合并多个LiveData源？

Question

我正在使用RoomDatabase、存储库、Viewmodel和LiveData。区域与网关有1到n的关系，网关与项有1到n的关系。我创建了一个AreaWithGateways实体和一个GatewayWithItems实体。

项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项目在哪些区域，我不知道如何做到这一点。

我想过将每个网关的LiveData合并在一起，并使用MediatorLiveData来观察这一点，但我并不真正理解如何使用它。或者也许可以创建一个实体areaWithGatewaysWithItems？

编辑：我正在添加一些代码以使问题更加清楚

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;
    }
}

@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;
    }


}

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;
    }
}

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;
    }
}

Answer 1

或者也许可以创建一个实体areaWithGatewaysWithItems？

不是一个实体，因为这些实体用于定义表，而是通过POJO使用@Embedded和@Relations注释（例如，您的GatewayWithCows是一个POJO）。

我觉得我应该以某种方式使用AreaWithGateways将LiveData项添加到一起，但我无法通过gateways到达这些项，必须反过来。

class AreaWithGatewayWithCows {

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

}

>

注意：我错过了类名的s后网关（在下面的查询中也是如此）

注意，需要使用entity=Gateway.class，因为关系是通过网关而不是通过GatewayWithCows（不是表）。

查询Dao可以简单到：-

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

• 为LiveData进行了相应修改。
• 注意，如果在查询中使用联接，那么any子句（如WHERE）只会影响区域，而不会影响底层网关和COW。也就是说，不管查询如何，@relations用与该区域相关的所有网关构建每个区域；每个网关获取与该网关相关的所有cow。