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。我被困在如何使用包含混合类型结构的 slice 和映射上。这样做的要点是，我希望开发人员根据需要在各个字段中创建尽可能多的实现 IEntity 和 IEvent 的结构。

我来自 JavaScript/Node.js 背景，具有一些 C/C++/Java 的基本知识，我可能在这里寻找类/继承模式并需要一些帮助来了解如何在 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)

结构类型可能丢失了，所以我需要另一种存储它们的方法，以便将它们放入映射和 slice 中，但是如何呢？

最佳答案

在 golang 中，当你声明一个像

type IEntity interface {
}

您可以定义无需类型转换即可使用此接口(interface)完成的所有操作。所以在这里你没有为这个接口(interface)定义任何功能。如果你想要功能而不是你必须给它一个方法，比如

type IEntity interface {
    Time() time.Time
}

任何想要与该接口(interface)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

然后你可以使用这些方法中的任何一个

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

还有2条笔记

• 遍历 map 时，您可以使用 k, v := range my_map
• *projection.Get(event.ID) 尝试引用非指针类型

关于inheritance - 具有混合结构类型的 Go 映射和 slice ，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/42924518/