Caliburn.Micro: Grid not getting binded/linked to x:Name

0
votes

So I'm developing a Windows Phone 8 app with the Caliburn.Micro framework. I'm trying to create a grid where I, at runtime add/remove elements such as TextBlock's at runtime. I've tried a few things to bind my code to the x:Name but nothing has worked so far.

So one of the things i tried was having a placeholder grid in my xaml aka View:

    <Grid x:Name="ContentPanel" Margin="0,97,0,0" Grid.RowSpan="2">
    </Grid>

And then i my ViewModel i use the following to bind my ContentPanel Grid:

    private Grid contentPanel;
    public Grid ContentPanel
    {
        get
        {
            return contentPanel;
        }
        set
        {
            contentPanel = value;
            NotifyOfPropertyChange(() => ContentPanel);
        }
    }

I then created a TextBlock to add to the grid:

TextBlock txt1 = new TextBlock();
txt1.Text = "2005 Products Shipped";
txt1.FontSize = 20;
txt1.FontWeight = FontWeights.Bold;
Grid.SetRow(txt1, 1);

And finally i added the TextBlock to my Grid:

ContentPanel.Children.Add(txt1);

When i run this code ContentPanel turn out to be equals null, why is that? Shouldn't Caliburn auto bind ContentPanel x:Name="ContentPanel" with the property ContentPanel?

I would appreciate your help in this matter.

My core problem, that i need solved is this: I got a login page in my app where i show some pictures and text loaded from a server. As you can see below this is done with Image and a TextBlock When that server is offline or the wi-fi simply aren't enabled i want to replace this picture+text with a static image. Aka i want to remove the TextBlock from the StackPanel.

The part where i load and show the stuff form my server works great and looks like this in my xaml:

<StackPanel Orientation="Horizontal" Background="White" DataContext="{Binding FeedItemsAnnounce,Mode=TwoWay}" >
<Image delay:LowProfileImageLoader.UriSource="{Binding ImagePath,Mode=TwoWay}" Margin="5" Width="170" Height="138">
    <i:Interaction.Triggers>
        <i:EventTrigger
        EventName="Tap">
            <cm:ActionMessage
        MethodName="LoadAnnouncement">
                <cm:Parameter Value="{Binding Link}"></cm:Parameter>
            </cm:ActionMessage>
        </i:EventTrigger>
    </i:Interaction.Triggers>
</Image>
<TextBlock Text="{Binding Title}" TextWrapping="Wrap" Width="160" Foreground="Black" FontSize="24" VerticalAlignment="Center" Margin="25,0,0,0"></TextBlock>
<i:Interaction.Triggers>
    <i:EventTrigger
        EventName="Tap">
        <cm:ActionMessage
        MethodName="LoadAnnouncement">
            <cm:Parameter Value="{Binding Link}"></cm:Parameter>
        </cm:ActionMessage>
    </i:EventTrigger>
</i:Interaction.Triggers>

So when the server is offline/wifi disabled i want to replace that with. so that the TextBlock is no longer there:

<Image  delay:LowProfileImageLoader.UriSource="{Binding ImagePath,Mode=TwoWay}" DataContext="{Binding FeedItemsAdvertisement,Mode=TwoWay}" Margin="0,20,0,39" Width="380" Height="128">
<i:Interaction.Triggers>
    <i:EventTrigger
        EventName="Tap">
        <cm:ActionMessage
        MethodName="LoadAdvertisement" >
            <cm:Parameter Value="{Binding Link}"></cm:Parameter>
        </cm:ActionMessage>
    </i:EventTrigger>
</i:Interaction.Triggers>

Is this even possible? If not what would the best semi-solution be?

EDIT 1: I've managed to setup the flow following the instructions from the accepted answer. But my BooleanToVisibilityConverter is not called, though my NotifyOfPropertyChange(() => IsConnectionAvailable); is getting called.

My Property:

private bool _isConnectionAvailable;

public bool IsConnectionAvailable
{
    get { return _isConnectionAvailable; }
    set
    {
        if (_isConnectionAvailable != value)
        {
            _isConnectionAvailable = value;
            NotifyOfPropertyChange(() => IsConnectionAvailable);
        }
    }
}

How i change the bool: This code is called in my constructor for my ViewModel(just as a test to see if it was working):

IsConnectionAvailable = false;

TextBlock (without trigger code cause its the same as previous):

<TextBlock Text="{Binding Title}" Visibility="{Binding IsConnectionAvailable, Converter={StaticResource BoolToVisibility}}" TextWrapping="Wrap" Width="160" Foreground="Black" FontSize="24" VerticalAlignment="Center" Margin="25,0,0,0"></TextBlock>

It's like the Binding IsConnectionAvailable isn't working because i can change the name IsConnectionAvailable in my Xaml to anything and my NotifyOfPropertyChange(() => IsConnectionAvailable); will still be called.

Any ideas?

I can't even do a normal bind Visibility="{Binding Path=IsVisibil,Mode=TwoWay} to a public Visibility IsVisibil property. I've done this in other classes, but even this won't work??

EDIT 2: The problem that course the binding not to work, seems to lie somewhere in this code:

<StackPanel Orientation="Horizontal" Background="White" DataContext="{Binding FeedItemsAnnounce,Mode=TwoWay}" >
                <Image delay:LowProfileImageLoader.UriSource="{Binding ImagePath,Mode=TwoWay}" Margin="5" Width="170" Height="138">
                    <i:Interaction.Triggers>
                        <i:EventTrigger
                        EventName="Tap">
                            <cm:ActionMessage
                        MethodName="LoadAnnouncement">
                                <cm:Parameter Value="{Binding Link}"></cm:Parameter>
                            </cm:ActionMessage>
                        </i:EventTrigger>
                    </i:Interaction.Triggers>
                </Image>
                <TextBlock Text="{Binding Title}" Visibility="{Binding Path=IsVisibil,Mode=TwoWay}" TextWrapping="Wrap" Width="160" Foreground="Black" FontSize="24" VerticalAlignment="Center" Margin="25,0,0,0"></TextBlock>
                <i:Interaction.Triggers>
                    <i:EventTrigger
                        EventName="Tap">
                        <cm:ActionMessage
                        MethodName="LoadAnnouncement">
                            <cm:Parameter Value="{Binding Link}"></cm:Parameter>
                        </cm:ActionMessage>
                    </i:EventTrigger>
                </i:Interaction.Triggers>
            </StackPanel>

Solution to EDIT 1 and 2: I created an x:Name"Root" at the top of my xaml structure. Then changed the binding to:

ElementName=Root, Path=DataContext.IsVisibil

This is needed because the binding to visibility that I'm trying to set is inside another DataContxt.

1
c#xamlwindows-phone-8caliburn.micro

1 Answers

5
votes

This isn't the correct way to use CM, there are a number of areas where you are confusing the model and viewmodel and the binding functionality in CM.

What you are doing currently

You are attempting to have the CM framework look for a property called ContentPanel on your ViewModel and automatically figure out what properties on Grid to bind it to...

This won't work because of a few reasons:

  1. I don't think there is a convention for Grid in CM - it's not really bindable in an obvious way (it's a layout container)
  2. Grid is not a data enabled control - it doesn't know how to consume a collection and display dynamic rows out the box (it's a layout container)
  3. What you are doing doesn't really make any sense (you have an instance of a grid in your UserControl and you have also instantiated a grid in your ViewModel - these are two separate instances of a control - you can't 'bind' them together - that's not how it all works)

CM and Bindings

When you using element name bindings e.g. x:Name with CM, it attempts to find a property on the ViewModel which matches the element name. At this point, depending on the conventions setup for the source control in question, CM will attempt to automagically wire up all the bits and pieces.

There are default conventions contained in ConventionManager which determine which properties to bind when you use element name bindings - e.g. for TextBlock, the Text property on the TextBlock is bound to the target property on the ViewModel.

http://caliburnmicro.codeplex.com/SourceControl/latest#src/Caliburn.Micro.Platform/ConventionManager.cs - look at the class constructor on ConventionManager to see the out of the box conventions - there isn't one for Grid

Once a target property is found, CM will bind it up.

(As an aside: it's worth noting that if the control type is a ContentControl CM will do some composition magic so you can have viewmodels that contain other viewmodels and have a composition all bound up at runtime - great for screens which have multiple sub-windows etc)

The problem you have is that there is no convention setup for Grid out of the box - this is most likely because a Grid in SL/WPF is primarily used for layout, and is not really a 'data container' or data aware in any way (apart from the few dependency properties you can bind to) - i.e. I don't think it's possible to bind to a grid and get a dynamic number of columns/rows without some customisation to the control, hence the omission of any conventions

(think about it - if you are binding a grid to a collection, what should the grid do... add rows or columns? It can't really be supported in a sensible way)

Now bringing it back to SL/WPF for a sec:

Usually if you want a variable list of items you will need to bind to the ItemsSource property of a control which inherits from ItemsControl (or ItemsControl itself).

Many controls do this: if they need to display a dynamic number of items they will usually inherit from ItemsControl.

How does this tie in with CM?

Caliburn Micro knows how to bind up ItemsControl out of the box. This means you can have a property on your ViewModel containing a collection of items and after binding you get a dynamic view of these at runtime

For example - a CM bound ItemsControl might look like this:

<ItemsControl x:Name="TextItems">
  <!-- host the items generated by this ItemsControl in a grid -->
  <ItemsControl.ItemsPanel>
    <ItemsPanelTemplate>
      <Grid/>
    </ItemsPanelTemplate>
  </ItemsControl.ItemsPanel>
  <!-- render each bound item using a TextBlock-->
  <ItemsControl.ItemTemplate>
    <DataTemplate>
      <TextBlock Text="{Binding SomeTextualProperty}"/>
    </DataTemplate>
  </ItemsControl.ItemTemplate>
</ItemsControl>

Now you just need a collection of objects to bind this to - each item in the collection becomes a new item in the control with its DataContext pointing to the bound item. I've made the assumption that you would want each item to be a ViewModel which contained the property SomeTextualProperty - I've defined that here...

// Provides a viewmodel for a textual item
public class TextItemViewModel
{
    public string SomeTextualProperty { get; set;}
}

The VM that should contain the list of items would need to have a collection to bind against.

(Note: Since you are adding items to it at runtime you need to tell the UI when the collection changes - ObservableCollection gives you this for free as it implements collection changed notification events)

// This is the viewmodel that contains the list of text items
public class ScreenViewModel 
{
    public ObservableCollection<TextItemViewModel> TextItems { get; set; }
}

What else I would consider the incorrect approach

Your ViewModels shouldn't know about your View implementation i.e. they shouldn't reference any type of controls unless absolutely necessary (I can't think of a time when I had to put a control in a VM). ViewModels should model the view - but they shouldn't really need to know any specifics about what that view contains - this way they are more easily testable and they are easily reused

If you follow the above approach, you can get away with providing an application which re-uses the set of viewmodels, but provides different views for each. You can try this by replacing ItemsControl with another type of control in the view (as long as it's data aware such as a datagrid) and the VM will still work - the VM is view agnostic.

Your use of Grid in your VM is not ideal because Grid is a visual control, it is not data. Remember that the visuals are your View and the ViewModel should just contain data and events which notify the view of things happening

If I was doing this - the code would look more like the code I posted above.

To sum up

  • Model the information you wanted to show in a ViewModel (TextItemViewModel)
  • Add a collection of these objects to the main ViewModel (ScreenViewModel) using a change aware collection such as ObservableCollection
  • Add/remove items from the collection using the standard add/remove
  • Bind the ItemsControl in the view using x:Name bindings to the collection on your ScreenViewModel
  • Adding/removing items in the VM will fire property changed notifications. ItemsControl will watch for these events and update itself accordingly

Addendum

You could get away with just using an ObservableCollection<string> instead of a TextBlockViewModel but it's not clear if you want to add more properties to the items you are binding to the grid (such as IsHeading property for headings which you could then make bold/italic in the view)

If you want to just use strings just modify the DataTemplate to bind directly to the DataContext rather than a property on the DataContext

<ItemsControl x:Name="TextItems">
  <!-- host the items generated by this ItemsControl in a grid -->
  <ItemsControl.ItemsPanel>
    <ItemsPanelTemplate>
      <Grid/>
    </ItemsPanelTemplate>
  </ItemsControl.ItemsPanel>
  <!-- render each bound item using a TextBlock-->
  <ItemsControl.ItemTemplate>
    <DataTemplate>
      **<TextBlock Text="{Binding}"/> <!-- Bind direct -->**
    </DataTemplate>
  </ItemsControl.ItemTemplate>
</ItemsControl>

Edit:

Ok in your case it's quite simple - your ViewModel should simply model the state of the server:

public class LoginPageViewModel
{
    public bool IsConnectionAvailable { get; set; } // or whatever your variable should be called
}

Then bind the visibility of the textblock to this using a converter:

<TextBlock Visibility="{Binding IsConnectionAvailable, Converter={StaticResource BooleanToVisibilityConverter}}">

You will need to declare the static resource for the converter somewhere (in the control itself or your main resources dictionary for example)

It looks like there is a converter already defined in System.Windows.Controls somewhere, but in case you can't find it the implementation is pretty simple (you could probably do this a bit better to guard against invalid input but for brevity I've kept it tiny):

public class BooleanToVisibilityConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return (bool) value ? Visibility.Visible : Visibility.Collapsed;
    }

    public object ConvertBack(object value, Type targetType, object parameter,CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

You may also want to change the state from available/unavailable during the views lifecycle, so in that case you probably want to use the property changed events built in to PropertyChangedBase (which Screen also inherits) to let the view know when the property changes

    private bool _isConnectionAvailable;

    public bool IsConnectionAvailable
    {
        get { return _isConnectionAvailable; }
        set
        {
            if (_isConnectionAvailable != value)
            {
                _isConnectionAvailable = value;
                NotifyOfPropertyChange(() => IsConnectionAvailable);
            }
        }
    }

Addendum 2

I prefer the terse CM syntax instead of being explicit when binding action messages - so your XAML would change from:

<Image delay:LowProfileImageLoader.UriSource="{Binding ImagePath,Mode=TwoWay}" DataContext="{Binding FeedItemsAdvertisement,Mode=TwoWay}" Margin="0,20,0,39" Width="380" Height="128">
<i:Interaction.Triggers>
    <i:EventTrigger
        EventName="Tap">
        <cm:ActionMessage
        MethodName="LoadAdvertisement" >
            <cm:Parameter Value="{Binding Link}"></cm:Parameter>
        </cm:ActionMessage>
    </i:EventTrigger>
</i:Interaction.Triggers>
</Image>

To

<Image delay:LowProfileImageLoader.UriSource="{Binding ImagePath,Mode=TwoWay}" DataContext="{Binding FeedItemsAdvertisement,Mode=TwoWay}" Margin="0,20,0,39" Width="380" Height="128" cal:Message.Attach="[Tap] = [LoadAdvertisement($dataContext.Link)]"></Image>

(actually that might not be right with the $dataContext.Link part ... but then again it might be... see here: http://caliburnmicro.codeplex.com/wikipage?title=All%20About%20Actions&referringTitle=Documentation)