MvvmCross-based application is unable to load design-time view model

1
votes

I've found a topic describing NullReferenceException in Windows Phone designer, however I believe this is a somewhat different issue, since it happens in all platforms and not related to MvvmCross plugins.

  1. Designer (VS2012, Store/WP8/WPF apps) works fine for earlier versions of MvvmCross. I tried to compile a version that corresponds to 3.0.6 NuGet packages, and I can see that design-time model is loaded in all platforms.

  2. Recent MvvmCross version causes all designer to raise NULL reference exception ("Object reference not set to an instance of an object") on all platforms.

  3. Attempt to debug a VS instance didn't help: the call stack doesn't say much, and there is no MvvmCross code there.

  4. The error is not related to the actual view/view model: I can remove all controls from the view, and the error is stil raised. The view model constructor code is never reached.

So it looks like there's some MvvmCross code (not from MvxView) that is unsuccessfully executed in at design-time. I wonder if this is a known issue and/or if there is a workaround to get designer work.

UPDATE. I created a GitHub issue with steps to reproduce the problem: https://github.com/slodge/MvvmCross/issues/347

1
mvvmcross
I don't actually see a question here. If you think there's a bug, then please report it via GitHub issues - preferably with a test repo. If you have time, please also test with the latest beta nuget packages - as these are due to become the tip very soon. Alternatively, if there is a question you want to ask, could you possibly try editing your text here - so that it actually asks a question - currently it just reads to me as a set of statements. I don't see the actual question. - Stuart
OK. I will try to make two projects: one that works with design-time model, the other one that doesn't. I will create a GitHub issue for them. Thanks! - Vagif Abilov
@Stuart, the GitHib issue is created. - Vagif Abilov

1 Answers

2
votes

This question moved to GitHub where there was lots of discussion and a few samples - https://github.com/slodge/MvvmCross/issues/347

From these samples, I believe we concluded the Nuget release 3.0.8.1 :

  1. supports time data based on Blend-generated XML data files
  2. supports design-time data based on simple C# classes
  3. does not support design-time data where the data uses MvvmCross MvxViewModel as a base class

It isn't entirely obvious that developers should want to do the third thing (designtime data isn't real data?), but given that the overall MvvmCross manifesto broadly speaking aims to enable developers and to give them choice, then future MvvmCross will make this easier in the future.

In the meantime, if anyone wants to initialise objects at design-time, then there are a few helper classes around to enable this.

For example, the color plugin (and a small part of MvvmCross IoC) can be made available in the design-time environment using the MvxColorDesignTimeHelper - see: