一、分析案例
<?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><!--這里面就用來(lái)定義數(shù)據(jù)源-->
<variable
name="user"
type= "com.example.databindingdemo.User"/>
</data>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:textSize="50sp"
android:id="@+id/tv1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{user.name}"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:textSize="50sp"
android:id="@+id/tv2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@{user.pwd}"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</LinearLayout>
</layout>
public class User extends BaseObservable {
private String name;
private String pwd;
public User(String name, String pwd) {
this.name = name;
this.pwd = pwd;
}
@Bindable
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
notifyPropertyChanged(BR.name);
}
@Bindable
public String getPwd() {
return pwd;
}
public void setPwd(String pwd) {
this.pwd = pwd;
notifyPropertyChanged(BR.pwd);
}
}
在MainActivity中，調(diào)用下面的方法：
binding= DataBindingUtil.setContentView(this,R.layout.activity_main);
user=new User("zzq","778899");
binding.setUser(user);
二、DataBinding源碼分析
1.首先，看使用了DataBinding的布局變化：
app\build\intermediates\data_binding_layout_info_type_merge\debug\out
<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<Layout
directory="layout"
filePath="app\src\main\res\layout\activity_main.xml"
isBindingData="true"
isMerge="false"
layout="activity_main"
modulePackage="com.example.databindingdemo_20210117"
rootNodeType="android.widget.LinearLayout">
<Variables
name="user"
declared="true"
type="com.example.databindingdemo_20210117.User">
<location
endLine="7"
endOffset="62"
startLine="5"
startOffset="8" />
</Variables>
<Targets>
<Target
tag="layout/activity_main_0"
view="LinearLayout">
<Expressions />
<location
endLine="35"
endOffset="18"
startLine="9"
startOffset="4" />
</Target>
<Target
id="@+id/tv1"
tag="binding_1"
view="TextView">
<Expressions>
<Expression
attribute="android:text"
text="user.name">
<Location
endLine="19"
endOffset="38"
startLine="19"
startOffset="12" />
<TwoWay>false</TwoWay>
<ValueLocation
endLine="19"
endOffset="36"
startLine="19"
startOffset="28" />
</Expression>
</Expressions>
<location
endLine="23"
endOffset="55"
startLine="14"
startOffset="8" />
</Target>
<Target
id="@+id/tv2"
tag="binding_2"
view="TextView">
<Expressions>
<Expression
attribute="android:text"
text="user.pwd">
<Location
endLine="30"
endOffset="37"
startLine="30"
startOffset="12" />
<TwoWay>false</TwoWay>
<ValueLocation
endLine="30"
endOffset="35"
startLine="30"
startOffset="28" />
</Expression>
</Expressions>
<location
endLine="34"
endOffset="55"
startLine="25"
startOffset="8" />
</Target>
</Targets>
</Layout>
可以看到，這里定義了多個(gè)Target標(biāo)簽，這些Target的定義，其實(shí)就是定義對(duì)應(yīng)的tag，將tag與activity_main.xml布局中的對(duì)應(yīng)的View的id對(duì)應(yīng)起來(lái)
經(jīng)過(guò)DataBinding變化后的布局，會(huì)多出tag。
app/build/imtermediates/incremental/mergeDebugResources/stripped.dir/layout/activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" android:tag="layout/activity_main_0" 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">
<TextView
android:textSize="50sp"
android:id="@+id/tv1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:tag="binding_1"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
<TextView
android:textSize="50sp"
android:id="@+id/tv2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:tag="binding_2"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintLeft_toLeftOf="parent"
app:layout_constraintRight_toRightOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</LinearLayout>
2.DataBindingUtil.setContentView原理
binding= DataBindingUtil.setContentView(this,R.layout.activity_main);
（1）DataBindingUtil.setContentView()
其實(shí)DataBindingUtil的setContentView()方法，主要就是調(diào)用activity的setContentView設(shè)置布局，并且綁定添加對(duì)應(yīng)的View
public static <T extends ViewDataBinding> T setContentView(@NonNull Activity activity,
int layoutId) {
return setContentView(activity, layoutId, sDefaultComponent);
}
public static <T extends ViewDataBinding> T setContentView(@NonNull Activity activity,
int layoutId, @Nullable DataBindingComponent bindingComponent) {
// 調(diào)用Activity的setContentView設(shè)置布局
activity.setContentView(layoutId);
View decorView = activity.getWindow().getDecorView();
// 獲取DecorView中的content
ViewGroup contentView = (ViewGroup) decorView.findViewById(android.R.id.content);
return bindToAddedViews(bindingComponent, contentView, 0, layoutId);
}
（2）DataBindingUtil.bindToAddedViews()
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;
// 如果childrenAdded==1，則就只有一個(gè)子View
// 如果不等于1，則有多個(gè)
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);
}
}
（3）DataBindingUtil.bind()
static <T extends ViewDataBinding> T bind(DataBindingComponent bindingComponent, View[] roots,
int layoutId) {
return (T) sMapper.getDataBinder(bindingComponent, roots, layoutId);
}
static <T extends ViewDataBinding> T bind(DataBindingComponent bindingComponent, View root,
int layoutId) {
return (T) sMapper.getDataBinder(bindingComponent, root, layoutId);
}
這里的sMapper是一個(gè)DataBinderMapper對(duì)象，其實(shí)現(xiàn)類是DataBinderMapperImpl
DataBinderMapperImpl是通過(guò)apt注解處理器生成的。
這里的sMapper.getDataBinder()其實(shí)就是調(diào)用的MergedDataBinderMapper的getDataBinder()方法
而sMapper中的數(shù)據(jù)，其實(shí)就是DataBinderMapperImpl的構(gòu)造器中調(diào)用其父類MergedDataBinderMapper 的addMapper()方法添加的對(duì)象
public class DataBinderMapperImpl extends MergedDataBinderMapper {
DataBinderMapperImpl() {
addMapper(new com.example.databindingdemo.DataBinderMapperImpl());
}
}
在DataBinding中有兩個(gè)DataBinderMapperImpl類，一個(gè)是上面這個(gè)在androidx.databinding包下，繼承了MergedDataBinderMapper的，另一個(gè)是在com.example.databindingdemo應(yīng)用包下，直接繼承DataBinderMapper。其實(shí)MergedDataBinderMapper也是繼承自DataBinderMapper
（4）MergedDataBinderMapper.getDataBinder()
@Override
public ViewDataBinding getDataBinder(DataBindingComponent bindingComponent, View view,
int layoutId) {
// mMappers集合中的數(shù)據(jù)就是來(lái)源于androidx.databinding.DataBinderMapperImpl
// 的構(gòu)造器中調(diào)用addMapper方法傳入的對(duì)象添加的
// 所以這里的mapper就是com.example.databindingdemo.DataBinderMapperImpl對(duì)象
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;
}
（5）com.example.databindingdemo.DataBinderMapperImpl.getDataBinder
這里要注意兩點(diǎn)，就是如果是布局的頂層View，比如tag為layout/activity_main_0，那么就會(huì)new一個(gè)ActivityMainBindingImpl對(duì)象。這個(gè)tag，其實(shí)可以從前面看到的app/build/imtermediates/incremental/mergeDebugResources/stripped.dir/layout/activity_main.xml布局中的LinearLayout的tag知道
@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;
}
@Override
public ViewDataBinding getDataBinder(DataBindingComponent component, View[] views, int layoutId) {
if(views == null || views.length == 0) {
return null;
}
int localizedLayoutId = INTERNAL_LAYOUT_ID_LOOKUP.get(layoutId);
if(localizedLayoutId > 0) {
final Object tag = views[0].getTag();
if(tag == null) {
throw new RuntimeException("view must have a tag");
}
switch(localizedLayoutId) {
}
}
return null;
}
（6）ActivityMainBindingImpl的構(gòu)造器
在new出ActivityMainBindingImpl對(duì)象后，則進(jìn)行一些View的綁定操作，將通過(guò)tag取出的View與ActivityMainBindingImpl中對(duì)應(yīng)的View屬性進(jìn)行綁定。
public ActivityMainBindingImpl(@Nullable androidx.databinding.DataBindingComponent bindingComponent, @NonNull View root) {
this(bindingComponent, root, mapBindings(bindingComponent, root, 3, sIncludes, sViewsWithIds));
}
在這里，會(huì)調(diào)用了一個(gè)mapBindings方法，第三個(gè)參數(shù)是一個(gè)3，這個(gè)3的意思，就是activity_main.xml布局文件中有3個(gè)節(jié)點(diǎn)
mapBindings(bindingComponent, root, 3, sIncludes, sViewsWithIds)
mapBindings就會(huì)返回一個(gè)Object[] bindings數(shù)組。
（7）ViewDataBinding.mapBinding()
這里的主要工作，就是將布局中的View保存在對(duì)應(yīng)的bindings數(shù)組中，然后取出這個(gè)數(shù)組中的數(shù)據(jù)賦值給ActivityMainBindingImpl中的View
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;
}
private static void mapBindings(DataBindingComponent bindingComponent, View view,
Object[] bindings, IncludedLayouts includes, SparseIntArray viewsWithIds,
boolean isRoot) {
final int indexInIncludes;
// 判斷View是否已經(jīng)存在綁定，如果已經(jīng)綁定，則直接return
final ViewDataBinding existingBinding = getBinding(view);
if (existingBinding != null) {
return;
}
// 獲取View的tag標(biāo)簽
Object objTag = view.getTag();
final String tag = (objTag instanceof String) ? (String) objTag : null;
boolean isBound = false;
// 如果tag是根布局，并且是以layout開頭的tag
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);
// 將根布局標(biāo)簽對(duì)應(yīng)的View放在bindings數(shù)組中
if (bindings[index] == null) {
bindings[index] = view;
}
indexInIncludes = includes == null ? -1 : index;
isBound = true;
} else {
indexInIncludes = -1;
}
} else if (tag != null && tag.startsWith(BINDING_TAG_PREFIX)) {
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;
}
}
}
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;
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;
}
}
}
}
if (!isInclude) {
mapBindings(bindingComponent, child, bindings, includes, viewsWithIds, false);
}
}
}
}
ActivityMainBindingImpl的父類ActivityMainBinding是在Eapp\build\generated\data_binding_base_class_source_out\debug\out\com\example\databindingdemo\databinding包下
public abstract class ActivityMainBinding extends ViewDataBinding {
@NonNull
public final TextView tv1;
@NonNull
public final TextView tv2;
@Bindable
protected User mUser;
protected ActivityMainBinding(Object _bindingComponent, View _root, int _localFieldCount,
TextView tv1, TextView tv2) {
super(_bindingComponent, _root, _localFieldCount);
this.tv1 = tv1;
this.tv2 = tv2;
}
...
}
3.binding.setUser(user)解析
BR文件
BR的作用：其實(shí)BR的作用，就用BR中的屬性值來(lái)標(biāo)記不同的操作需要的監(jiān)聽在mLocalFieldObservers數(shù)組中的位置
// binding.setVariable(BR.name,"21341234123");
// binding.setUser(user);
// 這里的BR，其實(shí)代表的含義不同，如果是BR.user，則也是會(huì)修改整個(gè)User數(shù)據(jù)
// 如果是binding.setUser()，此時(shí)就是修改了整個(gè)DataBinding的數(shù)據(jù)
// 如果是BR.name，則是修改了name的數(shù)據(jù)
public class BR {
public static final int _all = 0;
public static final int name = 1;
public static final int pwd = 2;
public static final int user = 3;
}
（1）ActivityMainBindingImpl.setUser()
public void setUser(@Nullable com.example.databindingdemo_20210117.User User) {
// 更新注冊(cè)
updateRegistration(0, User);
this.mUser = User;
synchronized(this) {
mDirtyFlags |= 0x1L;
}
notifyPropertyChanged(BR.user);
super.requestRebind();
}
（2）ViewDataBinding.updateRegistration()
// 使用BR的屬性的值做index，存儲(chǔ)每個(gè)BR的屬性對(duì)應(yīng)的監(jiān)聽器
private WeakListener[] mLocalFieldObservers;
// 創(chuàng)建屬性的監(jiān)聽器
private static final CreateWeakListener CREATE_PROPERTY_LISTENER = new CreateWeakListener() {
@Override
public WeakListener create(ViewDataBinding viewDataBinding, int localFieldId) {
// 返回一個(gè)屬性監(jiān)聽
return new WeakPropertyListener(viewDataBinding, localFieldId).getListener();
}
};
protected boolean updateRegistration(int localFieldId, Observable observable) {
return updateRegistration(localFieldId, observable, CREATE_PROPERTY_LISTENER);
}
這里的localFieldId=0，這個(gè)id其實(shí)就BR文件中的id，就是BR文件中對(duì)應(yīng)的靜態(tài)final屬性的值。而第二個(gè)就是觀察者對(duì)象，比如傳入的ViewModel對(duì)象。
private boolean updateRegistration(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
if (observable == null) {
return unregisterFrom(localFieldId);
}
// 根據(jù)BR的每個(gè)屬性的屬性值做index，存儲(chǔ)每個(gè)BR屬性對(duì)應(yīng)的監(jiān)聽器
WeakListener listener = mLocalFieldObservers[localFieldId];
if (listener == null) {
registerTo(localFieldId, observable, listenerCreator);
return true;
}
if (listener.getTarget() == observable) {
return false;//nothing to do, same object
}
unregisterFrom(localFieldId);
registerTo(localFieldId, observable, listenerCreator);
return true;
}
// 這個(gè)registerTo方法，其實(shí)就是將Activity這個(gè)觀察者和User這個(gè)被觀察者
// 統(tǒng)一添加到ObservableReference中
protected void registerTo(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
if (observable == null) {
return;
}
WeakListener listener = mLocalFieldObservers[localFieldId];
if (listener == null) {
// 通過(guò)屬性監(jiān)聽器的創(chuàng)建器創(chuàng)建一個(gè)BR屬性值對(duì)應(yīng)的監(jiān)聽器
// listener就是WeakPropertyListener對(duì)象的getListener返回的
// 就是WeakListener對(duì)象
// 在創(chuàng)建WeakPropertyListener的時(shí)候，其構(gòu)造器內(nèi)部就會(huì)創(chuàng)建
// WeakListener對(duì)象，并且將WeakPropertyListener對(duì)象傳給WeakListener
// 這樣WeakListener和WeakPropertyListener就相互持有
listener = listenerCreator.create(this, localFieldId);
mLocalFieldObservers[localFieldId] = listener;
// 將監(jiān)聽器與觀察者做綁定，這里的觀察者其實(shí)就是Activity
if (mLifecycleOwner != null) {
listener.setLifecycleOwner(mLifecycleOwner);
}
}
// 將該監(jiān)聽器與被觀察者做綁定
// 比如這里的被觀察者就是User
listener.setTarget(observable);
}
這里通過(guò)WeakListener監(jiān)聽器中的ObservableReference對(duì)象保存觀察者與被觀察者，當(dāng)被觀察者發(fā)生改變的時(shí)候，就會(huì)找到對(duì)應(yīng)的WeakListener監(jiān)聽器，然后通知觀察者做修改。
而ObservableReference方法的實(shí)現(xiàn)，有多個(gè)，比如：WeakPropertyListener。
這里讓W(xué)eakListener.setTarget()其實(shí)就是通過(guò)WeakPropertyListener給被觀察者添加callback，然后當(dāng)被觀察者數(shù)據(jù)發(fā)生改變的時(shí)候，被觀察者通過(guò)遍歷其內(nèi)部的PropertyChangeRegistry中的OnPropertyChangedCallback回調(diào)（其實(shí)就是WeakPropertyListener），然后通過(guò)WeakPropertyListener監(jiān)聽通知給ViewDataBinding以及其實(shí)現(xiàn)類ActivityMainBindingImpl具體進(jìn)行數(shù)據(jù)的處理和設(shè)置。
（3）WeakListener.setTarget()給Observable添加回調(diào)
// 這里的mTarget其實(shí)是一個(gè)泛型T對(duì)象，而這個(gè)泛型是在WeakPropertyListener初始化WeakListener的時(shí)候傳入的一個(gè)Observable，這個(gè)是databinding中的Observable，其子類實(shí)現(xiàn)就是BaseObservable
// ViewDataBinding.java
// 這里的mObservable就是WeakPropertyListener對(duì)象
private final ObservableReference<T> mObservable;
public void setTarget(T object) {
unregister();
mTarget = object;
if (mTarget != null) {
mObservable.addListener(mTarget);
}
}
WeakPropertyListener中的addListener方法，就會(huì)給Observable添加一個(gè)callback回調(diào)，向Observable這個(gè)被觀察者中添加callback的目的，就是在Observable數(shù)據(jù)發(fā)生變化的時(shí)候，遍歷Observable中的mCallbacks這個(gè)callback集合，通知觀察者進(jìn)行修改。
// ViewDataBinding.java
private static class WeakPropertyListener extends Observable.OnPropertyChangedCallback
implements ObservableReference<Observable> {
final WeakListener<Observable> mListener;
public WeakPropertyListener(ViewDataBinding binder, int localFieldId) {
mListener = new WeakListener<Observable>(binder, localFieldId, this);
}
@Override
public WeakListener<Observable> getListener() {
return mListener;
}
@Override
public void addListener(Observable target) {
target.addOnPropertyChangedCallback(this);
}
@Override
public void removeListener(Observable target) {
target.removeOnPropertyChangedCallback(this);
}
@Override
public void setLifecycleOwner(LifecycleOwner lifecycleOwner) {
}
@Override
public void onPropertyChanged(Observable sender, int propertyId) {
// 當(dāng)被觀察者數(shù)據(jù)發(fā)生變化的時(shí)候，就會(huì)回調(diào)該方法說(shuō)明屬性發(fā)生變化
ViewDataBinding binder = mListener.getBinder();
if (binder == null) {
return;
}
Observable obj = mListener.getTarget();
if (obj != sender) {
return; // notification from the wrong object?
}
binder.handleFieldChange(mListener.mLocalFieldId, sender, propertyId);
}
}
// BaseObservable.java
@Override
public void addOnPropertyChangedCallback(@NonNull OnPropertyChangedCallback callback) {
synchronized (this) {
if (mCallbacks == null) {
// 這個(gè)PropertyChangeRegistry就是用來(lái)做屬性修改的時(shí)候的回調(diào)的
// 因?yàn)橥挥^察者中添加了callback
// 當(dāng)被觀察者發(fā)生修改的時(shí)候，就會(huì)遍歷這個(gè)mCallbacks進(jìn)行數(shù)據(jù)回調(diào)
mCallbacks = new PropertyChangeRegistry();
}
}
mCallbacks.add(callback);
}
// PropertyChangeRegistry.java
public class PropertyChangeRegistry extends
CallbackRegistry<Observable.OnPropertyChangedCallback, Observable, Void> {
private static final CallbackRegistry.NotifierCallback<Observable.OnPropertyChangedCallback, Observable, Void> NOTIFIER_CALLBACK = new CallbackRegistry.NotifierCallback<Observable.OnPropertyChangedCallback, Observable, Void>() {
@Override
public void onNotifyCallback(Observable.OnPropertyChangedCallback callback, Observable sender,
int arg, Void notUsed) {
callback.onPropertyChanged(sender, arg);
}
};
public PropertyChangeRegistry() {
// 將靜態(tài)屬性NOTIFIER_CALLBACK設(shè)置給其父類的mNotifier
// 設(shè)置這個(gè)的目的，就是在被觀察者數(shù)據(jù)發(fā)生改好的時(shí)候
// 能通過(guò)NOTIFIER_CALLBACK對(duì)象實(shí)現(xiàn)中的onNotifyCallback方法
// 接收回調(diào)處理，將處理過(guò)程回調(diào)給WeakPropertyListener
super(NOTIFIER_CALLBACK);
}
/**
* Notifies registered callbacks that a specific property has changed.
*
* @param observable The Observable that has changed.
* @param propertyId The BR id of the property that has changed or BR._all if the entire
* Observable has changed.
*/
public void notifyChange(@NonNull Observable observable, int propertyId) {
notifyCallbacks(observable, propertyId, null);
}
}
從這第三步可以知道：
- BaseObservable這個(gè)被觀察者，會(huì)被WeakListener持有，而WeakListener會(huì)被WeakPropertyListener持有，從而BaseObservable會(huì)間接的被WeakPropertyListener持有。
- 而BaseObservable會(huì)通過(guò)mCallbacks持有Observable.OnPropertyChangedCallback對(duì)象，而Observable.OnPropertyChangedCallback的子類是WeakPropertyListener，所以BaseObservable也會(huì)持有WeakPropertyListener對(duì)象
- 而BaseObservable會(huì)持有一個(gè)mCallbacks對(duì)象，這是一個(gè)PropertyChangeRegistry對(duì)象，是CallbackRegistry的子類
而WeakPropertyListener和WeakListener是相互持有的對(duì)方的引用。
在完成監(jiān)聽的相互綁定關(guān)系，并且給Observable添加了回調(diào)之后，就會(huì)回到ActivityMainBindingImpl的setUser()方法繼續(xù)執(zhí)行notifyPropertyChanged()方法。
但是這里的例子有個(gè)問(wèn)題，就是監(jiān)聽是添加在User這個(gè)BaseObservable的子類中的，但是更新的時(shí)候，并不是通過(guò)這個(gè)User來(lái)進(jìn)行通知，而是根據(jù)ActivityMainBindingImpl這個(gè)BaseObservable來(lái)通知，那么這個(gè)時(shí)候并不會(huì)通過(guò)ActivityMainBindingImpl的調(diào)用notifyPropertyChanged()最終拿到User中的PropertyChangeRegistry對(duì)象mCallbacks，所以起作用的并不是這句話。而最終ActivityMainBindingImpl在設(shè)置User起刷新作用，是因?yàn)閟uper.requestRebind()的調(diào)用也觸發(fā)了mRebindRunnable任務(wù)的執(zhí)行，其實(shí)就是沒(méi)有通過(guò)PropertyChange來(lái)觸發(fā)requestRebind()
（4）ActivityMainBindingImpl.notifyPropertyChanged()
這里其實(shí)就是調(diào)用的BaseObservable的notifyPropertyChanged()方法，因?yàn)锳ctivityMainBindingImpl是ViewDataBinding的子類，而ViewDataBinding繼承了BaseObservable類
// ViewDataBinding.java
public void notifyPropertyChanged(int fieldId) {
synchronized (this) {
if (mCallbacks == null) {
return;
}
}
mCallbacks.notifyCallbacks(this, fieldId, null);
}
// CallbackRegistry.java
public synchronized void notifyCallbacks(T sender, int arg, A arg2) {
mNotificationLevel++;
notifyRecurse(sender, arg, arg2);
mNotificationLevel--;
if (mNotificationLevel == 0) {
if (mRemainderRemoved != null) {
for (int i = mRemainderRemoved.length - 1; i >= 0; i--) {
final long removedBits = mRemainderRemoved[i];
if (removedBits != 0) {
removeRemovedCallbacks((i + 1) * Long.SIZE, removedBits);
mRemainderRemoved[i] = 0;
}
}
}
if (mFirst64Removed != 0) {
removeRemovedCallbacks(0, mFirst64Removed);
mFirst64Removed = 0;
}
}
}
private void notifyRecurse(T sender, int arg, A arg2) {
final int callbackCount = mCallbacks.size();
final int remainderIndex = mRemainderRemoved == null ? -1 : mRemainderRemoved.length - 1;
// Now we've got all callbakcs that have no mRemainderRemoved value, so notify the
// others.
notifyRemainder(sender, arg, arg2, remainderIndex);
// notifyRemainder notifies all at maxIndex, so we'd normally start at maxIndex + 1
// However, we must also keep track of those in mFirst64Removed, so we add 2 instead:
final int startCallbackIndex = (remainderIndex + 2) * Long.SIZE;
// The remaining have no bit set
notifyCallbacks(sender, arg, arg2, startCallbackIndex, callbackCount, 0);
}
private void notifyRemainder(T sender, int arg, A arg2, int remainderIndex) {
if (remainderIndex < 0) {
notifyFirst64(sender, arg, arg2);
} else {
final long bits = mRemainderRemoved[remainderIndex];
final int startIndex = (remainderIndex + 1) * Long.SIZE;
final int endIndex = Math.min(mCallbacks.size(), startIndex + Long.SIZE);
notifyRemainder(sender, arg, arg2, remainderIndex - 1);
notifyCallbacks(sender, arg, arg2, startIndex, endIndex, bits);
}
}
private void notifyFirst64(T sender, int arg, A arg2) {
final int maxNotified = Math.min(Long.SIZE, mCallbacks.size());
notifyCallbacks(sender, arg, arg2, 0, maxNotified, mFirst64Removed);
}
private void notifyCallbacks(T sender, int arg, A arg2, final int startIndex,
final int endIndex, final long bits) {
long bitMask = 1;
for (int i = startIndex; i < endIndex; i++) {
if ((bits & bitMask) == 0) {
// mCallbacks是PropertyChangeRegistry對(duì)象
// mCallbacks持有的就是WeakPropertyListener對(duì)象
// 這里其實(shí)就是遍歷執(zhí)行所有的WeakPropertyListener的onPropertyChanged方法
// mNotifier是靜態(tài)對(duì)象，是由PropertyChangeRegistry中創(chuàng)建
// 并且由其構(gòu)造器中傳給其PropertyChangeRegistry的父類
mNotifier.onNotifyCallback(mCallbacks.get(i), sender, arg, arg2);
}
bitMask <<= 1;
}
}
這里的mNotifier.notifyCallback其實(shí)就會(huì)調(diào)用到下面的PropertyChangeRegistry中定義的NOTIFIER_CALLBACK 屬性中的onNotifyCallback實(shí)現(xiàn)，而這里的callback其實(shí)就是WeakPropertyListener，因?yàn)閃eakPropertyListener是OnPropertyChangedCallback的子類，這里其實(shí)會(huì)回調(diào)給mLocalFieldObservers數(shù)組中所有的WeakListener
public class PropertyChangeRegistry extends CallbackRegistry<OnPropertyChangedCallback, Observable, Void> {
private static final NotifierCallback<OnPropertyChangedCallback, Observable, Void> NOTIFIER_CALLBACK = new NotifierCallback<OnPropertyChangedCallback, Observable, Void>() {
public void onNotifyCallback(OnPropertyChangedCallback callback, Observable sender, int arg, Void notUsed) {
// WeakPropertyListener是OnPropertyChangedCallback的子類
callback.onPropertyChanged(sender, arg);
}
};
public PropertyChangeRegistry() {
super(NOTIFIER_CALLBACK);
}
public void notifyChange(@NonNull Observable observable, int propertyId) {
this.notifyCallbacks(observable, propertyId, (Object)null);
}
}
// ViewDataBinding.java中的WeakPropertyListener類
@Override
public void onPropertyChanged(Observable sender, int propertyId) {
ViewDataBinding binder = mListener.getBinder();
if (binder == null) {
return;
}
Observable obj = mListener.getTarget();
if (obj != sender) {
return; // notification from the wrong object?
}
// 從mListener中取出target，而這里的mListener其實(shí)就是
// WeakListener，而每個(gè)被觀察者，其實(shí)都是有一個(gè)對(duì)應(yīng)的LocalFieldId
// 這個(gè)id就是BR文件中定義的，剛才的流程中，我們傳入的是0
// 所以這里的mLocalFieldId=0
binder.handleFieldChange(mListener.mLocalFieldId, sender, propertyId);
}
從mListener中取出target，而這里的mListener其實(shí)就是，WeakListener，而每個(gè)被觀察者，其實(shí)都是有一個(gè)對(duì)應(yīng)的LocalFieldId，這個(gè)id就是BR文件中定義的，剛才的流程中，我們傳入的是0，所以這里的mLocalFieldId=0
// ViewDataBinding.java
private void handleFieldChange(int mLocalFieldId, Object object, int fieldId) {
if (mInLiveDataRegisterObserver) {
// We're in LiveData registration, which always results in a field change
// that we can ignore. The value will be read immediately after anyway, so
// there is no need to be dirty.
return;
}
// mLocalFieldId=0
boolean result = onFieldChange(mLocalFieldId, object, fieldId);
if (result) {
requestRebind();
}
}
這里的onFieldChange的方法的實(shí)現(xiàn)，就是在ActivityMainBindingImpl.java中
// ActivityMainBindingImpl.java
@Override
protected boolean onFieldChange(int localFieldId, Object object, int fieldId) {
switch (localFieldId) {
case 0 :
return onChangeUser((com.example.databindingdemo_20210117.User) object, fieldId);
}
return false;
}
private boolean onChangeUser(com.example.databindingdemo_20210117.User User, int fieldId) {
// mDirtyFlags的初始值 0xffffffffffffffffL
if (fieldId == BR._all) {
synchronized(this) {
mDirtyFlags |= 0x1L;
}
return true;
}
else if (fieldId == BR.name) {
synchronized(this) {
mDirtyFlags |= 0x2L;
}
return true;
}
else if (fieldId == BR.pwd) {
synchronized(this) {
mDirtyFlags |= 0x4L;
}
return true;
}
return false;
}
這里因?yàn)閒ieldId=0，所以會(huì)進(jìn)入第一個(gè)if條件if (fieldId == BR._all)，所以會(huì)返回true，所以就會(huì)返回到ViewDataBinding.java中的handleFieldChange方法中，繼續(xù)執(zhí)行requestRebind()
// ViewDataBinding.java
protected void requestRebind() {
if (mContainingBinding != null) {
mContainingBinding.requestRebind();
} else {
final LifecycleOwner owner = this.mLifecycleOwner;
if (owner != null) {
Lifecycle.State state = owner.getLifecycle().getCurrentState();
if (!state.isAtLeast(Lifecycle.State.STARTED)) {
return; // wait until lifecycle owner is started
}
}
synchronized (this) {
if (mPendingRebind) {
return;
}
mPendingRebind = true;
}
if (USE_CHOREOGRAPHER) {
mChoreographer.postFrameCallback(mFrameCallback);
} else {
mUIThreadHandler.post(mRebindRunnable);
}
}
}
這里最終都會(huì)執(zhí)行mRebindRunnable的run()方法。只不過(guò)SDK版本大于等于16的時(shí)候，會(huì)采用Choreographer編舞者來(lái)處理，而之前的版本則是采用Handler來(lái)執(zhí)行。
// ViewDataBinding.java
private final Runnable mRebindRunnable = new Runnable() {
@Override
public void run() {
synchronized (this) {
mPendingRebind = false;
}
processReferenceQueue();
if (VERSION.SDK_INT >= VERSION_CODES.KITKAT) {
// Nested so that we don't get a lint warning in IntelliJ
if (!mRoot.isAttachedToWindow()) {
// Don't execute the pending bindings until the View
// is attached again.
mRoot.removeOnAttachStateChangeListener(ROOT_REATTACHED_LISTENER);
mRoot.addOnAttachStateChangeListener(ROOT_REATTACHED_LISTENER);
return;
}
}
executePendingBindings();
}
};
public void executePendingBindings() {
if (mContainingBinding == null) {
executeBindingsInternal();
} else {
mContainingBinding.executePendingBindings();
}
}
private void executeBindingsInternal() {
if (mIsExecutingPendingBindings) {
requestRebind();
return;
}
if (!hasPendingBindings()) {
return;
}
mIsExecutingPendingBindings = true;
mRebindHalted = false;
if (mRebindCallbacks != null) {
mRebindCallbacks.notifyCallbacks(this, REBIND, null);
// The onRebindListeners will change mPendingHalted
if (mRebindHalted) {
mRebindCallbacks.notifyCallbacks(this, HALTED, null);
}
}
if (!mRebindHalted) {
executeBindings();
if (mRebindCallbacks != null) {
mRebindCallbacks.notifyCallbacks(this, REBOUND, null);
}
}
mIsExecutingPendingBindings = false;
}
在這里最終就會(huì)執(zhí)行到executeBindings()方法，而該方法的實(shí)現(xiàn)，又是在ActivityMainBindingImpl.java中
// ActivityMainBindingImpl.java
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
// mDirtyFlags在之前的onChangeUser方法中
// 因?yàn)閒ieldId=0，所以會(huì)執(zhí)行mDirtyFlags |= 0x1L
// 而初始值為private long mDirtyFlags = 0xffffffffffffffffL
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
java.lang.String userName = null;
com.example.databindingdemo_20210117.User user = mUser;
java.lang.String userPwd = null;
// 因?yàn)閙DirtyFlags或了一個(gè)0x1L，此時(shí)再執(zhí)行一個(gè)與操作
// 不為0
if ((dirtyFlags & 0xfL) != 0) {
// 0xbL為1011，與上0001，則不會(huì)為0
if ((dirtyFlags & 0xbL) != 0) {
if (user != null) {
// read user.name
userName = user.getName();
}
}
// 0xdL=1101，與上0001，也不會(huì)為0
if ((dirtyFlags & 0xdL) != 0) {
if (user != null) {
// read user.pwd
userPwd = user.getPwd();
}
}
}
// batch finished
// 這里最終再執(zhí)行setText的操作，其實(shí)就是調(diào)用了頁(yè)面布局的View的setText
if ((dirtyFlags & 0xbL) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setText(this.tv1, userName);
}
if ((dirtyFlags & 0xdL) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setText(this.tv2, userPwd);
}
}
三、總結(jié)
1.實(shí)現(xiàn)了BaseObservable的類數(shù)據(jù)更新
如果自定義類繼承了BaseObservable類，則會(huì)更新注冊(cè)監(jiān)聽。即BaseObservable保存PropertyChangeRegistry對(duì)象，該對(duì)象中會(huì)保存WeakPropertyListener監(jiān)聽，而WeakPropertyListener監(jiān)聽會(huì)持有WeakListener，WeakListener也會(huì)持有WeakPropertyListener，并且持有一個(gè)BaseObservable的target，這個(gè)target就是自定義的BaseObservable子類實(shí)現(xiàn)對(duì)象，在設(shè)置target的時(shí)候就會(huì)將WeakPropertyListener監(jiān)聽給保存在這個(gè)target中的PropertyChangeRegistry對(duì)象中，當(dāng)使用自定義的BaseObservable進(jìn)行更新的時(shí)候，就可以通過(guò)監(jiān)聽回調(diào)的方式通知到ActivityMainBindingImpl這些ViewDataBinding中，然后向ActivityMainBindingImpl解析得到的View實(shí)體中設(shè)置對(duì)應(yīng)的數(shù)據(jù)。
通過(guò)WeakListener監(jiān)聽器中的ObservableReference對(duì)象保存觀察者與被觀察者，當(dāng)被觀察者發(fā)生改變的時(shí)候，就會(huì)找到對(duì)應(yīng)的WeakListener監(jiān)聽器，然后通知觀察者做修改。
而ObservableReference方法的實(shí)現(xiàn)，有多個(gè)，比如：WeakPropertyListener。
這里讓W(xué)eakListener.setTarget()其實(shí)就是通過(guò)WeakPropertyListener給被觀察者添加callback，然后當(dāng)被觀察者數(shù)據(jù)發(fā)生改變的時(shí)候，被觀察者通過(guò)遍歷其內(nèi)部的PropertyChangeRegistry中的OnPropertyChangedCallback回調(diào)（其實(shí)就是WeakPropertyListener），然后通過(guò)WeakPropertyListener監(jiān)聽通知給ViewDataBinding以及其實(shí)現(xiàn)類ActivityMainBindingImpl具體進(jìn)行數(shù)據(jù)的處理和設(shè)置。
其實(shí)就是向ViewModel或者自定義的Observable（是databinding中的Observable）的子類實(shí)現(xiàn)中的mCallbacks中添加監(jiān)聽WeakPropertyListener，用于數(shù)據(jù)變化回調(diào)。而在WeakPropertyListener中的WeakListener對(duì)象會(huì)保存這個(gè)Observable為target，用于在更新的時(shí)候取出。
2.一般數(shù)據(jù)更新
比如在xml布局的data中直接使用Boolean、Integer、String等類型
如果ActivityMainBindingImpl中設(shè)置的是比如ViewModel，那么就需要看是否使用了LiveData修飾的屬性，如果沒(méi)有使用LiveData的，則并不會(huì)去更新注冊(cè)監(jiān)聽信息，而只是重新保存ViewDataBinding中保存的數(shù)據(jù)實(shí)體，并且直接調(diào)用ActivityMainBinding中保存的View實(shí)體進(jìn)行設(shè)置新的數(shù)據(jù)
3.LiveData數(shù)據(jù)更新
如果是使用了LiveData的話，則會(huì)在調(diào)用LiveDataListener（這是一個(gè)Observer子類與上面的WeakPropertyListener類似）的addListener的時(shí)候，就會(huì)給LiveData注冊(cè)觀察者LiveDataListener，然后在LiveDataListener中的onChanged實(shí)現(xiàn)中通過(guò)調(diào)用ViewDataBinding的handleFieldChange方法觸發(fā)數(shù)據(jù)變化修改，進(jìn)而更新View顯示的數(shù)據(jù)
