android – Room：来自Dao的LiveData会在每次更新时触发Observer.onChanged,即使LiveData值没有变化

我发现Dao返回的LiveData会在DB中更新行时调用它的观察者,即使LiveData值显然没有改变.

考虑类似以下示例的情况：

示例实体

@Entity
public class User {
    public long id;
    public String name;
    // example for other variables
    public Date lastActiveDateTime;
}

示例道

@Dao
public interface UserDao {
    // I am only interested in the user name
    @Query("SELECT name From User")
    LiveData<List<String>> getAllNamesOfUser();

    @Update(onConflict = OnConflictStrategy.REPLACE)
    void updateUser(User user);
}

某个地方在后台线程

UserDao userDao = //.... getting the dao
User user = // obtain from dao....
user.lastActiveDateTime = new Date(); // no change to user.name
userDao.updateUser(user);

UI中的某个地方

// omitted ViewModel for simplicity
userDao.getAllNamesOfUser().observe(this, new Observer<List<String>> {
    @Override
    public void onChanged(@Nullable List<String> userNames) {
        // this will be called whenever the background thread called updateUser. 
        // If user.name is not changed, it will be called with userNames 
        // with the same value again and again when lastActiveDateTime changed.
    }
});

在此示例中,ui仅对用户名感兴趣,因此LiveData的查询仅包括名称字段.但是,即使只更新了其他字段,仍会在Dao Update上调用observer.onChanged.
(事实上​​,如果我没有对User实体进行任何更改并调用UserDao.updateUser,则仍会调用observer.onChanged)

这是Dao LiveData在室内设计的行为吗？我是否有机会解决这个问题,以便只在更新所选字段时才会调用观察者？

编辑：我更改为使用以下查询将lastActiveDateTime值更新为评论建议中的KuLdip PaTel.仍然会调用用户名LiveData的观察者.

@Query("UPDATE User set lastActiveDateTime = :lastActiveDateTime where id = :id")
void updateLastActiveDateTime(Date lastActiveDateTime, int id);

解决方法:

这种情况被称为观察者的误报通知.
请检查link中提到的第7点以避免此类问题.

Below example is written in kotlin but you can use its java version to get it work.

fun <T> LiveData<T>.getDistinct(): LiveData<T> {
    val distinctLiveData = MediatorLiveData<T>()
    distinctLiveData.addSource(this, object : Observer<T> {
        private var initialized = false
        private var lastObj: T? = null
        override fun onChanged(obj: T?) {
            if (!initialized) {
                initialized = true
                lastObj = obj
                distinctLiveData.postValue(lastObj)
            } else if ((obj == null && lastObj != null) 
                       || obj != lastObj) {
                lastObj = obj
                distinctLiveData.postValue(lastObj)
            }
        }
    })
    return distinctLiveData
}

标签：android-room,android,android-livedata
来源： https://codeday.me/bug/20191001/1840335.html