Properly manage the lifetime of polling in the AppView and PcView

app/backend/computermanager.cpp

@@ -269,7 +269,7 @@ bool NvComputer::update(NvComputer& that)
 
 ComputerManager::ComputerManager(QObject *parent)
     : QObject(parent),
-      m_Polling(false),
+      m_PollingRef(0),
       m_MdnsBrowser(nullptr)
 {
     QSettings settings;
@@ -302,12 +302,10 @@ void ComputerManager::startPolling()
 {
     QWriteLocker lock(&m_Lock);
 
-    if (m_Polling) {
+    if (++m_PollingRef > 1) {
         return;
     }
 
-    m_Polling = true;
-
     // Start an MDNS query for GameStream hosts
     m_MdnsBrowser = new QMdnsEngine::Browser(&m_MdnsServer, "_nvstream._tcp.local.", &m_MdnsCache);
     connect(m_MdnsBrowser, &QMdnsEngine::Browser::serviceAdded,
@@ -400,12 +398,11 @@ void ComputerManager::stopPollingAsync()
 {
     QWriteLocker lock(&m_Lock);
 
-    if (!m_Polling) {
+    Q_ASSERT(m_PollingRef > 0);
+    if (--m_PollingRef > 0) {
         return;
     }
 
-    m_Polling = false;
-
     // Delete machines that haven't been resolved yet
     while (!m_PendingResolution.isEmpty()) {
         MdnsPendingComputer* computer = m_PendingResolution.first();
@@ -456,7 +453,7 @@ ComputerManager::handleComputerStateChanged(NvComputer* computer)
 void
 ComputerManager::startPollingComputer(NvComputer* computer)
 {
-    if (!m_Polling) {
+    if (m_PollingRef == 0) {
         return;
     }
 

app/backend/computermanager.h

@@ -279,7 +279,7 @@ private:
 
     void startPollingComputer(NvComputer* computer);
 
-    bool m_Polling;
+    int m_PollingRef;
     QReadWriteLock m_Lock;
     QMap<QString, NvComputer*> m_KnownHosts;
     QMap<QString, QThread*> m_PollThreads;

app/gui/AppView.qml

@@ -17,14 +17,20 @@ GridView {
     cellWidth: 225; cellHeight: 350;
     focus: true
 
-    Component.onCompleted: {
-        // Start polling when this view is shown
-        ComputerManager.startPolling()
-    }
+    // The StackView will trigger a visibility change when
+    // we're pushed onto it, causing our onVisibleChanged
+    // routine to run, but only if we start as invisible
+    visible: false
 
-    Component.onDestruction: {
-        // Stop polling when this view is destroyed
-        ComputerManager.stopPollingAsync()
+    onVisibleChanged: {
+        if (visible) {
+            // Start polling when this view is shown
+            ComputerManager.startPolling()
+        }
+        else {
+            // Stop polling when this view is not on top
+            ComputerManager.stopPollingAsync()
+        }
     }
 
     function createModel()

app/gui/PcView.qml

@@ -19,17 +19,25 @@ GridView {
     focus: true
     objectName: "Computers"
 
-    Component.onCompleted: {
-        // Start polling when this view is shown
-        ComputerManager.startPolling()
+    // The StackView will trigger a visibility change when
+    // we're pushed onto it, causing our onVisibleChanged
+    // routine to run, but only if we start as invisible
+    visible: false
 
+    Component.onCompleted: {
         // Setup signals on CM
         ComputerManager.computerAddCompleted.connect(addComplete)
     }
 
-    Component.onDestruction: {
-        // Stop polling when this view is destroyed
-        ComputerManager.stopPollingAsync()
+    onVisibleChanged: {
+        if (visible) {
+            // Start polling when this view is shown
+            ComputerManager.startPolling()
+        }
+        else {
+            // Stop polling when this view is not top-most
+            ComputerManager.stopPollingAsync()
+        }
     }
 
     function pairingComplete(error)