[Linux] Use DBus for following media playback change

linux/media/playerstatuswatcher.cpp

@@ -0,0 +1,47 @@
+#include "playerstatuswatcher.h"
+#include <QDBusConnection>
+#include <QDBusPendingReply>
+#include <QVariantMap>
+#include <QDBusReply>
+
+PlayerStatusWatcher::PlayerStatusWatcher(const QString &playerService, QObject *parent)
+    : QObject(parent),
+      m_playerService(playerService),
+      m_iface(new QDBusInterface(playerService, "/org/mpris/MediaPlayer2",
+                                 "org.mpris.MediaPlayer2.Player", QDBusConnection::sessionBus(), this)),
+      m_serviceWatcher(new QDBusServiceWatcher(playerService, QDBusConnection::sessionBus(),
+                                               QDBusServiceWatcher::WatchForOwnerChange, this))
+{
+    QDBusConnection::sessionBus().connect(
+        playerService, "/org/mpris/MediaPlayer2", "org.freedesktop.DBus.Properties",
+        "PropertiesChanged", this, SLOT(onPropertiesChanged(QString,QVariantMap,QStringList))
+    );
+    connect(m_serviceWatcher, &QDBusServiceWatcher::serviceOwnerChanged,
+            this, &PlayerStatusWatcher::onServiceOwnerChanged);
+    updateStatus();
+}
+
+void PlayerStatusWatcher::onPropertiesChanged(const QString &interface,
+                                              const QVariantMap &changed,
+                                              const QStringList &)
+{
+    if (interface == "org.mpris.MediaPlayer2.Player" && changed.contains("PlaybackStatus")) {
+        emit playbackStatusChanged(changed.value("PlaybackStatus").toString());
+    }
+}
+
+void PlayerStatusWatcher::updateStatus() {
+    QVariant reply = m_iface->property("PlaybackStatus");
+    if (reply.isValid()) {
+        emit playbackStatusChanged(reply.toString());
+    }
+}
+
+void PlayerStatusWatcher::onServiceOwnerChanged(const QString &name, const QString &, const QString &newOwner)
+{
+    if (name == m_playerService && newOwner.isEmpty()) {
+        emit playbackStatusChanged(""); // player disappeared
+    } else if (name == m_playerService && !newOwner.isEmpty()) {
+        updateStatus(); // player appeared/reappeared
+    }
+}