Event handlers when using a string as a data template for dataform in Silverlight

Question

I am attempting to form some xaml for a dataform programmatically using a string. I can get the combo box to appear. but when I attempt to use the code specifying the "MouseLeftButtonUp" or the "Loaded" event handler in the string; the page will turn white (no noticeable error) out after going into it. Please see relevant code below.

     StringBuilder editTemplate = new StringBuilder("");
     editTemplate.Append("<DataTemplate ");
     editTemplate.Append("xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' ");
     editTemplate.Append("xmlns:toolkit='http://schemas.microsoft.com/winfx/2006/xaml/presentation/toolkit' ");
     editTemplate.Append("xmlns:navigation='clr-namespace:System.Windows.Controls;assembly=System.Windows.Controls.Navigation' ");
     editTemplate.Append("xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml' >");
     editTemplate.Append("<StackPanel>");
     editTemplate.Append(@"  <toolkit:DataField Label='" + GetFieldWithoutNumber(theInfo, theDataContext) + "'>");
     /* Won't Work */ editTemplate.Append(@" <ComboBox MouseLeftButtonUp='ComboBox_MouseLeftButtonUp' />");
     /* Will Work  */ editTemplate.Append(@" <ComboBox />");
     editTemplate.Append(@" </toolkit:DataField>");
     editTemplate.Append("</StackPanel></DataTemplate>");
     dynamicDataForm.EditTemplate = XamlReader.Load(editTemplate.ToString()) as DataTemplate;

This doesn't surprise me, what you should do is hook up the event handler programmatically after you've created the datatemplate. — slugster, Jan 07 '13 at 02:24
@slugster Please advise... how do I do that? Just give it a name? — Kulingar, Jan 07 '13 at 02:25

score 1 · Answer 1 · answered Jan 17 '13 at 21:23

One solution is to handle the BeginningEdit event of the DataForm and use that to subscribe your event handler to the ComboBox's MouseLeftButtonUp event.

To do this, add to your code-behind a private field named isEventWiredUp. We'll use this field to keep track of whether we've subscribed to the event and prevent the event from being subscribed to more than once.

Next, add an x:Name="..." attribute to your ComboBox. We use this name to get at the combobox.

Once that is done, add the following two methods, which should do what you want. Replace yourComboBoxName with the x:Name you gave to your combobox:

    private void dynamicDataForm_BeginningEdit(object sender, CancelEventArgs e)
    {
        Dispatcher.BeginInvoke(OnBeginEdit);
    }

    private void OnBeginEdit()
    {
        if (!isEventWiredUp)
        {
            var combobox = dynamicDataForm.FindNameInContent("yourComboBoxName") as ComboBox;
            if (combobox != null)
            {
                combobox.MouseLeftButtonUp += combobox_MouseLeftButtonUp;
                isEventWiredUp = true;
            }
        }
    }

Subscribe the first of these two methods to the DataForm's BeginningEdit event.

I have to admit that I was unable to get the MouseLeftButtonUp event to fire on the ComboBox. I'm not sure why this happens, but it seems to be a general problem with the ComboBox as opposed to something that happens because of the way you're constructing XAML. I was able to get an event handler for the ComboBox's SelectionChanged event to work, however.

I also tried replacing the Dispatcher.BeginInvoke line with a direct call to the OnBeginEdit method, but I found that this approach didn't work. The events weren't quite wired up correctly; again, I'm not sure why.

Answers where 13k rep users say they aren't sure why something is happening makes me happy because it justifies my sanity. ;) Thanks for your input, I'm looking at your and Luke's suggestions. Will let you know shortly. :) But yours seems to be easier. — Kulingar, Jan 18 '13 at 15:30

score 1 · Accepted Answer · answered Jan 17 '13 at 21:30

Event handlers hooked up in XAML are required to be declared in the code-behind connected to the XAML file. In the case of a ResourceDictionary or anything loaded from XamlReader.Load there can't be any code-behind, so event handlers can't be set in the XAML. The easiest way to get around this restriction would be to not build your template from strings and just declare it in the Resources section of your XAML file which you can then do like:

Resources["MyTemplate"] as DataTemplate

to get the template and assign it in code like you're doing here, or just use StaticResource in XAML. As long as it stays in the same XAML file connected to this code the event handlers you have in it currently should work fine. The dynamic part of the strings would also need to be changed to use Bindings.

If you want to stick with the XamlReader method you have 2 problems to solve.

Locate the ComboBox instance inside the rendered template
Wait until the template is rendered to look for the ComboBox

To find the ComboBox you need to first give it an x:Name attribute in the template text (you can just replace the event code currently there). Next you need to be able to locate an item in the visual tree by name. This is fairly straightforward and you can find an example here to do that.

To call this code at the right time you either need to override OnApplyTemplate, which unfortunately won't work if you're in something like a UserControl, or use another trick to keep it from running until all the controls are rendered. Here's a full example that could go in a constructor and uses the find method linked from above:

DataTemplate template = Resources["MyTemplate"] as DataTemplate;
dynamicDataForm.ContentTemplate = template;

Dispatcher.BeginInvoke(() =>
{
    ComboBox button = FindVisualChildByName<ComboBox>(this, "MyControl");
    if (button != null)
        button.MouseLeftButtonUp += (s, _) => MessageBox.Show("Click");
});

In your case it looks like your template might need to wait to switch to an edit state before it renders in which case you'd need to hold off on connecting the event and find some other event on your dataform that happens when that state is changed.

A Aiston · Answer 3 · 2013-01-22T18:46:35.767

Rather than trying to hookup the event directly, you can use interactivity to bind up your events

e.g.

...
editTemplate.Append("xmlns:i='clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity' ");
...
editTemplate.Append(@"
<ComboBox>
<i:Interaction.Triggers>
    <i:EventTrigger EventName='MouseLeftButtonUp'>

        <i:InvokeCommandAction Command='{Binding  DataContext.YourCommand,                                                                                 
           RelativeSource={RelativeSource AncestorType=XXX}}'  
           CommandParameter='{Binding}'/>

    </i:EventTrigger>
</i:Interaction.Triggers>
</ComboBox>");

you might have to use some ancestor binding to get to the context on which your handler is defined. I use a custom implementation of InvokeCommandAction; basically a copy of System.Windows.Interactivity.InvokeCommandAction but extended so that it will pass the event args to the command, you might want to do the same.

score 0 · Answer 4 · answered May 01 '15 at 09:40

XamlReader.Load not allowed to attach eventHandlers in it. so use this technique to dynamically attach the eventHandlers to it.

1- Write your Xaml string without eventHandlers -But write the Name property of those Controls.

2- Load the string with XamlReader.Load(str);

3- Then load the content of DataTemplate from it. using Grid template = ((Grid)(dt.LoadContent()));

Note: here Grid is the parent Control in DataTemplate.

4- Find the Control by Name you want to attach the Event Handler. Button img = (Button)template.FindName("MyButtonInDataTemplate");

I hope it helps.

Event handlers when using a string as a data template for dataform in Silverlight

4 Answers4