Android：在 ViewModel 中获取 DAO

Question

在 ViewModel 中获得 Android Room DAO 的最佳方法是什么？

基于分页库示例，我编写了这个 ViewModel：

class MyViewModel(myDao: MyDao) : ViewModel() {

    val data = myDao.get().create(
            /* initial load position */ 0,
            PagedList.Config.Builder()
                    .setPageSize(50)
                    .setPrefetchDistance(50)
                    .build())
}

然后我尝试获取一个实例

val viewModel = ViewModelProviders.of(this).get(MyViewModel::class.java)

尝试运行此程序时出现异常：

java.lang.RuntimeException: Unable to start activity ComponentInfo{....MyActivity}: java.lang.RuntimeException: Cannot create an instance of class ...MyViewModel
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
    at android.app.ActivityThread.-wrap11(Unknown Source:0)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
    at android.os.Handler.dispatchMessage(Handler.java:105)
    at android.os.Looper.loop(Looper.java:164)
    at android.app.ActivityThread.main(ActivityThread.java:6541)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
 Caused by: java.lang.RuntimeException: Cannot create an instance of class ...MyViewModel
    at android.arch.lifecycle.ViewModelProvider$NewInstanceFactory.create(ViewModelProvider.java:145)
    at android.arch.lifecycle.ViewModelProviders$DefaultFactory.create(ViewModelProviders.java:158)
    at android.arch.lifecycle.ViewModelProvider.get(ViewModelProvider.java:128)
    at android.arch.lifecycle.ViewModelProvider.get(ViewModelProvider.java:96)
    ...
 Caused by: java.lang.InstantiationException: java.lang.Class<...MyViewModel> has no zero argument constructor

从分页库示例中，视图模型如何获取 DAO 的副本并不明显，显然它失败了。问题是我是否遗漏了什么，或者示例不完整？

谷歌搜索异常我发现建议使用 ViewModelProvider.Factory，只有示例没有使用它。在示例代码中，视图模型如下所示：

class MyViewModel extends ViewModel {
    public final LiveData<PagedList<User>> usersList;
    public MyViewModel(UserDao userDao) {
        usersList = userDao.usersByLastName().create(
                /* initial load position */ 0,
                new PagedList.Config.Builder()
                        .setPageSize(50)
                        .setPrefetchDistance(50)
                        .build());
    }
}

并像这样检索

MyViewModel viewModel = ViewModelProviders.of(this).get(MyViewModel.class);

我的依赖

def roomVersion = "1.0.0"
implementation "android.arch.persistence.room:runtime:$roomVersion"
annotationProcessor "android.arch.persistence.room:compiler:$roomVersion"
kapt "android.arch.persistence.room:compiler:$roomVersion"
implementation "android.arch.paging:runtime:1.0.0-alpha3"

Answer 1

我通过谷歌找到了这个分页示例。基于此，我编写了这个视图模型（请参阅CheeseViewModel）：

class MyViewModel(app: Application) : AndroidViewModel(app) {

    val data = MyDatabase.get(app).dao.get().create(
            /* initial load position */ 0,
            PagedList.Config.Builder()
                    .setPageSize(50)
                    .setPrefetchDistance(50)
                    .build())
}

我将其添加到我的数据库类中（请参阅CheeseDb）：

companion object {

    // Google example noted that this might not be the best
    // solution and to use a dependency injection framework instead.

    private var instance: MyDatabase? = null

    @Synchronized
    fun get(context: Context): MyDatabase {
        return instance ?: Room.databaseBuilder(context.applicationContext,
                MyDatabase::class.java, "myDB")
                .build()
                .also { instance = it }
    }
}

这就回答了如何在视图模型中获取 DAO 实例的问题。关于另一个问题，我猜分页库示例不完整。