Q: Qt - [c++] PropertyAnimation from WorkerThread

[kevin_d]

Hi everyone!

I am using a QPropertyAnimation from within a QFuture.
Unfortunately, my program dies from time to time, stating the following:

QObject::startTimer: Timers can only be used with threads started with QThread
QBasicTimer::start: Timers cannot be started from another thread
QPropertyAnimation::updateState (): Changing state of an animation without target

How can i make the PropertyAnimation thread-safe?

best regards, kevin

[Bondrusiek]

@kevin_d Please, paste the suspicious code. Other users can use this code and find a solution.

[jeremy_k]

@kevin_d said in Q: Qt - [c++] PropertyAnimation from WorkerThread:

Hi everyone!

I am using a QPropertyAnimation from within a QFuture.
Unfortunately, my program dies from time to time, stating the following:

QObject::startTimer: Timers can only be used with threads started with QThread
QBasicTimer::start: Timers cannot be started from another thread
QPropertyAnimation::updateState (): Changing state of an animation without target

How can i make the PropertyAnimation thread-safe?

It's not clear whether the C++ QPropertyAnimation or QML PropertyAnimation is under discussion. Either way, the simple explanation is that neither class can be used from a different thread than the target object.

Reading https://doc.qt.io/qt-6/threads-qobject.html explains why and may reveal some options for this use case. It also helps to explain the first warning: QtConcurrent threads have no event dispatcher, and therefore can't play host to a timer.

Answering the question, make a property animation safe by not using it with threads. Threads are frequently unnecessary and always complicate a program's logic.

[kevin_d]

Hello!

Sorry for the delay and thanks for answering.

I am using a QPropertyAnimation to move the Camera on a Qt3D scene during a sequence, triggered in a QFuture.

1. Move the camera upwards (QPropertyAnimation on the Camera)
2. Wait for the animation to finish : QThread::msleep(1000)
3. Place a figure
4. Move the camera back down
5. Wait for the Animation to finish : QThread::msleep(1000)
6. Release a semaphore to trigger waiting action

futureSwitch = QtConcurrent::run([this, party]
    {
        semAnimation->acquire();
        QThread::msleep(300);
        MoveCameraToTop();
        QThread::msleep(400);
        DropFigure();
        QThread::msleep(600);
        MoveCameraToBottom();
        QThread::msleep(1800);
        return 1;
    });

Animation:

void MoveCameraToBottom()
{
    // class CameraAnimation : public QPropertyAnimation
    camPosAnimation = new CameraAnimation(camera, 
            CameraAnimation::CamPropertyType::Position);

    camPosAnimation->setParent(obj);
    camPosAnimation->startPos = pos_start;
    camPosAnimation->endPos = position;
    camPosAnimation->trigger();
}

Why using a future for the animation sequence?
... because when triggered in the main thread, the UI does not update the scene, and animations are not updating live (especially with QThread::msleep()).

[JonB]

@kevin_d
So, since accessing the UI from threads is flawed, get rid of things like msleep() and do it on a QTimer in the UI thread.

[jeremy_k]

@kevin_d If I understand the description, the goal is:

1. an animation that moves the camera up
2. an action that occurs at the end of step 1
3. an animation that moves the camera down

The correct way to determine when a QPropertyAnimation completes is to use the finished() signal. No sleeping, threading, semaphores, or explicit timers are required.

QPropertyAnimation upAnimator, downAnimator;
QObject::connect(&upAnimator, &QAbstractAnimation::finished, &midpoint);
QObject::connect(&downAnimator, &QAbstractAnimation::finished, []() { qDebug() << "Done!"; });

void beginAnimation() {
    upAnimator.start();
}

void midpoint() {
    DropFigure();
    downAnimator.start();
}

[kevin_d]

Thank you. This way the application is clearly more stable.

This works also fine, when i wait for all the animations:

QEventLoop loopPos;
connect(myAnimation, &QParallelAnimationGroup::finished, &loopPos, &QEventLoop::quit );
loopPos.exec();

... and if needed use a

QParallelAnimationGroup

best regards!