Binding Command of Button in AppBar does not work

Question

Why does the Command in the following example not execute?

I have a named Page with an AppBar and a Button in it:

   <Page
    x:Class="App13.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"
    mc:Ignorable="d" 
    x:Name="myPage"
    >
    <Page.BottomAppBar>
        <AppBar>
           <Button Content="OK" Command="{Binding OkCommand, ElementName=myPage}" />
        </AppBar>
    </Page.BottomAppBar>
</Page>

The Command "OkCommand" is defined in the code-behind like this (using MVVM light framework):

public RelayCommand OkCommand
{
    get
    {
        return m_OkCommand
            ?? (m_OkCommand = new RelayCommand(
                                  async () =>
                                  {
                                      await new MessageDialog("OkCommand").ShowAsync();
                                  }));
    }
}

There are no binding-errors nor any other hints in the output-window that give me an idea why this does not work. (Aside: if the button is placed outside the AppBar, everything works fine)

Does anyone have an idea what is wrong here?

Answer 1

There should be no reason for the command binding not working for a button in AppBar if it works for a button elsewhere on the page.

I suspect the problem is related to how DataContext is set. You should have it set at the page level instead of lower in the control tree. All the other buttons are inside the top content control of the page while the AppBar is outside of it causing the binding not to work if DataContext is set at the top content control or below.

You can try it with the following working example:

MainPage.xaml:

<common:LayoutAwarePage
    x:Class="App16.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:App16"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:common="using:App16.Common"
    x:Name="MyPageName"
    mc:Ignorable="d">

    <StackPanel x:Name="MainGrid" Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
        <Button Content="Ok" Command="{Binding OkCommand}" />
    </StackPanel>

    <Page.BottomAppBar>
        <AppBar x:Name="bottomAppBar" Padding="10,10,10,10"  >
            <StackPanel Orientation="Horizontal" HorizontalAlignment="Right">
                <Button Style="{StaticResource SkipBackAppBarButtonStyle}" Command="{Binding OkCommand}" >
                </Button>
            </StackPanel>
        </AppBar>
    </Page.BottomAppBar>
</common:LayoutAwarePage>

ViewModel.cs:

public class ViewModel
{
    private RelayCommand _okCommand;
    public RelayCommand OkCommand
    {
        get
        {
            return _okCommand
                ?? (_okCommand = new RelayCommand(
                                      async _ =>
                                      {
                                          await new MessageDialog("OkCommand").ShowAsync();
                                      }));
        }
    }
}

MainPage.xaml.cs:

public sealed partial class MainPage : LayoutAwarePage
{
    public MainPage()
    {
        this.InitializeComponent();
        DataContext = new ViewModel();
    }
}