上一章说明了DataBinding生存的类之间关系，现在这里来看看布局是如何加载的以及view是如何映射的。
这里把之前的代码重新贴下方便说明，代码如下：
class MainActivity : AppCompatActivity() {
private val viewModel: SimpleViewModel by viewModels()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val binding: ActivityMainBinding = DataBindingUtil.setContentView(this, R.layout.activity_main)
binding.lifecycleOwner = this
binding.viewModel = viewModel
}
}其中布局加载就这一行：DataBindingUtil.setContentView(this, R.layout.activity_main)，所以进入到DataBindingUtil中，代码如下：
public static <T extends ViewDataBinding> T setContentView(@NonNull Activity activity,
int layoutId) {
return setContentView(activity, layoutId, sDefaultComponent);
}就是简单的调用转发而已，继续下一步，如下：
public static <T extends ViewDataBinding> T setContentView(@NonNull Activity activity,
int layoutId, @Nullable DataBindingComponent bindingComponent) {
activity.setContentView(layoutId);
View decorView = activity.getWindow().getDecorView();
ViewGroup contentView = (ViewGroup) decorView.findViewById(android.R.id.content);
return bindToAddedViews(bindingComponent, contentView, 0, layoutId);
}activity.setContentView(layoutId)，这和我们不用DataBinding写的一样啊，所以Databinding在这里就帮我们加载了布局。
接下来，看DataBinding是如何实现view映射的。
然后拿到decorView 并找到contentView ，最后调用bindToAddedViews，bindToAddedViews的函数如下：
private static <T extends ViewDataBinding> T bindToAddedViews(DataBindingComponent component,
ViewGroup parent, int startChildren, int layoutId) {
final int endChildren = parent.getChildCount();
final int childrenAdded = endChildren - startChildren;
if (childrenAdded == 1) {
final View childView = parent.getChildAt(endChildren - 1);
return bind(component, childView, layoutId);
} else {
final View[] children = new View[childrenAdded];
for (int i = 0; i < childrenAdded; i++) {
children[i] = parent.getChildAt(i + startChildren);
}
return bind(component, children, layoutId);
}
}在我们的场景里面，endChildren 应该为1，childrenAdded 也为1，所以走了第一个分支，继续调用bind函数，如下：
private static DataBinderMapper sMapper = new DataBinderMapperImpl();
static <T extends ViewDataBinding> T bind(DataBindingComponent bindingComponent, View root,
int layoutId) {
return (T) sMapper.getDataBinder(bindingComponent, root, layoutId);
}调用了sMapper的getDataBinder函数，这里的sMapper类型为DataBinderMapperImpl，还记得上一章说过有两个DataBinderMapperImpl吗？为了便于说明，这里再把之前的类图贴下：
额，这就尴尬了，所以这里的Mapper到底是哪个呢？之前说过左边的是android提供的，右边的是我们自己包下面的；其实这里的sMapper属于左边这个行列，也就是androidx这个包下面的。那他们有什么区别呢？你可以认为左边的提供了一个简单的代理功能，其实它就是简单对右边的Mapper类进行包装而已。
这里需要说明下sMapper对象的初始化过程，我们知道类加载会触发类变量（静态变量）的初始化，这个时候sMapper就会被初始化，这个时候会调用DataBinderMapperImpl（左边那个mapper）的构建函数，代码如下：
package androidx.databinding;//位于androidx包下面
public class DataBinderMapperImpl extends MergedDataBinderMapper {
DataBinderMapperImpl() {
//这个DataBinderMapperImpl就是我们自己包下面的了
addMapper(new com.zfang.databindingstudy.DataBinderMapperImpl());
}
}正如前面所说，androidx下面的mapper类包装了项目中的mapper类，addMapper代码如下：
public void addMapper(DataBinderMapper mapper) {
Class<? extends DataBinderMapper> mapperClass = mapper.getClass();
if (mExistingMappers.add(mapperClass)) {
mMappers.add(mapper);
final List<DataBinderMapper> dependencies = mapper.collectDependencies();
for(DataBinderMapper dependency : dependencies) {
addMapper(dependency);
}
}
}这里会把项目中的mapper（即DataBinderMapperImpl）加入到mMappers这个CopyOnWriteArrayList中，后面会用到。
此时可以继续看看getDataBinder的实现了（其实现位于MergedDataBinderMapper中），代码如下：
@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;
}
}
if (loadFeatures()) {
return getDataBinder(bindingComponent, view, layoutId);
}
return null;
}这里就是从mMappers中把mapper拿出来，再根据传递进来的参数view、layoutId找到相应的ViewDataBinding对象；这里的mMappers就是刚刚提到的那个CopyOnWriteArrayList，所以会调用到我们的DataBinderMapperImpl，其中的getDataBinder实现如下：
private static final int LAYOUT_ACTIVITYMAIN = 1;
private static final SparseIntArray INTERNAL_LAYOUT_ID_LOOKUP = new SparseIntArray(1);
static {
INTERNAL_LAYOUT_ID_LOOKUP.put(com.zfang.databindingstudy.R.layout.activity_main, LAYOUT_ACTIVITYMAIN);
}
@Override
public ViewDataBinding getDataBinder(DataBindingComponent component, View view, int layoutId) {
int localizedLayoutId = INTERNAL_LAYOUT_ID_LOOKUP.get(layoutId);
if(localizedLayoutId > 0) {
final Object tag = view.getTag();
if(tag == null) {
throw new RuntimeException("view must have a tag");
}
switch(localizedLayoutId) {
case LAYOUT_ACTIVITYMAIN: {
if ("layout/activity_main_0".equals(tag)) {
return new ActivityMainBindingImpl(component, view);
}
throw new IllegalArgumentException("The tag for activity_main is invalid. Received: " + tag);
}
}
}
return null;
}这里有个SparseIntArray ，它定义了我们的布局与一个整数的映射关系，上面的代码首先拿到view的tag，这里返回的tag为layout/activity_main_0（回忆下：上一章说过DataBinding会生存两个xml，其中一个加了tag，那里说的tag正是和这里对应上了，其作用就体现在这里），所以会返回ActivityMainBindingImpl，这正是需要的ViewDataBinding类。
继续进入ActivityMainBindingImpl的构建函数中，代码如下：
public ActivityMainBindingImpl(@Nullable androidx.databinding.DataBindingComponent bindingComponent, @NonNull View root) {
this(bindingComponent, root, mapBindings(bindingComponent, root, 3, sIncludes, sViewsWithIds));
}
private ActivityMainBindingImpl(androidx.databinding.DataBindingComponent bindingComponent, View root, Object[] bindings) {
super(bindingComponent, root, 2
, (android.widget.TextView) bindings[1]
, (android.widget.TextView) bindings[2]
);
this.first.setTag(null);
this.mboundView0 = (androidx.constraintlayout.widget.ConstraintLayout) bindings[0];
this.mboundView0.setTag(null);
this.second.setTag(null);
setRootTag(root);
// listeners
invalidateAll();
}先调用了第一个构造函数，然后进入第二个。第二个构造函数又调用了父类的相应构造函数，代码如下：
protected ActivityMainBinding(Object _bindingComponent, View _root, int _localFieldCount,
TextView first, TextView second) {
super(_bindingComponent, _root, _localFieldCount);
this.first = first;
this.second = second;
}没错，上面的bindings数组中的bindings[1]、bindings[2]正是对应到了我们这个场景中的first和second两个view。现在的问题是bindings数组中的值是怎么来的呢？
我们继续看看ActivityMainBindingImpl类中第一个构建数据中调用的函数mapBindings，看来在mapBindings中会填充bindings数组，mapBindings代码如下：
protected static Object[] mapBindings(DataBindingComponent bindingComponent, View root,
int numBindings, IncludedLayouts includes, SparseIntArray viewsWithIds) {
Object[] bindings = new Object[numBindings];
mapBindings(bindingComponent, root, bindings, includes, viewsWithIds, true);
return bindings;
}这里 根据numBindings新建了一个数组，继续：
private static void mapBindings(DataBindingComponent bindingComponent, View view,
Object[] bindings, IncludedLayouts includes, SparseIntArray viewsWithIds,
boolean isRoot) {
final int indexInIncludes;
final ViewDataBinding existingBinding = getBinding(view);
if (existingBinding != null) {
return;
}
Object objTag = view.getTag();
final String tag = (objTag instanceof String) ? (String) objTag : null;
boolean isBound = false;
//第一次进来isRoot为true，tag为根据布局所以是以layout开头，因此这进入第一个if
if (isRoot && tag != null && tag.startsWith("layout")) {
final int underscoreIndex = tag.lastIndexOf('_');
if (underscoreIndex > 0 && isNumeric(tag, underscoreIndex + 1)) {
final int index = parseTagInt(tag, underscoreIndex + 1);
if (bindings[index] == null) {
bindings[index] = view;//放入bindings数组，这里的view代表根布局
}
//处理包含布局中有include标签的情况
indexInIncludes = includes == null ? -1 : index;
isBound = true;
} else {
indexInIncludes = -1;
}
} else if (tag != null && tag.startsWith(BINDING_TAG_PREFIX)) {
//如何不是根布局，对应到我们的场景则会走到这里,我们的两个TextView的
//tag刚是以binding开头的，其实只要写了绑定表达式就会到这里。
int tagIndex = parseTagInt(tag, BINDING_NUMBER_START);
if (bindings[tagIndex] == null) {
bindings[tagIndex] = view;
}
isBound = true;
indexInIncludes = includes == null ? -1 : tagIndex;
} else {
// Not a bound view
indexInIncludes = -1;
}
if (!isBound) {
final int id = view.getId();
if (id > 0) {
int index;
if (viewsWithIds != null && (index = viewsWithIds.get(id, -1)) >= 0 &&
bindings[index] == null) {
bindings[index] = view;
}
}
}
//如果是ViewGroup则递归处理找到相应的view
if (view instanceof ViewGroup) {
final ViewGroup viewGroup = (ViewGroup) view;
final int count = viewGroup.getChildCount();
int minInclude = 0;
for (int i = 0; i < count; i++) {
final View child = viewGroup.getChildAt(i);
boolean isInclude = false;
//处理include标签
if (indexInIncludes >= 0 && child.getTag() instanceof String) {
String childTag = (String) child.getTag();
if (childTag.endsWith("_0") &&
childTag.startsWith("layout") && childTag.indexOf('/') > 0) {
// This *could* be an include. Test against the expected includes.
int includeIndex = findIncludeIndex(childTag, minInclude,
includes, indexInIncludes);
if (includeIndex >= 0) {
isInclude = true;
minInclude = includeIndex + 1;
final int index = includes.indexes[indexInIncludes][includeIndex];
final int layoutId = includes.layoutIds[indexInIncludes][includeIndex];
int lastMatchingIndex = findLastMatching(viewGroup, i);
if (lastMatchingIndex == i) {
bindings[index] = DataBindingUtil.bind(bindingComponent, child,
layoutId);
} else {
final int includeCount = lastMatchingIndex - i + 1;
final View[] included = new View[includeCount];
for (int j = 0; j < includeCount; j++) {
included[j] = viewGroup.getChildAt(i + j);
}
bindings[index] = DataBindingUtil.bind(bindingComponent, included,
layoutId);
i += includeCount - 1;
}
}
}
}
//非include
if (!isInclude) {
mapBindings(bindingComponent, child, bindings, includes, viewsWithIds, false);
}
}
}
}这里就是实现view数组映射的关键，主要功能就是填充了bindings数组，思路就是找到包含绑定表达式的控件，然后把它们记录下来放到一个数组中，方便在相应控件的数据变化的时候能够通知到控件， 这里其实就是找到如下布局中的两个TextView然后加入到bindings中。
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:tag="layout/activity_main_0"
tools:context=".MainActivity">
<TextView
android:id="@+id/first"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginVertical="12dp"
android:tag="binding_1"
android:textColor="#333333"
android:textSize="18sp"
app:layout_constraintBottom_toTopOf="@id/second"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_chainStyle="packed" />
<TextView
android:id="@+id/second"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:tag="binding_2"
android:textColor="#999"
android:textSize="14sp"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toBottomOf="@id/first" />
</androidx.constraintlayout.widget.ConstraintLayout>上面那段代码的逻辑就是找到ConstraintLayout（android:tag="layout/activity_main_0"） 以及两个TextView（tag分别为binding_1和binding_2），总共三个控件。ConstraintLayout就是根布局，两个TextView就是我们需要操作的View。
好了，布局view映射完成，简单总结下：首先就是DataBinding会帮我们调用setContentView，所以我们不用调用这个方法；其次DataBinding会帮我们找到包含有数据绑定表达式的View其后帮我们存起来，方便在数据变化的时候操作我们的View。
下一章继续分析数据是如何与控件进行绑定的。
如果你对DataBinding生存的类关系有疑问，可以返回上一章DataBinding原理----类关系进行参考。
