Wednesday, March 22, 2017

WPF Tip #5 - Data Binding with IMultiValueConverter

In Tip #4, we examined the use of IValueConverter with WPF binding. If you are using MulitBinding, a converter class that implements IMultiValueConverter is needed.

Like the IValueConverter, the class must implement Convert and ConvertBack methods. The difference being the multiple values come in as an object[] parameter in Convert() and go back out as an object[] return value in ConvertBack(). The following example takes two string values as input and returns them concatenated with a pipe '|'. It will convert them back using the Split function, assuming your text value contains no other pipes.

public class TextAppendMultiConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        return String.Concat(values[0], "|", values[1]);
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        return (value as string).Split('|');
    }
}

This code will use the converter in a WPF TextBox's Text property MultiBinding.

<TextBox>
    <TextBox.Text>
        <MultiBinding Converter="{StaticResource TextAppendMultiConverter}" FallbackValue="Busy loading...">
            <Binding Path="IntroText"/>
            <Binding Path="ContentText"/>
        </MultiBinding>
    </TextBox.Text>
</TextBox>

The FallbackValue will be displayed if the converter fails or data is unavailable in the attached DataContext.

Next time, we will shift gears away from data binding. Happy coding!

 

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Thursday, March 16, 2017

WPF Tip #4 - Data Binding with IValueConverter

The first three tips have all been related to WPF data binding. Let's continue the theme with IValueConverter. IValueConverter is used in conjunction with the Converter parameter on a property's Binding.

Converters are used whenever the type of the property being bound does not match the type of the property in the binding source. Converting a boolean value to Visibility enum value is the most common conversion throughout all Xaml code. It is so common that the .NET Framework has its own BooleanToVisibilityConverter class that implements IValueConverter.

To implement your own converter, simply create a class that implements the IValueConverter interface. The interface has two methods, Convert() and ConvertBack(). If your property will never update its binding source, the ConvertBack() method does not need to be implemented. If you leave the default code Visual Studio puts in newly generated methods to throw a NotImplementedException, you will quickly find out if the ConvertBack() method is necessary for your converter.

This simple example is a variation on the BooleanToVisibilityConverter. It is an IndecisiveEnumToVisibilityConverter. Here is what my IndecisiveEnum looks like:

public enum IndecisiveEnum
{
    Yes,
    Probably,
    ThinkSo,
    NotSure,
    No,
    WhyWouldAnyoneDoThat,
    NotUnlessEveryoneElseIsDoingIt
}

The converter needs to take one of these values and return a Visibility. Here's the implementation of Convert() and ConvertBack().

public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    if (!(value is IndecisiveEnum))
        return Visibility.Collapsed;

    var typedValue = (IndecisiveEnum)value;

    switch (typedValue)
    {
        case IndecisiveEnum.Yes:
        case IndecisiveEnum.Probably:
        case IndecisiveEnum.ThinkSo:
            return Visibility.Visible;
        case IndecisiveEnum.No:
        case IndecisiveEnum.WhyWouldAnyoneDoThat:
            return Visibility.Collapsed;
        case IndecisiveEnum.NotUnlessEveryoneElseIsDoingIt:
            return IsEveryoneElseDoingIt() ? Visibility.Visible : Visibility.Collapsed;
        case IndecisiveEnum.NotSure:  // return a 50/50 random chance of yes/no
            Random gen = new Random();
            int prob = gen.Next(100);
            return prob <= 50 ? Visibility.Visible : Visibility.Collapsed;
        default:
            return Visibility.Collapsed;
    }
}

public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
    if (!(value is Visibility))
        return IndecisiveEnum.No;

    var typedValue = (Visibility)value;

    switch (typedValue)
    {
        case Visibility.Collapsed:
        case Visibility.Hidden:
            return IndecisiveEnum.No;
        case Visibility.Visible:
            return IndecisiveEnum.Yes;
        default:
            return IndecisiveEnum.No;
    }
}

For the IndecisiveEnum.NotUnlessEveryoneElseIsDoingIt value, the case calls a method which we'll pretend calls a web service to find out if everyone else is actually doing it or not. If the input value is NotSure, then a random call will return either Visible or Collapsed. Converting back is a little more straightforward.

Consuming this converter in the WPF Xaml consists of two parts. First, the converter must be defined as a resource.

<Window.Resources>
    <local:IndecisiveEnumToVisibilityConverter x:Key="IndecisiveEnumToVisibilityConverter" />
</Window.Resources>

Finally, the resource can be used in the binding for our TextBox Visibility property.

<TextBox Visibility="{Binding IsVisible, Converter={StaticResource IndecisiveEnumToVisibilityConverter}}"/>

That's it. Next time we'll look at the IMultiValueConverter, which is used with MultiBinding.

 

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Monday, March 13, 2017

WPF Tip #3 - Using FallbackValue with MultiBinding

Let's take another look at using a FallbackValue in WPF. In Tip #2, it was used to provide a temporary value while waiting for an async data binding value to return from a slow-running ViewModel property getter.

A more common use of FallbackValue is in conjunction with MultiBinding. MultiBinding allows a single element's property to be bound to multiple objects on your data source (ViewModel, relative sources, etc.) If the values of the MultiBinding are unavailable or invalid, or use converters that return invalid results, the MultiBinding's FallbackValue will be displayed. This StackOverflow answer provides a great explanation with some examples.

Here is a simple example using the same Xaml file from Tip #2.

<TextBox Grid.Row="1">
     <TextBox.Text>
         <MultiBinding FallbackValue="Busy loading." StringFormat="Plain text is {0} and rich text is {1}.">
             <Binding Path="SlowText" IsAsync="True"/>
             <Binding Converter="{StaticResource RtfToPlainTextConverter}" Path="RichText"/>
         </MultiBinding>
     </TextBox.Text>
</TextBox>


The TextBox is now using a MultiBinding to return the original SlowText from last time as async plus some RichText property being converted to plain text with an RtfToPlainTextConverter that has been added to the project. If the bindings fail, the converters fail, or our async binding is really slow, the FallbackValue will be displayed in the TextBox.Text.

What is a converter, you ask? This sounds like a great subject for our next couple of WPF Tips. Stay tuned!

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Sunday, March 5, 2017

WPF Tip #2 - Using FallbackValue with Async WPF Bindings

In Tip #1, we examined how an IsAsync attribute can be added to WPF bindings to allow an application's UI to remain responsive while waiting for long-running ViewModel property getters.

It was mentioned that setting the FallbackValue on an object provides a way to put a default value into a property while waiting for the live data to populate through the ViewModel. This can be used to populate actual default values or to display some placeholder text to indicate to users that the live data is still loading.

Here is a simple example of using a FallbackValue with IsAsync on a standard WPF TextBox bound to a ViewModel property named SlowText.

<TextBox Text="{Binding Path=SlowText, IsAsync=True, FallbackValue='Loading data from a slow connection...'}"/>

This will display "Loading data from a slow connection…" until the real data is returned from the SlowText property.

In Tip #3, we will look at the use of FallbackValue with MultiBinding.

 

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Saturday, March 4, 2017

WPF Tip #1 - IsAsync and WPF Bindings

Welcome to a new series of quick tips for .NET desktop developers using WPF. Expect new tips (at least) weekly on this site. For links to other great resources for .NET developers, please check out my daily link blog, the Morning Dew.

IsAsync

The IsAsync attribute on a WPF data binding allows your UI thread to remain responsive to user activity while the property to which a control is bound is retrieving data. This can be useful if the application must fetch large amounts of data or when a slower database or web data source is invoked.

Here's a sample usage of the attribute on a simple ListView. This ListView gets a list of items from a user's media catalog and presents them in a simple grid format with headers.

<ListView HorizontalAlignment="Left" Margin="10" VerticalAlignment="Top" ItemsSource="{Binding Path=MediaCatalog, IsAsync=True}">
    <ListView.View>
        <GridView>
            <GridViewColumn Header="Artist" DisplayMemberBinding="{Binding Path=Artist}" Width="80"/>
            <GridViewColumn Header="Title" DisplayMemberBinding="{Binding Path=Title}" Width="100"/>
            <GridViewColumn Header="Category" DisplayMemberBinding="{Binding Path=Category}" Width="80"/>
            <GridViewColumn Header="Purchase Date" DisplayMemberBinding="{Binding Path=PurchaseDate}"/>
            <GridViewColumn Header="Number of Views" DisplayMemberBinding="{Binding Path=NumberOfViews}"/>
            <GridViewColumn Header="Year" DisplayMemberBinding="{Binding Path=Year}"/>
        </GridView>
    </ListView.View>
</ListView>

While the data is being retrieved by the MediaCatalog's getter, a FallbackValue can be specified. In this case, a data row with a Title of "Loading…" could be used to indicate to the application's user that live data is on its way.

More information on WPF data binding on MSDN.

Sunday, March 6, 2016

Welcome to WPF Tips

Hello.

Welcome to WPF Tips. I will be posting regular quick tips to help you get started with WPF, learn best practices, and to work through common pitfalls.

Visit my other site, Alvin Ashcraft's Morning Dew, to get daily links to great .NET and web development resources.

Cheers!

Alvin