Data Bondage in WPF

Bruce Johnson, ObjectSharp Consulting bjohnson@objectsharp.com

Data Bondage

The Basic Functionality .Title .Text Data Book Class TitleTextBox Binding Target Source Source Data Target Element Property Property Object

DataContext Matters • A DataContext in WPF is similar to a DataSource in WinForms/ASP.NET • Provides the basis for data binding Book b = new Book() { Title=quot;The Firmquot;, Author=quot;John Grishamquot; }; this.DataContext = b;

Binding Expressions • We saw a simple binding expression in our walkthrough Text=quot;{Binding Path=Title}quot; • Binding expressions can be more complex Text=quot;{Binding Path=DateCreated.Month, Mode=OneWay}quot;

Data bindings can have different modes • One way • Two way • One time • One way to source • Default – The characteristics of the target property determines the mode

Formatting strings during binding • StringFormat property of Binding class • Formats strings during transfer to target property • Uses standard formatting strings, previously defined in .NET <TextBlock Text=quot;{Binding Path=Price, StringFormat=Only {0:c} with purchase!}quot; />

Bindings have silent failure • If a binding fails, no exception is generated • The program’s output will note the binding failure

Collections as the DataContext  It’s common to use a list of data items instead of a single one  For example, a collection of books instead of a single book: Private Books As New List(Of Book) Private Sub Window1_Loaded(ByVal sender As Object, … Books.Add( _ New Book(quot;Ender's Gamequot;, quot;Orson Scott Cardquot;)) Books.Add( _ New Book(quot;Old Man's Warquot;, quot;John Scalziquot;)) Me.DataContext = Books End Sub

Navigating with a ListBox • Add an attribute to the XAML definition of the Listbox <ListBox ItemsSource=quot;{Binding}quot; DisplayMemberPath=quot;Titlequot; IsSynchronizedWithCurrentItem=quot;Truequot; … > • Listbox now navigates the Books collection If your ListBox fails to navigate the list, always check the IsSynchronizedWithCurrentItem property

Lots of possible data sources…  XML data (XPath attribute instead of Path)  ADO.NET Datasets, DataTables  Resource lists defined in XAML You can bind to any control property that is a dependency property IF you can match data types But what if you can’t?

Value converters • WPF has some built-in converters – For example, a string name of a color will be converted to a brush • You can write your own converters – Convert one type to another – Reformat strings (if StringFormat not sufficient) • Implement the IValueConverter interface – Convert method – data source property  WPF element property – ConvertBack method – WPF element property  data source property

Interesting applications • Bind data field to visibility of element – Built-in BooleanToVisibilityConverter • Bind month number to month name • Bind graphic to data field – Have image vary based on value of data field

MultiBinding • A MultiBinding instance binds several data fields in the source to one target property • Collection of Binding instances gets the data fields • Converter assembles and parses the fields

Validation during binding • Binding can have a collection of validation rules • Built-in rules handle exceptions and IDataErrorInfo – IDataErrorInfo support is new in FX3.5 SP1 – Not available in Silverlight (even 3) • You can write your own validation rules – Inherit from ValidationRule class – Override the Validate method – Return a ValidationResult object • ValidationResult has a static ValidResult when there's no error

Displaying validation errors • By default, all you get is a red rectangle – Yes, I hate it too. It’s ugly. – It can be overridden with a control template – You’ll see this in the Control Templates section • Validation class has a static GetErrors method to return errors on an element • Validation class has an Errors attached property for an element – You can use some fancy binding on this property to display error messages in, e.g., tooltips – However, binding information must be present in every binding that is being validated

Another option – Error event • Validation class has an attached event named Error – Set this event on some container that holds all of your fields • Event args tells you if error is being added or removed • Event args includes an Error object with ErrorContent property

Binding element-to-element • Use ElementName property of binding instead of setting a DataContext • Allows you to tie user interface elements together • Example – zooming with Slider and TextBox

No-code zooming <Grid> <Grid.RowDefinitions> <RowDefinition Height=quot;*quot; /> <RowDefinition Height=quot;8*quot; /> </Grid.RowDefinitions> <StackPanel Name=quot;StackPanel1quot; Orientation=quot;Horizontalquot;> <Slider Height=quot;22quot; Name=quot;ZoomSliderquot; Width=quot;100quot; Minimum=quot;10quot; Maximum=quot;100quot; Value=quot;50quot; /> <TextBox Height=quot;23quot; Name=quot;TextBox1quot; Text=quot;{Binding Path=Value, ElementName=ZoomSlider}quot; Width=quot;120quot; Margin=quot;5quot; /> </StackPanel> <Ellipse Width=quot;{Binding Path=Value, ElementName=ZoomSlider}quot; Height=quot;{Binding Path=Value, ElementName=ZoomSlider}quot; Grid.Row=quot;1quot; Name=quot;Ellipse1quot; Stroke=quot;Blackquot; Fill=quot;Redquot;> </Grid>

RelativeSource binding • Allows properties of an element to be bound to other properties of that element Height= quot;{Binding RelativeSource={RelativeSource Self}, Path=Width}quot; • Allows properties of an element to be bound to properties further up in the tree of elements

Questions? • My contact information – EMail: bjohnson@objectsharp.com – Twitter: LACanuck – Blog: http://www.objectsharp.com/blogs/bruce – MSN: lacanadians@hotmail.com

Data Bondage in WPF

by Bruce Johnson on Apr 25, 2009

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Data Bondage in WPF — Presentation Transcript