MvvmCross ViewTypeResolver doesn't resolve Tag (fragment or custom type)

Question

Actual situation is, that I added to the MvxViewTypeResolver class the "Fragment"-Case, so it does look like this:

 #region Copyright
// <copyright file="MvxViewTypeResolver.cs" company="Cirrious">
// (c) Copyright Cirrious. http://www.cirrious.com
// This source is subject to the Microsoft Public License (Ms-PL)
// Please see license.txt on http://opensource.org/licenses/ms-pl.html
// All other rights reserved.
// </copyright>
// 
// Project Lead - Stuart Lodge, Cirrious. http://www.cirrious.com
#endregion

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Android.Views;
using Cirrious.MvvmCross.Binding.Android.Interfaces.Binders;

namespace Cirrious.MvvmCross.Binding.Android.Binders
{
    public class MvxViewTypeResolver : IMvxViewTypeResolver
    {
        private Dictionary<string, Type> _cache = new Dictionary<string, Type>();

        public IDictionary<string, string> ViewNamespaceAbbreviations { get; set; }

        #region IMvxViewTypeResolver Members

        public virtual Type Resolve(string tagName)
        {
            Type toReturn;
            if (_cache.TryGetValue(tagName, out toReturn))
                return toReturn;

            var unabbreviatedTagName = UnabbreviateTagName(tagName);

            var longLowerCaseName = GetLookupName(unabbreviatedTagName);
            var viewType = typeof(View);

#warning AppDomain.CurrentDomain.GetAssemblies is only the loaded assemblies - so we might miss controls if not already loaded
            var query = from assembly in AppDomain.CurrentDomain.GetAssemblies()
                        from type in assembly.GetTypes()
                        where viewType.IsAssignableFrom(type)
                        where (type.FullName ?? "-").ToLowerInvariant() == longLowerCaseName
                        select type;

            toReturn = query.FirstOrDefault();
            _cache[tagName] = toReturn;

            return toReturn;
        }

        private string UnabbreviateTagName(string tagName)
        {
            var filteredTagName = tagName;
            if (ViewNamespaceAbbreviations != null)
            {
                var split = tagName.Split(new char[] {'.'}, 2, StringSplitOptions.RemoveEmptyEntries);
                if (split.Length == 2)
                {
                    var abbreviate = split[0];
                    string fullName;
                    if (ViewNamespaceAbbreviations.TryGetValue(abbreviate, out fullName))
                    {
                        filteredTagName = fullName + "." + split[1];
                    }
                }
            }
            return filteredTagName;
        }

        #endregion

        protected string GetLookupName(string tagName)
        {
            var nameBuilder = new StringBuilder();

            switch (tagName)
            {
                case "View":
                case "ViewGroup":
                    nameBuilder.Append("android.view.");
                    break;
                case "fragment":
                    nameBuilder.Append("android.app.");
                    break;
                default:
                    if (!IsFullyQualified(tagName))
                        nameBuilder.Append("android.widget.");
                    break;
            }

            nameBuilder.Append(tagName);
            return nameBuilder.ToString().ToLowerInvariant();
        }

        private static bool IsFullyQualified(string tagName)
        {
            return tagName.Contains(".");
        }
    }
}

Now it is submitting the correct longLowerCaseTagName (android.app.fragment) but in the query it isn't able to resolve the type.

My suggestion is, that the fragment-control isn't loaded when the type should be resolved. Maybe there is an other way to get the type resolved?

Also if I add a custom type (giving the tag Mvx.MyCustomType in the axml) it doesn't get resolved. Do I have to add something in the MvxBindingAttributes.xml in this case?

Thanks for the help!

Answer 1

First an explanation of the code:

The custom XML inflater factory used by the MvvmCross Binder tries to load Views in a very similar way to the standard 2.x Android XML inflater.

The default code for the view type resolution is indeed in: https://github.com/slodge/MvvmCross/blob/master/Cirrious/Cirrious.MvvmCross.Binding/Android/Binders/MvxViewTypeResolver.cs

If your xml contains a name such as <MyCompany.MyProject.MyViews.MyFirstView /> then the view type resolver:

• first checks for abbreviations and expands these into full namespaces - by default the only known abbreviation is Mvx. which is expanded to: Cirrious.MvvmCross.Binding.Android.Views.. If you want to add more abbrevations then override ViewNamespaceAbbreviations in https://github.com/slodge/MvvmCross/blob/master/Cirrious/Cirrious.MvvmCross.Binding/Android/MvxBaseAndroidBindingSetup.cs

• then checks to see if the unabbreviated name is a non-namespaced name. If it is, then it assumes that the class is the Android namespace and prepends it with android.view. or android.widget.

• then converts the fully namespaced name to all lowercase as a case-insensitive lookup key

• uses that lowercase key to search all Types which derive from View in all loaded assemblies.

• caches the result (whether its null or not) in order to speed up subsequent inflations.

All of this behaviour was designed to match the default Android xml view inflation code in http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/2.3.6_r1/android/view/LayoutInflater.java#LayoutInflater.createViewFromTag%28java.lang.String%2Candroid.util.AttributeSet%29

---

With that explanation out of the way - here's an answer to your questions:

---

MvvmCross does not yet currently contain any Fragment support. The official MonoDroid fragment support itself was only released last week, and I've not yet had anybody request fragments - Android "fragmentation" seems to have kept most people back on Activity and Dialog based code.

Briefly ;ooking at the documentation, fragment isn't an Android View - it looks like Fragment inherits directly from Java.Lang.Object - see http://developer.android.com/reference/android/app/Fragment.html

Because of this, there's no way that the MvvmCross ViewTypeResolver will currently work with fragments.

I would suggest that if you need both mvvmcross and fragments today, then your best bet is to replace the default resolver (using IoC) with your own resolver - but I can't offer much advice on this as I haven't yet fully read and understood the droid docs on http://developer.android.com/guide/topics/fundamentals/fragments.html

From my experience in creating the current inflation code, then I think you will find the source essential reading when you do this - e.g. see : http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/4.0.1_r1/android/view/LayoutInflater.java#LayoutInflater.createViewFromTag%28android.view.View%2Cjava.lang.String%2Candroid.util.AttributeSet%29

I can't give you any information on when official mvvmcross fragment support will be available - it's not something that is currently scheduled.

---

Custom views are supported, but will not normally live in the Mvx. abbreviated namespace.

They are much more likely to live in your UI application namespace, or in some shared library.

To see a custom view in action, see the PullToRefresh example in the tutorial - https://github.com/slodge/MvvmCross/blob/master/Sample%20-%20Tutorial/Tutorial/Tutorial.UI.Droid/Resources/Layout/Page_PullToRefreshView.axml