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projector.go
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projector.go
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// Copyright (c) 2017 - The Event Horizon authors.
//
// 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 todo
import (
"context"
"errors"
"fmt"
eh "github.com/looplab/eventhorizon"
"github.com/looplab/eventhorizon/eventhandler/projector"
)
// Projector is a projector of todo list events on the TodoList read model.
type Projector struct{}
// ProjectorType implements the ProjectorType method of the
// eventhorizon.Projector interface.
func (p *Projector) ProjectorType() projector.Type {
return projector.Type(AggregateType.String())
}
// Project implements the Project method of the eventhorizon.Projector interface.
func (p *Projector) Project(ctx context.Context,
event eh.Event, entity eh.Entity) (eh.Entity, error) {
model, ok := entity.(*TodoList)
if !ok {
return nil, errors.New("model is of incorrect type")
}
switch event.EventType() {
case Created:
// Set the ID when first created.
model.ID = event.AggregateID()
model.Items = []*TodoItem{} // Prevents "null" in JSON.
model.CreatedAt = TimeNow()
case Deleted:
// Return nil as the entity to delete the model.
return nil, nil
case ItemAdded:
data, ok := event.Data().(*ItemAddedData)
if !ok {
return nil, errors.New("invalid event data")
}
model.Items = append(model.Items, &TodoItem{
ID: data.ItemID,
Description: data.Description,
})
case ItemRemoved:
data, ok := event.Data().(*ItemRemovedData)
if !ok {
return nil, errors.New("invalid event data")
}
for i, item := range model.Items {
if item.ID == data.ItemID {
model.Items = append(model.Items[:i], model.Items[i+1:]...)
break
}
}
case ItemDescriptionSet:
data, ok := event.Data().(*ItemDescriptionSetData)
if !ok {
return nil, errors.New("invalid event data")
}
for _, item := range model.Items {
if item.ID == data.ItemID {
item.Description = data.Description
}
}
case ItemChecked:
data, ok := event.Data().(*ItemCheckedData)
if !ok {
return nil, errors.New("invalid event data")
}
for _, item := range model.Items {
if item.ID == data.ItemID {
item.Completed = data.Checked
}
}
default:
// Also return the modele here to not delete it.
return model, fmt.Errorf("could not project event: %s", event.EventType())
}
// Always increment the version and set update time on successful updates.
model.Version++
model.UpdatedAt = TimeNow()
return model, nil
}