EvalExpression
| EvalExpression | |
|---|---|
| 220px | |
| imagecaption | |
| Name | EvalExpression |
| Namespace | "using:UBIK.WinX.Controls" in UBIK.UWP "clr-namespace:UBIK.CPL.Controls;assembly=UBIK.CPL" in UBIK.Xamarin |
| Purpose | Evaluate a C# expression in XAML |
| Version | 3.2+ in UBIK.UWP 1.0+ in UBIK.Xamarin |
Contents
Concept
The EvalExpression control allows to evaluate a C# expression from within XAML markup. The Expression has to be a single-line, valid C# expression ("Lambda") and has to return a single value; expressions can also reference names of subordinate EvalExpressionParameter items.
Examples
 | When writing expressions in XAML code, you have to avoid (escape) special characters. There are useful online tools for this. |
Evaluation without parameters
UWP
The following example shows how to evaluate a simple expression without using any parameters and then use the result for visibility binding.
<ctrls:EvalExpression
x:Name="ExpressionEvaluator"
Context="{Binding Self}"
Expression="Context.Values["LK_OFFLINE"]!=null || Context.Values["GUIDREF"]!=null" />
<TextBlock Foreground="White" Visibility="{Binding ElementName=ExpressionEvaluator, Path=Result, Converter={StaticResource BoolToVisConverter}}" />
</StackPanel>
The TextBlock should be visible as long as at least one of the context object's two named properties has a value.
Xamarin
The following example shows how to evaluate a simple expression without using any parameters and then use the result for visibility binding.
<ctrls:EvalExpression
x:Name="ControllingVisibility"
Context="{Binding}"
Expression="Context.Values["LK_EXAMPLE"]!=null || Context.Values["GUIDREF"]!=null" />
<Label
TextColor="#00000"
IsVisible="{Binding Path=Result, Source={x:Reference ControllingVisibility}, Converter={StaticResource NullToBoolConverter}}" />
</StackLayout>
Simple Calculation
UWP
The following example shows how to use the control with three parameters, where the first two are user input (Textbox Param0 and Param1) and the third one is a property from its DataContext (ViewModel). The evaluated Result is then bound to a Textblock for output in the UI.
HorizontalAlignment="Center"
Orientation="Vertical">
<TextBox x:Name="Expression" Width="200" />
<TextBox x:Name="Param0" Width="200" />
<TextBox x:Name="Param1" Width="200" />
<ctrls:EvalExpression x:Name="Evaluator" Expression="{Binding ElementName=Expression, Path=Text}" Context="{Binding}">
<ctrls:EvalExpressionParameter Name="P0" Value="{Binding ElementName=Param0, Path=Text, Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
<ctrls:EvalExpressionParameter Name="P1" Value="{Binding ElementName=Param1, Path=Text, Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
<ctrls:EvalExpressionParameter Name="P2" Value="{Binding IsLoggedIn}" />
</ctrls:EvalExpression>
<TextBlock Foreground="White" Text="{Binding ElementName=Evaluator, Path=Result}" />
</StackPanel>
Lets assume that the Textbox Param0 contains a text of 42 and Param1 contains a text of 43. If Expression now contains (P0 + P1) *2 then the result would display 170.
Xamarin example under construction
The following example shows how to use the control with three parameters, where the first two are user input (Editor Param0 and Param1) and the third one is a property from its DataContext (ViewModel). The evaluated Result is then bound to an Label for output in the UI.
<Editor x:Name="Expression"/>
<Editor x:Name="P0" WidthRequest="32"/>
<Editor x:Name="P1" WidthRequest="43"/>
<ctrls:EvalExpression x:Name="Evaluator" Expression="{Binding Path=Text, Source={x:Reference Expression}}" Context="{Binding}">
<ctrls:EvalExpressionParameter Name="P0" Value="{Binding Path=WidthRequest, Source={x:Reference P0}}" />
<ctrls:EvalExpressionParameter Name="P1" Value="{Binding Path=WidthRequest, Source={x:Reference P1}}" />
</ctrls:EvalExpression>
<Label Text="{Binding Path=Result, Source={x:Reference Evaluator}}" />
</StackLayout>
(P0 + P1) then the result would display 75.Setting a calculated Property Value
UWP
The following example shows how to create a button that adds and stores +5 to the value of a a numeric property named MP_EXAMPLE, every time it is pressed:
<ctrls:EvalExpression x:Name="Evaluator" Expression=""MP_EXAMPLE|" + (P0 + 5)">
<ctrls:EvalExpressionParameter Name="P0" Value="{Binding Values[MP_EXAMPLE], Converter={StaticResource ToType}, ConverterParameter='System.Int32'}" />
</ctrls:EvalExpression>
<Button Content="Tap for 5 more" Command="{Binding SetPropertyValueAndValidateCommand}" CommandParameter="{Binding ElementName=Evaluator, Path=Result}"/>
</Grid>
Xamarin
The following example shows how to create a button that adds and stores +5 to the value of a a numeric property named MP_EXAMPLE, every time it is pressed:
<ctrls:EvalExpression x:Name="Evaluator2" Expression=""MP_EXAMPLE|" + (P0 + 2)" Context="{Binding}">
<ctrls:EvalExpressionParameter Name="P0" Value="{Binding Values[MP_EXAMPLE]}" />
</ctrls:EvalExpression>
<Button Text="Tap to add 2 more"
Command="{Binding SetPropertyValueAndValidateCommand}" CommandParameter="{Binding Path=Result, Source={x:Reference Evaluator2}}"/>
</Grid>
Conditional Statement
UWP
If / Or statements can be evaluated using the C# syntax P0 ? P1 : P2. In this case, if the result of the P0 expression is True, P1 will be effected. Otherwise, P2 will be.
<Controls:EvalExpressionParameter Name="P0" Value="{Binding StoredProfileParameters[WPType1]}" />
<Controls:EvalExpressionParameter Name="P1" Value="WPType1=1" />
<Controls:EvalExpressionParameter Name="P2" Value="{Binding ElementName=InitialiseParameter, Path=Result}" />
</Controls:EvalExpression>
Xamarin
If / Or statements can also be evaluated using various C# syntax. In this case, if the result of the P0 expression is true P00 will be used, if P1 is true P10 will be used.
<controls:EvalExpression x:Name="StatusAutomator" Expression="P0 == true ? P00 : (P1 == true ? P10)" Context="{Binding}">
<controls:EvalExpressionParameter Name="P0" Value="{Binding Values[MP_STATUS], Converter={StaticResource EqualityToBool}, ConverterParameter=0}" />
<controls:EvalExpressionParameter Name="P00" Value="State1" />
<controls:EvalExpressionParameter Name="P1" Value="{Binding Values[MP_STATUS], Converter={StaticResource EqualityToBool}, ConverterParameter=10}" />
<controls:EvalExpressionParameter Name="P10" Value="State2" />
</controls:EvalExpression>
<Label
Text="{Binding Path=Result, Source={x:Reference StatusAutomator}}"
FontSize="16"/>
</Grid>
Parameters
EvalExpressionParameters can be added as child objects to an EvalExpression control. Each parameter object needs a unique Name and a Value, where the latter can be either a constant or dynamic value supplied through a binding.
