snapshot of https://github.com/ThreeDotsLabs/watermill.git (master, 2026-09-15; blobs > 131072B omitted)

components/cqrs/command_handler.go

@@ -0,0 +1,59 @@
+package cqrs
+
+import (
+	"context"
+)
+
+// CommandHandler receives a command defined by NewCommand and handles it with the Handle method.
+// If using DDD, CommandHandler may modify and persist the aggregate.
+//
+// In contrast to EventHandler, every Command must have only one CommandHandler.
+//
+// One instance of CommandHandler is used during handling messages.
+// When multiple commands are delivered at the same time, Handle method can be executed multiple times at the same time.
+// Because of that, Handle method needs to be thread safe!
+type CommandHandler interface {
+	// HandlerName is the name used in message.Router while creating handler.
+	//
+	// It will be also passed to CommandsSubscriberConstructor.
+	// May be useful, for example, to create a consumer group per each handler.
+	//
+	// WARNING: If HandlerName was changed and is used for generating consumer groups,
+	// it may result with **reconsuming all messages**!
+	HandlerName() string
+
+	NewCommand() any
+
+	Handle(ctx context.Context, cmd any) error
+}
+
+type genericCommandHandler[Command any] struct {
+	handleFunc  func(ctx context.Context, cmd *Command) error
+	handlerName string
+}
+
+// NewCommandHandler creates a new CommandHandler implementation based on provided function
+// and command type inferred from function argument.
+func NewCommandHandler[Command any](
+	handlerName string,
+	handleFunc func(ctx context.Context, cmd *Command) error,
+) CommandHandler {
+	return &genericCommandHandler[Command]{
+		handleFunc:  handleFunc,
+		handlerName: handlerName,
+	}
+}
+
+func (c genericCommandHandler[Command]) HandlerName() string {
+	return c.handlerName
+}
+
+func (c genericCommandHandler[Command]) NewCommand() any {
+	tVar := new(Command)
+	return tVar
+}
+
+func (c genericCommandHandler[Command]) Handle(ctx context.Context, cmd any) error {
+	command := cmd.(*Command)
+	return c.handleFunc(ctx, command)
+}