In SceneKit SCNAction hangs when called from completion handler of RunAction

Question

Calling an SCNAction from the completion handler of RunAction seems to hang SceneKit.

A touch event or rotating the device seems to unblock the hang.

To reproduce:

1) Take the default SceneKit project you get on startup with the rotating spaceship.

2) Replace the animation code:

ship.RunAction(SCNAction.RepeatActionForever(SCNAction.RotateBy(0, 2, 0, 1)));

with:

        ship.RunAction(SCNAction.RotateBy(0, 2, 0, durationInSeconds: 3.0f), delegate
        {
            Console.WriteLine("DONE ROTATE");
            ship.RunAction(SCNAction.MoveBy(1, 0, 0, durationInSeconds: 3.0f), delegate
            {
                Console.WriteLine("DONE MOVEBY");
            });
        });

3) Run on the simulator or real device (the problem is the same on both)

4) The results is:

Spaceship rotates OK
DONE ROTATE is printed out OK
Now it's hung
Tap the screen (or rotate the device to landscape) and then the move happens OK and DONE MOVEBY is printed out.

I'm using C# and Visual Studio for Mac, but I suspect it happens using Xcode too.

Is this a bug in SceneKit? How can a workaround be done?

Maybe this is the same issue as described here:

SCNAction completion handler awaits gesture to execute

Instead of running a new SCNAction in the completion handler, have you tried the SCNAction.sequence([SCNAction]) function. It may help but I’m not sure — Brandon, Jun 02 '19 at 23:01
Thanks for the idea, but I have now discovered the underlying cause of the problem which is the callback is not on the main thread(!). Also using Sequence with Run to execute a block also calls the block on a different thread. So a solution is to invoke the callback on the main thread using eg: SynchronizationContext Post... — Bbx, Jul 04 '19 at 11:42

score 3 · Answer 1 · answered May 21 '19 at 20:30

This happens because by default SceneKit is not rendering continuously. When tapping the screen the scene is changed and a new frame will be rendered. That's why the moveBy action is not triggered immediately after the rotateBy action.

Try setting SCNView's renderContinuously property to true like so:

scnView.rendersContinuously = true
ship.runAction(SCNAction.rotateBy(x: 0, y: 2, z: 0, duration: 3.0)) {
    print("DONE ROTATE")
    ship.runAction(SCNAction.moveBy(x: 1, y: 0, z: 0, duration: 3.0), completionHandler: {
        print("DONE MOVEBY")
        scnView.rendersContinuously = false
    })
}

Thanks for that explanation. It does indeed fix the above example, but I think it will be bad for battery life (my game does not need continuous rendering) and rendersContinuously is iOS 11+ only. I have also discovered that using SCNTransaction or CABasicAnimation work fine (I will add an answer showing how). Why do you think they work, but RunAction doesn't? — Bbx, May 23 '19 at 08:26

score 1 · Answer 2 · answered May 23 '19 at 08:25

Using SCNTransaction with a completionBlock does not suffer from the same problem, so this works fine:

        SCNTransaction.Begin();
        SCNTransaction.AnimationDuration = 3.0f;
        SCNTransaction.SetCompletionBlock(() =>
        {
            Console.WriteLine("DONE ROTATE");
            SCNTransaction.Begin();
            SCNTransaction.AnimationDuration = 3.0f;
            SCNTransaction.SetCompletionBlock(() =>
            {
                Console.WriteLine("DONE MOVEBY");
            });
            ship.Position = new SCNVector3(1.0f, 0.0f, 0.0f);
            SCNTransaction.Commit();
        });
        ship.EulerAngles = new SCNVector3(0.0f, (float)Math.PI / 2, 0.0f);
        SCNTransaction.Commit();

(Also using CABasicAnimation with CAAnimationDelegate to do the callback works OK.)

Since SCNTransaction and CABasicAnimation work, but RunAction doesn't, it really looks like an Apple bug in RunAction.

score 0 · Answer 3 · answered Mar 16 '23 at 09:02

0

My solution is to use DispatchQueue to lunch inner actions:

piece.runAction(action0, completionHandler: {
    DispatchQueue.main.asyncAfter(deadline: .now()) {
       ship.runAction(action1)
    }
}

answered Mar 16 '23 at 09:02

Tony

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In SceneKit SCNAction hangs when called from completion handler of RunAction

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