Android-Jetpack笔记-DataBinding

DataBinding即数据绑定，能够实现数据和UI的双向绑定。数据改变时，驱动UI刷新；操做UI时，也能够同步给数据。一般在开发界面时，总有findViewById的重复工做，DataBinding能够免去这些操做。同时，DataBinding还能够直接在xml中绑定数据，免去相似setText的操做，让数据来驱动UI刷新。java

Jetpack笔记代码android

使用

在app/build.gradle中开启：git

android {
    dataBinding {
        enabled = true
    }
}
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在布局文件中，将光标定位在根布局，alt+enter，而后convert to data binding layout：github

布局外层会多出一层layout标签：markdown

<layout>
    <!--数据描述-->
    <data>
    </data>
    <!--布局描述-->
    <ScrollView>
    </ScrollView>
</layout>
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在数据描述内，能够导入类和声明变量：app

<data>
    <import type="com.holiday.jetpackstudy.model.User" />
    <variable name="user" type="User" />
</data>
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在布局描述内，定义一个TextView并绑定数据：ide

<TextView android:id="@+id/tv_name" android:text="@{user.name}" />
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在activity中，经过DataBindingUtil获得binding对象：oop

void onCreate(Bundle savedInstanceState) {
    mBinding = DataBindingUtil.setContentView(this, R.layout.activity_main);
}
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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，而后就能够经过binding对象直接访问到view：布局

mBinding.tvName.setTextColor(xxx);
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经过binding对象设置数据，驱动UI刷新：post

mBinding.setUser(user);
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原理

以DataBindingUtil.setContentView做为入口跟进去，

//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); } 复制代码

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;
}
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那么mMappers的值是在何时设置的呢？发现只有一处进行add，

//MergedDataBinderMapper.java
public void addMapper(DataBinderMapper mapper) {
    Class<? extends DataBinderMapper> mapperClass = mapper.getClass();
    //若是不在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);
        }
    }
}
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再来看看谁调了addMapper，发现有一个生成类DataBinderMapperImpl（data binding经过apt建立了一些类），

//DataBinderMapperImpl.java
package androidx.databinding;//注意包名
public class DataBinderMapperImpl extends MergedDataBinderMapper {
    DataBinderMapperImpl() {
        //构造的时候把另外一个包下的生成类DataBinderMapperImpl添加进去
        addMapper(new com.holiday.jetpackstudy.DataBinderMapperImpl());
    }
}
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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);
              }
          }
      }
  }
}
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这里出现了tag，须要知道的是，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" >
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activity_main-layout.xml在app/build/intermediates/data_binding_layout_info_type_merge/debug/mergeDebugResources/out/目录下，里面能够看到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>
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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本身写的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); } }  复制代码

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，从这里能够找到binding能直接引用view的缘由：

//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;
    }
}
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优缺点

优势：
- DataBinding会对绑定的数据进行判空，减小判空代码和空指针异常
- 省去了找id操做，不会再出现id找不着的状况
缺点：
- apt建立了不少类，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

本文使用 mdnice 排版