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扩展RoomDatabase的Singleton Kotlin类只调用一次

android-room kotlin android

fermoga · 技术社区 · 6 年前

我正在使用一个应用程序的Java代码库在Kotlin中构建另一个应用程序。我写了一个班 AppDatabase 延伸 RoomDatabase 那是一个单子。它只工作一次,其他时间类不被调用。这是班级:

@Database(entities = [Classes::class], version = 1, exportSchema = false)
abstract class AppDatabase : RoomDatabase() {
    abstract fun classesDao() : ClassesDao
}

object DatabaseProvider {
    private var database: String = "db_classes"
    private var sInstance: AppDatabase? = null

    fun getInstance(context: Context) : AppDatabase {
        if (sInstance == null) {
            sInstance = Room.databaseBuilder(
                context.applicationContext,
                AppDatabase::class.java,
                database
            ).build()
        }

        return sInstance!!
    }
}

我的 MainActivity 显示 CalendarView 它将按选定的日期获取数据(到目前为止,它是在我按下日期之后获取的)。

...
    ...
    mCalendar = findViewById(R.id.calendar)
    mCalendar?.setOnDateChangeListener { _, y, m, d ->
        mSelectedDate = DateUtils.convertToDate(y.toString(), m.toString(), d.toString())
        fetchClasses()
    }
    ...
...

private fun fetchClasses() {
    val modelFactory = MainViewModelFactory(this.applicationContext, mSelectedDate!!)
    val classesViewModel = ViewModelProviders.of(this, modelFactory).get(MainViewModel::class.java)

    classesViewModel.getClasses().observe(this, Observer<List<Classes>> {
        if (it!!.isEmpty()) {
            mRvClasses?.visibility = View.GONE
            mTvNoClasses?.visibility = View.VISIBLE
            mImgNoClasses?.visibility = View.VISIBLE
        }

        mAdapter?.setClasses(it)
    })
}

这个 MainViewModel :

class MainViewModel(context: Context, date: String) : ViewModel() {
    private var classes: LiveData<List<Classes>>?  = null
    private var db: AppDatabase? = null

    init {
        db = DatabaseProvider.getInstance(context)
        classes = db?.classesDao()?.getClassesByDate(date)
    }

    fun getClasses() : LiveData<List<Classes>> {
        return classes!!
    }
}

它工作,但我注意到,如果我切换日期,它将不再被调用。例如,如果我在应用程序打开后只选择一个日期,我可以存储该日期的任意多个记录,但当我切换日期时,它将停止被调用和插入。

这是我基于的Java代码:

@Database(entities = { TaskEntry.class }, version = 1, exportSchema = false)
@TypeConverters(DateConverter.class)
public abstract class AppDatabase extends RoomDatabase {

    private static final Object LOCK = new Object();
    private static final String DATABASE_NAME = "todolist";
    private static AppDatabase sInstance;

    public static AppDatabase getInstance(Context context) {
        if (sInstance == null) {
            synchronized (LOCK) {
                sInstance = Room.databaseBuilder(context.getApplicationContext(), AppDatabase.class, AppDatabase.DATABASE_NAME).build();
            }
        }

        return sInstance;
    }

    public abstract TaskDao taskDao();
}

1 回复 | 直到 6 年前

Anatolii 6 年前

你应该调用一个新的 SQL 修改日期时查询。首先,修改 ViewModel 所以它会对 Date 变化:

class MainViewModel(context: Context, date: String) : ViewModel() {
    private var db: AppDatabase? = null
    private val dateInput = MutableLiveData<String>()
    val classes: LiveData<List<Classes>>

    init {
        db = DatabaseProvider.getInstance(context)
        dateInput.value = date
        classes = Transformations.switchMap(dateInput, {
            db?.classesDao()?.getClassesByDate(it)
        })
    }

    fun setDate(date: String) {
        dateInput.value = date
    }
}

然后,在你的 MainActivity onCreate(...) 每次 日期 改变只是打电话 classesViewModel.setDate(mSelectedDate) . 我删除了 fetchClasses() 因为没必要每次都打电话 onCreate :

override fun onCreate(savedInstanceState: Bundle?) {
    ...
    mCalendar = findViewById(R.id.calendar)
    mCalendar?.setOnDateChangeListener { _, y, m, d ->
        mSelectedDate = DateUtils.convertToDate(y.toString(), m.toString(), d.toString())
        // just update the date and nothing else is needed 
        classesViewModel.setDate(mSelectedDate)
    }
    val modelFactory = MainViewModelFactory(this.applicationContext, mSelectedDate)
    val classesViewModel = ViewModelProviders.of(this, modelFactory).get(MainViewModel::class.java)    
    classesViewModel.classes.observe(this, Observer<List<Classes>> {
        if (it!!.isEmpty()) {
            mRvClasses?.visibility = View.GONE
            mTvNoClasses?.visibility = View.VISIBLE
            mImgNoClasses?.visibility = View.VISIBLE
        }

        mAdapter?.setClasses(it)
    })
    ...
}

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