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Android viewBinding

FranzLiszt1847 人气:0

viewBinding

优点

当一个页面布局出现多个控件时,使用findViewById去进行控件绑定,过于冗长,且存在NULL指针异常风险。viewBinding直接创建对视图的引用,不存在因控件ID不存在而引发的NULL指针异常。并且在绑定类中对控件添加@NonNull注解

findViewByIdviewBinding
冗长简短
NULLNULL安全

配置

3.6之前的版本在build.gradle文件中声明如下定义

 viewBinding {
            enabled = true
        }

4.0以上的版本在build.gradle文件中声明如下定义

 buildFeatures {
            viewBinding = true
        }

声明如上定义之后,点击同步(Sync Now)按钮,系统会自动生成viewBinding类,例如MainActivity会生成名为ActivityMainBinding的类,ReceiveActivity会生成名为ActivityReceiveBinding的类,以此类推;
以上viewBinding类会生成在如下路径文件中

build//generated//data_binding_base_class_source_out//debug//out//com.你的包名//databinding

使用

使用步骤很简单,需要被调用的控件声明id就行,然后声明viewBinding类对象

private ActivityMainBinding binding;

绑定视图:

binding = ActivityMainBinding.inflate(getLayoutInflater());
setContentView(binding.getRoot());

控件引用:

binding.postMes.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(MainActivity.this,ReceiverActivity.class));
            }
        });

源码解析

如上所示,我们使用了ActivityMainBinding.inflate()方法进行视图绑定和binding.getRoot()方法获取视图。
首先我们在外部通过调用ActivityMainBinding.inflate()方法。

  @NonNull
  public static ActivityMainBinding inflate(@NonNull LayoutInflater inflater) {
    return inflate(inflater, null, false);
  }

然后内部进行重载,添加我们的Avcivity的布局文件,并调研bind(root)方法

  @NonNull
  public static ActivityMainBinding inflate(@NonNull LayoutInflater inflater,
      @Nullable ViewGroup parent, boolean attachToParent) {
    View root = inflater.inflate(R.layout.activity_main, parent, false);
    if (attachToParent) {
      parent.addView(root);
    }
    return bind(root);
  }

在bind方法中进行控件绑定,通过其findChildViewById()方法

@NonNull
  public static ActivityMainBinding bind(@NonNull View rootView) {
    // The body of this method is generated in a way you would not otherwise write.
    // This is done to optimize the compiled bytecode for size and performance.
    int id;
    missingId: {
      id = R.id.content;
      TextView content = ViewBindings.findChildViewById(rootView, id);
      if (content == null) {
        break missingId;
      }

      id = R.id.postMes;
      Button postMes = ViewBindings.findChildViewById(rootView, id);
      if (postMes == null) {
        break missingId;
      }
      return new ActivityMainBinding((LinearLayout) rootView, content, postMes);
    }
    String missingId = rootView.getResources().getResourceName(id);
    throw new NullPointerException("Missing required view with ID: ".concat(missingId));
  }

然后在findChildViewById()方法中最终也使用到了findViewById()方法,但差距在于跳过视图本身

 /**
     * Like `findViewById` but skips the view itself.
     *
     * @hide
     */
    @Nullable
    public static <T extends View> T findChildViewById(View rootView, @IdRes int id) {
        if (!(rootView instanceof ViewGroup)) {
            return null;
        }
        final ViewGroup rootViewGroup = (ViewGroup) rootView;
        final int childCount = rootViewGroup.getChildCount();
        for (int i = 0; i < childCount; i++) {
            final T view = rootViewGroup.getChildAt(i).findViewById(id);
            if (view != null) {
                return view;
            }
        }
        return null;
    }

最后通过将获取到的控件定义与内部定义的字段进行缝合,以暴露给外部使用

  @NonNull
  private final LinearLayout rootView;

  @NonNull
  public final TextView content;

  @NonNull
  public final Button postMes;
  private ActivityMainBinding(@NonNull LinearLayout rootView, @NonNull TextView content,
      @NonNull Button postMes) {
    this.rootView = rootView;
    this.content = content;
    this.postMes = postMes;
  }

DataBinding

配置

依旧在build.gradle文件中配置如下定义:

 dataBinding {
            enabled = true
        }

创建实体类

实体类通过继承BaseObservable类,而BaseObservable又实现了Observable,从而获取添加和移除监听的机制。
在get()方法中使用@Bindable注解,会自动生成BR类,此类中将添加@Bindable的字段声明成常量,然后在set()方法使用notifyPropertyChanged()配合使用,当数据发生变化时,dataBinding会自动修改该字段的值。

public class EventMessage extends BaseObservable {
    public String title;
    public EventMessage(){

    }
    public EventMessage(String title){
        this.title = title;
    }
    @Bindable
    public String getTitle() {
        return title;
    }
    /**
     * @param title*/
    public void setTitle(String title) {
        this.title = title;
        notifyPropertyChanged(BR.title);
    }
}

创建布局

创建layout标签布局才会生成ActivityMainBinding(以及布局文件名而定)
EditText通过使用如下定义进行绑定,

 android:text="@={viewModel.message.title}"

Button通过如下定义进行点击事件监听

 android:onClick="@{viewModel.setText}"

以上两者的存在差距,EditText多了一个=,而Button没有,并且Button绑定监听事件,不需要加()

<?xml version="1.0" encoding="utf-8"?>
<layout 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">

    <data>
        <variable
            name="viewModel"
            type="com.franzliszt.databinding.ViewModel" />

    </data>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context=".MainActivity"
        android:orientation="vertical"
        android:gravity="center">
        <EditText
            android:id="@+id/inputText"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:hint="default"
            android:text="@={viewModel.message.title}"/>
        <TextView
            android:id="@+id/ShowText"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text=""/>
        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="update"
            android:onClick="@{viewModel.setText}"/>
    </LinearLayout>
</layout>

创建viewModel

通过监听Button点击事件,在其中监听EditText输入事件,并将其输入的字符串显示在TextView中

public class ViewModel {
    private ActivityMainBinding binding;
    public EventMessage message;
    public ViewModel(ActivityMainBinding binding, EventMessage message){
        this.binding = binding;
        this.message = message;
    }
    public void setText(View view){
        String str = message.getTitle();
        binding.ShowText.setText(str);
    }
}

dataBinding绑定

 private ActivityMainBinding binding;
binding = DataBindingUtil.setContentView( this,R.layout.activity_main );
binding.setViewModel(new ViewModel(binding,new EventMessage()));

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