feat(core): specify event listener API

core/appmodule/event.go

@@ -0,0 +1,37 @@
+package appmodule
+
+import (
+	"context"
+
+	"google.golang.org/protobuf/runtime/protoiface"
+)
+
+// HasEventListeners is the extension interface that modules should implement to register
+// event listeners.
+type HasEventListeners interface {
+	AppModule
+
+	// RegisterEventListeners registers the module's events listeners.
+	RegisterEventListeners(registrar *EventListenerRegistrar)
+}
+
+// EventListenerRegistrar allows registering event listeners.
+type EventListenerRegistrar struct {
+	listeners []any
+}
+
+// GetListeners gets the event listeners that have been registered
+func (e *EventListenerRegistrar) GetListeners() []any {
+	return e.listeners
+}
+
+// RegisterEventListener registers an event listener for event type E.
+func RegisterEventListener[E protoiface.MessageV1](registrar *EventListenerRegistrar, listener func(context.Context, E)) {
+	registrar.listeners = append(registrar.listeners, listener)
+}
+
+// RegisterEventInterceptor registers an event interceptor for event type E. Event interceptors can return errors
+// to cause the process which emitted the event to fail.
+func RegisterEventInterceptor[E protoiface.MessageV1](registrar *EventListenerRegistrar, interceptor func(context.Context, E) error) {
+	registrar.listeners = append(registrar.listeners, interceptor)
+}

core/appmodule/event_test.go

@@ -0,0 +1,22 @@
+package appmodule
+
+import (
+	"context"
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/types/known/structpb"
+	"google.golang.org/protobuf/types/known/timestamppb"
+)
+
+func TestEventListenerRegistrar(t *testing.T) {
+	registrar := &EventListenerRegistrar{}
+	RegisterEventListener(registrar, func(ctx context.Context, dummy *timestamppb.Timestamp) {})
+	RegisterEventInterceptor(registrar, func(ctx context.Context, dummy *structpb.Struct) error {
+		return nil
+	})
+	require.Len(t, registrar.listeners, 2)
+	require.Equal(t, reflect.Func, reflect.TypeOf(registrar.listeners[0]).Kind())
+	require.Equal(t, reflect.Func, reflect.TypeOf(registrar.listeners[1]).Kind())
+}