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

问题描述

LiveData/ViewModel是复杂的Loader.

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

AsyncTask是 的成员LiveData。

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

  public JsonLiveData(Context context) {

    loadData();
  }


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

    }.execute();
  }
}

然而，根据Lyla Fujiwara 的介绍：

在此处输入图像描述

我应该成为班级AsyncTask成员吗？Repository

标签： androidandroid-architecture-componentsandroid-jetpackandroid-livedata

解决方案

你应该避免运行你AsyncTask的LiveData. LiveData真的应该只关心数据的观察。不是更改数据的行为。

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

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

在这种情况下，我会避免使用 AsyncTask。AsyncTask 的好处是你可以在工作后在 UI 线程上得到结果。但在这种情况下，这不是必需的。LiveData 将为您做到这一点。

这是一个（未经测试的）示例：

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;
    }
}

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();
    }

}

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