Question

我正在尝试通过创建一个能够创建投影的简单事件存储来学习如何使用Go。我不知道如何使用包含混合类型结构的切片和贴图。与此相关的一点是，我希望开发人员根据需要在各个领域创建尽可能多的实现IEntity和IEvent的结构。

我来自JavaScript / Node.js背景，有一些C / C ++ / Java的基础知识，我可能在这里寻找类/继承模式，并需要一些帮助来解决如何在Go中获得相同的功能

package main

import (
    "sync"
    "time"

    uuid "github.com/satori/go.uuid"
)

// IEntity describes an entity, a struct that is the sum of all events resolved in a chronological order
type IEntity interface {
}

// IProjection describes the interface for a projection struct
type IProjection interface {
    Lock()
    Unlock()
    Get(id uuid.UUID) IEntity
    Set(id uuid.UUID, entity IEntity)
    GetLastEventTime() time.Time
    Append(event IEvent)
}

// IEvent describes the interface for a event, any event added to the system needs to implement this interface
type IEvent interface {
    Resolve(projection IProjection)
}

// EventVault is the base struct that keeps all the events and allows for projections to be created
type EventVault struct {
    sync.Mutex
    events []IEvent
}

// Append adds a IEvent to the projection and runs that events IEvent.Resolve method
func (vault *EventVault) Append(event IEvent) {
    vault.Lock()
    defer vault.Unlock()
    vault.events = append(vault.events, event)
}

// Project creates a projection with entities from all events up until the choosen end time
func (vault *EventVault) Project(endTime time.Time, projection IProjection) IProjection {
    lastEventTime := projection.GetLastEventTime()
    for index := range vault.events {
        event := vault.events[index]
        if event.Time.After(lastEventTime) && event.Time.Before(endTime) {
            projection.Append(event)
        }
    }
    return projection
}

// Projection caculates and stores a projection of the events appended to it with the Append method
type Projection struct {
    sync.Mutex
    events   []IEvent
    entities map[uuid.UUID]IEntity
}

// Get returns an IEntity struct from an id (for use in the IEvent.Resolve method)
func (projection *Projection) Get(id uuid.UUID) IEntity {
    return projection.entities[id]
}

// Set add a IEntity to the projection (for use in the IEvent.Resolve method)
func (projection *Projection) Set(id uuid.UUID, entity IEntity) {
    projection.entities[id] = entity
}

// GetLastEventTime returns the time for the event that was added last
func (projection *Projection) GetLastEventTime() time.Time {
    event := projection.events[len(projection.events)-1]

    if event == nil {
        return time.Unix(0, 0)
    }

    return event.Time
}

// Append adds a IEvent to the projection and runs that events IEvent.Resolve method
func (projection *Projection) Append(event IEvent) {
    projection.Lock()
    projection.events = append(projection.events, event)
    event.Resolve(projection)
    projection.Unlock()
}

// ------------------ Below is usage of above system ------------------

// PlayerEntity is a sample entity that can be used for testing
type PlayerEntity struct {
    ID        uuid.UUID
    Name      string
    Birthday  time.Time
    UpdatedAt time.Time
}

// AddPlayerEvent is a sample event that can be used for testing
type AddPlayerEvent struct {
    ID   uuid.UUID
    Time time.Time
    Name string
}

// Resolve will create a new PlayerEntity and add that to the projection
func (event *AddPlayerEvent) Resolve(projection IProjection) {
    player := PlayerEntity{ID: uuid.NewV4(), Name: event.Name, UpdatedAt: event.Time}
    projection.Set(player.ID, &player)
}

// SetBirthdayPlayerEvent is a sample event that can be used for testing
type SetBirthdayPlayerEvent struct {
    ID       uuid.UUID
    Time     time.Time
    Birthday time.Time
}

// Resolve will change the name on a PlayerEntity
func (event *SetBirthdayPlayerEvent) Resolve(projection IProjection) {
    player := *projection.Get(event.ID)
    player.Birthday = event.Birthday
    player.UpdatedAt = event.Time
}

func main() {
    vault := EventVault{}
    event1 := AddPlayerEvent{ID: uuid.NewV4(), Time: time.Now(), Name: "Lisa"}
    vault.Append(&event1)
    birthday, _ := time.Parse("2006-01-02", "2017-03-04")
    event2 := SetBirthdayPlayerEvent{ID: event1.ID, Time: time.Now(), Birthday: birthday}
    vault.Append(&event2)
}

我得到的错误是

./main.go:47: event.Time undefined (type IEvent has no field or method Time)
./main.go:79: event.Time undefined (type IEvent has no field or method Time)
./main.go:122: invalid indirect of projection.Get(event.ID) (type IEntity)

结构类型可能已丢失，所以我需要另一种方法来存储它们以将它们放入地图和切片中但是如何？

Answer 1

在golang中声明类似

的界面时

type IEntity interface {
}

您可以定义在没有类型转换的情况下可以使用此接口完成的所有操作。所以在这里你没有为这个界面定义任何功能。如果你想要的功能，你需要给它一个像

这样的方法

type IEntity interface {
    Time() time.Time
}

任何想要与该接口一起使用的类型都必须实现这些功能，即

func (a AddPlayerEvent) Time() time.Time {
    return a.Time 
}

See the docs

然后你可以使用任何这些方法

func (projection *Projection) Append(event IEvent) {
    ...
    event.Time()
    ...
}

另外2个笔记

在迭代地图时，您可以使用k，v：= range my_map
* projection.Get（event.ID）尝试使用非指针类型

使用混合结构类型转换贴图和切片

1 个答案: