Connected.Components/Components/Form/FormComponent.cs

using System.ComponentModel.DataAnnotations;
using System.Diagnostics.CodeAnalysis;
using System.Globalization;
using System.Linq.Expressions;
using System.Reflection;
using Connected.Annotations;
using Microsoft.AspNetCore.Components;
using Microsoft.AspNetCore.Components.Forms;
using static System.String;

namespace Connected.Components;

public abstract class FormComponent<T, U> : UIComponent, IFormComponent, IDisposable
{
	 private Converter<T, U> _converter;

	 /// <summary>
	 /// Invoked whenever the string value cannot be converted
	 /// </summary>
	 public event EventHandler<string> ConversionErrorOccured;

	 protected FormComponent(Converter<T, U> converter)
	 {
		  _converter = converter ?? throw new ArgumentNullException(nameof(converter));
		  _converter.ErrorOccured += (s, e) => OnConversionError(e);
	 }

	 [CascadingParameter]
	 internal IForm? Form { get; set; }

	 /// <summary>
	 /// If true, this is a top-level form component. If false, this input is a sub-component of another input (i.e. TextField, Select, etc).
	 /// If it is sub-component, it will NOT do form validation!!
	 /// </summary>
	 [CascadingParameter(Name = "SubscribeToParentForm")]
	 internal bool SubscribeToParentForm { get; set; } = true;

	 /// <summary>
	 /// If true, this form input is required to be filled out.
	 /// </summary>
	 [Parameter]
	 public bool Required { get; set; }

	 /// <summary>
	 /// The error text that will be displayed if the input is not filled out but required.
	 /// </summary>
	 [Parameter]
	 public string RequiredError { get; set; } = "Required";

	 /// <summary>
	 /// The ErrorText that will be displayed if <see cref="HasError"/> is set to true.
	 /// </summary>
	 [Parameter]
	 public string ErrorText { get; set; }

	 /// <summary>
	 /// If true, the label will be displayed in an error state.
	 /// </summary>
	 [Parameter]
	 public bool HasError { get; set; }

	 /// <summary>
	 /// The ErrorId that will be used by aria-describedby if <see cref="HasError"/> is true
	 /// </summary>
	 [Parameter]
	 public string ErrorId { get; set; }

	 /// <summary>
	 /// The generic converter of the component.
	 /// </summary>
	 public Converter<T, U> Converter => _converter;

	 /// <summary>
	 /// The culture of the component. Also sets the culture of the <see cref="Converter"/> .
	 /// </summary>
	 [Parameter]
	 public CultureInfo Culture
	 {
		  get => _converter.Culture;
		  set => SetCulture(value);
	 }

	 private string _conversionError { get; set; }

	 protected virtual bool SetConverter(Converter<T, U> value)
	 {
		  var changed = _converter != value;

		  if (changed)
		  {
				/*
             * Converter is mandatory at all times
             */
				_converter = value ?? throw new ArgumentNullException(nameof(value));
				_converter.ErrorOccured += (s, e) => OnConversionError(e);
		  }

		  return changed;
	 }

	 protected virtual bool SetCulture(CultureInfo value)
	 {
		  var changed = _converter.Culture != value;

		  if (changed)
				_converter.Culture = value;

		  return changed;
	 }

	 private void OnConversionError(string error)
	 {
		  // note: we need to update the form here because the conversion error might lead to not updating the value
		  // ... which leads to not updating the form

		  //TODO Why does the form need to be updated?
		  Modified = true;

		  _conversionError = error;

		  Form?.Update(this);

		  ConversionErrorOccured?.Invoke(this, error);
	 }

	 /// <summary>
	 /// True if the conversion from string to T failed
	 /// </summary>
	 public bool ConversionError => !string.IsNullOrWhiteSpace(_conversionError);

	 /// <summary>
	 /// The error message of the conversion error from string to T. Null otherwise
	 /// </summary>
	 public string ConversionErrorMessage => _conversionError;

	 /// <summary>
	 /// True if the input has any of the following errors: An error set from outside, a conversion error or
	 /// one or more validation errors
	 /// </summary>
	 public bool HasErrors => HasError || ConversionError || ValidationErrors.Count > 0;

	 /// <summary>
	 /// Return the validation error text or the conversion error message.
	 /// </summary>
	 /// <returns>Error text/message</returns>
	 public string? GetErrorText()
	 {
		  // ErrorText is either set from outside or the first validation error
		  if (!IsNullOrWhiteSpace(ErrorText))
				return ErrorText;

		  if (!IsNullOrWhiteSpace(ConversionErrorMessage))
				return ConversionErrorMessage;

		  return null;
	 }

	 /// <summary>
	 /// This manages the state of having been modified by the user. A form control always starts out unmodified
	 /// but becomes modified when the user performed input or the blur event was raised.
	 ///
	 /// The modified state is only relevant for inputs that have no value (i.e. empty text fields). Being unmodified will
	 /// suppress the display of the  <see cref="RequiredError"/>
	 /// </summary>
	 public bool Modified { get; protected set; }

	 #region MudForm Validation

	 public List<string> ValidationErrors { get; set; } = new List<string>();

	 /// <summary>
	 /// A validation func or a validation attribute. Supported types are:
	 /// <para>Func&lt;T, bool&gt; ... will output the standard error message "Invalid" if false</para>
	 /// <para>Func&lt;T, string&gt; ... outputs the result as error message, no error if null </para>
	 /// <para>Func&lt;T, IEnumerable&lt; string &gt;&gt; ... outputs all the returned error messages, no error if empty</para>
	 /// <para>Func&lt;object, string, IEnumerable&lt; string &gt;&gt; input Form.Model, Full Path of Member ... outputs all the returned error messages, no error if empty</para>
	 /// <para>Func&lt;T, Task&lt; bool &gt;&gt; ... will output the standard error message "Invalid" if false</para>
	 /// <para>Func&lt;T, Task&lt; string &gt;&gt; ... outputs the result as error message, no error if null</para>
	 /// <para>Func&lt;T, Task&lt;IEnumerable&lt; string &gt;&gt;&gt; ... outputs all the returned error messages, no error if empty</para>
	 /// <para>Func&lt;object, string, Task&lt;IEnumerable&lt; string &gt;&gt;&gt; input Form.Model, Full Path of Member ... outputs all the returned error messages, no error if empty</para>
	 /// <para>System.ComponentModel.DataAnnotations.ValidationAttribute instances</para>
	 /// </summary>
	 [Parameter]
	 [Category(CategoryTypes.FormComponent.Validation)]
	 public object Validation { get; set; }

	 private T __value;

	 /// <summary>
	 /// This is the form component's value.
	 /// </summary>
	 protected T _value
	 {
		  get => __value;
		  set
		  {
				__value = value;
				_conversionError = null;
		  }
	 }

	 // These are the fire-and-forget methods to launch an async validation process.
	 // After each async step, we make sure the current Value of the component has not changed while
	 // async code was executed to avoid race condition which could lead to incorrect validation results.
	 protected void BeginValidateAfter(Task task)
	 {
		  Func<Task> execute = async () =>
		  {
				var value = _value;

				await task;

				// we validate only if the value hasn't changed while we waited for task.
				// if it has in fact changed, another validate call will follow anyway
				if (EqualityComparer<T>.Default.Equals(value, _value))
				{
					 BeginValidate();
				}
		  };
		  execute().AndForget();
	 }

	 protected void BeginValidate()
	 {
		  Func<Task> execute = async () =>
		  {
				var value = _value;

				await ValidateValue();

				if (EqualityComparer<T>.Default.Equals(value, _value))
				{
					 EditFormValidate();
				}
		  };
		  execute().AndForget();
	 }

	 /// <summary>
	 /// Cause this component to validate its value.
	 /// </summary>
	 public Task Validate()
	 {
		  // when a validation is forced, we must set Touched to true, because for untouched fields with
		  // no value, validation does nothing due to the way forms are expected to work (display errors
		  // only after fields have been touched).
		  Modified = true;
		  return ValidateValue();
	 }

	 protected virtual async Task ValidateValue()
	 {
		  var changed = false;
		  var errors = new List<string>();
		  try
		  {
				// conversion error
				if (ConversionError)
					 errors.Add(ConversionErrorMessage);
				// validation errors
				if (Validation is ValidationAttribute)
					 ValidateWithAttribute(Validation as ValidationAttribute, _value, errors);
				else if (Validation is Func<T, bool>)
					 ValidateWithFunc(Validation as Func<T, bool>, _value, errors);
				else if (Validation is Func<T, string>)
					 ValidateWithFunc(Validation as Func<T, string>, _value, errors);
				else if (Validation is Func<T, IEnumerable<string>>)
					 ValidateWithFunc(Validation as Func<T, IEnumerable<string>>, _value, errors);
				else if (Validation is Func<object, string, IEnumerable<string>>)
					 ValidateModelWithFullPathOfMember(Validation as Func<object, string, IEnumerable<string>>, errors);
				else
				{
					 var value = _value;

					 if (Validation is Func<T, Task<bool>>)
						  await ValidateWithFunc(Validation as Func<T, Task<bool>>, _value, errors);
					 else if (Validation is Func<T, Task<string>>)
						  await ValidateWithFunc(Validation as Func<T, Task<string>>, _value, errors);
					 else if (Validation is Func<T, Task<IEnumerable<string>>>)
						  await ValidateWithFunc(Validation as Func<T, Task<IEnumerable<string>>>, _value, errors);
					 else if (Validation is Func<object, string, Task<IEnumerable<string>>>)
						  await ValidateModelWithFullPathOfMember(Validation as Func<object, string, Task<IEnumerable<string>>>, errors);

					 changed = !EqualityComparer<T>.Default.Equals(value, _value);
				}

				// Run each validation attributes of the property targeted with `For`
				if (_validationAttrsFor != null)
				{
					 foreach (var attr in _validationAttrsFor)
					 {
						  ValidateWithAttribute(attr, _value, errors);
					 }
				}

				// required error (must be last, because it is least important!)
				if (Required)
				{
					 if (Modified && !HasValue(_value))
						  errors.Add(RequiredError);
				}
		  }
		  finally
		  {
				// If Value has changed while we were validating it, ignore results and exit
				if (!changed)
				{
					 // this must be called in any case, because even if Validation is null the user might have set Error and ErrorText manually
					 // if Error and ErrorText are set by the user, setting them here will have no effect.
					 // if Error, create an error id that can be used by aria-describedby on input control
					 ValidationErrors = errors;
					 HasError = errors.Count > 0;
					 ErrorText = errors.FirstOrDefault();
					 ErrorId = HasErrors ? Guid.NewGuid().ToString() : null;
					 Form?.Update(this);
					 StateHasChanged();
				}
		  }
	 }

	 protected virtual bool HasValue(T value)
	 {
		  if (typeof(T) == typeof(string))
				return !IsNullOrWhiteSpace(value as string);

		  return value != null;
	 }

	 [UnconditionalSuppressMessage("Trimming", "IL2026:Members annotated with 'RequiresUnreferencedCodeAttribute' require dynamic access otherwise can break functionality when trimming application code", Justification = "In the context of EditContext.Model / FieldIdentifier.Model they won't get trimmed.")]
	 protected virtual void ValidateWithAttribute(ValidationAttribute attr, T value, List<string> errors)
	 {
		  try
		  {
				// The validation context is applied either on the `EditContext.Model`, '_fieldIdentifier.Model', or `this` as a stub subject.
				// Complex validation with fields references (like `CompareAttribute`) should use an EditContext or For when not using EditContext.
				var validationContextSubject = EditContext?.Model ?? _fieldIdentifier.Model ?? this;
				var validationContext = new ValidationContext(validationContextSubject);
				if (validationContext.MemberName is null && _fieldIdentifier.FieldName is not null)
					 validationContext.MemberName = _fieldIdentifier.FieldName;
				var validationResult = attr.GetValidationResult(value, validationContext);
				if (validationResult != ValidationResult.Success)
					 errors.Add(validationResult.ErrorMessage);
		  }
		  catch (Exception e)
		  {
				// Maybe conditionally add full error message if `IWebAssemblyHostEnvironment.IsDevelopment()`
				// Or log using proper logger.
				errors.Add($"An unhandled exception occurred: {e.Message}");
		  }
	 }

	 protected virtual void ValidateWithFunc(Func<T, bool> func, T value, List<string> errors)
	 {
		  try
		  {
				if (!func(value))
					 errors.Add("Invalid");
		  }
		  catch (Exception e)
		  {
				errors.Add("Error in validation func: " + e.Message);
		  }
	 }

	 protected virtual void ValidateWithFunc(Func<T, string> func, T value, List<string> errors)
	 {
		  try
		  {
				var error = func(value);
				if (error != null)
					 errors.Add(error);
		  }
		  catch (Exception e)
		  {
				errors.Add("Error in validation func: " + e.Message);
		  }
	 }

	 protected virtual void ValidateWithFunc(Func<T, IEnumerable<string>> func, T value, List<string> errors)
	 {
		  try
		  {
				foreach (var error in func(value))
					 errors.Add(error);
		  }
		  catch (Exception e)
		  {
				errors.Add("Error in validation func: " + e.Message);
		  }
	 }

	 protected virtual void ValidateModelWithFullPathOfMember(Func<object, string, IEnumerable<string>> func, List<string> errors)
	 {
		  try
		  {
				if (Form?.Model == null)
				{
					 return;
				}

				if (For == null)
				{
					 errors.Add($"For is null, please set parameter For on the form input component of type {GetType().Name}");
					 return;
				}

				foreach (var error in func(Form.Model, For.GetFullPathOfMember()))
					 errors.Add(error);
		  }
		  catch (Exception e)
		  {
				errors.Add("Error in validation func: " + e.Message);
		  }
	 }

	 protected virtual async Task ValidateWithFunc(Func<T, Task<bool>> func, T value, List<string> errors)
	 {
		  try
		  {
				if (!await func(value))
					 errors.Add("Invalid");
		  }
		  catch (Exception e)
		  {
				errors.Add("Error in validation func: " + e.Message);
		  }
	 }

	 protected virtual async Task ValidateWithFunc(Func<T, Task<string>> func, T value, List<string> errors)
	 {
		  try
		  {
				var error = await func(value);
				if (error != null)
					 errors.Add(error);
		  }
		  catch (Exception e)
		  {
				errors.Add("Error in validation func: " + e.Message);
		  }
	 }

	 protected virtual async Task ValidateWithFunc(Func<T, Task<IEnumerable<string>>> func, T value, List<string> errors)
	 {
		  try
		  {
				foreach (var error in await func(value))
					 errors.Add(error);
		  }
		  catch (Exception e)
		  {
				errors.Add("Error in validation func: " + e.Message);
		  }
	 }

	 protected virtual async Task ValidateModelWithFullPathOfMember(Func<object, string, Task<IEnumerable<string>>> func, List<string> errors)
	 {
		  try
		  {
				if (Form?.Model == null)
				{
					 return;
				}

				if (For == null)
				{
					 errors.Add($"For is null, please set parameter For on the form input component of type {GetType().Name}");
					 return;
				}

				foreach (var error in await func(Form.Model, For.GetFullPathOfMember()))
					 errors.Add(error);
		  }
		  catch (Exception e)
		  {
				errors.Add("Error in validation func: " + e.Message);
		  }
	 }

	 /// <summary>
	 /// Notify the Form that a field has changed if SubscribeToParentForm is true
	 /// </summary>
	 protected void FieldChanged(object newValue)
	 {
		  if (SubscribeToParentForm)
				Form?.FieldChanged(this, newValue);
	 }

	 /// <summary>
	 /// Reset the value and the validation.
	 /// </summary>
	 public void Reset()
	 {
		  ResetValue();
		  ResetValidation();
	 }

	 protected virtual void ResetValue()
	 {
		  /* to be overridden */
		  _value = default;
		  _conversionError = null;
		  Modified = false;
		  StateHasChanged();
	 }

	 /// <summary>
	 /// Reset the validation.
	 /// </summary>
	 public void ResetValidation()
	 {
		  HasError = false;
		  ValidationErrors.Clear();
		  ErrorText = null;
		  StateHasChanged();
	 }

	 #endregion


	 #region --> Blazor EditForm validation support

	 /// <summary>
	 /// This is the form validation context for Blazor's <EditForm></EditForm> component
	 /// </summary>
	 [CascadingParameter]
	 EditContext EditContext { get; set; } = default!;

	 /// <summary>
	 /// Triggers field to be validated.
	 /// </summary>
	 internal void EditFormValidate()
	 {
		  if (_fieldIdentifier.FieldName != null)
		  {
				EditContext?.NotifyFieldChanged(_fieldIdentifier);
		  }
	 }

	 /// <summary>
	 /// Specify an expression which returns the model's field for which validation messages should be displayed.
	 /// </summary>
	 [Parameter]
	 public Expression<Func<T>>? For { get; set; }


	 public bool IsForNull => For == null;


	 /// <summary>
	 /// Stores the list of validation attributes attached to the property targeted by <seealso cref="For"/>. If <seealso cref="For"/> is null, this property is null too.
	 /// </summary>
	 private IEnumerable<ValidationAttribute>? _validationAttrsFor;


	 private void OnValidationStateChanged(object sender, ValidationStateChangedEventArgs e)
	 {
		  if (EditContext != null && !_fieldIdentifier.Equals(default(FieldIdentifier)))
		  {
				var error_msgs = EditContext.GetValidationMessages(_fieldIdentifier).ToArray();
				HasError = error_msgs.Length > 0;
				ErrorText = (HasError ? error_msgs[0] : null);
				StateHasChanged();
		  }
	 }

	 /// <summary>
	 /// Points to a field of the model for which validation messages should be displayed.
	 /// </summary>
	 private FieldIdentifier _fieldIdentifier;

	 /// <summary>
	 /// To find out whether or not For parameter has changed we keep a separate reference
	 /// </summary>
	 private Expression<Func<T>>? _currentFor;


	 /// <summary>
	 /// To find out whether or not EditContext parameter has changed we keep a separate reference
	 /// </summary>
	 private EditContext? _currentEditContext;

	 protected override void OnParametersSet()
	 {
		  if (For != null && For != _currentFor)
		  {
				// Extract validation attributes
				// Sourced from https://stackoverflow.com/a/43076222/4839162 
				// and also https://stackoverflow.com/questions/59407225/getting-a-custom-attribute-from-a-property-using-an-expression
				var expression = (MemberExpression)For.Body;
				var propertyInfo = (PropertyInfo)expression.Expression?.Type.GetProperty(expression.Member.Name);
				_validationAttrsFor = propertyInfo?.GetCustomAttributes(typeof(ValidationAttribute), true).Cast<ValidationAttribute>();

				_fieldIdentifier = FieldIdentifier.Create(For);
				_currentFor = For;
		  }

		  if (EditContext != null && EditContext != _currentEditContext)
		  {
				DetachValidationStateChangedListener();
				EditContext.OnValidationStateChanged += OnValidationStateChanged;
				_currentEditContext = EditContext;
		  }
	 }

	 private void DetachValidationStateChangedListener()
	 {
		  if (_currentEditContext != null)
				_currentEditContext.OnValidationStateChanged -= OnValidationStateChanged;
	 }

	 #endregion


	 protected override Task OnInitializedAsync()
	 {
		  RegisterAsFormComponent();
		  return base.OnInitializedAsync();
	 }

	 protected virtual void RegisterAsFormComponent()
	 {
		  if (SubscribeToParentForm)
		  {
				Form?.Add(this);
		  }
	 }

	 /// <summary>
	 /// Called to dispose this instance.
	 /// </summary>
	 /// <param name="disposing"><see langword="true"/> if called within <see cref="IDisposable.Dispose"/>.</param>
	 protected virtual void Dispose(bool disposing)
	 {
	 }

	 void IDisposable.Dispose()
	 {
		  try
		  {
				Form?.Remove(this);
		  }
		  catch { /* ignore */ }
		  DetachValidationStateChangedListener();
		  Dispose(disposing: true);
	 }
}