作用

1. 将视图绑定到Activity不再需要findViewById()
2. DataBinding 与 ViewModle+LiveData 将可观察数据反向绑定到View，此时可以实现一个结构化较好的程序

使用

一、将视图绑定到Activity

1. build.gradle 启用数据绑定

android {
    ......
    dataBinding {
        enabled = true
    }
}

2. xml中增加layout标签

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android">

    <data>

    </data>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">
        <TextView
            android:id="@+id/tv"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content" />
        <Button
            android:id="@+id/but"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"/>
    </LinearLayout>

</layout>

此时会自动生成与xml文件名相对应的类例如 activity_main.xml 会生成 ActivityMainBinding 类

3. Activity中创建binding对象

private LayoutBinding binding;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    binding = DataBindingUtil.setContentView(this, R.layout.layout);
    binding.tv.setText("文本");
    binding.but.setOnClickListener(v -> {
        //...
    });
}

省略了findViewById 通过binding.id直接调用view，但这样还不够简洁还需要.setText、setOnClickListener等操作设置数据，通过下一步可以反向绑定将数据直接绑定到界面上

二、DataBinding 与 ViewModle+LiveData 将可观察数据反向绑定

• ViewModel

public class MyViewModle extends ViewModel {
    // public int index = 0;
    private MutableLiveData<Integer> liveDataIndex;

    //获取liveDataIndex类型
    public MutableLiveData<Integer> getLiveDataIndex() {
        if (null == liveDataIndex) {
            liveDataIndex = new MutableLiveData<>();
            liveDataIndex.setValue(0);
        }
        return liveDataIndex;
    }

    //liveDataIndex 数据加1
    public void addLiveDataIndex() {
        getLiveDataIndex().setValue(liveDataIndex.getValue() + 1);
    }
}

• xml

<?xml version="1.0" encoding="utf-8"?>
<layout xmlns:android="http://schemas.android.com/apk/res/android">

    <data>

        <variable
            name="data"
            type="com.example.modle.MyViewModle" />
      
    </data>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <TextView
            android:id="@+id/tv"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@{String.valueOf(data.liveDataIndex)}" />

        <Button
            android:id="@+id/but"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:onClick="@{()->data.addLiveDataIndex(1)}" />
    </LinearLayout>

</layout>

variable标签中引用了一个ViewModel，给TextView绑定数据liveDataIndex，在Button中绑定点击事件调用addLiveDataIndex方法，addLiveDataIndex执行liveDataIndex数据变更，则界面自动更新。此时Activity不在需要setText、setListener()

• Activity

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    MyViewModle myViewModle = new ViewModelProvider(this).get(MyViewModle.class);
    LayoutBinding binding = DataBindingUtil.setContentView(this, R.layout.layout);
    binding.setData(myViewModle);
    binding.setLifecycleOwner(this);
}

• Fragment

public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
    MyViewModle myViewModle = new ViewModelProvider(this).get(MyViewModle.class);
    binding = DataBindingUtil.inflate(inflater, R.layout.fragment_about, container, false);
    binding.setModel(myViewModle );
    binding.setLifecycleOwner(this); //委托当前类管理生命周期
    return binding.getRoot();
}

• 如果希望Fragment获取Activity的ViewModel 使用requireActivity()

MyViewModle myViewModle = new ViewModelProvider(requireActivity()).get(MyViewModle.class);