Commands in MVVM

<UserControl x:Class="MVVMSample.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:local="clr-namespace:MVVMSample"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <!-- The datacontext for the usercontrol is an instance of our viewmodel. -->
    <UserControl.DataContext>
        <local:MainViewModel x:Name="viewModel" />
    </UserControl.DataContext>

    <Grid x:Name="LayoutRoot" Background="White">
        <StackPanel>
            <TextBlock Text="First Name" />
            <TextBox Margin="4" Text="{Binding FirstName, Mode=TwoWay}" />
            <TextBlock Text="Second Name" />
            <TextBox Margin="4" Text="{Binding SecondName, Mode=TwoWay}" />
            <Button Margin="4" Width="80" Content="Build Name" Command="{Binding BuildNameCommand}" />
            <TextBox Margin="4" Text="{Binding FullName}" />
        </StackPanel>
    </Grid>
</UserControl>