Windows 8: Binding

Windows 8 Binding – Part 1

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Objectives
• Learn to use Binding objects to bind data
sources and targets
• Add data converters to manage conversion
during the binding process
• Use data templates to modify the layout of
bound data in lists


Agenda
• Introducing Binding
• Working with Type Converters
• Binding Lists and Data Templates
• Using Binding and Data Templates


When to Use Binding?
• Display the number of characters in a text box,
in a TextBlock control
• Display the value of Slider control in a TextBlock
• Display list of customers in ListBox
• Display customer’s photo in Image control
• Display and edit customer’s name in TextBox
• Of course, there are an infinite number of
reasons

Connecting Sources and Targets
• Every time you use binding
• Must supply source for data, and target
• Normally, target is dependency property of some
user interface element


Connecting Sources and Targets
• Binding source: any property of any object
• Binding object connects source to binding target
• Must be dependency property of some
DependencyObject instance


Under the Hood
• When using binding in XAML
• Create instance of Binding class, setting various
properties that affect its behavior
• XAML provides binding markup extension
• Hides this fact, but still working with Binding object
• Completely transparent if you create Binding object
in code
• Can set Mode property of Binding to control
direction of data flow (one/two directions?)

Value Converters
• Value Converter provides instance of class that
implements
Windows.UI.Xaml.Data.IValueConverter
interface
• Binding declaratively often requires value
converter


Value Converters
• Select a customer from a ListBox, display
combined FirstName and LastName fields in
TextBlock
• Select an integer, bind to BorderThickness of
Border
• Can’t bind directly—BorderThickness is a Thickness
structure
• Infinite reasons to use a value converter
• But only if binding declaratively

A Few Simple Examples
• SimpleBinding1
• SimpleBinding2


Binding Details
• Standard Binding markup extension includes
ElementName and Path attributes:
• Text="{Binding ElementName=DemoSlider,
Path=Value}"
• Path attribute not required:
• Text="{Binding Value,
ElementName=DemoSlider}"
• Choose whichever you like


Binding Details
• Path property can refer to property, or
property of a property, or indexed property
• Need to refer to an attached property?
• Grid.Row, for example
• Set Path property to (Grid.Row) (in parentheses)
• Parentheses force evaluation—won’t work without
• Parentheses not necessary for property of a property


Binding Details
• Binding Markup extension shortcut for child
element syntax:
<TextBox Width="40"
Name="DemoTextBox"
Height="23">
<TextBox.Text>
<Binding ElementName="DemoSlider"
Path="Value" />
</TextBox.Tet>
</TextBox>


Setting the Binding Mode
• In demo, data moves from Slider to TextBox
• Changes in TextBox have no effect on slider
• Binding is one-way: Data moves in one
direction
• Set Mode property of Binding to change
• OneTime
• OneWay
• TwoWay


DEMO
• Two-way binding, SimpleBinding3


A Simple Example
• Enter text into TextBox, update TextBlock with
length of the text
• Could react to TextChanged event of TextBox
• Better: Bind Text property of TextBlock to
Text.Length property of TextBox
• Points out that binding can use expressions


DEMO
• SimpleBinding4


Using a Type Converter
• Determine source and target data types
• Write code to perform the conversion
• Perhaps write code to convert back (for two-way
binding)
• Sample binds integer from combo box to
BorderThickness property
• What’s the problem? Thickness structure has four
values


Creating the Type Converter
• Attempt to bind SelectedValue of ComboBox to
BorderThickness of Border, and doesn’t work
• Need type (or value) converter


Converters
• Implement IValueConverter interface
• Requires Convert and ConvertBack methods
• Parameters:
• value (System.Object)
• targetType (System.Type)
• parameter (System.Object)
• Language (System.String)


Handle Null Case
• Always possible that value will be null
• Code runs at design time!
• Always verify that value isn’t null before
performing conversion


Converter Warning
• Converter doesn’t trap exceptions
• Treated as runtime errors
• You must trap and handle all runtime errors
• Return DependencyProperty.UnsetValue on
error
• Not handled in this simple demo
• Worth considering!


DEMO
• IntegerToThicknessConverter


Referencing the Type Converter
• Need way to refer to type converter in XAML
• Like any other resource, declare instance in
resources; Page.Resources in demo
• Need namespace reference
• xmlns:local="using:Binding"
• Then, within Page.Resources:
• <local:IntegerToThicknessConverter
x:Key="thicknessConverter" />


Using the Type Converter
• Add a setting in Binding markup extension
• <Border BorderThickness="{Binding
ElementName=ThicknessComboBox,
Path=SelectedValue,
Converter={StaticResource thicknessConverter}}"
BorderBrush="Black"
Margin="5">


Using the Visual Studio Designer
• DEMO


End of Part 1

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Windows 8: Binding

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