DataBinding 和 LiveData:两个实现(Kotlin 和 Java)并且无法使 Java impl 工作

DataBinding & LiveData : two implementations (Kotlin & Java) and can't make the Java impl working

我在 Java 项目中使用 DataBinding 和 LiveData 时遇到问题。我学习了之前的 Kotlin 课程,当我尝试实现相同的行为时,我就是无法让它发挥作用。我显然在理解方面遗漏了一些东西,所以我想听听你的想法。

我将粘贴 Kotlin(有效)示例中的代码,然后粘贴 Java(无效)示例中的代码。

KOTLIN

score_fragment.xml

...
<data>

    <variable
        name="scoreViewModel"
        type="com.example.android.guesstheword.screens.score.ScoreViewModel" />
</data>

<androidx.constraintlayout.widget.ConstraintLayout
    android:id="@+id/score_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".screens.score.ScoreFragment">

    <TextView
        android:id="@+id/score_text"
        ...
        android:text="@{String.valueOf(scoreViewModel.score)}"
        .../>
...

ScoreFragment.kt

class ScoreFragment : Fragment() {

private lateinit var viewModelFactory: ScoreViewModelFactory
private lateinit var viewModel: ScoreViewModel

override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
): View? {

        // Inflate view and obtain an instance of the binding class.
        val binding: ScoreFragmentBinding = DataBindingUtil.inflate(
                inflater,
                R.layout.score_fragment,
                container,
                false
        )

        // Get args using by navArgs property delegate
        val scoreFragmentArgs by navArgs<ScoreFragmentArgs>()

        viewModelFactory = ScoreViewModelFactory(scoreFragmentArgs.score)
        viewModel = ViewModelProviders.of(this, viewModelFactory)
                .get(ScoreViewModel::class.java)

        binding.scoreViewModel = viewModel
        binding.lifecycleOwner = this

        return binding.root
    }
}

ScoreViewModel.kt

class ScoreViewModel(finalScore: Int) : ViewModel() {

    private val _score = MutableLiveData<Int>()
    val score: LiveData<Int>
        get() = _score

    private val _eventPlayAgain = MutableLiveData<Boolean>()
    val eventPlayAgain: LiveData<Boolean>
        get() = _eventPlayAgain

    init {
        Timber.i("ScoreViewModel created")
        _score.value = finalScore
    }

    fun onPlayAgain() {
        _eventPlayAgain.value = true
    }

    fun onPlayAgainComplete() {
        _eventPlayAgain.value = false
    }

    override fun onCleared() {
        super.onCleared()
        Timber.i("ScoreViewModel cleared")
    }
}

说明 : 让我们只关注分数值。在 ScoreViewModel 中,该值属于 LiveData 类型。当片段启动时,该值通过“@{String.valueOf(scoreViewModel.score)}”正确显示在屏幕上。这工作正常。

JAVA

activity_main.xml

<data>
    <variable
        name="noteViewModel"
        type="com.example.architectureapp.viewModel.NoteViewModel" />
</data>

<androidx.constraintlayout.widget.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textview"
        android:text="@{noteViewModel.test}" />

</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity

public class MainActivity extends AppCompatActivity {
    private ActivityMainBinding binding;
    private NoteViewModel mNoteViewModel;

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

        binding = ActivityMainBinding.inflate(getLayoutInflater());
        mNoteViewModel = ViewModelProviders.of(this).get(NoteViewModel.class);
        binding.setNoteViewModel(mNoteViewModel);
        binding.setLifecycleOwner(this);
    }
}

NoteViewModel

public class NoteViewModel extends AndroidViewModel {
    public MutableLiveData<String> test = new MutableLiveData<>("TesT");

    public NoteViewModel(@NonNull Application application) {
        super(application);
    }
}

说明 :这里我设置了一个值为 "TesT" 的 MutableLiveData 测试,然后我打算使用 android:text="@ 显示它{noteViewModel.test}”。但是文本永远不会显示并保持空白。

显然有问题,但尽管两种实现之间存在语法差异,但我还是无法弄清楚为什么 Java 版本没有在 Textview 中显示值。

编辑

感谢 Rajnish suryavanshi,我没有以正确的方式绑定,我不得不只使用 :

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

这个设置内容视图与提供的布局和 return 绑定。 或者你可以这样做:

binding = ActivityMainBinding.inflate(getLayoutInflater());(return绑定但不设置内容视图)

setContentView(binding.getRoot());(用绑定根视图设置内容视图)

我发现这具有误导性 -> https://developer.android.com/topic/libraries/data-binding/expressions

说明我们可以替换

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

来自

ActivityMainBinding 绑定 = ActivityMainBinding.inflate(getLayoutInflater());

这不一样!

很高兴现在得到了敏锐度!

看看这两行。

setContentView(R.layout.activity_main);
binding = ActivityMainBinding.inflate(getLayoutInflater());

您在扩充布局时并未创建绑定。而不是 setContentView 使用 DataBindingUtil.setContentView(this, R.layout.activity_main);

现在您可以使用 layout inflater 获取视图

binding = ActivityMainBinding.inflate(getLayoutInflater());