ConcurrentModificationException when setting ScatterLineSeries.ItemsSource in UI for Xamarin

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ConcurrentModificationException when setting ScatterLineSeries.ItemsSource

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Chart

This is a migrated thread and some comments may be shown as answers.

9 Answers, 1 is accepted

answered on 12 Dec 2018, 03:23 PM

Hi Jim,

Without the rest of the code I can't give you a definitive answer. What happens when you marshal the task's work back to the UI thread when its done:

Task ShowData()

{

return Task.Factory.StartNew(() => {

// do calculations here...

Device.BeginInvokeOnMainThread(()=> { LineSeries1.ItemsSource = _chartLines; };

}

If this doesn't help, can you please reply with the rest of the code, I particularly need to see the following:

- The chart definition (e.g. XAML)
- The methods that invoke the task (e.g. is ShowData in OnAppearing?)
- Any other code that will help me build the reprodicible (e.g. model class, view model class, etc)

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

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answered on 12 Dec 2018, 03:37 PM

Here's my chart xaml:

<chart:RadCartesianChart x:Name="ChartResults" VerticalOptions="CenterAndExpand" HorizontalOptions="CenterAndExpand" Margin="1,1,1,1" Grid.Row="0" Grid.Column="0">

<chart:RadCartesianChart.HorizontalAxis>

<chart:NumericalAxis x:Name="HorizontalAxis" />

</chart:RadCartesianChart.HorizontalAxis>

<chart:RadCartesianChart.VerticalAxis>

<chart:NumericalAxis x:Name="YAxis2" LabelFormat="0.##" />

</chart:RadCartesianChart.VerticalAxis>

<chart:RadCartesianChart.Grid>

<chart:CartesianChartGrid MajorLinesVisibility="XY" />

</chart:RadCartesianChart.Grid>

<chart:RadCartesianChart.Series>

<chart:ScatterLineSeries x:Name="LineSeries1" Stroke="CornflowerBlue" StrokeThickness="1" />

</chart:RadCartesianChart.Series>

</chart:RadCartesianChart>

ShowData is called by a Timer. The Timer is initialised at the end of the OnAppearing method:

ShowWaveformDelegate showWaveform = DoShowWaveform

MyEvent.Callback = showWaveform

Globals.MyTimer = new Timer { Interval = 1000 };

Globals.MyTimer.Elapsed += OnTimedEvent;

Globals.MyTimer.Start();

private static void OnTimedEvent(object source, ElapsedEventArgs e

{

Globals.MyTimer.Stop();

MyEvent.Run();

}

private async void DoShowWaveform(Data data)

{

await ShowWaveform(data);

}

The idea here is to update the chart every second, but when the chart is being updated, stop the timer, then restart the timers after it has been updated.

answered on 12 Dec 2018, 08:05 PM

MainPage.zip

Hi Bill,

Unfortunately, the code is not complete enough for me to recreate the exception as there are missing pieces and infrastructure. Regardless, I suspect the reason for the error is the code is multiple threads are trying to update the series simultaneously.

It's recommended that you use Device.StartTimer in Xamarin.Forms as it will use the proper clock for the device. Here's a good discussion on StackOverflow about System Timer vs Device.StartTimer.

Demo

You could use the PropertyChanged event of the series to know when the last time the ItemsSource was set before starting the 1 second timer again.

// subscribe to Propertychanged

LineSeries1.PropertyChanged += LineSeries1_PropertyChanged;

// the event handler that will fire off another 1 second timer

private void LineSeries1_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)

{

// this will be true is the ItemsSource was reset

if (e.PropertyName == nameof(ScatterLineSeries.ItemsSource))

{

// Get the next data in 1 second

Device.StartTimer(TimeSpan.FromMilliseconds(1000), OnTick);

}

To stop each timer, you return false (returning True continues the timer)

private bool OnTick()

{

Task.Run(async () =>

{

// Get Data From sensors

await Task.Delay(200);

// Set the ItemsSource

Device.BeginInvokeOnMainThread(() =>

{

LineSeries1.ItemsSource = _lineSeriesData;

});

// Stop the timer

return false;

}

With this approach you have a reliable order of operations. Here's the debug output from my attached demo to verify everything is occurringas intended:

[0:] OnTick

[0:] Reading Sensor Data

[0:] Setting ItemsSource

[0:] OnTick

[0:] Reading Sensor Data

[0:] Setting ItemsSource

[0:] OnTick

[0:] Reading Sensor Data

[0:] Setting ItemsSource

[0:] OnTick

[0:] Reading Sensor Data

[0:] Setting ItemsSource

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

answered on 13 Dec 2018, 12:08 PM

Thanks for the example. It works, but the timer isn't being retriggered after the LineSeries1.ItemSource is set for the second time. Here's my debug output:

[0:] OnTick
[0:] stop timer
[0:] do reading
[0:] show waveform
[0:] start timer
[0:] OnTick
[0:] stop timer
[0:] do reading
[0:] show waveform

My code is the same as your, except I do await ShowWaveform(data) in the OnTick method, and I set LineSeries1 in my ShowWaveform method, like this:

Device.BeginInvokeOnMainThread(() =>

{

LineSeries1.ItemsSource?.Clear();

LineSeries1.ItemsSource = _chartLines;

});

I also tried omitting the Clear line, but still no joy.

answered on 13 Dec 2018, 03:50 PM

TimerUpdatedSeries.zip

Hello Bill,

You don't need to call Clear on the ItemsSource because you're resetting the value in the next line. Since ItemsSource is a DependencyProperty, setting it replaces the previous value in the control.

I can confirm that the LineSeries's ItemsSource is working as expected (as demonstrated in my attached demo). I've reattached it here as a runnable solution so you can see this first-hand. When the page launches, the ItemsSource is updated continuously.

I'm unable to directly diagnose the issue you're having, but as you suspect it might be due to using await in the tick instead of my recommended approach (you would do the async call where I have Task.Delay).

Further Assistance using Task

Telerik Forums specifically covers the Telerik controls and their APIs, the problem you're having isn't related to the Chart's APIs. For example, if you swapped out the Chart for a plain Xamarin ListView, you'd have the same problem.

If you'd like assistance in redesigning or debugging the architecture, I recommend posting in StackOverflow or the Xamarin Forums. This isn't an uncommon threading issue and you'll be able to get assistance from the Xamarin and .NET community who've previously built solutions for this scenario.

Professional Services

If you do get completely stuck, and are unable to find any solution, we have partners who are specialized work on development problems like this and help you build out the app. You can contact them here: Professional Services - Architecture Review or Professional Services - Custom App Dev

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik

answered on 14 Dec 2018, 03:02 PM

Hello Bill,

This is what I'm referring to. The fact that PropertyChanged event is not firing, means that the logic in the task is not actually setting ItemsSource.

If you can isolate this in a separate runnable project, send that project to us in a support ticket (you can create one here). I'll ask the development team to to take a look and offer suggestions on how to restructure.

Regards,
Lance | Tech Support Engineer, Sr.
Progress Telerik