LiveData 和 ViewModel 不更新 textview

Question

我有一个小问题，当我注释掉binding.setData(dataContainer); 在我的 Activity 中观察 Livedata 时，在onChanged中，我无法更新 UI，当我取消注释时，UI 会更新。 如果您觉得需要，请指导并进行一些代码审查。

我有一个 Runnable 每隔 x 秒重复运行一次。这是我在活动中观察到的 Livedata。

谢谢。

public class MainActivity extends AppCompatActivity {

  TextOnScreen dataContainer;
  ActivityMainBinding binding;
  ViewModelClass modelClass;

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

    modelClass = ViewModelProviders.of(this).get(ViewModelClass.class);
    binding = DataBindingUtil.setContentView(this, R.layout.activity_main);
    dataContainer = new TextOnScreen("hello");
    binding.setData(dataContainer);

    modelClass.getLiveData().observe(this, new Observer<String>() {
      @Override
      public void onChanged(String s) {
        Log.e("TAG", "--onChanged--" + s);
        dataContainer.onscreen.set(s);

        // this line allows the textview to update
        // other wise no change on UI is seen
        binding.setData(dataContainer);
      }
    });
  }
}


public class TextOnScreen {

  public final ObservableField<String> onscreen=new ObservableField<>();

  public TextOnScreen(String t) {
    onscreen.set(t);
  }

  public String getOnscreen() {
    return onscreen.get();
  }

  public void setOnscreen(String onscreen) {
    this.onscreen.set(onscreen);
  }
}


    public class ViewModelClass extends ViewModel{

      private MutableLiveData<String> liveData;

      public ViewModelClass() {
        ModelClass modelClass = new ModelClass();
        liveData= modelClass.generateName();
      }

      public LiveData<String> getLiveData() {
        return liveData;
      }
    }


public class ModelClass {

  MutableLiveData<String > data =new MutableLiveData<>();


  MutableLiveData<String> generateName(){
    final android.os.Handler handler=new android.os.Handler();
    Runnable runnable=new Runnable() {
      @Override
      public void run() {

        data.setValue(data.getValue()+"*");
        handler.postDelayed(this,1000);
      }
    };
    handler.postDelayed(runnable,2000);
    return data;
  }

}

<?xml version="1.0" encoding="utf-8"?>
<layout>
  <data>
    <variable
      name="data"
      type="dsfa.drish.com.livedatasample.TextOnScreen"/>
  </data>
<android.support.constraint.ConstraintLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  tools:context=".MainActivity">

  <TextView
    android:id="@+id/textview"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@{data.onscreen}"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toTopOf="parent"/>

</android.support.constraint.ConstraintLayout>
</layout>

Answer 1

对我来说，它只是缺少：

binding.setLifecycleOwner(this);

Answer 2

通过删除TextOnScreen并将MainActivity修改为：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    ViewModelClass modelClass = ViewModelProviders.of(this).get(ViewModelClass.class);

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

    binding.setData(modelClass); // <-- set the ViewModel to be binded
    binding.setLifecycleOwner(this); // <-- this enables MutableLiveData to be update on your UI

    modelClass.getLiveData().setValue("hello");

    modelClass.getLiveData().observe(this, new Observer<String>() {
        @Override
        public void onChanged(String s) {
            Log.e("TAG", "--onChanged--" + s);
        }
    });
}

并在您的activity_main.xml 上：

<?xml version="1.0" encoding="utf-8"?>
<layout>
<data>
    <variable
        name="data"
        type="com.rafaelfukuda.stackoverflow.ViewModelClass"/>
</data>
    <android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textview"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@{data.liveData}"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"/>
    </android.support.constraint.ConstraintLayout>
</layout> 

其他代码改进：

我会保持我的模型非常简单（没有 MutableLiveData 属性的 POJO）并将generateName移动到 ViewModel。这将保持适当的MVVM结构，并且将来很容易理解和扩展您的代码。

结果：