我正在为roomDatabase创建一个liveData和ViewModel类，并且对如何调用SQL命令更改数据库内容感到困惑。
Room持久性库在SQLite上提供了一个抽象层，以在利用SQLite的全部功能的同时允许更健壮的数据库访问。有关更多信息，请参见参考文档。
修复了一个错误，即如果添加了内容表和外部内容FTS表的观察者，则无效跟踪器将停止观察内容表。 b / 128508917 更新了Room的SQLite语法以匹配SQLite 3.24.0。 b / 110883668
LIVEDATA AND VIEWMODEL WITH ROOMDATABASE
Defining Table in roomDatabase with corresponding column names and a unique primaryKey
@Entity(tableName = "note_table")
public class Note {
Variables below are names of the column names
@PrimaryKey(autoGenerate = true)
private int id;
private String title;
private String description;
private int priority;
public Note(String title, String description, int priority) {
this.title = title;
this.description = description;
this.priority = priority;
}
public void setId(int id) {
this.id = id;
}
public int getId() {
return id;
}
public String getTitle() {
return title;
}
public String getDescription() {
return description;
}
public int getPriority() {
return priority;
}
}
This is an interface class where methods are created with corresponding SQL statements
@Dao
public interface NoteDao {
@Insert
void insert(Note note);
@Update
void update(Note note);
@Delete
void delete(Note note);
@Query("DELETE FROM note_table")
void deleteAllNotes();
@Query("SELECT * FROM note_table ORDER BY priority DESC")
(Comment: LiveData acts as an observer for any changes in the DB) LiveData<List<Note>> getAllNotes();
}
Repository class executes sql commands to insert, delete, update or deleteAll data in the Room Database
public class NoteRepository {
private NoteDao noteDao;
private LiveData<List<Note>> allNotes;
public NoteRepository(Application application) {
NoteDatabase database = NoteDatabase.getInstance(application);
noteDao = database.noteDao();
allNotes = noteDao.getAllNotes();
}
public void insert(Note note) {
new InsertNoteAsyncTask(noteDao).execute(note);
}
public void update(Note note) {
new UpdateNoteAsyncTask(noteDao).execute(note);
}
public void delete(Note note) {
new DeleteNoteAsyncTask(noteDao).execute(note);
}
public void deleteAllNotes() {
new DeleteAllNotesAsyncTask(noteDao).execute();
}
public LiveData<List<Note>> getAllNotes() {
return allNotes;
}
private static class InsertNoteAsyncTask extends AsyncTask<Note, Void, Void> {
private NoteDao noteDao;
private InsertNoteAsyncTask(NoteDao noteDao) {
this.noteDao = noteDao;
}
@Override
protected Void doInBackground(Note... notes) {
noteDao.insert(notes[0]);
return null;
}
}
private static class UpdateNoteAsyncTask extends AsyncTask<Note, Void, Void> {
private NoteDao noteDao;
private UpdateNoteAsyncTask(NoteDao noteDao) {
this.noteDao = noteDao;
}
@Override
protected Void doInBackground(Note... notes) {
noteDao.update(notes[0]);
return null;
}
}
private static class DeleteNoteAsyncTask extends AsyncTask<Note, Void, Void> {
private NoteDao noteDao;
private DeleteNoteAsyncTask(NoteDao noteDao) {
this.noteDao = noteDao;
}
@Override
protected Void doInBackground(Note... notes) {
noteDao.delete(notes[0]);
return null;
}
}
private static class DeleteAllNotesAsyncTask extends AsyncTask<Void, Void, Void> {
private NoteDao noteDao;
private DeleteAllNotesAsyncTask(NoteDao noteDao) {
this.noteDao = noteDao;
}
@Override
protected Void doInBackground(Void... voids) {
noteDao.deleteAllNotes();
return null;
}
}
}
ViewModel class updates the UI in the mainActivity when changes occur in room database through repository class
public class NoteViewModel extends AndroidViewModel {
private NoteRepository repository;
private LiveData<List<Note>> allNotes;
public NoteViewModel(@NonNull Application application) {
super(application);
repository = new NoteRepository(application);
allNotes = repository.getAllNotes();
}
public void insert(Note note) {
repository.insert(note);
}
public void update(Note note) {
repository.update(note);
}
public void delete(Note note) {
repository.delete(note);
}
public void deleteAllNotes() {
repository.deleteAllNotes();
}
public LiveData<List<Note>> getAllNotes() {
return allNotes;
}
public class NoteViewModel extends AndroidViewModel {
private NoteRepository repository;
private LiveData<List<Note>> allNotes;
public NoteViewModel(@NonNull Application application) {
super(application);
repository = new NoteRepository(application);
allNotes = repository.getAllNotes();
}
public void insert(Note note) {
repository.insert(note);
}
public void update(Note note) {
repository.update(note);
}
public void delete(Note note) {
repository.delete(note);
}
public void deleteAllNotes() {
repository.deleteAllNotes();
}
public LiveData<List<Note>> getAllNotes() {
return allNotes;
}
}
MainActivity has the UI whose data can be updated using the viewModel class(instance of view model class needs to be created in MainActivity)
public class MainActivity extends AppCompatActivity {
private NoteViewModel noteViewModel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
RecyclerView recyclerView = findViewById(R.id.recycler_view);
recyclerView.setLayoutManager(new LinearLayoutManager(this));
recyclerView.setHasFixedSize(true);
final NoteAdapter adapter = new NoteAdapter();
recyclerView.setAdapter(adapter);
noteViewModel = ViewModelProviders.of(this).get(NoteViewModel.class);
noteViewModel.getAllNotes().observe(this, new Observer<List<Note>>() {
@Override
public void onChanged(@Nullable List<Note> notes) {
adapter.setNotes(notes);
}
});
}
}
H’s LiveData and ViewModel class
LiveData class with setValue and getValue methods
public class MutabxleLiveData<T> extends LiveData<T> {
@Override
protected void setValue(T value) {
super.setValue(value);
}
@Override
public T getValue() {
return super.getValue();
}
}
ViewModel class
public class createNumber extends ViewModel {
private MutableLiveData<String> myRandomNumber;
public MutableLiveData<String> getNumber() {
Log.i("viewmodel", "Create new number");
if (myRandomNumber == null) {
myRandomNumber = new MutableLiveData<>();
createNumber();
}
return myRandomNumber;
}
public void createNumber() {
Log.i("viewmodel", "Create new number");
Random random = new Random();
myRandomNumber.setValue("Number: " + (random.nextInt(10 - 1) + 1));
}
@Override
protected void onCleared() {
super.onCleared();
Log.i("viewmodel", "ViewModel: onCleared");
}
}
MainActivity
public class MainActivity extends AppCompatActivity {
TextView mytext;
Button btn01;
createNumber myviewmodel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mytext = (TextView) findViewById(R.id.textView01);
btn01 = (Button)findViewById(R.id.button01);
myviewmodel = ViewModelProviders.of(this).get(createNumber.class);
LiveData<String> myrandomnumber = myviewmodel.getNumber();
myrandomnumber.observe(this, new Observer<String>() {
@Override
public void onChanged(@Nullable String s) {
mytext.setText(s);
}
});
btn01.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
myviewmodel.createNumber();
}
});
}
}
MainActivity
public class MainActivity extends AppCompatActivity {
TextView mytext;
Button btn01;
createNumber myviewmodel;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mytext = (TextView) findViewById(R.id.textView01);
btn01 = (Button)findViewById(R.id.button01);
myviewmodel = ViewModelProviders.of(this).get(createNumber.class);
LiveData<String> myrandomnumber = myviewmodel.getNumber();
myrandomnumber.observe(this, new Observer<String>() {
@Override
public void onChanged(@Nullable String s) {
mytext.setText(s);
}
});
btn01.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
myviewmodel.createNumber();
}
});
}
}
This should update the UI in the mainActivity