MVVMCross changing ViewModel within a MvxBindableListView

Question

Little problem with my Android application and I don't know how to solve it with MVVM Cross.

Here is my Model

public class Article 
{
    string Label{ get; set; }
    string Remark { get; set; }
}

My ViewModel

public class ArticleViewModel: MvxViewModel
{
    public List<Article> Articles;
    ....

}

My layout.axml ...

    <LinearLayout
        android:layout_width="0dip"
        android:layout_weight="6"
        android:layout_height="fill_parent"
        android:orientation="vertical"
        android:id="@+id/layoutArticleList">
        <EditText
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:id="@+id/editSearch"
            android:text=""
            android:singleLine="True"
            android:selectAllOnFocus="true"
            android:capitalize="characters"
            android:drawableLeft="@drawable/ic_search_24"
            local:MvxBind="{'Text':{'Path':'Filter','Mode':'TwoWay'}}"
            />
      <Mvx.MvxBindableListView
            android:id="@+id/listviewArticle"
            android:choiceMode="singleChoice"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            android:orientation="vertical" 
            local:MvxItemTemplate="@layout/article_rowlayout"
            local:MvxBind="{'ItemsSource':{'Path':'Articles'}}" />                
    </LinearLayout>
...

And here comes my problem, the "article_rowlayout"

...
<TableRow
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@color/blue">
        <TextView
            android:id="@+id/rowArticleLabel"
            android:layout_width="0dip"
            android:layout_weight="14"
            android:layout_height="wrap_content"
            android:textSize="28dip"
            local:MvxBind="{'Text':{'Path':'Label'}}" />
        <ImageButton
            android:src="@drawable/ic_modify"
            android:layout_width="0dip"
            android:layout_weight="1"
            android:layout_height="wrap_content"
            android:id="@+id/rowArticleButtonModify"
            android:background="@null" 
            android:focusable="false"
            android:clickable="true"    
            local:MvxBind="{'Click':{'Path':'MyTest'}}"
          />
...

The "Click" command called "MyTest" is linked on the item given by the MvxBindableListView. In other words, Click search for a command "MyTest" in my model "Article", instead of my ViewModel. How can I change that behaviour in order to link my ViewModel "ArticleViewModel" which is responsible of my MvxBindableListView?

Any suggestions?

Answer 1

Your analysis is definitely correct about where the click event is trying to bind.

There are two approaches I generally take:

1. Use ItemClick on the List
2. Continuing using Click but do some redirection on the ViewModel side.

---

So...1

The Main Menu in the tutorial has a ViewModel a bit like:

public class MainMenuViewModel
    : MvxViewModel
{
    public List<T> Items { get; set; }

    public IMvxCommand ShowItemCommand
    {
        get
        {
            return new MvxRelayCommand<T>((item) => /* do action with item */ );
        }
    }
}

This is used in axml as:

<Mvx.MvxBindableListView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:local="http://schemas.android.com/apk/res/Tutorial.UI.Droid"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    local:MvxBind="{'ItemsSource':{'Path':'Items'},'ItemClick':{'Path':'ShowItemCommand'}}"
    local:MvxItemTemplate="@layout/listitem_viewmodel"
  />

This approach can only be done for ItemClick on the whole list item - not on individual subviews within the list items.

---

Or...2

Since we don't have any RelativeSource binding instructions in mvx, this type of redirection can be done in the ViewModel/Model code.

This can be done by presenting a behaviour-enabled wrapper of the Model object rather than the Model object itself - e.g. using a List<ActiveArticle>:

public ActiveArticle
{
   Article _article;
   ArticleViewModel _parent;

   public WrappedArticle(Article article, ArticleViewModel parent)
   {
       /* assignment */
   }

   public IMvxCommand TheCommand { get { return MvxRelayCommand(() -> _parent.DoStuff(_article)); } }

   public Article TheArticle { get { return _article; } } 
}

Your axml would then have to use bindings like:

    <TextView            ...
        local:MvxBind="{'Text':{'Path':'TheArticle.Label'}}" />

and

    <ImageButton
        ...
        local:MvxBind="{'Click':{'Path':'TheCommand.MyTest'}}" />

One example of this approach is the Conference sample which uses WithCommand

However... please note that when using WithCommand<T> we discovered a memory leak - basically the GarbageCollection refused to collect the embedded MvxRelayCommand - which is why WithCommand<T> is IDisposable and why BaseSessionListViewModel clears the list and disposes the WithCommand elements when views are detached.

---

Update after comment:

If your data list is large - and your data is fixed (your articles are models without PropertyChanged) and you don't want to incur the overhead of creating a large List<WrappedArticle> then one way around this might be to use a WrappingList<T> class.

This is very similar to the approach taken in Microsoft code - e.g. in virtualizing lists in WP7/Silverlight - http://shawnoster.com/blog/post/Improving-ListBox-Performance-in-Silverlight-for-Windows-Phone-7-Data-Virtualization.aspx

For your articles this might be:

public class ArticleViewModel: MvxViewModel
{
    public WrappingList<Article> Articles;

    // normal members...
}

public class Article
{
    public string Label { get; set; }
    public string Remark { get; set; }
}

public class WrappingList<T> : IList<WrappingList<T>.Wrapped>
{
    public class Wrapped
    {
        public IMvxCommand Command1 { get; set; }
        public IMvxCommand Command2 { get; set; }
        public IMvxCommand Command3 { get; set; }
        public IMvxCommand Command4 { get; set; }
        public T TheItem { get; set; }
    }

    private readonly List<T> _realList;
    private readonly Action<T>[] _realAction1;
    private readonly Action<T>[] _realAction2;
    private readonly Action<T>[] _realAction3;
    private readonly Action<T>[] _realAction4;

    public WrappingList(List<T> realList, Action<T> realAction)
    {
        _realList = realList;
        _realAction = realAction;
    }

    private Wrapped Wrap(T item)
    {
        return new Wrapped()
            {
                Command1 = new MvxRelayCommand(() => _realAction1(item)),
                Command2 = new MvxRelayCommand(() => _realAction2(item)),
                Command3 = new MvxRelayCommand(() => _realAction3(item)),
                Command4 = new MvxRelayCommand(() => _realAction4(item)),
                TheItem = item
            };
    }

    #region Implementation of Key required methods

    public int Count { get { return _realList.Count; } }

    public Wrapped this[int index]
    {
        get { return Wrap(_realList[index]); }
        set { throw new NotImplementedException(); }
    }

    #endregion

    #region NonImplementation of other methods

    public IEnumerator<Wrapped> GetEnumerator()
    {
        throw new NotImplementedException();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public void Add(Wrapped item)
    {
        throw new NotImplementedException();
    }

    public void Clear()
    {
        throw new NotImplementedException();
    }

    public bool Contains(Wrapped item)
    {
        throw new NotImplementedException();
    }

    public void CopyTo(Wrapped[] array, int arrayIndex)
    {
        throw new NotImplementedException();
    }

    public bool Remove(Wrapped item)
    {
        throw new NotImplementedException();
    }

    public bool IsReadOnly { get; private set; }

    #endregion

    #region Implementation of IList<DateFilter>

    public int IndexOf(Wrapped item)
    {
        throw new NotImplementedException();
    }

    public void Insert(int index, Wrapped item)
    {
        throw new NotImplementedException();
    }

    public void RemoveAt(int index)
    {
        throw new NotImplementedException();
    }

    #endregion
}