display widget is gone before we swap over.
       g_emu_thread->setSurfaceless(true);
-      while (m_display_widget)
-        QApplication::processEvents(QEventLoop::ExcludeUserInputEvents, 1);
+      QtUtils::ProcessEventsWithSleep(QEventLoop::ExcludeUserInputEvents,
+                                      [this]() { return static_cast<bool>(m_display_widget); });
     }
   }
 
@@ -2915,12 +2913,11 @@ MainWindow::SystemLock MainWindow::pauseAndLockSystem()
     g_emu_thread->setFullscreen(false, false);
 
     // Container could change... thanks Wayland.
-    QWidget* container;
-    while (s_system_valid &&
-           (g_emu_thread->isFullscreen() || !(container = getDisplayContainer()) || container->isFullScreen()))
-    {
-      QApplication::processEvents(QEventLoop::ExcludeUserInputEvents);
-    }
+    QtUtils::ProcessEventsWithSleep(QEventLoop::ExcludeUserInputEvents, [this]() {
+      QWidget* container;
+      return (s_system_valid &&
+              (g_emu_thread->isFullscreen() || !(container = getDisplayContainer()) || container->isFullScreen()));
+    });
   }
 
   if (!was_paused)
@@ -2928,8 +2925,7 @@ MainWindow::SystemLock MainWindow::pauseAndLockSystem()
     g_emu_thread->setSystemPaused(true);
 
     // Need to wait for the pause to go through, and make the main window visible if needed.
-    while (!s_system_paused)
-      QApplication::processEvents(QEventLoop::ExcludeUserInputEvents, 1);
+    QtUtils::ProcessEventsWithSleep(QEventLoop::ExcludeUserInputEvents, []() { return !s_system_paused; });
 
     // Ensure it's visible before we try to create any dialogs parented to us.
     QApplication::sync();

src/duckstation-qt/qthost.cpp

@@ -311,11 +311,13 @@ std::optional<bool> QtHost::DownloadFile(QWidget* parent, const QString& title,
     &progress);
 
   // Block until completion.
-  while (http->HasAnyRequests())
-  {
-    QApplication::processEvents(QEventLoop::AllEvents, HTTP_POLL_INTERVAL);
-    http->PollRequests();
-  }
+  QtUtils::ProcessEventsWithSleep(
+    QEventLoop::AllEvents,
+    [http = http.get()]() {
+      http->PollRequests();
+      return http->HasAnyRequests();
+    },
+    HTTP_POLL_INTERVAL);
 
   return download_result;
 }
@@ -806,9 +808,8 @@ void EmuThread::stopFullscreenUI()
     QMetaObject::invokeMethod(this, &EmuThread::stopFullscreenUI, Qt::QueuedConnection);
 
     // wait until the host display is gone
-    while (!QtHost::IsSystemValid() && g_gpu_device)
-      QApplication::processEvents(QEventLoop::ExcludeUserInputEvents, 1);
-
+    QtUtils::ProcessEventsWithSleep(QEventLoop::ExcludeUserInputEvents,
+                                    []() { return (!QtHost::IsSystemValid() && g_gpu_device); });
     return;
   }
 
@@ -1742,8 +1743,7 @@ void EmuThread::stop()
   AssertMsg(!g_emu_thread->isOnThread(), "Not called on the emu thread");
 
   QMetaObject::invokeMethod(g_emu_thread, &EmuThread::stopInThread, Qt::QueuedConnection);
-  while (g_emu_thread->isRunning())
-    QApplication::processEvents(QEventLoop::ExcludeUserInputEvents, 1);
+  QtUtils::ProcessEventsWithSleep(QEventLoop::ExcludeUserInputEvents, []() { return (g_emu_thread->isRunning()); });
 }
 
 void EmuThread::stopInThread()

src/duckstation-qt/qtutils.h

@@ -8,8 +8,11 @@
 #include "common/types.h"
 
 #include <QtCore/QByteArray>
+#include <QtCore/QCoreApplication>
+#include <QtCore/QEventLoop>
 #include <QtCore/QMetaType>
 #include <QtCore/QString>
+#include <QtCore/QTimer>
 #include <QtGui/QIcon>
 #include <QtWidgets/QWidget>
 #include <functional>
@@ -121,4 +124,30 @@ bool SaveWindowGeometry(std::string_view window_name, QWidget* widget, bool auto
 /// Restores a window's geometry from configuration. Returns false if it was not found in the configuration.
 bool RestoreWindowGeometry(std::string_view window_name, QWidget* widget);
 
+/// CPU-friendly way of blocking the UI thread while some predicate holds true.
+template<typename Predicate>
+[[maybe_unused]] static void ProcessEventsWithSleep(QEventLoop::ProcessEventsFlags flags, const Predicate& pred,
+                                                    int sleep_time_ms = 10)
+{
+  if (sleep_time_ms == 0)
+  {
+    while (pred())
+      QCoreApplication::processEvents(flags);
+  }
+
+  if (!pred())
+    return;
+
+  QEventLoop loop;
+  QTimer timer;
+  QObject::connect(&timer, &QTimer::timeout, &timer, [&loop, &pred]() {
+    if (pred())
+      return;
+
+    loop.exit();
+  });
+  timer.start(sleep_time_ms);
+  loop.exec(flags);
+}
+
 } // namespace QtUtils