android - 我应该让 AsyncTask 成为 LiveData 或 Repository 类的成员 - 作为 Loader 的替代品

Question

LiveData/ViewModel 是复杂的 Loader 的良好替代品。

基于 https://medium.com/google-developers/lifecycle-aware-data-loading-with-android-architecture-components-f95484159de4 ,

AsyncTask 是 LiveData 的成员。

public class JsonLiveData extends LiveData<List<String>> {

  public JsonLiveData(Context context) {

    loadData();
  }


  private void loadData() {
    new AsyncTask<Void,Void,List<String>>() {

    }.execute();
  }
}

---

但是，基于presentation from Lyla Fujiwara :

我应该让 AsyncTask 成为 Repository 类的成员吗？

Answer 1

您应该避免在 LiveData 中运行您的 AsyncTask。 LiveData 应该只关心数据的观察。不是更改数据的行为。

处理这种情况的最佳方法是使用 ViewModel/Repository 模式。

Activity/Fragment 从ViewModel 观察LiveData，ViewModel 从Repository 观察LiveData。在存储库中进行更改，这些更改被推送到其 LiveData。这些更改将传送到 Activity/Fragment(通过 ViewModel)。

在这种情况下我会避免使用 AsyncTask。 AsyncTask 的好处是你可以在完成工作后在 UI 线程上获得结果。但在这种情况下，这不是必需的。 LiveData 会为您做到这一点。

这是一个(未经测试的)示例:

Activity

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();
    }
}

View 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;
    }
}

存储库

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();
    }

}