7

在使用新的架构组件时,我对数据绑定应该如何工作感到有些困惑。

假设我有一个带有列表、ProgressBar 和 TextView 的简单 Activity。Activity 应该负责控制所有视图的状态,而 ViewModel 应该保存数据和逻辑。例如,我的 Activity 现在看起来像这样:

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    binding = DataBindingUtil.setContentView(this, R.layout.activity_main);

    listViewModel = ViewModelProviders.of(this).get(ListViewModel.class);

    binding.setViewModel(listViewModel);

    list = findViewById(R.id.games_list);

    listViewModel.getList().observeForever(new Observer<List<Game>>() {
        @Override
        public void onChanged(@Nullable List<Game> items) {
            setUpList(items);
        }
    });

    listViewModel.loadGames();
}

private void setUpList(List<Game> items){
    list.setLayoutManager(new LinearLayoutManager(this));
    GameAdapter adapter = new GameAdapter();
    adapter.setList(items);
    list.setAdapter(adapter);
}

而 ViewModel 它只负责加载数据并在列表准备好时通知 Activity,以便它可以准备适配器并显示数据:

public int progressVisibility = View.VISIBLE;

private MutableLiveData<List<Game>> list;

public void loadGames(){

    Retrofit retrofit = GamesAPI.create();

    GameService service = retrofit.create(GameService.class);

    Call<GamesResponse> call = service.fetchGames();

    call.enqueue(this);
}


@Override
public void onResponse(Call<GamesResponse> call, Response<GamesResponse> response) {
    if(response.body().response.equals("success")){
        setList(response.body().data);

    }
}

@Override
public void onFailure(Call<GamesResponse> call, Throwable t) {

}

public MutableLiveData<List<Game>> getList() {
    if(list == null)
        list = new MutableLiveData<>();
    if(list.getValue() == null)
        list.setValue(new ArrayList<Game>());
    return list;
}

public void setList(List<Game> list) {
    this.list.postValue(list);
}

我的问题是:显示/隐藏列表、进度条和错误文本的正确方法是什么?

我应该为 ViewModel 中的每个视图添加一个整数,使其控制视图并像这样使用它:

<TextView
    android:id="@+id/main_list_error"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@{viewModel.error}"
    android:visibility="@{viewModel.errorVisibility}" />

或者 ViewModel 应该为每个属性实例化一个 LiveData 对象:

private MutableLiveData<Integer> progressVisibility = new MutableLiveData<>();
private MutableLiveData<Integer> listVisibility = new MutableLiveData<>();
    private MutableLiveData<Integer> errorVisibility = new MutableLiveData<>();

在需要时更新它们的值并让 Activity 观察它们的值?

viewModel.getProgressVisibility().observeForever(new Observer<Integer>() {
    @Override
    public void onChanged(@Nullable Integer visibility) {
        progress.setVisibility(visibility);
    }
});

viewModel.getListVisibility().observeForever(new Observer<Integer>() {
    @Override
    public void onChanged(@Nullable Integer visibility) {
        list.setVisibility(visibility);
    }
});

viewModel.getErrorVisibility().observeForever(new Observer<Integer>() {
    @Override
    public void onChanged(@Nullable Integer visibility) {
        error.setVisibility(visibility);
    }
});

我真的很难理解这一点。如果有人能澄清这一点,那就太好了。

谢谢

4

2 回答 2

6

以下是简单的步骤:

public class MainViewModel extends ViewModel {

    MutableLiveData<ArrayList<Game>> gamesLiveData = new MutableLiveData<>();
    // ObservableBoolean or ObservableField are classes from  
    // databinding library (android.databinding.ObservableBoolean)

    public ObservableBoolean progressVisibile = new ObservableBoolean();
    public ObservableBoolean listVisibile = new ObservableBoolean();
    public ObservableBoolean errorVisibile = new ObservableBoolean();
    public ObservableField<String> error = new ObservableField<String>();

    // ...


    // For example we want to change list and progress visibility
    // We should just change ObservableBoolean property
    // databinding knows how to bind view to changed of field

    public void loadGames(){
        GamesAPI.create().create(GameService.class)
            .fetchGames().enqueue(this);

        listVisibile.set(false); 
        progressVisibile.set(true);
    }

    @Override
    public void onResponse(Call<GamesResponse> call, Response<GamesResponse> response) {
        if(response.body().response.equals("success")){
            gamesLiveData.setValue(response.body().data);

            listVisibile.set(true);
            progressVisibile.set(false);
        }
    }

}

进而

<data>
    <import type="android.view.View"/>

    <variable
        name="viewModel"
        type="MainViewModel"/>
</data>

...

<ProgressBar
    android:layout_width="32dp"
    android:layout_height="32dp"
    android:visibility="@{viewModel.progressVisibile ? View.VISIBLE : View.GONE}"/>

<ListView
    android:layout_width="32dp"
    android:layout_height="32dp"
    android:visibility="@{viewModel.listVisibile ? View.VISIBLE : View.GONE}"/>

<TextView
    android:id="@+id/main_list_error"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@{viewModel.error}"
    android:visibility="@{viewModel.errorVisibile ? View.VISIBLE : View.GONE}"/>

另请注意,让视图观察是您的选择

ObservableBoolean : false / true 
    // or
ObservableInt : View.VISIBLE / View.INVISIBLE / View.GONE

但 ObservableBoolean 更适合 ViewModel 测试。

您还应该观察 LiveData 考虑生命周期:

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...
    listViewModel.getList().observe((LifecycleOwner) this, new Observer<List<Game>>() {
        @Override
        public void onChanged(@Nullable List<Game> items) {
            setUpList(items);
        }
    });
}
于 2017-12-08T00:44:37.447 回答
0

以下是实现您的观点的简单步骤。

首先,让你ViewModel公开一个LiveData对象,你可以从LiveData一个空值开始。

private MutableLiveData<List<Game>> list = new MutableLiveData<>();

public MutableLiveData<List<Game>> getList() {
    return list;
}

其次,让您的视图(活动/片段)观察LiveData并相应地更改 UI。

listViewModel = ViewModelProviders.of(this).get(ListViewModel.class);
listViewModel.data.observe(this, new Observer<List<Game>>() {
    @Override
    public void onChanged(@Nullable final List<Game> games) {
        setUpList(games);
    }
});

在这里使用observe(LifecycleOwner, Observer)变体很重要,这样您的观察者就不会收到LifecycleOwner不再活动的事件,基本上,这意味着当您的片段活动不再活动时,您不会泄漏该侦听器。

第三,由于数据变得可用,您需要更新您的LiveData对象。

@Override
public void onResponse(Call<GamesResponse> call, Response<GamesResponse> response) {
    if(response.body().response.equals("success")){
        List<Game> newGames = response.body().data; // Assuming this is a list
        list.setValue(newGames); // Update your LiveData object by calling setValue
    }
}

通过调用setValue()您的LiveData,这将导致onChanged您的视图的侦听器被调用,并且您的 UI 应该会自动更新。

于 2017-12-05T13:41:15.617 回答