![]() ![]() Therefore, every time the user enters a character in the Entry control for the username, validation of the entered data is performed. In turn, the ValidateUserNameCommand delegate executes the ValidateUserName method, which executes the Validate method on the ValidatableObject instance. This behavior executes the ValidateUserNameCommand in response to the TextChanged event firing on the Entry, which is raised when the text in the Entry changes. ![]() The Entry control binds to the UserName.Value property of the ValidatableObject instance, and the control's Behaviors collection has an EventToCommandBehavior instance added to it. The following code example demonstrates how this occurs: For example, when a two-way binding in the LoginView sets the UserName or Password property, validation is triggered. Validation is also automatically triggered whenever a bound property changes. Triggering validation when properties change Finally, the IsValid property is set, and its value is returned to the calling method, indicating whether validation succeeded or failed. The Check method for each retrieved validation rule is executed, and the ValidationMessage property value for any validation rule that fails to validate the data is added to the Errors collection of the ValidatableObject instance. This method retrieves any validation rules that were added to the object's Validations collection. The following code example shows the Validate method from the ValidatableObject class: public bool Validate() The Validate method performs validation of the username and password entered by the user on the LoginView, by invoking the Validate method on each ValidatableObject instance. The command delegate calls the MockSignInAsync method in the LoginViewModel, which invokes validation by executing the Validate method, which is shown in the following code example: private bool Validate()īool isValidPassword = ValidatePassword() For example, this occurs in the eShopOnContainers multi-platform app when the user taps the Login button on the LoginView, when using mock services. Validation can be triggered manually for a view model property. The validation approach used in the eShopOnContainers multi-platform app can manually trigger validation of a property, and automatically trigger validation when a property changes. This method adds the IsNotNullOrEmptyRule validation rule to the Validations collection of each ValidatableObject instance, specifying values for the validation rule's ValidationMessage property, which specifies the validation error message that will be displayed if validation fails. ValidationMessage = "A password is required." ValidationMessage = "A username is required." Public List> Validations įor validation to occur, validation rules must be added to the Validations collection of each ValidatableObject instance, as demonstrated in the following code example: private void AddValidations() Public class ValidatableObject : ObservableObject, IValidity The following code shows the implementation of the ValidatableObject: using Any validation errors are placed into the Errors property of the ValidatableObject instance, and the IsValid property of the ValidatableObject instance is updated to indicate whether the validation succeeded or failed. Validation is invoked from the view model by calling the Validate method of the ValidatableObject instance, which retrieves the validation rules and executes them against the ValidatableObject.Value property. View model properties that require validation are of type ValidatableObject, and each ValidatableObject instance has validation rules added to its Validations property. The image below shows the classes involved in performing validation in the eShopOnContainers multi-platform app. The eShopOnContainers multi-platform app performs synchronous client-side validation of view model properties and notifies the user of any validation errors by highlighting the control that contains the invalid data, and by displaying error messages that inform the user of why the data is invalid. In the context of the Model-View-ViewModel (MVVM) pattern, a view model or model will often be required to perform data validation and signal any validation errors to the view so that the user can correct them. Proper validation enforces business rules and could help to prevent an attacker from injecting malicious data. Without validation, a user can supply data that causes the app to fail. An app could, for example, check for input that contains only characters in a particular range, is of a certain length, or matches a particular format. Any app that accepts input from users should ensure that the input is valid.
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