在ViewModel中获取
Android Room DAO的最佳方法是什么?
基于paging library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)
试图运行这个我得到一个例外:
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paging library示例中可以看出,视图模型如何获取DAO的副本并且显然它失败了.问题是,如果我错过了什么,或者示例是不完整的?
谷歌搜索异常我发现使用ViewModelProvider.Factory的建议,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());
}
}
并且像这样被收回
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"
最佳答案 我通过谷歌找到了这个
paging example.基于此我写了这个视图模型(见
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())
}
我把它添加到我的DB类(见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 }
}
}
这样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paging library的例子不完整.