Dynamic Xamarin Forms From XAML and JSON

Yes, I’m well aware that the official API set for Xamarin.Forms only allows loading the compiled XAML into its “owning” view, using the LoadFromXaml<TView>(this TView view, Type callingType) extension method from the Xamarin.Forms.Xaml.Extensions class. And this is of course the actual real and supported way of doing it (for whatever reason, I’m not part of the Xamarin.Forms team :)).

But, being just a .NET assembly, I got curious as to how they loaded that XAML in that case. So a bit of .NET Reflector revealed the internal Xamarin.Forms.Xaml.XamlLoader class, with the following nice overload:

public static void Load(BindableObject view, string xaml)

So nothing prevents us from invoking that via reflection, you know? ;). Of course that is hardly a well-performing way of developing an entire app, but if you happen to need it for something, it’s totally doable and works acceptably well IMHO. I turned it into an extension method and was back in business in no time.

Another thing I was pretty excited to try out with this new knowledge, was a way to dynamically bind data too, so that I could load a dynamic view, then a dynamic data/view model (using JSON, but why not also any arbitrary key/value dictionary?), and have the app display both, bi-directional data binding and all.

For this I did have to send a PR to the Xamarin.Forms team for a little feature contribution, and they thankfully merged it :). It’s currently available in the pre-release nuget package for Xamarin.Forms:

PM> Install-Package Xamarin.Forms -Pre

This feature allows the BindingContext to provide its own reflection via the System.Reflection.IReflectableType interface. The implementation of that interface isn’t exactly trivial (I’d have liked it better if ICustomTypeDescriptor was available for PCLs) but it’s like 99% boilerplace. So I’ve made it generic enough that it can be reused for other dynamic models. I implemented an IDictionary-based model as well as a Linq to JSON-based JsonModel.

For convenience, I’ve put it all in a Xamarin.Forms.Dynamic package that takes a dependency on the pre-release Xamarin.Forms package:

PM> Install-Package Xamarin.Forms.Dynamic -Pre

With that in place, you can simpy do:

var page = new ContentPage();
string xaml = // load XAML string from wherever.
string json = // load JSON string from wherever.
page.LoadFromXaml(xaml);
page.BindingContext = JsonModel.Parse(json);

And that will do exactly what my Xamarin Forms Player demo does :). Of course, there is a tiny bit more involving trivial SignalR + Azure in there too, to send the payloads from VS to the app on the device, but that’s also available to explore if you want to see how I did it. Quite a few asked about this at the Xamarin event we did a few days ago at Microsoft Argentina, so there you go ;)