Implement initialization of video devices on the first connection

The problem is that VideoDevices are created and managed by
QPlatformMediaIntegration, but signals are connected to
QPlatformMediaDevices.
A possible redesign is here
codereview.qt-project.org/c/qt/qtmultimedia/+/484647,
but the concern is that it's a bit of controversy with the initial idea.

Current patchs solves the poblem directly and covers with UTs.
Idea of the patch: let's handle connection to
QMediaDevices::videoInputsChanged and initialize
QPlatformMediaIntegration and video devices connection.
I propose postpone initialization because users don't expect
us doing any serious magic just on the signal connection.

The UTs were slightly refactored in order to test the case.

Pick-to: 6.6 6.5
Change-Id: I0ca3dfb2126daf69746380fa65096d9d05327f5e
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>

6 files changed, 60 insertions, 41 deletions

diff --git a/src/multimedia/platform/qplatformmediadevices.cpp b/src/multimedia/platform/qplatformmediadevices.cpp
index 09c26a670..f971fb4c3 100644
--- a/src/multimedia/platform/qplatformmediadevices.cpp
+++ b/src/multimedia/platform/qplatformmediadevices.cpp
@@ -2,11 +2,13 @@
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
 #include "qplatformmediadevices_p.h"
+#include "qplatformmediaintegration_p.h"
 #include "qmediadevices.h"
 #include "qaudiodevice.h"
 #include "qcameradevice.h"
 #include "qaudiosystem_p.h"
 #include "qaudiodevice.h"
+#include "qplatformvideodevices_p.h"
 
 #include <qmutex.h>
 #include <qloggingcategory.h>
@@ -75,8 +77,18 @@ QPlatformMediaDevices *QPlatformMediaDevices::instance()
 }
 
 
-QPlatformMediaDevices::QPlatformMediaDevices()
-{}
+QPlatformMediaDevices::QPlatformMediaDevices() = default;
+
+void QPlatformMediaDevices::initVideoDevicesConnection() {
+    std::call_once(m_videoDevicesConnectionFlag, [this]() {
+        QMetaObject::invokeMethod(this, [this]() {
+            auto videoDevices = QPlatformMediaIntegration::instance()->videoDevices();
+            if (videoDevices)
+                connect(videoDevices, &QPlatformVideoDevices::videoInputsChanged, this,
+                        &QPlatformMediaDevices::videoInputsChanged);
+        }, Qt::QueuedConnection);
+    });
+}
 
 void QPlatformMediaDevices::setDevices(QPlatformMediaDevices *devices)
 {
diff --git a/src/multimedia/platform/qplatformmediadevices_p.h b/src/multimedia/platform/qplatformmediadevices_p.h
index d475495d1..0d7ba9306 100644
--- a/src/multimedia/platform/qplatformmediadevices_p.h
+++ b/src/multimedia/platform/qplatformmediadevices_p.h
@@ -18,6 +18,7 @@
 #include <private/qtmultimediaglobal_p.h>
 #include <qlist.h>
 #include <qobject.h>
+#include <mutex>
 
 QT_BEGIN_NAMESPACE
 
@@ -52,10 +53,15 @@ public:
 
     virtual void prepareAudio();
 
+    void initVideoDevicesConnection();
+
 Q_SIGNALS:
     void audioInputsChanged();
     void audioOutputsChanged();
     void videoInputsChanged();
+
+private:
+    std::once_flag m_videoDevicesConnectionFlag;
 };
 
 QT_END_NAMESPACE
diff --git a/src/multimedia/platform/qplatformmediaintegration.cpp b/src/multimedia/platform/qplatformmediaintegration.cpp
index 7adfbcf54..b094d549c 100644
--- a/src/multimedia/platform/qplatformmediaintegration.cpp
+++ b/src/multimedia/platform/qplatformmediaintegration.cpp
@@ -73,25 +73,24 @@ static QString defaultBackend(const QStringList &backends)
 
 QT_BEGIN_NAMESPACE
 
-namespace {
-struct InstanceHolder {
-    ~InstanceHolder()
-    {
-        QMutexLocker locker(&mutex);
-        instance = nullptr;
-    }
+struct QPlatformMediaIntegration::InstanceHolder
+{
+    // TODO: replace the mutex with std::once
     QBasicMutex mutex;
-    QPlatformMediaIntegration *instance = nullptr;
-    QPlatformMediaIntegration *nativeInstance = nullptr;
+    std::unique_ptr<QPlatformMediaIntegration> instance;
+    Factory factory;
 } instanceHolder;
 
-}
-
 QPlatformMediaIntegration *QPlatformMediaIntegration::instance()
 {
     QMutexLocker locker(&instanceHolder.mutex);
     if (instanceHolder.instance)
-        return instanceHolder.instance;
+        return instanceHolder.instance.get();
+
+    if (instanceHolder.factory) {
+        instanceHolder.instance = instanceHolder.factory();
+        return instanceHolder.instance.get();
+    }
 
     const auto backends = availableBackends();
     QString backend = QString::fromUtf8(qgetenv("QT_MEDIA_BACKEND"));
@@ -99,27 +98,25 @@ QPlatformMediaIntegration *QPlatformMediaIntegration::instance()
         backend = defaultBackend(backends);
 
     qCDebug(qLcMediaPlugin) << "loading backend" << backend;
-    instanceHolder.nativeInstance =
-            qLoadPlugin<QPlatformMediaIntegration, QPlatformMediaPlugin>(loader(), backend);
+    instanceHolder.instance.reset(
+            qLoadPlugin<QPlatformMediaIntegration, QPlatformMediaPlugin>(loader(), backend));
 
-    if (!instanceHolder.nativeInstance) {
+    if (!instanceHolder.instance) {
         qWarning() << "could not load multimedia backend" << backend;
-        instanceHolder.nativeInstance = new QDummyIntegration;
+        instanceHolder.instance = std::make_unique<QDummyIntegration>();
     }
 
-    instanceHolder.instance = instanceHolder.nativeInstance;
-    return instanceHolder.instance;
+    return instanceHolder.instance.get();
 }
 
 /*
     This API is there to be able to test with a mock backend.
 */
-void QPlatformMediaIntegration::setIntegration(QPlatformMediaIntegration *integration)
+void QPlatformMediaIntegration::setPlatformFactory(Factory factory)
 {
-    if (integration)
-        instanceHolder.instance = integration;
-    else
-        instanceHolder.instance = instanceHolder.nativeInstance;
+    Q_ASSERT((factory == nullptr) ^ (instanceHolder.factory == nullptr));
+    instanceHolder.instance.reset();
+    instanceHolder.factory = std::move(factory);
 }
 
 QList<QCameraDevice> QPlatformMediaIntegration::videoInputs()
diff --git a/src/multimedia/platform/qplatformmediaintegration_p.h b/src/multimedia/platform/qplatformmediaintegration_p.h
index debd68485..728425355 100644
--- a/src/multimedia/platform/qplatformmediaintegration_p.h
+++ b/src/multimedia/platform/qplatformmediaintegration_p.h
@@ -58,9 +58,6 @@ class Q_MULTIMEDIA_EXPORT QPlatformMediaIntegration
 public:
     static QPlatformMediaIntegration *instance();
 
-    // API to be able to test with a mock backend
-    static void setIntegration(QPlatformMediaIntegration *);
-
     QPlatformMediaIntegration();
     virtual ~QPlatformMediaIntegration();
     virtual QPlatformMediaFormatInfo *formatInfo() = 0;
@@ -84,6 +81,15 @@ public:
     QList<QCapturableWindow> capturableWindows();
     bool isCapturableWindowValid(const QCapturableWindowPrivate &);
 
+    QPlatformVideoDevices *videoDevices() { return m_videoDevices.get(); }
+
+private:
+    friend class QMockIntegrationFactory;
+    // API to be able to test with a mock backend
+    using Factory = std::function<std::unique_ptr<QPlatformMediaIntegration>()>;
+    struct InstanceHolder;
+    static void setPlatformFactory(Factory factory);
+
 protected:
     std::unique_ptr<QPlatformVideoDevices> m_videoDevices;
     std::unique_ptr<QPlatformCapturableWindows> m_capturableWindows;
diff --git a/src/multimedia/platform/qplatformvideodevices.cpp b/src/multimedia/platform/qplatformvideodevices.cpp
index e556bb899..bcf664cd2 100644
--- a/src/multimedia/platform/qplatformvideodevices.cpp
+++ b/src/multimedia/platform/qplatformvideodevices.cpp
@@ -2,18 +2,11 @@
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
 #include "qplatformvideodevices_p.h"
-#include "qplatformmediadevices_p.h"
 
 QT_BEGIN_NAMESPACE
 
-QPlatformVideoDevices::~QPlatformVideoDevices()
-{
-
-}
-
-void QPlatformVideoDevices::videoInputsChanged()
-{
-    emit QPlatformMediaDevices::instance()->videoInputsChanged();
-}
+QPlatformVideoDevices::~QPlatformVideoDevices() = default;
 
 QT_END_NAMESPACE
+
+#include "moc_qplatformvideodevices_p.cpp"
diff --git a/src/multimedia/platform/qplatformvideodevices_p.h b/src/multimedia/platform/qplatformvideodevices_p.h
index 0ec049c3c..008322be2 100644
--- a/src/multimedia/platform/qplatformvideodevices_p.h
+++ b/src/multimedia/platform/qplatformvideodevices_p.h
@@ -16,23 +16,28 @@
 
 #include <private/qtmultimediaglobal_p.h>
 #include <qmediarecorder.h>
+#include <qobject.h>
 
 QT_BEGIN_NAMESPACE
 
 class QPlatformMediaIntegration;
 
-class Q_MULTIMEDIA_EXPORT QPlatformVideoDevices
+class Q_MULTIMEDIA_EXPORT QPlatformVideoDevices : public QObject
 {
+    Q_OBJECT
 public:
     QPlatformVideoDevices(QPlatformMediaIntegration *integration)
         : m_integration(integration)
     {}
-    virtual ~QPlatformVideoDevices();
+
+    ~QPlatformVideoDevices() override;
 
     virtual QList<QCameraDevice> videoDevices() const = 0;
 
-protected:
+Q_SIGNALS:
     void videoInputsChanged();
+
+protected:
     QPlatformMediaIntegration *m_integration = nullptr;
 };