Damian Mehers' Blog Android, VR and Wearables from Geneva, Switzerland.

6Jul/160

Xamarin Media Plugin error: Only one operation can be active at at time

I've been getting System.InvalidOperationException: Only one operation can be active at a time in a Xamarin app I've created which uses the Media Plugin, and finally figured out why I was getting it. I was being spectacularly stupid.

I was triggering the taking of a photo in a form's Appearing event handler

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:behaviors="clr-namespace:QuickNote.Behaviors;assembly=QuickNote"
             xmlns:viewModels="clr-namespace:QuickNote.Shared.ViewModels;assembly=QuickNote"
             BindingContext="{x:Static viewModels:Locator.ExecuteQuickNote}"
             x:Class="QuickNote.ExecuteQuickNotePage" >
  <ContentPage.Behaviors>
    <behaviors:EventToCommandBehavior EventName="Appearing" Command="{Binding LoadCommand}" />
  </ContentPage.Behaviors>
  <StackLayout Orientation="Vertical">
      ...

The relevant line is the binding to the LoadCommand in the view model, which looked like this:

      LoadCommand = new Command(async () => {
        var options = new StoreCameraMediaOptions();
        using(var file = await CrossMedia.Current.TakePhotoAsync(options))
        {
          if (file == null) {
            Debug.WriteLine("No photo");
            return;
          }
          Debug.WriteLine("Got a photo");
        }
      });

The behavior I was seeing was that when the form loaded, the camera started, I took a photo, tapped the Use Photo button and then the app crashed with System.InvalidOperationException: Only one operation can be active at a time.

Can you guess why? I finally realized that after taking the photo it was re-displaying the form, causing the appearing event to be fired again, and thus causing a new photo to be taken while the old one was being taken. Hence the crash. D'oh.

My clue was that I discovered that by inserting a await Task.Yield(); at the start of the LoadCommand delegate, it stopped the crash, but started the camera again after I'd finished taking a photo.

The solution was to add a flag which I checked to ensure I didn't run the command more than once:

      LoadCommand = new Command(async () => {
        if(_loaded) return;
        _loaded = true;
            ...

The error was perfectly correct, I was causing more than one "take photo" operation to be active at the same time, I just didn't realize why.

Comments (0) Trackbacks (0)

No comments yet.


Leave a comment

No trackbacks yet.