合并多个 LiveData 源？

Question

为了更容易形象化我的问题，我画了以下内容：

我正在使用 RoomDatabase、Repository、Viewmodel 和 Livedata。区域与网关具有 1 对 n 的关系，网关与项目具有 1 对 n 的关系。我创建了 AreaWithGateways 实体和 GatewayWithItems 实体。

物品可以从一个网关移动到另一个网关，这就是为什么我用 Livedata 观察它们以跟踪它们在哪个网关中。我现在的问题是我发现自己也需要跟踪哪些物品在哪些区域，我不知道该怎么做。

我想过将每个网关的 LiveData 合并在一起并使用 MediatorLiveData 进行观察，但我不太了解如何使用它。或者也许可以创建实体 AreaWithGatewayswithItems？

如有任何见解，我们将不胜感激

编辑：我正在添加一些代码以使问题更清楚一些

这是区域实体

@Entity(tableName = "area_table")
public class Area {

    @PrimaryKey(autoGenerate = false)
    private int areaId;

    private String areaName;
    private float wertX;
    private float wertY;
    private Boolean isDrawn;


    public int getAreaId() {
        return areaId;
    }

    public void setAreaId(int areaId) {
        this.areaId = areaId;
    }

    public String getAreaName() {
        return areaName;
    }

    public float getWertX() {
        return wertX;
    }

    public float getWertY() {
        return wertY;
    }

    public Boolean getDrawn() {
        return isDrawn;
    }

    public Area(int areaId, String areaName, float wertX, float wertY, Boolean isDrawn) {
        this.areaId = areaId;
        this.areaName = areaName;
        this.wertX = wertX;
        this.wertY = wertY;
        this.isDrawn = isDrawn;
    }
}

网关实体：

@Entity(tableName = "gateway_table")
public class Gateway {
    
    private float temp;
    private String title;
    private int areaId;


    @PrimaryKey(autoGenerate = false)
    private int gatewayId;

    public int getGatewayId() {
        return gatewayId;
    }

    public void setGatewayId(int gatewayId) {
        this.gatewayId = gatewayId;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public float getTemp() {
        return temp;
    }

    public void setTemp(float temp) {
        this.temp = temp;
    }

    public int getAreaId() {
        return areaId;
    }

    public Gateway(int areaId, int gatewayId, String title) {
        this.title = title;
        this.areaId = areaId;
        this.gatewayId = gatewayId;
    }
}

和“项目”实体：

@Entity(tableName = "cow_table")
public class Cow {

    private int age;
    private String sex;
    private String name;
    private boolean drawn;
    private int gatewayId;
    private String raceId;


    @PrimaryKey(autoGenerate = false)
    private int cowId;

    public int getCowId() {
        return cowId;
    }

    public void setCowId(int cowId) {
        this.cowId = cowId;
    }

    public String getName() {
        return name;
    }


    public boolean isDrawn() {
        return drawn;
    }

    public int getAge() {
        return age;
    }

    public String getSex() {
        return sex;
    }

    public void setDrawn(boolean drawn) {
        this.drawn = drawn;
    }

    public int getGatewayId() {
        return gatewayId;
    }

    public String getRaceId() {
        return raceId;
    }

    public Cow(int age, int cowId, int gatewayId,String name, String raceId, String sex, boolean drawn) {

        this.age = age;
        this.cowId = cowId;
        this.sex= sex;
        this.name = name;
        this.drawn = drawn;
        this.gatewayId = gatewayId;
        this.raceId = raceId;
    }


}

这是关系“AreaWithGateways”：

public class AreaWithGateways {

    @Embedded
    private Area area;

    @Relation(parentColumn = "areaId",
    entityColumn = "areaId")
    private List<Gateway> gatewayList;

    public Area getArea() {
        return area;
    }

    public List<Gateway> getGatewayList() {
        return gatewayList;
    }

    public AreaWithGateways(Area area, List<Gateway> gatewayList) {
        this.area = area;
        this.gatewayList = gatewayList;
    }
}

以及 GatewaysWithCows：

public class GatewayWithCows {

    @Embedded
    private Gateway gateway;

    @Relation(parentColumn = "gatewayId",
            entityColumn = "gatewayId")
    private List<Cow> cowList;

    public Gateway getGateway() {
        return gateway;
    }

    public List<Cow> getCowList() {
        return cowList;
    }

    public GatewayWithCows(Gateway gateway, List<Cow> cowList) {
        this.gateway = gateway;
        this.cowList = cowList;
    }
}

我一直在尝试找到一种方法来将一个区域中的所有“项目”作为实时数据获取，但仍然无法弄清楚。我觉得我应该以某种方式使用 AreaWithGateways 将 LiveData 项目添加在一起，但我无法通过网关访问这些项目，它必须是相反的方式。

Answer 1

Or maybe it's possible to create an Entity AreaWithGatewayswithItems?

不是实体，因为它们用于定义 table，但是通过使用 @Embedded 和 @Relation 注释的 POJO（例如，您的 GatewayWithCows 是一个 POJO）。

I feel like I should somehow use AreaWithGateways to add the LiveData items together but I can't reach the items through the gateways, it has to be the other way around.

您基本上使用分层方法，但 POJO 是这样的，因为您有 GatewayWithCows 然后按照 :-

从 Area 与此相关

class AreaWithGatewayWithCows {

    @Embedded
    Area area;
    @Relation(entity = Gateway.class, parentColumn = "areaId",
            entityColumn = "areaId")
    List<GatewayWithCows> gatewayWithCowsList;

}

注意我错过了 s 网关后的 class 名称（因此在下面的查询）
注意需要使用 entity = Gateway.class 因为关系是通过网关而不是通过GatewayWithCows（不是 table）。

Query Dao 可以像这样简单：-

@Query("SELECT * FROM area_table")
List<AreaWithGatewayWithCows> getAreaWithGatewaysWithCows();

针对 LiveData 进行了相应修改。
请注意，如果您在查询中使用 JOINS，则 WHERE 等任何子句只会影响 Area 的，而不影响底层的 Gateways 和 Cows。这与查询无关 @Relation 构建每个 Area 与 ALL Gateways 相关 Area;每个 Gateway 得到 ALL 与 Gateway.

Cow

合并多个 LiveData 源？

Merging multiple LiveData sources?

java

android

repository

android-room

android-livedata