android– 如何在ViewModel中将LiveData从Room“分配”到MutableLiveData

概述最近,我坚持使用以下代码.publicclassNoteViewModelextendsViewModel{privatefinalMutableLiveData<List<Note>>notesLiveData=newMutableLiveData<>();publicNoteViewModel(){LiveData<List<Note>>notesLiveDataFro

最近,我坚持使用以下代码.

public class Noteviewmodel extends viewmodel {    private final mutablelivedata<List<Note>> notesliveData = new mutablelivedata<>();    public Noteviewmodel() {        liveData<List<Note>> notesliveDataFromrepository = NoteRepository.INSTANCE.getNotes();        // How can I "assign" liveData from Room, to mutablelivedata?    }}

我想知道,如何将Room中的liveData“分配”到mutablelivedata？

使用transformation.map和transformation.switchMap不起作用,因为它们都返回liveData,而不是mutablelivedata.

可能的解决方法

其中一个可能的解决方案是,而不是

@Daopublic abstract class NoteDao {    @Transaction    @query("SELECT * FROM plain_note")    public abstract liveData<List<Note>> getNotes();

我会用的

@Daopublic abstract class NoteDao {    @Transaction    @query("SELECT * FROM plain_note")    public abstract List<Note> getNotes();

然后,在我的viewmodel中,我会写

public class Noteviewmodel extends viewmodel {    private final mutablelivedata<List<Note>> notesliveData = new mutablelivedata<>();    public Noteviewmodel() {        new Thread(() -> {            List<Note> notesliveDataFromrepository = NoteRepository.INSTANCE.getNotes();            notesliveData.postValue(notesliveDataFromrepository);        }).start();    }}

我真的不喜欢这种方法,因为我被迫明确处理线程化问题.

有没有更好的方法,以避免明确处理线程？

解决方法:

诀窍是不在视图模型中进行任何实际的提取.

从网络或数据库获取数据应该在存储库中完成.在这方面,viewmodel应该是不可知的.

在viewmodel中,使用liveData类,而不是mutablelivedata.除非你真的找到它的用例.

// In your constructor, no extra threadnotesliveData = notesliveDataFromrepository.getAllNotes();

然后在您的存储库中,您可以使用getAllNotes()方法中的逻辑来确定这些注释的来源.在存储库中,您拥有mutablelivedata.然后,您可以从获取数据的线程postValue.这对于房间来说不是必需的,这是为你处理的.

因此,在您的存储库中,您将返回另一个直接从DAO方法支持的liveData.

在这种情况下,您需要坚持使用公共抽象liveData< List< Note>> getNotes();.

活动

public class MyActivity extends AppCompatActivity {    private Myviewmodel viewmodel;    @OverrIDe    protected voID onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        // Set up your vIEw model        viewmodel = viewmodelProvIDers.of(this).get(Myviewmodel.class);        // Observe the vIEw model        viewmodel.getMyliveData().observe(this, s -> {            // You work with the data provIDed through the vIEw model here.            // You should only really be delivering UI updates at this point. Updating            // a RecyclerVIEw for example.            Log.v("liVEDATA", "The livedata changed: "+s);        });        // This will start the off-the-UI-thread work that we want to perform.        MyRepository.getInstance().doSomeStuff();    }}

视图模型

public class Myviewmodel extends AndroIDviewmodel {    @NonNull    private MyRepository repo = MyRepository.getInstance();    @NonNull    private liveData<String> myliveData;    public Myviewmodel(@NonNull Application application) {        super(application);        // The local live data needs to reference the repository live data        myliveData = repo.getMyliveData();    }    @NonNull    public liveData<String> getMyliveData() {        return myliveData;    }}

知识库

public class MyRepository {    private static MyRepository instance;    // Note the use of mutablelivedata, this allows changes to be made    @NonNull    private mutablelivedata<String> myliveData = new mutablelivedata<>();    public static MyRepository getInstance() {        if(instance == null) {            synchronized (MyRepository.class) {                if(instance == null) {                    instance = new MyRepository();                }            }        }        return instance;    }    // The getter upcasts to liveData, this ensures that only the repository can cause a change    @NonNull    public liveData<String> getMyliveData() {        return myliveData;    }    // This method runs some work for 3 seconds. It then posts a status update to the live data.    // This would effectively be the "doInBackground" method from AsyncTask.    public voID doSomeStuff() {        new Thread(() -> {            try {                Thread.sleep(3000);            } catch (InterruptedException ignored) {            }            myliveData.postValue("Updated time: "+System.currentTimeMillis());        }).start();    }}

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