The other approaches to set the data-binding

Using the inherent Command of TextCell

The data-binding mechanism of XAML world is flexible. Actually, we have more than one way to achieve our goal. For example, if you only use TextCell to show the menu item, there is a simple way to do the navigation. As I said in the previous section, TextCell supports ICommand inherently. So we can use the data-binding syntax like this:

<DataTemplate>
<TextCell Text="{Binding}" Command="{Binding BindingContext.DataContext.ShowDetailPageAsyncCommand, Source={x:Reference MainContent}}" CommandParameter="{Binding}"></TextCell>
</DataTemplate>

This way is easier. When the user taps the TextCell, it will trigger the Command. But the disadvantage is that you cannot customize the UI of the menu item. TextCell only supports the text. If you would like to add some images or define a complex item layout, you must use ViewCell.

Using Bahaviors

Also, you might think we can use ItemSelectedor ItemTapped events. Definitely, we can! But unfortunately, these events do not implement ICommand interface, so we cannot use the data-binding directly. To use the ICommand binding, we need to use a Behavior to convert an event to a command, which is described here: Reusable EventToCommandBehavior.

You might be not familiar with Behaviors. Behavior comes from Blend SDK, which is a very useful library in XAML world. The behaviors can be attached to some controls and listen to some events, then invoke some commands in the ViewModel. It is a good way to add the support for Command pattern for those controls that were not designed to interact with commands. So we can use MVVM pattern gracefully, instead of using event handlers in the code-behind files.

You can follow the instructions in the official documentation to create your EventToCommandBehavior, but we can leverage a third-party library to do it quickly: Behaviors.Xamarin.Forms.Netstandard. It is not the official project, but is easy to use. You can install it to the MvxFormsMasterDetailApp.UI project by searching Behaviors.Xamarin.Forms through the NuGet Package Manager:

1546892619292

We can use this library to enable the ListView control to trigger our command in the ViewModel when selecting the item. To do this, add a bindable property called SelectedMenuItem in the MenuViewModel.cs file, which is used to indicate the current selected item, like this:

#region SelectedMenuItem;
private string _selectedMenuItem;
public string SelectedMenuItem
{
get => _selectedMenuItem;
set => SetProperty(ref _selectedMenuItem, value);
}
#endregion

Replace the ShowDetailPageAsyncCommand region we created in the previous section by the code shown below:

#region ShowDetailPageAsyncCommand;
private IMvxAsyncCommand _showDetailPageAsyncCommand;
public IMvxAsyncCommand ShowDetailPageAsyncCommand
{
get
{
_showDetailPageAsyncCommand = _showDetailPageAsyncCommand ?? new MvxAsyncCommand(ShowDetailPageAsync);
return _showDetailPageAsyncCommand;
}
}
private async Task ShowDetailPageAsync()
{
// Implement your logic here.
switch (SelectedMenuItem)
{
case "Contacts":
await _navigationService.Navigate<ContactsViewModel>();
break;
case "Todo":
await _navigationService.Navigate<TodoViewModel>();
break;
default:
break;
}
if (Application.Current.MainPage is MasterDetailPage masterDetailPage)
{
masterDetailPage.IsPresented = false;
}
else if (Application.Current.MainPage is NavigationPage navigationPage
&& navigationPage.CurrentPage is MasterDetailPage nestedMasterDetail)
{
nestedMasterDetail.IsPresented = false;
}
}
#endregion

Did you find the difference? I removed the parameter from the command and use SelectedMenuItem property in the ShowDetailPageAsync method. Next, we need to set the data-binding for the SelectedItem of the ListView. Open MenuPage.xaml file in the Pages folder in the MvxFormsMasterDetailApp.UI project, remove current ListView control, and add a new ListView as shown below:

<ListView x:Name="MenuList" ItemsSource="{Binding MenuItemList}"
SelectedItem="{Binding SelectedMenuItem, Mode=TwoWay}">
<ListView.ItemTemplate>
<DataTemplate>
<TextCell Text="{Binding}"></TextCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>

By the following code SelectedItem="{Binding SelectedMenuItem, Mode=TwoWay}", we can set a TwoWay data-binding between the SelectedItem of the ListView and the SelectedMenuItem property in the ViewModel.

Import the namespace for the Behavior in the views:MvxContentPage definition by adding the following code: xmlns:behaviors="clr-namespace:Behaviors;assembly=Behaviors". Now we can use the behaviors prefix to use the behaviors in the library. Update the XMAL of the ListView as shown below:

<ListView x:Name="MenuList" ItemsSource="{Binding MenuItemList}"
SelectedItem="{Binding SelectedMenuItem, Mode=TwoWay}">
<ListView.Behaviors>
<behaviors:EventHandlerBehavior EventName="ItemSelected">
<behaviors:InvokeCommandAction
Command="{Binding BindingContext.DataContext.ShowDetailPageAsyncCommand,
Source={x:Reference MainContent}}"></behaviors:InvokeCommandAction>
</behaviors:EventHandlerBehavior>
</ListView.Behaviors>
<ListView.ItemTemplate>
<DataTemplate>
<TextCell Text="{Binding}"></TextCell>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>

I place a Behaviors section for the ListView control. There are one behavior called EventHandlerBehavior, which will be trigged by the ItemSelected event. In the behavior, there is a InvokeCommandAction, which will invoke the ShowDetailPageAsyncCommand in the ViewModel. Notice the data-binding syntax. We need to specify the Source and the Path for the binding. If you just use {Binding ShowDetailPageAsyncCommand}, it will not work. So be careful the current BindingContext for your control.

Run the apps for three platforms and you will see it works as expected. You can choose any approach to implement the menu function. I just want to show you how to do it in different ways. Maybe you will use them for other scenarios.