CQRS與MediatR(五)

有些時候，我們想在命令執行前後統一做額外的事情，例如每個命令執行前驗證資料欄位、紀錄請求者，或是命令執行後，統一執行資料庫交易等。若是使用自訂的Command，通常會使用裝飾者模式 (Decorator Pattern) 做處理，而 MediatR 則使用管線的概念協助。

MediatR管線：

IRequestPreProcessor<> 請求執行前的預處理
IRequestPostProcessor<,> 請求執行後的再處理
IPipelineBehavior<,> 自訂管線行為

以下是我們目前的程式碼，每次執行命令前，我們都會使用 FluentValidation，協助我們驗證資料欄位是否正確，並使用方法注入，更好的顯示程式碼意圖。

然而隨著時間流逝，相似的程式碼，會逐步在每個請求中重複出現，所以我們希望能透過 MediatR 管線，來協助我們自動執行驗證。

[ApiController]
[Route("[controller]")]
public class WeatherForecastController : ControllerBase
{
    private readonly IMediator _mediator;

    public WeatherForecastController(IMediator mediator)
    {
        _mediator = mediator;
    }

    [HttpPost]
    public async Task<IActionResult> CreateWeatherForecast(
        [FromBody] CreateWeatherForecastRequest request,
        [FromServices] IValidator<CreateWeatherForecastCommand> validator)
    {
        var command = new CreateWeatherForecastCommand
        {
            Nation = request.Nation,
            City = request.City,
            Date = request.Date,
            TemperatureC = request.TemperatureC,
            Summary = request.Summary,
        };

        var validate = await validator.ValidateAsync(command);
        if (validate.IsValid is false) return BadRequest(validate.ToDictionary());

        var result = await _mediator.Send(command);

        return result ? Ok() : BadRequest();
    }

    [HttpPut("{id:int}")]
    public async Task<IActionResult> UpdateWeatherForecast(int id,
        [FromBody] UpdateWeatherForecastRequest request,
        [FromServices] IValidator<UpdateWeatherForecastCommand> validator)
    {
        if (id != request.WeatherForecastId) return BadRequest();

        var command = new UpdateWeatherForecastCommand
        {
            WeatherForecastId = request.WeatherForecastId,
            Nation = request.Nation,
            City = request.City,
            Date = request.Date,
            TemperatureC = request.TemperatureC,
            Summary = request.Summary,
        };

        var validate = await validator.ValidateAsync(command);
        if (validate.IsValid is false) return BadRequest(validate.ToDictionary());

        var result = await _mediator.Send(command);

        return result ? Ok() : BadRequest();
    }
}

新增`ValidationBehavior`並實作`IPipelineBehavior`

// 1.只有當透過管道的請求是 Command 命令時，我們才允許執行此 IPipelineBehavior，所以我們使用where進行條件拘束。
public class ValidationBehavior<TRequest, TReponse> : IPipelineBehavior<TRequest, TReponse> where TRequest : ICommand<TReponse>
{
    // 2.查詢所有實作AbstractValidator類別
    private readonly IEnumerable<IValidator<TRequest>> _validators;

    public ValidationBehavior(IEnumerable<IValidator<TRequest>> validators)
    {
        _validators = validators;
    }

    public async Task<TReponse> Handle(
        TRequest request,
        RequestHandlerDelegate<TReponse> next,
        CancellationToken cancellationToken)
    {
        if (!_validators.Any()) return await next();

        var context = new ValidationContext<TRequest>(request);

        // 3.執行驗證，並只取出驗證錯誤的部分
        var errorsDictionary = _validators
            .Select(validator => validator.Validate(context))
            .Where(validationResult => !validationResult.IsValid)
            .SelectMany(validationResult => validationResult.Errors)
            .GroupBy(
                x => x.PropertyName,
                x => x.ErrorMessage,
                (propertyName, errorMessages) => new
                {
                    Key = propertyName,
                    Values = errorMessages.Distinct().ToArray()
                })
            .ToDictionary(x => x.Key, x => x.Values);

        // 4.如果有任何驗證錯誤，拋出ValidationException，阻止進一步執行
        if (errorsDictionary.Any())
        {
            throw new Exceptions.ValidationException(errorsDictionary);
        }

        return await next();
    }
}

新增`ValidationException`記錄錯誤

public class ValidationException : Exception
{
    public IDictionary<string, string[]> Errros { get; }

    public ValidationException(IDictionary<string, string[]> errors)
    {
        Errros = errors;
    }
}

public record ValidationError(string PropertyName, string ErrorMessage);

新增`ExceptionHandlingMiddleware`處理驗證異常

public class ExceptionHandlingMiddleware : IMiddleware
{
    private readonly ILogger<ExceptionHandlingMiddleware> _logger;

		public ExceptionHandlingMiddleware(ILogger<ExceptionHandlingMiddleware> logger)
    {
        _logger = logger;
    }

    public async Task InvokeAsync(HttpContext context, RequestDelegate next)
    {
        try
        {
            await next(context);
        }
        catch (Exception e)
        {
            _logger.LogError(e, e.Message);
            await HandleExceptionAsync(context, e);
        }
    }

    private static async Task HandleExceptionAsync(HttpContext context, Exception exception)
    {
        var exceptionDetails = GetExceptionDetails(exception);

        var problemDetails = new ProblemDetails
        {
            Status = exceptionDetails.StatusCode,
            Type = exceptionDetails.Type,
            Title = exceptionDetails.Title,
            Detail = exceptionDetails.Details,
        };

        if (exceptionDetails.Errors != null)
        {
            problemDetails.Extensions["errors"] = exceptionDetails.Errors;
        }

        context.Response.ContentType = "application/json";
        context.Response.StatusCode = exceptionDetails.StatusCode;
        await context.Response.WriteAsJsonAsync(problemDetails);
    }

    private static ExceptionDetails GetExceptionDetails(Exception exception)
    {
				// 驗證錯誤統一使用400錯誤，其他則使用500，顯示伺服器錯誤
        return exception switch
        {
            ValidationException validationException => new ExceptionDetails(
                StatusCodes.Status400BadRequest,
                "ValidationFailure",
                "Validation error",
                "One or more validation errors has occurred",
                validationException.Errros),
            _ => new ExceptionDetails(
                StatusCodes.Status500InternalServerError,
                "ServerError",
                "Server error",
                "An unexpected error has occured",
                null)
        };
    }
}

internal record ExceptionDetails(int StatusCode, string Type, string Title, string Details, IDictionary<string, string[]> Errors);

在`IServiceCollection`註冊

builder.Services.AddMediatR(cf =>
{
    cf.RegisterServicesFromAssembly(applicationAssemblies);

    cf.AddOpenBehavior(typeof(ValidationBehavior<,>));
});
builder.Services.AddTransient<ExceptionHandlingMiddleware>();

// ...
app.UseMiddleware<ExceptionHandlingMiddleware>();

新增ValidationBehavior並實作IPipelineBehavior

新增ValidationException記錄錯誤

新增ExceptionHandlingMiddleware處理驗證異常

在IServiceCollection註冊

Controller移除原本舊的驗證

新增`ValidationBehavior`並實作`IPipelineBehavior`

新增`ValidationException`記錄錯誤

新增`ExceptionHandlingMiddleware`處理驗證異常

在`IServiceCollection`註冊