如何在后台线程中使用ViewModel的保存状态模块

Question

如何在后台线程中使用ViewModel的Saved State模块

对于MutableLiveData我们可以选择使用setvalue和postvalue，其中Postvalue可以在后台线程中使用。

如何使用BACKGROUND THREADFOR保存状态模块的ViewModel

这是我正在尝试的代码

public class CommonViewModel extends ViewModel {
private SavedStateHandle mState;
public CommonViewModel(SavedStateHandle savedStateHandle) {
mState = savedStateHandle;
}
private static final String NAME_KEY = "name";
private Executor mExecutor = Executors.newSingleThreadExecutor();
public LiveData<ArrayList<CommonOwn>> getCart() {
if (mState.getLiveData(NAME_KEY) == null) {
initCart();
}
return mState.getLiveData(NAME_KEY);
}
public void initCart() {
mState.set(NAME_KEY, new ArrayList<CommonOwn>());
}
public void addItemToCart(CommonOwn commonOwn) {
if (getCart().getValue() == null) {
initCart();
}
ArrayList<CommonOwn> cartItemList = new ArrayList<CommonOwn>(getCart().getValue());
if (cartItemList.contains(commonOwn)) {
int a = cartItemList.indexOf(commonOwn);
cartItemList.remove(a);
} else {
cartItemList.add(commonOwn);
}
// mState.set(NAME_KEY, cartItemList);
mExecutor.execute(new Runnable() {
@Override
public void run() {
mState.set(NAME_KEY, cartItemList);
}
});
}
}

使用后台线程时出现以下错误

java.lang.IllegalStateException: Cannot invoke setValue on a background thread
at androidx.lifecycle.LiveData.assertMainThread(LiveData.java:487)
at androidx.lifecycle.LiveData.setValue(LiveData.java:306)
at androidx.lifecycle.MutableLiveData.setValue(MutableLiveData.java:50)
at androidx.lifecycle.SavedStateHandle$SavingStateLiveData.setValue(SavedStateHandle.java:367)
at androidx.lifecycle.SavedStateHandle.set(SavedStateHandle.java:256)
at com.example.CommonViewModel$1.run(CommonViewModel.java:63)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
at java.lang.Thread.run(Thread.java:818)

我们如何解决这个问题?

Answer 1

以下行永远不能为null:

mState.getLiveData(NAME_KEY) == null

希望这说明了解决方案。你只需要依赖mutableelivedata从SavedStateHandle:

public class CommonViewModel extends ViewModel {
private MutableLiveData<ArrayList<CommonOwn>> cart;
public CommonViewModel(SavedStateHandle savedStateHandle) {
cart = savedStateHandle.getLiveData(NAME_KEY);
}
private static final String NAME_KEY = "name";
private Executor mExecutor = Executors.newSingleThreadExecutor();
public MutableLiveData<ArrayList<CommonOwn>> getCart() {
return cart;
}
public void addItemToCart(CommonOwn commonOwn) {
ArrayList<CommonOwn> cartItemList;
if(cart.getValue() == null) {
cartItemList = new ArrayList<CommonOwn>();
} else {
cartItemList = cart.getValue();
}
if (cartItemList.contains(commonOwn)) {
int a = cartItemList.indexOf(commonOwn);
cartItemList.remove(a);
} else {
cartItemList.add(commonOwn);
}
// mState.set(NAME_KEY, cartItemList);
mExecutor.execute(new Runnable() {
@Override
public void run() {
cart.postValue(cartItemList);
}
});
}
}