Namespaces

Variants
Actions

Please note that as of October 24, 2014, the Nokia Developer Wiki will no longer be accepting user contributions, including new entries, edits and comments, as we begin transitioning to our new home, in the Windows Phone Development Wiki. We plan to move over the majority of the existing entries over the next few weeks. Thanks for all your past and future contributions.

(Difference between revisions)

How to debug data binding problems on Windows Phone

From Wiki
Jump to: navigation, search
hamishwillee (Talk | contribs)
m (Hamishwillee - Bot update - Fix metadata)
hamishwillee (Talk | contribs)
m (Hamishwillee - Subedit)
Line 1: Line 1:
 
[[Category:Windows Phone]][[Category:Silverlight]][[Category:Debugging]]
 
[[Category:Windows Phone]][[Category:Silverlight]][[Category:Debugging]]
{{Abstract|Often XAML elements with data binding show no data during development. This article shows one method how to find the source of this problem. }}
+
{{Abstract|This article explains how you can debug data binding issues by inspecting the elements before they are due to be displayed by the control.}}
  
 
{{ArticleMetaData <!-- v1.2 -->
 
{{ArticleMetaData <!-- v1.2 -->
Line 26: Line 26:
  
 
== Introduction ==
 
== Introduction ==
If you use data binding in your Windows Phone application, during development you'll come to the point where you have linked everything together but data doesn't display correctly in your control.
+
[http://msdn.microsoft.com/en-us/library/ms752347.aspx Data binding] allows you to link a data source defined in code to a UI description in XAML (target). While defining a binding is quite straightforward, if something does go wrong it is useful to be able to tell whether the problem is in your source data or in the binding definition.
  
== Solution ==
+
The easiest way to do this is to create a custom value converter ([http://msdn.microsoft.com/en-us/library/system.windows.data.ivalueconverter.aspx IValueConverter]), which will be called when the data from the binding source arrives - before it gets delivered to the control. By setting a breakpoint in the converter you can confirm that the source is sending the expected data and contains the correct properties.
What can you do? You can use an [http://msdn.microsoft.com/en-us/library/system.windows.data.ivalueconverter.aspx IValueConverter] and set a breakpoint there. This way you can see what data arrives at your element and check the available properties.
+
  
Say you have a ListBox like this:
+
{{Tip|If the converter doesn't get called then this is a sign that the binding has failed earlier than expected, perhaps in the definition of the source property.}}
  
 +
== Worked example ==
 +
 +
First, consider the case where you have a {{Icode|ListBox}} definition in XAML as shown below:
 
<code xml>
 
<code xml>
        <ListBox x:Name="MainListBox" Margin="0,0,-12,0" ItemsSource="{Binding Items}" SelectionChanged="MainListBox_SelectionChanged">
+
<ListBox x:Name="MainListBox" Margin="0,0,-12,0" ItemsSource="{Binding Items}" SelectionChanged="MainListBox_SelectionChanged">
            <ListBox.ItemTemplate>
+
  <ListBox.ItemTemplate>
              <DataTemplate>
+
    <DataTemplate>
                  <StackPanel Margin="0,0,0,17" Width="432" Height="78">
+
      <StackPanel Margin="0,0,0,17" Width="432" Height="78">
                    <TextBlock Text="{Binding LineOne}" TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
+
        <TextBlock Text="{Binding LineOne}" TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
                    <TextBlock Text="{Binding LineTwo}" TextWrapping="Wrap" Margin="12,-6,12,0" Style="{StaticResource PhoneTextSubtleStyle}"/>
+
        <TextBlock Text="{Binding LineTwo}" TextWrapping="Wrap" Margin="12,-6,12,0" Style="{StaticResource PhoneTextSubtleStyle}"/>
                  </StackPanel>
+
      </StackPanel>
              </DataTemplate>
+
    </DataTemplate>
            </ListBox.ItemTemplate>
+
  </ListBox.ItemTemplate>
        </ListBox>
+
</ListBox>
 
</code>
 
</code>
  
You can add a converter that gets called when the data from the binding source arrives and before it gets delivered to the control. Here we have one condition when this debugging technique can't help: if the binding fails beforehand, the converter doesn't get called.
+
To implement the converter you first have to derive a class from {{Icode|IValueConverter}} and implement it's methods:
 
+
To implement the converter you have to derive a class from IValueConverter and implement it's methods:
+
 
+
 
<code csharp>
 
<code csharp>
  public class DatabindingDebugConverter : IValueConverter
+
public class DatabindingDebugConverter : IValueConverter
  {
+
  {
  public object Convert(object value, Type targetType,
+
  public object Convert(object value, Type targetType, object parameter, string language)
        object parameter, string language)
+
    {
      {
+
    return value;
      return value;
+
    }
      }
+
  
  public object ConvertBack(object value, Type targetType,
+
  public object ConvertBack(object value, Type targetType, object parameter, string language)
        object parameter, string language)
+
    {
      {
+
    return value;
      return value;
+
    }
      }
+
  }
  }
+
 
</code>
 
</code>
  
Then you have to declare the converter as a static resource in your page's xaml:
+
Then you have to declare the converter as a static resource in your page's XAML:
 
+
 
<code xml>
 
<code xml>
  <phone:PhoneApplicationPage.Resources>
+
<phone:PhoneApplicationPage.Resources>
      <local:DataBindingDebugConverter x:Key="DatabindingDebugConverter"/>
+
  <local:DataBindingDebugConverter x:Key="DatabindingDebugConverter"/>
  </phone:PhoneApplicationPage.Resources>
+
</phone:PhoneApplicationPage.Resources>
 
</code>
 
</code>
  
assuming you have declared the local namespace
+
You also have to change the XAML data binding statement to call the converter for a particular binding:
 
+
 
<code xml>
 
<code xml>
     xmlns:local="clr-namespace:EncryptionSample"
+
<TextBlock Text="{Binding LineOne, Converter={StaticResource DataBindingDebugConverter}}"
 +
     TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
 
</code>
 
</code>
  
in the page tag and call it in the data binding statement:
+
Note that this assumes you have declared the local namespace in the page tag:
 +
<code xml>xmlns:local="clr-namespace:EncryptionSample"</code>
  
<code xml>
+
Next you set a breakpoint in the {{Icode|Convert()}} method and voilà, you can see the values that arrive from binding, if any.
      <TextBlock Text="{Binding LineOne, Converter={StaticResource DataBindingDebugConverter}}" TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
+
</code>
+
 
+
Next you set a breakpoint in the Convert method and voilà, you can see the values that arrive from binding, if any.
+
  
 
== Sample code ==
 
== Sample code ==
A running sample is contained in the sample code for the [[How to encrypt your application data in Windows Phone]] article, please refer there.
+
The sample code for the [[How to encrypt your application data in Windows Phone]] article uses this technique.
 +
 
 
{{VersionHint|Versions will be visible from this template when viewed in preview. You can delete this or leave it in the page as it is not displayed in final version}}
 
{{VersionHint|Versions will be visible from this template when viewed in preview. You can delete this or leave it in the page as it is not displayed in final version}}

Revision as of 06:23, 15 October 2012

This article explains how you can debug data binding issues by inspecting the elements before they are due to be displayed by the control.

WP Metro Icon File.png
WP Metro Icon Bug.png
SignpostIcon XAML 40.png
WP Metro Icon WP8.png
SignpostIcon WP7 70px.png
Article Metadata
CompatibilityArticle
Created: influencer (12 Oct 2012)
Last edited: hamishwillee (15 Oct 2012)

Contents

Introduction

Data binding allows you to link a data source defined in code to a UI description in XAML (target). While defining a binding is quite straightforward, if something does go wrong it is useful to be able to tell whether the problem is in your source data or in the binding definition.

The easiest way to do this is to create a custom value converter (IValueConverter), which will be called when the data from the binding source arrives - before it gets delivered to the control. By setting a breakpoint in the converter you can confirm that the source is sending the expected data and contains the correct properties.

Tip.pngTip: If the converter doesn't get called then this is a sign that the binding has failed earlier than expected, perhaps in the definition of the source property.

Worked example

First, consider the case where you have a ListBox definition in XAML as shown below:

<ListBox x:Name="MainListBox" Margin="0,0,-12,0" ItemsSource="{Binding Items}" SelectionChanged="MainListBox_SelectionChanged">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Margin="0,0,0,17" Width="432" Height="78">
<TextBlock Text="{Binding LineOne}" TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
<TextBlock Text="{Binding LineTwo}" TextWrapping="Wrap" Margin="12,-6,12,0" Style="{StaticResource PhoneTextSubtleStyle}"/>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>

To implement the converter you first have to derive a class from IValueConverter and implement it's methods:

public class DatabindingDebugConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, string language)
{
return value;
}
 
public object ConvertBack(object value, Type targetType, object parameter, string language)
{
return value;
}
}

Then you have to declare the converter as a static resource in your page's XAML:

<phone:PhoneApplicationPage.Resources>
<local:DataBindingDebugConverter x:Key="DatabindingDebugConverter"/>
</phone:PhoneApplicationPage.Resources>

You also have to change the XAML data binding statement to call the converter for a particular binding:

<TextBlock Text="{Binding LineOne, Converter={StaticResource DataBindingDebugConverter}}" 
TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>

Note that this assumes you have declared the local namespace in the page tag:

xmlns:local="clr-namespace:EncryptionSample"

Next you set a breakpoint in the Convert() method and voilà, you can see the values that arrive from binding, if any.

Sample code

The sample code for the How to encrypt your application data in Windows Phone article uses this technique.

Version Hint

Windows Phone: [[Category:Windows Phone]]
[[Category:Windows Phone 7.5]]
[[Category:Windows Phone 8]]

Nokia Asha: [[Category:Nokia Asha]]
[[Category:Nokia Asha Platform 1.0]]

Series 40: [[Category:Series 40]]
[[Category:Series 40 1st Edition]] [[Category:Series 40 2nd Edition]]
[[Category:Series 40 3rd Edition (initial release)]] [[Category:Series 40 3rd Edition FP1]] [[Category:Series 40 3rd Edition FP2]]
[[Category:Series 40 5th Edition (initial release)]] [[Category:Series 40 5th Edition FP1]]
[[Category:Series 40 6th Edition (initial release)]] [[Category:Series 40 6th Edition FP1]] [[Category:Series 40 Developer Platform 1.0]] [[Category:Series 40 Developer Platform 1.1]] [[Category:Series 40 Developer Platform 2.0]]

Symbian: [[Category:Symbian]]
[[Category:S60 1st Edition]] [[Category:S60 2nd Edition (initial release)]] [[Category:S60 2nd Edition FP1]] [[Category:S60 2nd Edition FP2]] [[Category:S60 2nd Edition FP3]]
[[Category:S60 3rd Edition (initial release)]] [[Category:S60 3rd Edition FP1]] [[Category:S60 3rd Edition FP2]]
[[Category:S60 5th Edition]]
[[Category:Symbian^3]] [[Category:Symbian Anna]] [[Category:Nokia Belle]]

329 page views in the last 30 days.

Was this page helpful?

Your feedback about this content is important. Let us know what you think.

 

Thank you!

We appreciate your feedback.

×