ControlIndicatorTimer: Add support for legacy vsyncthread mode

Due to concerns that a regular QTimer that fires during vsync could
cause jerking legacy waveforms, this implements a way to use the
legacy vsync thread/GuiTick infrastructure instead of a `QTimer` in the
`ControlIndicatorTimer` (although I'm still not convinced that
this actually makes a difference).

This can be toggled explicitly using `setLegacyVsyncEnabled` when
switching between a legacy and a QML skin (the latter has no vsync
thread, so a regular timer needs to be used).

Relevant discussion can be found here:
#4157 (comment)

src/control/controlindicatortimer.cpp

@@ -2,6 +2,7 @@
 
 #include "control/controlobject.h"
 #include "moc_controlindicatortimer.cpp"
+#include "util/math.h"
 
 namespace mixxx {
 
@@ -11,7 +12,9 @@ ControlIndicatorTimer::ControlIndicatorTimer(QObject* pParent)
                   ConfigKey("[Master]", "indicator_250millis"))),
           m_pCOIndicator500millis(std::make_unique<ControlObject>(
                   ConfigKey("[Master]", "indicator_500millis"))),
-          m_toggleIndicator500millisOnNextTimeout(true) {
+          m_toggleIndicator500millisOnNextTimeout(true),
+          m_nextSwitchTime(0.0),
+          m_pCPGuiTick50ms(nullptr) {
     m_pCOIndicator250millis->setReadOnly();
     m_pCOIndicator500millis->setReadOnly();
     connect(&m_timer, &QTimer::timeout, this, &ControlIndicatorTimer::slotTimeout);
@@ -29,4 +32,37 @@ void ControlIndicatorTimer::slotTimeout() {
     m_toggleIndicator500millisOnNextTimeout = !m_toggleIndicator500millisOnNextTimeout;
 }
 
+// TODO: Everything below this comment only added for compatiblity with the
+// legacy waveform vsync thread. It should be removed when the legacy skin
+// system is dropped.
+
+void ControlIndicatorTimer::setLegacyVsyncEnabled(bool enabled) {
+    const bool isLegacyVsyncEnabled = (m_pCPGuiTick50ms != nullptr);
+    if (isLegacyVsyncEnabled == enabled) {
+        return;
+    }
+
+    if (enabled) {
+        m_timer.stop();
+        m_pCPGuiTick50ms = std::make_unique<ControlProxy>(ConfigKey("[Master]", "guiTick50ms"));
+        m_pCPGuiTick50ms->connectValueChanged(this, &ControlIndicatorTimer::slotGuiTick50ms);
+    } else {
+        m_pCPGuiTick50ms->disconnect(this);
+        m_pCPGuiTick50ms.reset();
+        m_timer.start(250);
+    }
+}
+
+void ControlIndicatorTimer::slotGuiTick50ms(double cpuTime) {
+    if (m_nextSwitchTime > cpuTime) {
+        return;
+    }
+
+    constexpr double duration = 0.25;
+    const double tickTime = ControlObject::get(ConfigKey("[Master]", "guiTickTime"));
+    const double toggles = floor(tickTime / duration);
+    m_nextSwitchTime = (toggles + 1) * duration;
+    slotTimeout();
+}
+
 } // namespace mixxx

src/control/controlindicatortimer.h

@@ -5,6 +5,7 @@
 #include <memory>
 
 #include "control/controlobject.h"
+#include "control/controlproxy.h"
 
 namespace mixxx {
 
@@ -28,6 +29,19 @@ class ControlIndicatorTimer : public QObject {
     std::unique_ptr<ControlObject> m_pCOIndicator250millis;
     std::unique_ptr<ControlObject> m_pCOIndicator500millis;
     bool m_toggleIndicator500millisOnNextTimeout;
+
+    /// TODO: Everything below this comment only added for compatiblity with the
+    /// legacy waveform vsync thread. It should be removed when the legacy skin
+    /// system is dropped.
+  public:
+    void setLegacyVsyncEnabled(bool enabled);
+
+  private slots:
+    void slotGuiTick50ms(double cpuTime);
+
+  private:
+    double m_nextSwitchTime;
+    std::unique_ptr<ControlProxy> m_pCPGuiTick50ms;
 };
 
 } // namespace mixxx

src/coreservices.cpp

@@ -188,7 +188,7 @@ void CoreServices::initialize(QApplication* pApp) {
         exit(-1);
     }
 
-    m_pControlIndicatorTimer = std::make_unique<mixxx::ControlIndicatorTimer>(this);
+    m_pControlIndicatorTimer = std::make_shared<mixxx::ControlIndicatorTimer>(this);
 
     auto pChannelHandleFactory = std::make_shared<ChannelHandleFactory>();
 

src/coreservices.h

@@ -55,6 +55,10 @@ class CoreServices : public QObject {
         return m_pKbdConfig;
     }
 
+    std::shared_ptr<mixxx::ControlIndicatorTimer> getControlIndicatorTimer() const {
+        return m_pControlIndicatorTimer;
+    }
+
     std::shared_ptr<SoundManager> getSoundManager() const {
         return m_pSoundManager;
     }
@@ -119,7 +123,7 @@ class CoreServices : public QObject {
     bool initializeDatabase();
 
     std::shared_ptr<SettingsManager> m_pSettingsManager;
-    std::unique_ptr<mixxx::ControlIndicatorTimer> m_pControlIndicatorTimer;
+    std::shared_ptr<mixxx::ControlIndicatorTimer> m_pControlIndicatorTimer;
     std::shared_ptr<EffectsManager> m_pEffectsManager;
     // owned by EffectsManager
     LV2Backend* m_pLV2Backend;

src/mixxxmainwindow.cpp

@@ -28,6 +28,7 @@
 #ifdef __BROADCAST__
 #include "broadcast/broadcastmanager.h"
 #endif
+#include "control/controlindicatortimer.h"
 #include "control/controlpushbutton.h"
 #include "controllers/controllermanager.h"
 #include "controllers/keyboard/keyboardeventfilter.h"
@@ -132,6 +133,8 @@ MixxxMainWindow::MixxxMainWindow(
 
     m_pCoreServices->initialize(pApp);
 
+    m_pCoreServices->getControlIndicatorTimer()->setLegacyVsyncEnabled(true);
+
     UserSettingsPointer pConfig = m_pCoreServices->getSettings();
 
     // Set the visibility of tooltips, default "1" = ON
@@ -426,6 +429,8 @@ MixxxMainWindow::~MixxxMainWindow() {
     qDebug() << t.elapsed(false).debugMillisWithUnit() << "deleting DlgPreferences";
     delete m_pPrefDlg;
 
+    m_pCoreServices->getControlIndicatorTimer()->setLegacyVsyncEnabled(false);
+
     WaveformWidgetFactory::destroy();
 
     delete m_pGuiTick;