PulltoRefresh + await FadeTo+TranslateTo animation combination Crash on Xamarin.forms - xamarin

Question

I'm making app with using Xamarin.forms PCL.
I'm having really hard time to solve this issue.
I've spent several days but couldn't solve.
I'm using this nuget
PullToRefresh
https://github.com/jamesmontemagno/Xamarin.Forms-PullToRefreshLayout
xabre ble plugin
https://github.com/xabre/xamarin-bluetooth-le
When I use this at the same time AND Add some animation on the page.
It gives this runtime exception when I try to refresh. (It happens very rare like 1 time on 10 times)
The Crash happens ONLY iOS.
It's very rare. You can try to refresh more than 10 times. You will see.
It happens only actual iOS Device.
Foundation.MonoTouchException: Objective-C exception thrown. Name:
NSGenericException Reason: *** Collection <__NSSetM: 0x14feb8b0> was
mutated while being enumerated.
I know well what this is and when this happen.
It happens when I try to access deleted item on list or different thread.
So I made very simple source code to look this issue simply.
I don't have any list or array on my code.
Well, it happens again.
https://github.com/myallb/test_pulltorefresh
This is my sample source code for reproducing this issue. If you can help me, please look this code.
The Crash happens ONLY iOS.
It's very rare. You can try to refresh more than 10 times. You will see.
It happens only actual iOS Device.
Thanks so much.
Full source code
using Xamarin.Forms;
using Plugin.BLE.Abstractions.Contracts;
using Plugin.BLE;
using Plugin.BLE.Abstractions.EventArgs;
using System;
using System.Diagnostics;
using Refractored.XamForms.PullToRefresh;
using System.Threading.Tasks;
namespace test
{
public partial class testPage : ContentPage
{
public static IAdapter Adapter { set; get; }
public PullToRefreshLayout RefreshView = null;
AbsoluteLayout layout;
public testPage()
{
InitializeComponent();
Adapter = CrossBluetoothLE.Current.Adapter;
if (Adapter != null)
{
Adapter.DeviceAdvertised += OnEvent_DeviceAdvertised;
Adapter.DeviceConnected += OnEvent_DeviceConnected;
Adapter.DeviceConnectionLost += OnEvent_DeviceConnectionLost;
Adapter.DeviceDisconnected += OnEvent_DeviceDisconnected;
Adapter.DeviceDiscovered += OnEvent_DeviceDiscovered;
Device.StartTimer(TimeSpan.FromSeconds(5), Timer_ScanDevice);
}
else {
}
layout = new AbsoluteLayout()
{
BackgroundColor = Color.Purple,
};
ScrollView scrollview = new ScrollView()
{
VerticalOptions = LayoutOptions.FillAndExpand,
HorizontalOptions = LayoutOptions.FillAndExpand,
Content = layout
};
RefreshView = new PullToRefreshLayout
{
VerticalOptions = LayoutOptions.FillAndExpand,
HorizontalOptions = LayoutOptions.FillAndExpand,
Content = scrollview,
RefreshColor = Color.Red,
RefreshCommand = new Command(RefreshStart)
};
RefreshView.IsPullToRefreshEnabled = true;
Content = RefreshView;
Device.StartTimer(new TimeSpan(0, 0, 1), ani);
}
bool ani()
{
Label z = new Label()
{
Text = "Z",
TextColor = Color.White,
FontAttributes = FontAttributes.Bold,
FontSize = new Random().Next(22, 35)
};
AbsoluteLayout.SetLayoutBounds(z, new Rectangle(0.67 + new Random().Next(0, 10) / 100.0, 0.13 + new Random().Next(0, 10) / 100.0, 40, 40));
AbsoluteLayout.SetLayoutFlags(z, AbsoluteLayoutFlags.PositionProportional);
layout.Children.Add(z);
Device.BeginInvokeOnMainThread(async () =>
{
Task t1 = z.FadeTo(0, 3500);
Task t2 = z.TranslateTo(0, -70, 3500, Easing.SinInOut);
await Task.WhenAll(t1, t2);
layout.Children.Remove(z);
});
return true;
}
void RefreshStart()
{
Debug.WriteLine("RefreshStart");
if (RefreshView != null)
RefreshView.IsRefreshing = true;
Device.BeginInvokeOnMainThread(async () =>
{
await Task.Delay(20);
Debug.WriteLine("RefreshEnd");
RefreshView.IsRefreshing = false;
});
}
bool Timer_ScanDevice()
{
Adapter.StartScanningForDevicesAsync();
return true;
}
void OnEvent_DeviceAdvertised(object sender, DeviceEventArgs a)
{
Debug.WriteLine("OnEvent_DeviceAdvertised");
}
void OnEvent_DeviceDiscovered(object sender, DeviceEventArgs a)
{
Debug.WriteLine("OnEvent_DeviceDiscovered");
}
void OnEvent_DeviceConnected(object sender, DeviceEventArgs a)
{
Debug.WriteLine("OnEvent_DeviceConnected");
}
void OnDeviceProcessError(IDevice device, string message)
{
Debug.WriteLine("OnDeviceProcessError");
}
void OnEvent_DeviceConnectionLost(object sender, DeviceErrorEventArgs a)
{
Debug.WriteLine("OnEvent_DeviceConnectionLost");
}
void OnEvent_DeviceDisconnected(object sender, DeviceEventArgs a)
{
Debug.WriteLine("OnEvent_DeviceDisconnected");
}
}
}