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
findViewById | viewBinding |
---|---|
冗长 | 简短 |
NULL | NULL安全 |
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));
}
});
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;
}
依旧在build.gradle文件中配置如下定义
dataBinding {
enabled = true
}
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>
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);
}
}
private ActivityMainBinding binding;
binding = DataBindingUtil.setContentView( this,R.layout.activity_main );
binding.setViewModel(new ViewModel(binding,new EventMessage()));