ViewModel validations
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Validations exist to enforce a set of rules that apply to a specific use case. A use case is implemented by a ViewModel, so the ViewModel needs Validations.
Validations exist to enforce a set of rules that apply to a specific use case. A use case is implemented by a ViewModel, so the ViewModel needs validations.


This is how you add them in the ViewModelEditor:
This is how to add them in the ViewModelEditor:


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[[File:Valid -1.png|frameless|334x334px]]
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== Validation Rules in the ViewModel ==
== Validation Rules in the ViewModel ==
In a ViewModel, you can add validation rules that will generate error messages when some data is entered improperly.  You can name and connect this rule to any number of controls you want to be affected by this.     
You can add validation rules in a ViewModel that will generate error messages when some data is improperly entered. Name and connect this rule to any number of controls you want to be affected by this.     


If you have constraints, you should not repeat expressions. Look up previously established constraints to ensure that they are not broken.     
If you have constraints, you should not repeat expressions.     
* Look up the constraint to ensure that it is not broken.     
* Create a method to verify the validity of the constraint by providing the constraint name as a string and returning a boolean.     
This helps in linking the constraints in the class with the validations in the ViewModel.      


We could also take this expression and add a method to a thing called check constraint valid.  Some of the constraint which is a string and it should return a boolean.  So constraint valid name of constraint which is a string and returns a boolean.  We make it a query, otherwise, we can't use it in OCL.     
If we look up the name of the constraint, it should not be empty because we need to find the constraint to be sure we specified the right constraint. We should also check that it's not broken. If it is incorrectly specified, the whole expression will be false and we will get notified with constant problems, which can be seen in the user interface.     


We take the name of the constraint.  So now we're looking this up and we are returning if it's not broken.  We could also phrase this as this constraint is not broken because then we could say that this is empty.  Now, self.constraints and the name equal to the name of the constraint are not broken.  We should also add here for extra protection.       
This is a good way to link together constraints in the class with validations in the ViewModel.        
 
If we look up the name of the constraint, it should not be empty, because we need to find the constraint to be sure that we had specified the right constraint. We should also check that it's not broken. If this is incorrectly specified, the whole expression will be false and we will get notified, with constant problems showing in the user interface.     
 
This is a good way to link together constraints in the class with validations in the ViewModel.    
 
The text explains how to create validation rules in a ViewModel, which can generate error messages when data is entered incorrectly. The validation rule can be named and connected to multiple controls. The author stresses the significance of avoiding expression repetition and   
 
It is recommended to create a method to verify the validity of constraints by providing the constraint name as a string and returning a boolean. The use of this approach helps in linking the constraints in the class with the validations in the ViewModel. The author also highlights the importance of checking the constraint's name and whether it is broken or not. If it is not correctly specified, it can lead to a false expression and create constant problems, which can be seen in the user interface. Therefore, by using this approach, developers can easily manage and ensure the accuracy of constraints and validations, resulting in better overall software quality.    
 
'''To learn more about this, watch the video below.'''   


'''To learn more about this, watch this video:'''
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<html>
<br>
<div class="video">
<div class="video">
   <div class="video__wrapper">
   <div class="video__wrapper">
     <iframe src="https://www.youtube.com/embed/nkPR9P4i3P0" title="YouTube video player" frameborder="0" allowfullscreen></iframe>
     <iframe src="https://www.youtube.com/embed/CkA8qQWqSUg?si=C5aN_7XBfzD3E8Ct" title="YouTube video player" frameborder="0" allowfullscreen></iframe>
   </div>
   </div>
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</html>
</html>   
 
 
[[Category:View Model]]
[[Category:View Model]]
[[Category:Validation rules]]
[[Category:Validation rules]]
{{Edited|July|12|2024}}

Latest revision as of 06:05, 12 March 2024

Validations exist to enforce a set of rules that apply to a specific use case. A use case is implemented by a ViewModel, so the ViewModel needs validations.

This is how to add them in the ViewModelEditor:

Valid -1.png

You will get a new row in the Validations list. Fill in the rule name and the expression that should evaluate to true when the rule is fulfilled, and false when data breaks the rule. Also, provide a text with the error message you want to show.

The context of the rule is the context of the Main ViewModelClass; this means that you reach variables etc.

Valid -2.png

You can have as many rules as you need.

The rule state (true or false) can then be displayed by one or multiple columns. Right-click a ViewModelColumn and check the correct rule to make an association between the two:

Valid -4.png

Once the association is made and a rule break is discovered in runtime, a standard WPF ValidationError will occur. How this ValidationError is displayed is defined by styles.

All these apply to the web interface.

WPF Implementation

In WPF, I added a default style – override it if you need to – that brings up a tooltip, draws an exclamation mark behind the offending control, and draws a red border around the value.

The default style is defined like this:

Code Snippet

<ControlTemplate x:Key="validationTemplate">
  <DockPanel>
    <Border BorderBrush="Red" BorderThickness="2" CornerRadius="4,4,4,4"  >
      <AdornedElementPlaceholder/>
    </Border>
    <TextBlock Foreground="Red" VerticalAlignment="Center" FontWeight="ExtraBold" Margin="4">!</TextBlock>
  </DockPanel>
</ControlTemplate>

<Style TargetType="TextBox">
  <Setter Property="Validation.ErrorTemplate" Value="{StaticResource validationTemplate}">
  </Setter>
  <Style.Triggers>
    <Trigger Property="Validation.HasError" Value="true">
      <Setter Property="TextBox.ToolTip" Value="{Binding RelativeSource={RelativeSource Self}, Path=(Validation.Errors)[0].ErrorContent}"/>
    </Trigger>
  </Style.Triggers>
</Style>

When the rule is broken, it looks like this in runtime:

Valid -5.png

Validation Rules in the ViewModel

You can add validation rules in a ViewModel that will generate error messages when some data is improperly entered. Name and connect this rule to any number of controls you want to be affected by this.   

If you have constraints, you should not repeat expressions.     

  • Look up the constraint to ensure that it is not broken.    
  • Create a method to verify the validity of the constraint by providing the constraint name as a string and returning a boolean.    

This helps in linking the constraints in the class with the validations in the ViewModel.    

If we look up the name of the constraint, it should not be empty because we need to find the constraint to be sure we specified the right constraint. We should also check that it's not broken. If it is incorrectly specified, the whole expression will be false and we will get notified with constant problems, which can be seen in the user interface.   

This is a good way to link together constraints in the class with validations in the ViewModel.       

To learn more about this, watch this video:

    

This page was edited 71 days ago on 03/12/2024. What links here