将LiveData转换为MutableLiveData [英] Convert LiveData to MutableLiveData

问题描述

显然，Room无法处理MutableLiveData，我们必须坚持使用LiveData，因为它会返回以下错误:

Apparently, Room is not able to handle MutableLiveData and we have to stick to LiveData as it returns the following error:

error: Not sure how to convert a Cursor to this method's return type

我通过这种方式在数据库帮助器中创建了一个自定义" MutableLiveData:

I created a "custom" MutableLiveData in my DB helper this way:

class ProfileRepository @Inject internal constructor(private val profileDao: ProfileDao): ProfileRepo{

    override fun insertProfile(profile: Profile){
        profileDao.insertProfile(profile)
    }

    val mutableLiveData by lazy { MutableProfileLiveData() }
    override fun loadMutableProfileLiveData(): MutableLiveData<Profile> = mutableLiveData

    inner class MutableProfileLiveData: MutableLiveData<Profile>(){

        override fun postValue(value: Profile?) {
            value?.let { insertProfile(it) }
            super.postValue(value)
        }

        override fun setValue(value: Profile?) {
            value?.let { insertProfile(it) }
            super.setValue(value)
        }

        override fun getValue(): Profile? {
            return profileDao.loadProfileLiveData().getValue()
        }
    }
}

这样，我从数据库获取更新并可以保存Profile对象，但是我不能修改属性.

This way, I get the updates from DB and can save the Profile object but I cannot modify attributes.

例如: mutableLiveData.value = Profile()可以工作. mutableLiveData.value.userName = "name"会调用getValue()而不是postValue()，并且不起作用.

For example: mutableLiveData.value = Profile() would work. mutableLiveData.value.userName = "name" would call getValue() instead postValue() and wouldn't work.

有人找到解决方案了吗?

Did anyone find a solution for this?

推荐答案

叫我疯了，但AFAIK没有理由为从DAO接收的对象使用MutableLiveData.

Call me crazy but AFAIK there is zero reason to use a MutableLiveData for the object that you received from the DAO.

想法是您可以通过LiveData<List<T>>

@Dao
public interface ProfileDao {
    @Query("SELECT * FROM PROFILE")
    LiveData<List<Profile>> getProfiles();
}

现在您可以观察它们:

profilesLiveData.observe(this, (profiles) -> {
    if(profiles == null) return;

    // you now have access to profiles, can even save them to the side and stuff
    this.profiles = profiles;
});

因此，如果要使此实时数据发出新数据并对其进行修改"，则需要将概要文件插入数据库中.写入操作将重新评估该查询，并且将新的配置文件值写入db后将发出该查询.

So if you want to make this live data "emit a new data and modify it", then you need to insert the profile into the database. The write will re-evaluate this query and it will be emitted once the new profile value is written to db.

dao.insert(profile); // this will make LiveData emit again

因此，没有理由使用getValue/setValue，只需写入数据库即可.

So there is no reason to use getValue/setValue, just write to your db.

这篇关于将LiveData转换为MutableLiveData的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！