forked from looplab/eventhorizon
/
commandhandler.go
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/
commandhandler.go
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// Copyright (c) 2014 - Max Ekman <max@looplab.se>
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package eventhorizon
import (
"errors"
"reflect"
"time"
)
// ErrNilRepository is when a dispatcher is created with a nil repository.
var ErrNilRepository = errors.New("repository is nil")
// ErrAggregateAlreadySet is when an aggregate is already registered for a command.
var ErrAggregateAlreadySet = errors.New("aggregate is already set")
// ErrAggregateNotFound is when no aggregate can be found.
var ErrAggregateNotFound = errors.New("no aggregate for command")
// CommandFieldError is returned by Dispatch when a field is incorrect.
type CommandFieldError struct {
Field string
}
func (c CommandFieldError) Error() string {
return "missing field: " + c.Field
}
// AggregateCommandHandler dispatches commands to registered aggregates.
//
// The dispatch process is as follows:
// 1. The handler receives a command
// 2. An aggregate is created or rebuilt from previous events by the repository
// 3. The aggregate's command handler is called
// 4. The aggregate stores events in response to the command
// 5. The new events are stored in the event store by the repository
// 6. The events are published to the event bus when stored by the event store
type AggregateCommandHandler struct {
repository Repository
aggregates map[string]string
}
// NewAggregateCommandHandler creates a new AggregateCommandHandler.
func NewAggregateCommandHandler(repository Repository) (*AggregateCommandHandler, error) {
if repository == nil {
return nil, ErrNilRepository
}
h := &AggregateCommandHandler{
repository: repository,
aggregates: make(map[string]string),
}
return h, nil
}
// SetAggregate sets an aggregate as handler for a command.
func (h *AggregateCommandHandler) SetAggregate(aggregate Aggregate, command Command) error {
// Check for already existing handler.
if _, ok := h.aggregates[command.CommandType()]; ok {
return ErrAggregateAlreadySet
}
// Add aggregate type to command type.
h.aggregates[command.CommandType()] = aggregate.AggregateType()
return nil
}
// HandleCommand handles a command with the registered aggregate.
// Returns ErrAggregateNotFound if no aggregate could be found.
func (h *AggregateCommandHandler) HandleCommand(command Command) error {
err := h.checkCommand(command)
if err != nil {
return err
}
var aggregateType string
var ok bool
if aggregateType, ok = h.aggregates[command.CommandType()]; !ok {
return ErrAggregateNotFound
}
var aggregate Aggregate
if aggregate, err = h.repository.Load(aggregateType, command.AggregateID()); err != nil {
return err
}
if aggregate == nil {
return ErrAggregateNotFound
}
if err = aggregate.HandleCommand(command); err != nil {
return err
}
if err = h.repository.Save(aggregate); err != nil {
return err
}
return nil
}
func (h *AggregateCommandHandler) checkCommand(command Command) error {
rv := reflect.Indirect(reflect.ValueOf(command))
rt := rv.Type()
for i := 0; i < rt.NumField(); i++ {
field := rt.Field(i)
if field.PkgPath != "" {
continue // Skip private field.
}
tag := field.Tag.Get("eh")
if tag == "optional" {
continue // Optional field.
}
if isZero(rv.Field(i)) {
return CommandFieldError{field.Name}
}
}
return nil
}
func isZero(v reflect.Value) bool {
switch v.Kind() {
case reflect.Func, reflect.Map, reflect.Slice:
return v.IsNil()
case reflect.Struct:
// Special case to get zero values by method.
switch obj := v.Interface().(type) {
case time.Time:
return obj.IsZero()
}
// Check public fields for zero values.
z := true
for i := 0; i < v.NumField(); i++ {
if v.Type().Field(i).PkgPath != "" {
continue // Skip private fields.
}
z = z && isZero(v.Field(i))
}
return z
}
// Compare other types directly:
z := reflect.Zero(v.Type())
return v.Interface() == z.Interface()
}