无法从片段中的 ViewModel 观察 LiveData
Can't Observe LiveData from ViewModel in Fragment
我的 ViewPager
中有一个 RecyclerView
Fragment
。此 RecyclerView
显示从服务器获取的数据。它通常可以工作,但我无法使用 ViewModel
和 LiveData
实现相同的功能,因为 livedata.observe
方法在 livedata
被调用时没有被调用从 ViewModel
.
更改
这是我的 MonthlyOrderViewModel.java class
的源代码
public class MonthlyOrderViewModel extends AndroidViewModel {
private Retrofit retrofit;
private RetrofitServices retrofitServices;
private MutableLiveData<MonthlyOrderHistory> monthlyOrderHistoryMutableLiveData = new MutableLiveData<>();
public MonthlyOrderViewModel(@NonNull Application application) {
super(application);
retrofit = RetrofitFactory.getRetrofit();
retrofitServices = retrofit.create(RetrofitServices.class);
}
public MutableLiveData<MonthlyOrderHistory> getMonthlyOrderHistoryMutableLiveData() {
return monthlyOrderHistoryMutableLiveData;
}
public void fetchMonthlyOrders() {
Call<MonthlyOrderHistory> call = retrofitServices.getAllMonthlyOrderHistory(IpharmaApplication.getInstance().getLoggedInUser().getId());
call.enqueue(new Callback<MonthlyOrderHistory>() {
@Override
public void onResponse(Call<MonthlyOrderHistory> call, Response<MonthlyOrderHistory> response) {
MonthlyOrderHistory monthlyOrderHistory = response.body();
if(monthlyOrderHistory.getStatus().equalsIgnoreCase("success")) {
List<UserOrder> userOrders = monthlyOrderHistory.getOrders().getUserOrder();
if(userOrders != null && userOrders.size() != 0) {
DefaultScheduler.INSTANCE.execute(() -> {
monthlyOrderHistoryMutableLiveData.postValue(monthlyOrderHistory);
return Unit.INSTANCE;
});
}
}
}
@Override
public void onFailure(Call<MonthlyOrderHistory> call, Throwable t) {
t.printStackTrace();
}
});
}
}
DefaultScheduler
是一个 Scheduler
,默认情况下处理 AsyncScheduler
中的操作。
这是我的 Scheduler.kt class
的源代码
interface Scheduler {
fun execute(task: () -> Unit)
fun postToMainThread(task: () -> Unit)
fun postDelayedToMainThread(delay: Long, task: () -> Unit)
}
//A shim [Scheduler] that by default handles operations in the [AsyncScheduler].
object DefaultScheduler : Scheduler {
private var delegate: Scheduler = AsyncScheduler
//Sets the new delegate scheduler, null to revert to the default async one.
fun setDelegate(newDelegate: Scheduler?) {
delegate = newDelegate ?: AsyncScheduler
}
override fun execute(task: () -> Unit) {
delegate.execute(task)
}
override fun postToMainThread(task: () -> Unit) {
delegate.postToMainThread(task)
}
override fun postDelayedToMainThread(delay: Long, task: () -> Unit) {
delegate.postDelayedToMainThread(delay, task)
}
}
//Runs tasks in a [ExecutorService] with a fixed thread of pools
internal object AsyncScheduler : Scheduler {
private val executorService: ExecutorService = Executors.newFixedThreadPool(NUMBER_OF_THREADS)
override fun execute(task: () -> Unit) {
executorService.execute(task)
}
override fun postToMainThread(task: () -> Unit) {
if (isMainThread()) {
task()
} else {
val mainThreadHandler = Handler(Looper.getMainLooper())
mainThreadHandler.post(task)
}
}
private fun isMainThread(): Boolean {
return Looper.getMainLooper().thread === Thread.currentThread()
}
override fun postDelayedToMainThread(delay: Long, task: () -> Unit) {
val mainThreadHandler = Handler(Looper.getMainLooper())
mainThreadHandler.postDelayed(task, delay)
}
}
//Runs tasks synchronously.
object SyncScheduler : Scheduler {
private val postDelayedTasks = mutableListOf<() -> Unit>()
override fun execute(task: () -> Unit) {
task()
}
override fun postToMainThread(task: () -> Unit) {
task()
}
override fun postDelayedToMainThread(delay: Long, task: () -> Unit) {
postDelayedTasks.add(task)
}
fun runAllScheduledPostDelayedTasks() {
val tasks = postDelayedTasks.toList()
clearScheduledPostdelayedTasks()
for (task in tasks) {
task()
}
}
fun clearScheduledPostdelayedTasks() {
postDelayedTasks.clear()
}
}
最后,我尝试使用以下方法观察片段 onCreateView
方法的变化:
MonthlyOrderViewModel monthlyOrderViewModel = new MonthlyOrderViewModel(getActivity().getApplication());
monthlyOrderViewModel.getMonthlyOrderHistoryMutableLiveData().observe(this, monthlyOrderHistory -> {
monthlyOrderRecyclerView = view.findViewById(R.id.monthlyOrderRecyclerView);
monthlyOrderRecyclerViewAdapter = new OrderHistoryRecyclerViewAdapter(getContext(), monthlyOrderHistory.getOrders().getUserOrder());
monthlyOrderRecyclerViewAdapter.notifyDataSetChanged();
monthlyOrderRecyclerView.setAdapter(monthlyOrderRecyclerViewAdapter);
monthlyOrderRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
});
我尝试调试了一下,发现retrofit取数据没有问题。但是当我尝试在我的片段中使用它们时,observe
方法永远不会被调用。
请建议我如何解决这个问题?任何帮助将不胜感激。
您不应该使用 new
实例化您的 ViewModel
。你必须这样做:
MonthlyOrderViewModel monthlyOrderViewModel = ViewModelProviders.of(this).get(MonthlyOrderViewModel.class);
如果您确定 postValue
正在您的 ViewModel 中执行,那么我认为此更改应该可以使其正常工作。还有一件事:每次观察到的 LiveData 通知您更改时,您不应该做所有这些。我会将下一行移动到 observe()
之前
monthlyOrderRecyclerView = view.findViewById(R.id.monthlyOrderRecyclerView);
monthlyOrderRecyclerViewAdapter = new OrderHistoryRecyclerViewAdapter(getContext(), monthlyOrderHistory.getOrders().getUserOrder());
monthlyOrderRecyclerView.setAdapter(monthlyOrderRecyclerViewAdapter);
monthlyOrderRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
更新
正如 Fahim 在评论中确认的那样,问题在于创建了相同 ViewModel
的多个实例。如果您需要共享 ViewModel
的相同实例,请确保传递 Fragment/Activity 的引用,即该 ViewModel 的 "owner"。例如,如果 MyActivity
有一个 MainActivityViewModel
并且它包含一个名为 MyFragment
的子 Fragment,那么如果该 Fragment 想要获得对其父 ViewModel 的引用,则必须在以下方式:
MainActivityViewModel mainActivityVM = ViewModelProviders
.of(getActivity())
.get(MainActivityViewModel.class);
我的 ViewPager
中有一个 RecyclerView
Fragment
。此 RecyclerView
显示从服务器获取的数据。它通常可以工作,但我无法使用 ViewModel
和 LiveData
实现相同的功能,因为 livedata.observe
方法在 livedata
被调用时没有被调用从 ViewModel
.
这是我的 MonthlyOrderViewModel.java class
的源代码public class MonthlyOrderViewModel extends AndroidViewModel {
private Retrofit retrofit;
private RetrofitServices retrofitServices;
private MutableLiveData<MonthlyOrderHistory> monthlyOrderHistoryMutableLiveData = new MutableLiveData<>();
public MonthlyOrderViewModel(@NonNull Application application) {
super(application);
retrofit = RetrofitFactory.getRetrofit();
retrofitServices = retrofit.create(RetrofitServices.class);
}
public MutableLiveData<MonthlyOrderHistory> getMonthlyOrderHistoryMutableLiveData() {
return monthlyOrderHistoryMutableLiveData;
}
public void fetchMonthlyOrders() {
Call<MonthlyOrderHistory> call = retrofitServices.getAllMonthlyOrderHistory(IpharmaApplication.getInstance().getLoggedInUser().getId());
call.enqueue(new Callback<MonthlyOrderHistory>() {
@Override
public void onResponse(Call<MonthlyOrderHistory> call, Response<MonthlyOrderHistory> response) {
MonthlyOrderHistory monthlyOrderHistory = response.body();
if(monthlyOrderHistory.getStatus().equalsIgnoreCase("success")) {
List<UserOrder> userOrders = monthlyOrderHistory.getOrders().getUserOrder();
if(userOrders != null && userOrders.size() != 0) {
DefaultScheduler.INSTANCE.execute(() -> {
monthlyOrderHistoryMutableLiveData.postValue(monthlyOrderHistory);
return Unit.INSTANCE;
});
}
}
}
@Override
public void onFailure(Call<MonthlyOrderHistory> call, Throwable t) {
t.printStackTrace();
}
});
}
}
DefaultScheduler
是一个 Scheduler
,默认情况下处理 AsyncScheduler
中的操作。
这是我的 Scheduler.kt class
interface Scheduler {
fun execute(task: () -> Unit)
fun postToMainThread(task: () -> Unit)
fun postDelayedToMainThread(delay: Long, task: () -> Unit)
}
//A shim [Scheduler] that by default handles operations in the [AsyncScheduler].
object DefaultScheduler : Scheduler {
private var delegate: Scheduler = AsyncScheduler
//Sets the new delegate scheduler, null to revert to the default async one.
fun setDelegate(newDelegate: Scheduler?) {
delegate = newDelegate ?: AsyncScheduler
}
override fun execute(task: () -> Unit) {
delegate.execute(task)
}
override fun postToMainThread(task: () -> Unit) {
delegate.postToMainThread(task)
}
override fun postDelayedToMainThread(delay: Long, task: () -> Unit) {
delegate.postDelayedToMainThread(delay, task)
}
}
//Runs tasks in a [ExecutorService] with a fixed thread of pools
internal object AsyncScheduler : Scheduler {
private val executorService: ExecutorService = Executors.newFixedThreadPool(NUMBER_OF_THREADS)
override fun execute(task: () -> Unit) {
executorService.execute(task)
}
override fun postToMainThread(task: () -> Unit) {
if (isMainThread()) {
task()
} else {
val mainThreadHandler = Handler(Looper.getMainLooper())
mainThreadHandler.post(task)
}
}
private fun isMainThread(): Boolean {
return Looper.getMainLooper().thread === Thread.currentThread()
}
override fun postDelayedToMainThread(delay: Long, task: () -> Unit) {
val mainThreadHandler = Handler(Looper.getMainLooper())
mainThreadHandler.postDelayed(task, delay)
}
}
//Runs tasks synchronously.
object SyncScheduler : Scheduler {
private val postDelayedTasks = mutableListOf<() -> Unit>()
override fun execute(task: () -> Unit) {
task()
}
override fun postToMainThread(task: () -> Unit) {
task()
}
override fun postDelayedToMainThread(delay: Long, task: () -> Unit) {
postDelayedTasks.add(task)
}
fun runAllScheduledPostDelayedTasks() {
val tasks = postDelayedTasks.toList()
clearScheduledPostdelayedTasks()
for (task in tasks) {
task()
}
}
fun clearScheduledPostdelayedTasks() {
postDelayedTasks.clear()
}
}
最后,我尝试使用以下方法观察片段 onCreateView
方法的变化:
MonthlyOrderViewModel monthlyOrderViewModel = new MonthlyOrderViewModel(getActivity().getApplication());
monthlyOrderViewModel.getMonthlyOrderHistoryMutableLiveData().observe(this, monthlyOrderHistory -> {
monthlyOrderRecyclerView = view.findViewById(R.id.monthlyOrderRecyclerView);
monthlyOrderRecyclerViewAdapter = new OrderHistoryRecyclerViewAdapter(getContext(), monthlyOrderHistory.getOrders().getUserOrder());
monthlyOrderRecyclerViewAdapter.notifyDataSetChanged();
monthlyOrderRecyclerView.setAdapter(monthlyOrderRecyclerViewAdapter);
monthlyOrderRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
});
我尝试调试了一下,发现retrofit取数据没有问题。但是当我尝试在我的片段中使用它们时,observe
方法永远不会被调用。
请建议我如何解决这个问题?任何帮助将不胜感激。
您不应该使用 new
实例化您的 ViewModel
。你必须这样做:
MonthlyOrderViewModel monthlyOrderViewModel = ViewModelProviders.of(this).get(MonthlyOrderViewModel.class);
如果您确定 postValue
正在您的 ViewModel 中执行,那么我认为此更改应该可以使其正常工作。还有一件事:每次观察到的 LiveData 通知您更改时,您不应该做所有这些。我会将下一行移动到 observe()
monthlyOrderRecyclerView = view.findViewById(R.id.monthlyOrderRecyclerView);
monthlyOrderRecyclerViewAdapter = new OrderHistoryRecyclerViewAdapter(getContext(), monthlyOrderHistory.getOrders().getUserOrder());
monthlyOrderRecyclerView.setAdapter(monthlyOrderRecyclerViewAdapter);
monthlyOrderRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
更新
正如 Fahim 在评论中确认的那样,问题在于创建了相同 ViewModel
的多个实例。如果您需要共享 ViewModel
的相同实例,请确保传递 Fragment/Activity 的引用,即该 ViewModel 的 "owner"。例如,如果 MyActivity
有一个 MainActivityViewModel
并且它包含一个名为 MyFragment
的子 Fragment,那么如果该 Fragment 想要获得对其父 ViewModel 的引用,则必须在以下方式:
MainActivityViewModel mainActivityVM = ViewModelProviders
.of(getActivity())
.get(MainActivityViewModel.class);