MvvmCross vnext: monodroid CommandParameter similar to wp7

Question

I use a Dictionary instead of a List in the MainMenuView from Tutorial sample. In wp7, I bind like this:

 <ListBox ItemsSource="{Binding Items}" x:Name="TheListBox">
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding Key}" Margin="12" FontSize="24" TextWrapping="Wrap">
                        <i:Interaction.Triggers>
                            <i:EventTrigger EventName="Tap">
                                <commandbinding:MvxEventToCommand Command="{Binding Path=DataContext.ShowItemCommand, ElementName=TheListBox}" CommandParameter="{Binding Value}" />
                            </i:EventTrigger>
                        </i:Interaction.Triggers>
                    </TextBlock>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

But with monodroid, I don't know where to put CommandParameter="{Binding Value}" in mvxListView, I get this error:"MvxBind:Error: 2,71 Problem seen during binding execution for from Items to ItemsSource - problem ArgumentException: failed to convert parameters" from my axml code:

<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"

/>

How can I set a CommandParameter property as in wp7 ?

Thanks in advance for your help.

Following your instruction 1, I change MainMenuViewModel in Tutorial.Core like this:

`public Dictionary Items { get; set; }

    public ICommand ShowItemCommand
    {
        get
        {
            return new MvxRelayCommand<KeyValuePair<string, Type>>((type) => DoShowItem(type.Value));
        }
    }

    public void DoShowItem(Type itemType)
    {
        this.RequestNavigate(itemType);
    }

    public MainMenuViewModel()
    {
        Items = new Dictionary<string, Type>()
                    {
                        {"SimpleTextProperty",  typeof(Lessons.SimpleTextPropertyViewModel)},
                        {"PullToRefresh",  typeof(Lessons.PullToRefreshViewModel)},
                        {"Tip",  typeof(Lessons.TipViewModel)},
                        {"Composite",typeof(Lessons.CompositeViewModel)},
                        {"Location",typeof(Lessons.LocationViewModel)}
                    };
    }`

The sample is working as expected in wp7, but with monodroid I get the same error as the previous one because I think KeyValuePair Key property cause the problem in:

<LinearLayout
        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="wrap_content"
          android:layout_margin="12dp"
        android:orientation="vertical">
<TextView
          android:layout_width="fill_parent"
          android:layout_height="wrap_content"
          android:text="View Model:"
        />
  <TextView
          android:layout_width="fill_parent"
          android:layout_height="wrap_content"
          android:textAppearance="?android:attr/textAppearanceLarge"
          local:MvxBind="{'Text':{'Path':'Key'}}"
        />
</LinearLayout>

Answer 1

Mvx doesn't currently have a separate CommandParameter dependency target so you can't currently tackle this problem in the same way.

The reason why CommandParameters aren't included is a design choice, and is tied to the lack of Behaviours. Because there isn't a behaviour object wrapping the command and the commandparameter together around a control event, then separate CommandParameter bindings would be needed for Click, LongClick, Swipe, etc. - and these could get quite verbose and ugly - so, so far, we've steered clear of this approach.

However, there are a couple of ways that you can achieve a similar effect to the one you are looking for.

---

Firstly, the ItemClick event on a list is always bound so that the parameter is always the object that has been clicked - so if you can do the .Value projection within your MvxRelayCommand action, then the code will work both in WP7 and in MonoDroid.

ie this can be achieved:

<ListBox ItemsSource="{Binding Items}" x:Name="TheListBox">
        <ListBox.ItemTemplate>
            <DataTemplate>
                <TextBlock Text="{Binding Key}" Margin="12" FontSize="24" TextWrapping="Wrap">
                    <i:Interaction.Triggers>
                        <i:EventTrigger EventName="Tap">
                            <commandbinding:MvxEventToCommand Command="{Binding Path=DataContext.ShowItemCommand, ElementName=TheListBox}" CommandParameter="{Binding}" />
                        </i:EventTrigger>
                    </i:Interaction.Triggers>
                </TextBlock>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

using:

<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"
/>

where your command handler does the .Value work:

public ShowItemCommand { get { return new MvxRelayCommand( item => { DoShowFor(item.Value); } ); } }

---

Secondly, you can instead choose to bind to the Click event on Views/Controls within each list Item, rather than binding to the List-level event. For some discussion of this, see the answers on MVVMCross changing ViewModel within a MvxBindableListView

---

Thirdly, you could write your own binding in this case if you really wanted to... I think that would be overkill for this situation, but it might be useful in other cases.

---

For more list selection examples, check out the BestSellers and CustomerManagement examples.