如果我正确理解您的架构，updateUsers 位于 AsyncTask 或类似的内部。
这是我提出的方法，其中包括调整你的道以获得最大的效率。您编写了大量代码来做出可以要求数据库做出的决定。
这也不是紧凑或高效的代码，但我希望它能够更有效地使用这些库。
后台线程（IntentService、AsyncTask 等）：
/*
* assuming this method is executing on a background thread
*/
private void updateUsers(/* from API call */List<User> serverUsers) {
for(User serverUser : serverUsers){
switch(appDatabase.userDao().userExistsSynchronous(serverUser.getId())){
case 0: //doesn't exist
saveShiftForUser(serverUser);
case 1: //does exist
UserWithShifts localUser = appDatabase.userDao().getOldUserSynchronous(serverUser.getId(), serverUser.getUpdatedAt());
if(localUser != null){ //there is a record that's too old
deleteEventsAndShifts(serverUser.getId());
saveShiftForUser(serverUser);
}
default: //something happened, log an error
}
}
}
如果在 UI 线程（Activity、Fragment、Service）上运行：
/*
* If you receive the IllegalStateException, try this code
*
* NOTE: This code is not well architected. I would recommend refactoring if you need to do this to make things more elegant.
*
* Also, RxJava is better suited to this use case than LiveData, but this may be easier for you to get started with
*/
private void updateUsers(/* from API call */List<User> serverUsers) {
for(User serverUser : serverUsers){
final LiveData<Integer> userExistsLiveData = appDatabase.userDao().userExists(serverUser.getId());
userExistsLiveData.observe(/*activity or fragment*/ context, exists -> {
userExistsLiveData.removeObservers(context); //call this so that this same code block isn't executed again. Remember, observers are fired when the result of the query changes.
switch(exists){
case 0: //doesn't exist
saveShiftForUser(serverUser);
case 1: //does exist
final LiveData<UserWithShifts> localUserLiveData = appDatabase.userDao().getOldUser(serverUser.getId(), serverUser.getUpdatedAt());
localUserLiveData.observe(/*activity or fragment*/ context, localUser -> { //this observer won't be called unless the local data is out of date
localUserLiveData.removeObservers(context); //call this so that this same code block isn't executed again. Remember, observers are fired when the result of the query changes.
deleteEventsAndShifts(serverUser.getId());
saveShiftForUser(serverUser);
});
default: //something happened, log an error
}
});
}
}
您需要根据您决定使用的任何方法修改 Dao
@Dao
public interface UserDao{
/*
* LiveData should be chosen for most use cases as running on the main thread will result in the error described on the other method
*/
@Query("SELECT * FROM users")
LiveData<List<UserWithShifts>> getAllUsers();
/*
* If you attempt to call this method on the main thread, you will receive the following error:
*
* Caused by: java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long periods of time.
* at android.arch.persistence.room.RoomDatabase.assertNotMainThread(AppDatabase.java:XXX)
* at android.arch.persistence.room.RoomDatabase.query(AppDatabase.java:XXX)
*
*/
@Query("SELECT * FROM users")
List<UserWithShifts> getAllUsersSynchronous();
@Query("SELECT EXISTS (SELECT * FROM users WHERE id = :id)")
LiveData<Integer> userExists(String id);
@Query("SELECT EXISTS (SELECT * FROM users WHERE id = :id)")
Integer userExistsSynchronous(String id);
@Query("SELECT * FROM users WHERE id = :id AND updatedAt < :updatedAt LIMIT 1")
LiveData<UserWithShifts> getOldUser(String id, Long updatedAt);
@Query("SELECT * FROM users WHERE id = :id AND updatedAt < :updatedAt LIMIT 1")
UserWithShifts getOldUserSynchronous(String id, Long updatedAt);
}
这能解决你的问题吗？
注意：我没有看到你的saveShiftForUserordeleteEventsAndShifts方法。插入、保存和更新由 Room 同步执行。如果您在主线程上运行任一方法（我猜这是您的错误的来源），您应该创建一个从 appDatabase 返回的 daoWrapper，如下所示：
public class UserDaoWrapper {
private final UserDao userDao;
public UserDaoWrapper(UserDao userDao) {
this.userDao = userDao;
}
public LiveData<Long[]> insertAsync(UserWithShifts... users){
final MutableLiveData<Long[]> keys = new MutableLiveData<>();
HandlerThread ht = new HandlerThread("");
ht.start();
Handler h = new Handler(ht.getLooper());
h.post(() -> keys.postValue(userDao.insert(users)));
return keys;
}
public void updateAsync(UserWithShifts...users){
HandlerThread ht = new HandlerThread("");
ht.start();
Handler h = new Handler(ht.getLooper());
h.post(() -> {
userDao.update(users);
});
}
public void deleteAsync(User... users){
HandlerThread ht = new HandlerThread("");
ht.start();
Handler h = new Handler(ht.getLooper());
h.post(() -> {
for(User e : users)
userDao.delete(e.getId());
});
}
}