Android-Jetpack笔记-DataBinding

DataBinding即数据绑定，可以实现数据和UI的双向绑定。数据改变时，驱动UI刷新；操作UI时，也可以同步给数据。通常在开发界面时，总有findViewById的重复工作，DataBinding可以免去这些操作。同时，DataBinding还可以直接在xml中绑定数据，免去类似setText的操作，让数据来驱动UI刷新。

Jetpack笔记代码

使用

在app/build.gradle中开启：

android {
    dataBinding {
        enabled = true
    }
}

在布局文件中，将光标定位在根布局，alt+enter，然后convert to data binding layout：

布局外层会多出一层layout标签：

<layout>
    <!--数据描述-->
    <data>
    </data>
    <!--布局描述-->
    <ScrollView>
    </ScrollView>
</layout>

在数据描述内，可以导入类和声明变量：

<data>
    <import type="com.holiday.jetpackstudy.model.User" />
    <variable name="user" type="User" />
</data>

在布局描述内，定义一个TextView并绑定数据：

<TextView
    android:id="@+id/tv_name"
    android:text="@{user.name}" />

在activity中，通过DataBindingUtil得到binding对象：

void onCreate(Bundle savedInstanceState) {
    mBinding = DataBindingUtil.setContentView(this, R.layout.activity_main);
}

其中xml文件名决定了生成的binding的类名，xml文件名+Binding，如activity_main.xml生成ActivityMainBinding.java，然后就可以通过binding对象直接访问到view：

mBinding.tvName.setTextColor(xxx);

通过binding对象设置数据，驱动UI刷新：

mBinding.setUser(user);

原理

以DataBindingUtil.setContentView作为入口跟进去，

//DataBindingUtil.java
public static <T extends ViewDataBinding> T setContentView(Activity activity,int layoutId,DataBindingComponent bindingComponent) {
    //这里设置了布局文件
    activity.setContentView(layoutId);
    return bindToAddedViews(bindingComponent, contentView, 0, layoutId);
}

//省略调用链：bindToAddedViews -> bind

static <T extends ViewDataBinding> T bind(DataBindingComponent bindingComponent, View root,int layoutId) {
    return (T) sMapper.getDataBinder(bindingComponent, root, layoutId);
}

来到MergedDataBinderMapper.java，

//MergedDataBinderMapper.java
@Override
public ViewDataBinding getDataBinder(DataBindingComponent bindingComponent, View view,int layoutId) {
    for(DataBinderMapper mapper : mMappers) {
        ViewDataBinding result = mapper.getDataBinder(bindingComponent, view, layoutId);
        if (result != null) {
            return result;
        }
    }
    return null;
}

那么mMappers的值是在什么时候设置的呢？发现只有一处进行add，

//MergedDataBinderMapper.java
public void addMapper(DataBinderMapper mapper) {
    Class<? extends DataBinderMapper> mapperClass = mapper.getClass();
    //如果不在mExistingMappers中，才添加进mMappers
    if (mExistingMappers.add(mapperClass)) {
        mMappers.add(mapper);
        final List<DataBinderMapper> dependencies = mapper.collectDependencies();
        for(DataBinderMapper dependency : dependencies) {
            addMapper(dependency);
        }
    }
}

再来看看谁调了addMapper，发现有一个生成类DataBinderMapperImpl（data binding通过apt创建了一些类），

//DataBinderMapperImpl.java
package androidx.databinding;//注意包名
public class DataBinderMapperImpl extends MergedDataBinderMapper {
    DataBinderMapperImpl() {
        //构造的时候把另一个包下的生成类DataBinderMapperImpl添加进去
        addMapper(new com.holiday.jetpackstudy.DataBinderMapperImpl());
    }
}

接着看业务包名下的生成类DataBinderMapperImpl，

//DataBinderMapperImpl.java
package com.holiday.jetpackstudy;//注意包名
public class DataBinderMapperImpl extends DataBinderMapper {
  @Override
  public ViewDataBinding getDataBinder(DataBindingComponent component, View view, int layoutId) {
      int localizedLayoutId = INTERNAL_LAYOUT_ID_LOOKUP.get(layoutId);
      switch(localizedLayoutId) {
          case  LAYOUT_ACTIVITYMAIN: {
              if ("layout/activity_main_0".equals(tag)) {
                  //返回了binding的具体实现类
                  return new ActivityMainBindingImpl(component, view);
              }
          }
      }
  }
}

这里出现了tag，需要知道的是，DataBinding将布局文件拆成了两个文件，activity_main.xml描述布局，activity_main-layout.xml描述数据，activity_main.xml在app/build/intermediates/incremental/mergeDebugResources/stripped.dir/layout/这个目录下，可见其被剔除了layout外壳和数据描述，同时根布局被加上了android:tag="layout/activity_main_0"，

<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:tag="layout/activity_main_0" >

activity_main-layout.xml在app/build/intermediates/data_binding_layout_info_type_merge/debug/mergeDebugResources/out/目录下，里面可以看到TextView被设置了一个tag="binding_1"，

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<Layout directory="layout" isMerge="false" layout="activity_main" modulePackage="com.holiday.jetpackstudy">
    <Variables name="user" declared="true" type="User">
    </Variables>
    <Imports name="User" type="com.holiday.jetpackstudy.model.User">
    </Imports>
    <Targets>
        <Target tag="layout/activity_main_0" view="ScrollView">
        </Target>
        <Target id="@+id/tv_name" tag="binding_1" view="TextView">
            <Expressions>
                <Expression attribute="android:text" text="user.name">
                    <TwoWay>false</TwoWay>
                </Expression>
            </Expressions>
        </Target>
    </Targets>
</Layout>

接下来跟进具体实现类ActivityMainBindingImpl，

//ActivityMainBindingImpl.java
public ActivityMainBindingImpl(androidx.databinding.DataBindingComponent bindingComponent,View root) {
    this(bindingComponent, root, mapBindings(bindingComponent, root, 2, sIncludes, sViewsWithIds));
    //mapBindings会解析xml里data binding相关的tag，返回Object[]
    //如：if (isRoot && tag != null && tag.startsWith("layout"))
    //如：if (tag != null && tag.startsWith("binding_"))
}

private ActivityMainBindingImpl(androidx.databinding.DataBindingComponent bindingComponent, View root, Object[] bindings) {
    //bindings存储了布局文件里含tag的view，如bindings[0]是根布局，bindings[1]是TextView
    //调用父类ActivityMainBinding的构造方法，为TextView赋值
    super(bindingComponent, root, 0, (android.widget.TextView) bindings[1]);
    this.mboundView0 = (android.widget.ScrollView) bindings[0];
    //这里把tag置空，就不会影响到开发者自己写的tag
    this.mboundView0.setTag(null);
    this.tvName.setTag(null);
    setRootTag(root);
    invalidateAll();
}

//省略调用链：invalidateAll -> requestRebind -> mUIThreadHandler.post(mRebindRunnable);
// -> executePendingBindings -> executeBindingsInternal -> executeBindings

@Override
protected void executeBindings() {
    long dirtyFlags = 0;
    synchronized(this) {
        dirtyFlags = mDirtyFlags;
        mDirtyFlags = 0;
    }
    java.lang.String userName = null;
    com.holiday.jetpackstudy.model.User user = mUser;
    if ((dirtyFlags & 0x3L) != 0) {
        //这里对数据进行了判空，避免了空指针
        if (user != null) {
            userName = user.getName();
        }
    }
    if ((dirtyFlags & 0x3L) != 0) {
        //这里把数据设置给了TextView
        androidx.databinding.adapters.TextViewBindingAdapter.setText(this.tvName, userName);
    }
}

最后补充一点，ActivityMainBinding这个类的位置在app/build/generated/data_binding_base_class_source_out/debug/dataBindingGenBaseClassesDebug/out/$业务包名/databinding/路径下，从这里可以找到binding能直接引用view的原因：

//ActivityMainBinding.java
public abstract class ActivityMainBinding extends ViewDataBinding {
    public final TextView tvName;
    protected ActivityMainBinding(Object _bindingComponent, View _root, int _localFieldCount,TextView tvName) {
        super(_bindingComponent, _root, _localFieldCount);
        this.tvName = tvName;
    }
}

优缺点

• 优点：
  • DataBinding会对绑定的数据进行判空，减少判空代码和空指针异常
  • 省去了找id操作，不会再出现id找不着的情况
• 缺点：
  • apt创建了很多类，增大包体积和编译时长

参考文章

• 简书-Android中的DataBinding的原理浅析