如何在Kotlin中使用ViewModelProviders

Question

我是Kotlin的新手，请帮助我如何在Kotlin中使用ViewModelProviders.of（this）

我在Java中的代码是

 mFavViewModel = ViewModelProviders.of(this).get(FavouritesViewModel.class);

我无法在Kotlin中找到ViewModelProviders类，尝试自动转换，但显示错误。谢谢。

这是我的DataModel类

class FavoritesDataViewModel:ViewModel{
    private var mFavHelper: DatabaseHelper
    private lateinit var mfav:ArrayList<Favorites>
     constructor(application: Application) {
        mFavHelper = DatabaseHelper(application)
    }
    fun getListFav():List<Favorites>{
        if (mfav==null){
            mfav =  arrayListOf<Favorites>()
            createDummyList()
            loadFav()
        }
        val clonedFavs = arrayListOf<Favorites>()
        for (i in 0 until mfav.size) {
            clonedFavs.add(Favorites(mfav.get(i)))
        }
        return clonedFavs
    }
    fun createDummyList(){
        addFav("https://www.journaldev.com", Date().getTime())
        addFav("https://www.medium.com", Date().getTime())
        addFav("https://www.reddit.com", Date().getTime())
        addFav("https://www.github.com", Date().getTime())
        addFav("https://www.hackerrank.com", Date().getTime())
        addFav("https://www.developers.android.com", Date().getTime())

    }
    fun addFav(url:String,date:Long):Favorites{
        val db:SQLiteDatabase = this.mFavHelper.getWritableDatabase();
        val values: ContentValues = ContentValues();
        values.put(DbSettings.DbEntry.COL_FAV_DATE,date)
        values.put(DbSettings.DbEntry.COL_FAV_URL,url)
        val id:Long = db.insertWithOnConflict(DbSettings.DbEntry.TABLE,
            null,
            values,SQLiteDatabase.CONFLICT_REPLACE)
        db.close()
        val fav:Favorites = Favorites(id,url,date)
        mfav.add(fav)
        return Favorites(fav)
    }
    fun loadFav(){
        mfav.clear()
        val db:SQLiteDatabase = mFavHelper.readableDatabase
        val cursor:Cursor = db.query(DbSettings.DbEntry.TABLE,
             arrayOf(
                DbSettings.DbEntry._ID,
                DbSettings.DbEntry.COL_FAV_URL,
                DbSettings.DbEntry.COL_FAV_DATE
             ),
            null, null, null, null, null);
        while (cursor.moveToNext()){
            val idxID:Int = cursor.getColumnIndex(DbSettings.DbEntry._ID)
            val idxURL:Int = cursor.getColumnIndex(DbSettings.DbEntry.COL_FAV_URL)
            val idxDate:Int = cursor.getColumnIndex(DbSettings.DbEntry.COL_FAV_DATE)
            mfav.add(Favorites(cursor.getLong(idxID),cursor.getString(idxURL),cursor.getLong(idxDate)))
        }
        cursor.close()
        db.close()
    }

}

尝试使用以下代码行

var mFavViewModel = ViewModelProviders.of(activity).get(FavouritesViewModel::class.java)
implementation 'android.arch.lifecycle:extensions:1.1.1'

这给出了错误：

  Caused by: java.lang.RuntimeException: Cannot create an instance of class com.nearex.mykotlinsample.viewmodel.FavoritesDataViewModel
            at android.arch.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:153)
            at android.arch.lifecycle.ViewModelProvider$AndroidViewModelFactory.create(ViewModelProvider.java:210)
            at android.arch.lifecycle.ViewModelProvider.get(ViewModelProvider.java:134)
            at android.arch.lifecycle.ViewModelProvider.get(ViewModelProvider.java:102)
            at com.nearex.mykotlinsample.view.MainActivity.onCreate(MainActivity.kt:29)

Answer 1

在Kotlin中，您可以编写如下内容

val mFavViewModel = ViewModelProviders.of(activity).get(FavouritesViewModel::class.java)

确保您已在Gradle中添加以下依赖项

implementation 'android.arch.lifecycle:extensions:1.1.1'

更新

implementation "androidx.lifecycle:lifecycle:extensions:2.2.0-alpha04"

在最新版本中，ViewModel可以如下声明

val mFavViewModel = ViewModelProvider(activity)[FavouritesViewModel::class.java]

Answer 2

它假设 kotlin 正在使用 - ViewModelProviders 类已被弃用，而是可以使用 ViewModelProvider。要使用此功能，需要在应用程序 build.gradle 中添加依赖项，如下所示 -

 implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'

创建一个视图模型类 -

class MyViewModel: ViewModel(){
}

可以从片段 - onCreateView 中找到相同的类对象。例如 -

class MyFragment: Fragment(){
    ...
    private lateint var myViewModel: MyViewModel
    ...
    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
         ...
         myViewModel = ViewModelProvider(this).get(MyViewModel::class.java)
         ...

     }

}