Android 房间数据库 - 不确定如何将 Cursor 转换为此方法的返回类型

Question

我正在尝试将此对象保存在 Room 数据库中，我读到了有关 Typeconverters 的信息，用于转换一个可以存储在数据库中的文件中的复杂对象。我收到此错误： 错误：不确定如何将游标转换为该方法的返回类型 (androidx.lifecycle.LiveData>).---- public abstract androidx.lifecycle.LiveData> queryQuestions(@org.jetbrains.annotations. NotNull() 我的代码基于我发现的将对象转换为房间的类似问题的解决方案，但这对我不起作用。

我的问题课：

@Entity

    data class Question(@PrimaryKey var questionId: String = "",
                        val uid: String,
                        val name: String,
                        val photo: String,
                        val question: String,
                        val points: Int,
                        @ServerTimestamp val timestamp: Date? = null,
                        val options: ArrayList<Option>){
        constructor(): this("", "", "", "", "", 0, null, ArrayList())
    }

    data class Option(val optionText: String,
                      val correct: Boolean,
                      var votes: Int = 0,
                      var usersVoted: ArrayList<UserVoted> = ArrayList()){
        constructor(): this("", false /*,0, ArrayList()*/)
    }

    data class UserVoted(val name: String,
                         val photo: String){
        constructor(): this("", "")
    }

我的 dao 类：

@Dao
interface QuestionDao {
    @Insert(onConflict = REPLACE)
    fun insertQuestions(question: ArrayList<Question>)

    @Query("SELECT * FROM question WHERE uid = :userId")
    fun queryQuestions(userId: String): LiveData<ArrayList<Question>>
}

我的数据库类：

@Database(entities = [Question::class], version = 1)
@TypeConverters(Converter::class)
abstract class AppDatabase : RoomDatabase(){
    abstract fun questionDao(): QuestionDao

}

我的转换器类：

class Converter {

    @TypeConverter
    fun fromTimestampToDate(value: Long?): Date? {
        return value?.let { Date(it) }
    }

    @TypeConverter
    fun fromDateToTimestamp(date: Date?): Long? {
        return date?.time?.toLong()
    }

    @TypeConverter
    fun fromStringToArrayList(value: String): ArrayList<Option> {
        Log.i("alengenije", "fromStringToArrayList string = $value")
        val listType = object:TypeToken<ArrayList<Option>>() {}.type
        return Gson().fromJson(value, listType)
    }

    @TypeConverter
    fun fromArrayLisrToString(list: ArrayList<Option>): String {
        val gson = Gson()
        return gson.toJson(list)
    }

}

Answer 1

您遇到的问题是由使用 LiveData 引起的。从 Android API 28 开始，支持库已移至 androidx，这意味着您必须将所有内容移至 androidx。您的问题出在 build.gradle 文件上。您可能有 andoidx 生命周期，但 Room lib 是旧的 1.1.1 Room lib。您应该更新所有支持库以使用 androidx，如下所示：

def lifecycle_version = "2.0.0"

implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'androidx.appcompat:appcompat:1.0.0'

// Add RecyclerView dependency; must match SDK version
implementation 'androidx.recyclerview:recyclerview:1.0.0'

// Add FAB dependency
implementation 'com.google.android.material:material:1.0.0-rc01'

def room_version = "2.1.0-alpha07"

implementation "androidx.room:room-runtime:$room_version"
annotationProcessor "androidx.room:room-compiler:$room_version"

implementation "androidx.lifecycle:lifecycle-extensions:$lifecycle_version"
annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version"