Xamarin - Make rotations with CoreGraphics.CGAffineTransform

Question

I want to make a UI Image rotate while content is downloading and I can't seem to make it rotate.

I have this event:

public override void ViewDidAppear(bool animated)
{
    NoConnectionView.Hidden = CheckConnectivityStatus();
    Task.Run(async () => await StartSpinningAnimation(true));
}

Which then fires this method:

protected async Task StartSpinningAnimation(bool IsSpinning) 
{
    do
    {
        UpdateActiveImage.Transform = CoreGraphics.CGAffineTransform.MakeRotation((float)Math.PI / 4);
    }
    while (IsSpinning);
    return;
}

The page will eventually change after files are downloaded so I just want it to spin forever. It does animate at all. Does anyone have any ideas?

Answer 1

Using C# and Xamarin.iOS to develop iOS is a letter bit different from the Native language, Whenever you want to invoke some UI element in your background thread(like you need make some UI do an animation), you must use:

InvokeOnMainThread(delegate {
    //Do something related UI stuff
});

If you add a try catch for it, you will get the exception like that "You are calling a method that can only be invoked in UI thread";

And by the way, you can not just use a do while to make an animation, I write a sample for you, you can take a look:

public partial class ViewController : UIViewController
{
    bool needAnimate = true;
    int count = 0;
    UIView animationView = new UIView();

    public ViewController (IntPtr handle) : base (handle)
    {
    }

    public override void ViewDidLoad ()
    {
        base.ViewDidLoad ();
        // Perform any additional setup after loading the view, typically from a nib.

        animationView.Frame = new CoreGraphics.CGRect (50, 50, 100, 100);
        animationView.BackgroundColor = UIColor.Red;
        this.Add (animationView);

        this.View.AddGestureRecognizer (new UITapGestureRecognizer (() => {
            Task.Run(async () => await StartAnimation());
        }));
    }

    private async Task StartAnimation() 
    {
        do
        {
            Console.WriteLine ("count = " + count++);
            InvokeOnMainThread(delegate {
                UIView.Animate(0.25,delegate {
                    animationView.Transform = CoreGraphics.CGAffineTransform.MakeRotation((float)Math.PI / 4 * (count/4 + 1));
                });
            });
            System.Threading.Thread.Sleep(250);
        }
        while (needAnimate);
        return;
    }

    public override void DidReceiveMemoryWarning ()
    {
        base.DidReceiveMemoryWarning ();
        // Release any cached data, images, etc that aren't in use.
    }
}

The animation is not smoothly, you need to optimize it yourself.

If you still have some questions, just leave here, I will check it latter.

Hope it can help you and welcome to Xamarin.