Validation for ReactiveUI based solutions, functioning in a reactive way. `ReactiveUI.Validation` was originally developed by [@jcmm33](https://github.com/jcmm33) as [Vistian.Reactive.Validation](https://github.com/jcmm33/ReactiveUI.Validation), and then refactored and updated by [Àlex Martínez Morón](https://github.com/alexmartinezm) and the [ReactiveUI Core Team](https://github.com/reactiveui/ReactiveUI#core-team). `ReactiveUI.Validation` supports all platforms, including .NET Framework, .NET Standard, .NET Core, Maui.
1. Decorate existing ViewModel with `IValidatableViewModel`, which has a single member, `ValidationContext`. The ValidationContext contains all of the functionality surrounding the validation of the ViewModel. Most access to the specification of validation rules is performed through extension methods on the `IValidatableViewModel` interface. Then, add validation to the ViewModel.
For more complex validation scenarios there are several more overloads of the `ValidationRule` extension method that accept observables. These allow validation to occur asynchronously, and allows complex chains of observables to be combined to produce validation results.
The simplest accepts an `IObservable<bool>` where the observed boolean indicates whether the `ValidationRule` is valid or not. The overload accepts a message which is used when the observable produces a `false` (_invalid_) result.
Any existing observables can be used to drive a `ValidationRule` using the extension method overload that accepts an arbitrary `IObservable<TState>` streams of events. The overload accepts a custom validation function that is supplied with the latest `TState`, and a custom error message function, responsible for formatting the latest `TState` object. The syntax for this looks as follows:
> **Note** The function to extract a message (`messageFunc`) is only invoked if the function to establish validity (`isValidFunc`) returns `false`, otherwise the message is set to `string.Empty`.
Finally, you can directly supply an observable that streams any object (or struct) that implements `IValidationState`; or you can use the `ValidationState` base class which already implements the interface. As the resulting object is stored directly against the context without further transformation, this can be the most performant approach:
> **Note** As a valid `ValidationState` does not really require a message, there is a singleton `ValidationState.Valid` property that you are encouraged to use to indicate a valid state whenever possible, to reduce memory allocations.
> **Note** `Name` is an `<Entry />`, `NameError` is a `<Label />`, and `FormErrors` is a `<Label />` as well. All these controls are from the [Xamarin.Forms](https://docs.microsoft.com/en-us/xamarin/xamarin-forms/) library.
There are extensions methods for Xamarin.Android and its Material design control `TextInputLayout`. These extensions use internally the `Error` property from the `TextInputLayout` control, allowing you to implement a fully native behavior to showing validation errors. To use these extensions you must import `ReactiveUI.Validation.Extensions` and install either `ReactiveUI.Validation.AndroidSupport` or `ReactiveUI.Validation.AndroidX`:
```
dotnet add package ReactiveUI.Validation.AndroidX
```
> **Note** In ReactiveUI.Validation **1.7 and lower**, the Android-specific extensions are available in [the main package](https://www.nuget.org/packages/reactiveui.validation) targeting `MonoAndroid90`, and you don't need to install `ReactiveUI.Validation.AndroidSupport`. In ReactiveUI.Validation **1.7 and lower**, add `using ReactiveUI.Validation.Platforms.Android` instead of `using ReactiveUI.Validation.Extensions`.
For those platforms that support the `INotifyDataErrorInfo` interface, ReactiveUI.Validation provides a helper base class named `ReactiveValidationObject`. The helper class implements both the `IValidatableViewModel` interface and the `INotifyDataErrorInfo` interface. It listens to any changes in the `ValidationContext` and invokes `INotifyDataErrorInfo` events.
When using a `ValidationRule` overload that accepts an observable, please remember to supply the property which the validation rule is targeting as the first argument. Otherwise it is not possible for `INotifyDataErrorInfo` to conclude which property the error message is for.
You can pass an instance of `IValidationTextFormatter<T>` to a call to `BindValidation` if you'd like to override the default `SingleLineFormatter` used in the validation library. The `SingleLineFormatter` accepts a separator char and uses whitespace by default, so the code snippet below shows how to use a non-default separator char:
If you'd like to override the `IValidationTextFormatter<string>` used in ReactiveUI.Validation by default, register an instance of `IValidationTextFormatter<string>` into `Locator.CurrentMutable` before your app starts. This could be useful in cases when your app needs localization and you wish to pass message keys instead of messages to `ValidationRule` calls.
```cs
// Register a singleton instance of IValidationTextFormatter<string> into Splat.Locator.
In essence, ReactiveUI.Validation is a relatively simple model of the `ValidationContext` containing a list of `IValidationComponent` instances. An `IValidationComponent` provides an observable of `IValidationState`. Whenever validation state changes (either a transition of validity) or `ValidationText` changes, then a new value is pushed out.
1. Rules can be composed of single or multiple properties along with more generic Observables.
2. Validation text can encapsulate both valid and invalid states.
3. Binding can occur to either a View or an action.
4. Validation text can reference either the ViewModel or properties which comprise the validation rule e.g. include text entered as part of validation message.
5. Validation text output can be adjusted using custom formatters, not only allowing for single & multiline output but also for platforms like Android it should be possible to achieve richer renderings i.e. Bold/italics.
ReactiveUI.Validation is developed under an OSI-approved open source license, making it freely usable and distributable, even for commercial use. We ❤ the people who are involved in this project, and we’d love to have you on board, especially if you are just getting started or have never contributed to open-source before.
