MvxRadioGroupSelectedIndexBinding - xamarin.android

Question

i created a RadioGroupSelectedIndexBinding from the source of MvxRadioGroupSelectedItemBinding.
It works ok, but not at viewmodel startup: the binding from viewmodel to view is called but at this time the RadioGroup has no child views. It seems they haven't been inflated yet.
This is a "bug" (or feature :p) in Mvvmcross custom inflater/binding ? Or is there something to overload in MvxAndroidTargetBinding ?
Edit: code of MvxRadioGroupSelectedIndexBinding (Index, not Item: different from MvxRadioGroupSelectedItemBinding).
public class MvxRadioGroupSelectedIndexBinding : MvxAndroidTargetBinding
{
bool stopListeningCheckChanged = false;
private int selectedIndex = -2;
public int SelectedIndex
{
get { return selectedIndex; }
set { if(value != selectedIndex) { selectedIndex = value; FireValueChanged(SelectedIndex); } }
}
public static void Register(IMvxTargetBindingFactoryRegistry registry)
{
registry.RegisterCustomBindingFactory<RadioGroup>("SelectedIndex", radioGroup => new MvxRadioGroupSelectedIndexBinding(radioGroup));
}
public MvxRadioGroupSelectedIndexBinding(RadioGroup radioGroup) : base(radioGroup)
{
if (radioGroup == null)
{
Mvx.Trace(MvxTraceLevel.Error, "RadioGroup SelectedIndex: radioGroup is null");
return;
}
radioGroup.CheckedChange += CheckedChange;
radioGroup.ChildViewAdded += RadioGroupOnChildViewAdded;
}
private void RadioGroupOnChildViewAdded(object sender, ViewGroup.ChildViewAddedEventArgs childViewAddedEventArgs)
{
var radioGroup = Target as RadioGroup;
if (selectedIndex == radioGroup.ChildCount-1)
{
stopListeningCheckChanged = true;
radioGroup.Check(radioGroup.GetChildAt(selectedIndex).Id);
stopListeningCheckChanged = false;
}
}
private void CheckedChange(object sender, RadioGroup.CheckedChangeEventArgs e)
{
if (stopListeningCheckChanged)
return;
var radioGroup = Target as RadioGroup;
var checkedId = e.CheckedId;
if (checkedId == View.NoId)
{
SelectedIndex = -1;
return;
}
for (var i = radioGroup.ChildCount - 1; i >= 0; i--)
{
if (checkedId == radioGroup.GetChildAt(i).Id)
{
SelectedIndex = i;
return;
}
}
SelectedIndex = -1;
Mvx.Trace(MvxTraceLevel.Error, "RadioGroup id not found: {0}", checkedId);
}
public override void SetValue(object index)
{
var radioGroup = Target as RadioGroup;
if (radioGroup == null)
return;
stopListeningCheckChanged = true;
selectedIndex = (int)index;
if (selectedIndex < 0 || selectedIndex >= radioGroup.ChildCount)
{
radioGroup.ClearCheck();
}
else
{
radioGroup.Check(radioGroup.GetChildAt(selectedIndex).Id);
}
stopListeningCheckChanged = false;
}
public override Type TargetType
{
get { return typeof(object); }
}
protected override void SetValueImpl(object target, object value)
{
}
public override MvxBindingMode DefaultMode
{
get { return MvxBindingMode.TwoWay; }
}
protected override void Dispose(bool isDisposing)
{
if (isDisposing)
{
var radioGroup = Target as RadioGroup;
if (radioGroup != null)
{
radioGroup.CheckedChange -= CheckedChange;
radioGroup.ChildViewAdded -= RadioGroupOnChildViewAdded;
}
}
base.Dispose(isDisposing);
}
}
And usage
<RadioGroup
android:orientation="horizontal"
local:MvxBind="SelectedIndex SelectedChoiceIndex">
<RadioButton
android:text="choice 1"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
<RadioButton
android:text="choice 2"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</RadioGroup>
The "bug" is that when SetValue is called, the RadioGroup has no childs. I suppose the custom inflater create and binds views at the same time. In fact it should bind views only when their childs are inflated. I may be wrong though, i've not checked the source code. And it could have other bad side effects.

Answer 1

This is a "bug" (or feature :p) in Mvvmcross custom inflater/binding ?
Neither - I think this is simply out of scope of what the MvxRadioGroup was designed to target.
MvxRadioGroupSelectedItemBinding was a user contribution and I believe it was designed to be used exactly as shown in https://github.com/MvvmCross/MvvmCross-Tutorials/pull/8:
<MvxRadioGroup
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:textSize="40dp"
local:MvxItemTemplate="#layout/item_radio"
local:MvxBind="ItemsSource Items;SelectedItem SelectedItem"
/>
=> so it is designed to work for lists of items where the ItemsSource is first set, and then the SelectedItem is also set.
I don't believe the author of that MvxRadioGroup had any intention of supporting AXML based lists of radio choices.
With that said, if anyone wants to author a more extensive RadioGroup solution - e.g. one that can cope with AXML defined lists or that can cope with ItemsSource changes after SelectedItem has been set, then I think this should be perfectly possible to do - and it looks like you've already gone a long way towards this :) To "perfectly" handle all combinations of dynamic and static list changes, would probably require using some kind of technique which rechecks the SelectedItem property each and every time the ItemsSource and/or static items have been added. For practical reasons I think this recheck would need to be performed within some custom RadioGroup and/or binding based code - I don't think there's any way to do this within the XML inflation handlers as Android simply doesn't present any suitable childrenInflatedAndAdded type callbacks during inflation.
It may also be interesting to note that XAML presents similar challenges in this area - e.g. see Silverlight XAML Attribute Definition Order Matters for a XAML ComboBox scenario where the items must be set before the selected item.