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Decoration
Using abilities of Microsoft's (and not only theirs) DI-container you could decorate your operations handlers with additional logic which will be applied in processing pipeline. This really almost the same functionality as with Filters but in some cases it could have it's own place. In example below we will add some decorators to process our queries and commands.
// Define interface for all decorators.
public interface ICommandDecorator<TCommand, TResult> where TCommand : ICommand<TResult>
{
Task<TResult> DecorateAsync(TCommand command, IOperationHandler<TCommand, TResult> handler, CancellationToken cancellationToken);
}
public interface IQueryDecorator<TQuery, TResult> where TQuery : IQuery<TResult>
{
Task<TResult> DecorateAsync(TQuery query, IOperationHandler<TQuery, TResult> handler, CancellationToken cancellationToken);
}
public interface ICommandDecorator<TCommand, TResult> where TCommand : ICommand<TResult>
{
Task<TResult> DecorateAsync(TCommand command, IOperationHandler<TCommand, TResult> handler, CancellationToken cancellationToken);
}
// Define base decorator for all your decorators
public abstract class BaseOperationDecorator<TOperation, TResult> where TOperation : IOperation<TResult>
{
public abstract Task<TResult> DecorateAsync(TOperation operation, IOperationHandler<TOperation, TResult> handler, CancellationToken cancellationToken);
}
// Define operation decorator that will dispatch command and query decorators
public class OperationDecorator<TOperation, TResult> : IOperationHandler<TOperation, TResult>
where TOperation : IOperation<TResult>
{
private readonly IOperationHandler<TOperation, TResult> _handler;
private readonly IServiceProvider _serviceProvider;
public OperationDecorator(IOperationHandler<TOperation, TResult> handler,
IServiceProvider serviceProvider)
{
if (handler == null)
{
throw new ArgumentNullException(nameof(handler));
}
if (serviceProvider == null)
{
throw new ArgumentNullException(nameof(serviceProvider));
}
_handler = handler;
_serviceProvider = serviceProvider;
}
public Task<TResult> HandleAsync(TOperation operation, CancellationToken cancellationToken)
{
if (operation is ICommand<TResult>)
{
return ((BaseOperationDecorator<TOperation, TResult>)_serviceProvider.GetService(typeof(ICommandDecorator<,>).MakeGenericType(operation.GetType(), typeo(TResult))))
.DecorateAsync(operation, _handler, cancellationToken);
}
else if (operation is IQuery<TResult>)
{
return ((BaseOperationDecorator<TOperation, TResult>)_serviceProvider.GetService(typeof(IQueryDecorator<,>).MakeGenericType(operation.GetType(), typeof(TResult))))
.DecorateAsync(operation, _handler, cancellationToken);
}
else
{
return _handler.HandleAsync(operation, cancellationToken);
}
}
}
// Create your own specific command decorator that have some business-logic payload.
public class CommandDecorator<TCommand, TResult> : BaseOperationDecorator<TCommand, TResult>, ICommandDecorator<TCommand, TResult> where TCommand : ICommand<TResult>
{
private readonly IStringWriter _writer;
public CommandDecorator(IStringWriter writer) => _writer = writer;
public override async Task<TResult> DecorateAsync(TCommand command, IOperationHandler<TCommand, TResult> handler, CancellationToken cancellationToken)
{
await _writer.WriteLineAsync(quot;Preprocess command `{typeof(TCommand).Name}` by `{GetType().Name}`");
var result = await handler.HandleAsync(command, cancellationToken);
await _writer.WriteLineAsync(quot;Postprocess command `{typeof(TCommand).Name}` by `{GetType().Name}`");
return result;
}
}
That's it! Now running of every command will be accompanied by writing some logs.
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