MvxImageViewLoader binding not working

Question

I use MvxImageViewLoader in pretty simple situation.

On ViewDidLoad:

        var pnlBackImage = new UIImageView(new RectangleF(0, 0, pnlBack.Frame.Width, pnlBack.Frame.Height));
        Add(pnlBackImage);
        pnlBackImageLoader = new MvxImageViewLoader(() => pnlBackImage);

        ...

        set.Bind(pnlBackImageLoader).To(x => x.UserBackgroundUrl);

where UserBackgroundUrl is a common string property.

But on set.Apply() I see the following in my log and image is not loading:

2013-06-24 13:51:26.999 SPBTouch[446:21b03] MvxBind: Error:   2.78 Problem seen during binding execution for from UserBackgroundUrl to ImageUrl - problem TargetInvocationException: Exception has been thrown by the target of an invocation.
      at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x000d9] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:236 
  at System.Reflection.MonoProperty.SetValue (System.Object obj, System.Object value, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] index, System.Globalization.CultureInfo culture) [0x00064] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System.Reflection/MonoProperty.cs:353 
  at System.Reflection.PropertyInfo.SetValue (System.Object obj, System.Object value, System.Object[] index) [0x00000] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System.Reflection/PropertyInfo.cs:93 
  at Cirrious.MvvmCross.Binding.Bindings.Target.MvxPropertyInfoTargetBinding.SetValue (System.Object value) [0x00000] in <filename unknown>:0 
  at Cirrious.MvvmCross.Binding.Bindings.MvxFullBinding.UpdateTargetFromSource (Boolean isAvailable, System.Object value) [0x00000] in <filename unknown>:0 
InnerException was NullReferenceException: Object reference not set to an instance of an object
      at Cirrious.MvvmCross.Binding.Views.MvxBaseImageViewLoader`1[MonoTouch.UIKit.UIImage].set_ImageUrl (System.String value) [0x00000] in <filename unknown>:0 
  at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
  at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) [0x000c0] in /Developer/MonoTouch/Source/mono/mcs/class/corlib/System.Reflection/MonoMethod.cs:228 

I've even defined the image loader and even the image panel as the view fields, but there is still no luck. What can cause that problem?

Thank you.

Answer 1

The inner exception error you've listed is:

NullReferenceException: Object reference not set to an instance of an object at Cirrious.MvvmCross.Binding.Views.MvxBaseImageViewLoader`1[MonoTouch.UIKit.UIImage].set_ImageUrl (System.String value) [0x00000] in :0

I guess the thing that would cause a NullReferenceException in the set of:

    public string ImageUrl
    {
        get { return _imageHelper.ImageUrl; }
        set { _imageHelper.ImageUrl = value; }
    }

is if _imageHelper is null

Looking in MvxBaseImageViewLoader.cs#L31 this can happen if this error message is displayed Unable to resolve the image helper - have you referenced and called EnsureLoaded on the DownloadCache plugin?

Is this error message displayed? If so, is the DownloadCache plugin (and it's dependent plugin File) loaded in your application?

---

If this doesn't help, try comparing your app so some of the apps in N+1. Successful use of the image views is covered in several of N+1 videos - http://mvvmcross.wordpress.com/ - e.g. N=2 or N=3

Answer 2

I come here when I had the same error (exception) mentioned in this question by following the tutorial on MvvmCross CollectionView example. I found that you get this error if you missed registering/installing your plugins. I resolved the error by installing the following plugins:

Install-Package MvvmCross.ITSparta.Plugin.DownloadCache

Install-Package MvvmCross.ITSparta.Plugin.File

When these plugins are installed, new files are created in your UI Project (iOS/Android) and these plugins are registered. Thus, you would not get this error.