目前工程使用了DataBinding，感觉还可以，但是有时候确实不好调试，运行的时候最好看下错误日志，可以使用gradle assembleDebug --info编译查看，可以看到比较明显的日志。然后呢，抽空看了下DataBinding的源码，也看了下其他同学对于DataBinding的一些分析，结合自己的一些看法，对DataBinding做个总结。
1.DataBinding的作用？
在MVC模式中，用不用DataBinding都可以，不会有明显的变化。因为UI的变化和数据的变化都在Activity中。
但是在MVP和MVVM模式中，如果使用了DataBinding，你会感觉很愉悦的。不是说不能用，配合LiveData也可以，但是感觉很麻烦，也是一些重复性的工作。【好的技术就是用来偷懒的！-鲁迅】
DataBinding的核心作用就一个，和UI组件相互绑定，能直接通知对方刷新，其他的作用都是顺带的。
2.DataBinding的原理？
下面来分析下DataBinding的原理，这里先做总结：
DataBinding通过apt技术生成编译期的代码，实现对View的绑定和对数据Observable的监听，而对View的监听则是通过setTextWatcher来实现的。
上面其实的说的比较笼统，我们下面来具体分析
一.DataBinding编译期间通过apt和javapoet技术生成代码
会生成几个重要的文件，先看生成的layout文件
image.png
说下它们的作用，先说下面那个
image.png
这个主要是生成layout文件，能被xml解析的。
再看第一个
image.png
可以看到，它是保存原layout文件保存可一些信息，比如是否有merge、时候双向绑定等。
再看生成的java文件，如下
image.png
里面有几个重要文件，DataBinding里面的核心代码就在它们里面
ActivityMainBinding、BR、DataBinderMapperImpl、ActivityMainBindingImpl等。
二.从DataBinding.setContentView分析源码：
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
val databind = DataBindingUtil.setContentView<ActivityMainBinding>(this,R.layout.activity_main)
val person = Person()
person.userName.set("1111111")
databind.person = person
// setContentView(R.layout.activity_main)
}
}
追踪DataBinding源码:
public class DataBindingUtil {
private static DataBinderMapper sMapper = new DataBinderMapperImpl();
private static DataBindingComponent sDefaultComponent = null;
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) {
activity.setContentView(layoutId);
View decorView = activity.getWindow().getDecorView();
ViewGroup contentView = (ViewGroup) decorView.findViewById(android.R.id.content);
return bindToAddedViews(bindingComponent, contentView, 0, layoutId);
}
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);
}
}
static <T extends ViewDataBinding> T bind(DataBindingComponent bindingComponent, View[] roots,
int layoutId) {
return (T) sMapper.getDataBinder(bindingComponent, roots, layoutId);
}
}
内部还是调用了Activity.setContentVIew()方法，然后返回一个ViewDataBinding。然后可以看到，遍历View的个数，调用bind()方法。而sMapper是DataBinderMapperImpl，所有我们看下其内部：
public class DataBinderMapperImpl extends DataBinderMapper {
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.example.databingtest2.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;
}
@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;
}
@Override
public int getLayoutId(String tag) {
if (tag == null) {
return 0;
}
Integer tmpVal = InnerLayoutIdLookup.sKeys.get(tag);
return tmpVal == null ? 0 : tmpVal;
}
@Override
public String convertBrIdToString(int localId) {
String tmpVal = InnerBrLookup.sKeys.get(localId);
return tmpVal;
}
@Override
public List<DataBinderMapper> collectDependencies() {
ArrayList<DataBinderMapper> result = new ArrayList<DataBinderMapper>(1);
result.add(new androidx.databinding.library.baseAdapters.DataBinderMapperImpl());
return result;
}
private static class InnerBrLookup {
static final SparseArray<String> sKeys = new SparseArray<String>(2);
static {
sKeys.put(0, "_all");
sKeys.put(1, "person");
}
}
private static class InnerLayoutIdLookup {
static final HashMap<String, Integer> sKeys = new HashMap<String, Integer>(1);
static {
sKeys.put("layout/activity_main_0", com.example.databingtest2.R.layout.activity_main);
}
}
}
最后返回的是ActivityMainBindingImpl构造方法，继续追踪：
public class ActivityMainBindingImpl extends ActivityMainBinding {
@Nullable
private static final androidx.databinding.ViewDataBinding.IncludedLayouts sIncludes;
@Nullable
private static final android.util.SparseIntArray sViewsWithIds;
static {
sIncludes = null;
sViewsWithIds = null;
}
// views
@NonNull
private final androidx.constraintlayout.widget.ConstraintLayout mboundView0;
// variables
// values
// listeners
// Inverse Binding Event Handlers
private androidx.databinding.InverseBindingListener text1androidTextAttrChanged = new androidx.databinding.InverseBindingListener() {
@Override
public void onChange() {
// Inverse of person.userName.get()
// is person.userName.set((java.lang.String) callbackArg_0)
java.lang.String callbackArg_0 = androidx.databinding.adapters.TextViewBindingAdapter.getTextString(text1);
// localize variables for thread safety
// person
com.example.databingtest2.Person person = mPerson;
// person.userName
androidx.databinding.ObservableField<java.lang.String> personUserName = null;
// person.userName.get()
java.lang.String personUserNameGet = null;
// person != null
boolean personJavaLangObjectNull = false;
// person.userName != null
boolean personUserNameJavaLangObjectNull = false;
personJavaLangObjectNull = (person) != (null);
if (personJavaLangObjectNull) {
personUserName = person.getUserName();
personUserNameJavaLangObjectNull = (personUserName) != (null);
if (personUserNameJavaLangObjectNull) {
personUserName.set(((java.lang.String) (callbackArg_0)));
}
}
}
};
public ActivityMainBindingImpl(@Nullable androidx.databinding.DataBindingComponent bindingComponent, @NonNull View root) {
this(bindingComponent, root, mapBindings(bindingComponent, root, 2, sIncludes, sViewsWithIds));
}
private ActivityMainBindingImpl(androidx.databinding.DataBindingComponent bindingComponent, View root, Object[] bindings) {
super(bindingComponent, root, 1
, (android.widget.TextView) bindings[1]
);
this.mboundView0 = (androidx.constraintlayout.widget.ConstraintLayout) bindings[0];// ---> ①
this.mboundView0.setTag(null);
this.text1.setTag(null);
setRootTag(root);
// listeners
invalidateAll();// ---->②
}
@Override
public void invalidateAll() {
synchronized(this) {
mDirtyFlags = 0x4L;
}
requestRebind();//----->三
}
@Override
public boolean hasPendingBindings() {
synchronized(this) {
if (mDirtyFlags != 0) {
return true;
}
}
return false;
}
@Override
public boolean setVariable(int variableId, @Nullable Object variable) {
boolean variableSet = true;
if (BR.person == variableId) {
setPerson((com.example.databingtest2.Person) variable);
}
else {
variableSet = false;
}
return variableSet;
}
public void setPerson(@Nullable com.example.databingtest2.Person Person) {
this.mPerson = Person;
synchronized(this) {
mDirtyFlags |= 0x2L;
}
notifyPropertyChanged(BR.person);
super.requestRebind();
}
@Override
protected boolean onFieldChange(int localFieldId, Object object, int fieldId) {
switch (localFieldId) {
case 0 :
return onChangePersonUserName((androidx.databinding.ObservableField<java.lang.String>) object, fieldId);
}
return false;
}
private boolean onChangePersonUserName(androidx.databinding.ObservableField<java.lang.String> PersonUserName, int fieldId) {
if (fieldId == BR._all) {
synchronized(this) {
mDirtyFlags |= 0x1L;
}
return true;
}
return false;
}
@Override
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
com.example.databingtest2.Person person = mPerson;
int personColor = 0;
androidx.databinding.ObservableField<java.lang.String> personUserName = null;
java.lang.String personUserNameGet = null;
if ((dirtyFlags & 0x7L) != 0) {
if ((dirtyFlags & 0x6L) != 0) {
if (person != null) {
// read person.color
personColor = person.getColor();
}
}
if (person != null) {
// read person.userName
personUserName = person.getUserName();
}
updateRegistration(0, personUserName);
if (personUserName != null) {
// read person.userName.get()
personUserNameGet = personUserName.get();
}
}
// batch finished
if ((dirtyFlags & 0x6L) != 0) {
// api target 1
com.example.databingtest2.DataBindingAdapterExtKt.setTextViewColor(this.text1, personColor);
}
if ((dirtyFlags & 0x7L) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setText(this.text1, personUserNameGet);
}
if ((dirtyFlags & 0x4L) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setTextWatcher(this.text1, (androidx.databinding.adapters.TextViewBindingAdapter.BeforeTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.OnTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.AfterTextChanged)null, text1androidTextAttrChanged);
}
}
// Listener Stub Implementations
// callback impls
// dirty flag
private long mDirtyFlags = 0xffffffffffffffffL;
/* flag mapping
flag 0 (0x1L): person.userName
flag 1 (0x2L): person
flag 2 (0x3L): null
flag mapping end*/
//end
}
ActivityMainBindingImpl继承ActivityMainBinding，所以可以返回ActivityMainBinding结束了。
三.DataBinding数据和View的相互绑定
ActivityMainBindingImpl.invalidateAll() -->ActivityMainBindingImpl.requestRebind() -->
ViewDataBinding.executePendingBindings() --->ViewDataBinding.executeBindingsInternal()--->
ActivityMainBindingImpl.executeBindings(),我们看下executeBindings()源码：
@Override
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
com.example.databingtest2.Person person = mPerson;
int personColor = 0;
androidx.databinding.ObservableField<java.lang.String> personUserName = null;
java.lang.String personUserNameGet = null;
if ((dirtyFlags & 0x7L) != 0) {
if ((dirtyFlags & 0x6L) != 0) {
if (person != null) {
// read person.color
personColor = person.getColor();
}
}
if (person != null) {
// read person.userName
personUserName = person.getUserName();
}
updateRegistration(0, personUserName);
if (personUserName != null) {
// read person.userName.get()
personUserNameGet = personUserName.get();
}
}
// batch finished
if ((dirtyFlags & 0x6L) != 0) {
// api target 1
com.example.databingtest2.DataBindingAdapterExtKt.setTextViewColor(this.text1, personColor);
}
if ((dirtyFlags & 0x7L) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setText(this.text1, personUserNameGet);
}
if ((dirtyFlags & 0x4L) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setTextWatcher(this.text1, (androidx.databinding.adapters.TextViewBindingAdapter.BeforeTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.OnTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.AfterTextChanged)null, text1androidTextAttrChanged);
}
}
可以看到，最终调用executeBindings()方法。实际上，View和BaseObservable数据的相互绑定就在这个方法里面。
- View变化，通知BaseObservable数据刷新
看源码倒数第三行，通过setTextWatcher()方法对view进行监听，看下text1androidTextAttrChanged是啥:
// variables
// values
// listeners
// Inverse Binding Event Handlers
private androidx.databinding.InverseBindingListener text1androidTextAttrChanged = new androidx.databinding.InverseBindingListener() {
@Override
public void onChange() {
// Inverse of person.userName.get()
// is person.userName.set((java.lang.String) callbackArg_0)
java.lang.String callbackArg_0 = androidx.databinding.adapters.TextViewBindingAdapter.getTextString(text1);
// localize variables for thread safety
// person
com.example.databingtest2.Person person = mPerson;
// person.userName
androidx.databinding.ObservableField<java.lang.String> personUserName = null;
// person.userName.get()
java.lang.String personUserNameGet = null;
// person != null
boolean personJavaLangObjectNull = false;
// person.userName != null
boolean personUserNameJavaLangObjectNull = false;
personJavaLangObjectNull = (person) != (null);
if (personJavaLangObjectNull) {
personUserName = person.getUserName();
personUserNameJavaLangObjectNull = (personUserName) != (null);
if (personUserNameJavaLangObjectNull) {
personUserName.set(((java.lang.String) (callbackArg_0)));
}
}
}
};
在InverseBindingListener中监听，当View的text改变时，刷新BaseObservable数据的值。
那么BaseObservable数据的值改变，怎么监听呢？我们先看下BaseObservable的源码：
public class BaseObservable implements Observable {
private transient PropertyChangeRegistry mCallbacks;
public BaseObservable() {
}
@Override
public void addOnPropertyChangedCallback(@NonNull OnPropertyChangedCallback callback) {
synchronized (this) {
if (mCallbacks == null) {
mCallbacks = new PropertyChangeRegistry();
}
}
mCallbacks.add(callback);
}
@Override
public void removeOnPropertyChangedCallback(@NonNull OnPropertyChangedCallback callback) {
synchronized (this) {
if (mCallbacks == null) {
return;
}
}
mCallbacks.remove(callback);
}
/**
* Notifies listeners that all properties of this instance have changed.
*/
public void notifyChange() {
synchronized (this) {
if (mCallbacks == null) {
return;
}
}
mCallbacks.notifyCallbacks(this, 0, null);
}
/**
* Notifies listeners that a specific property has changed. The getter for the property
* that changes should be marked with {@link Bindable} to generate a field in
* <code>BR</code> to be used as <code>fieldId</code>.
*
* @param fieldId The generated BR id for the Bindable field.
*/
public void notifyPropertyChanged(int fieldId) {
synchronized (this) {
if (mCallbacks == null) {
return;
}
}
mCallbacks.notifyCallbacks(this, fieldId, null);
}
}
继承Observable，我们看下Observable的源码：
public interface Observable {
/**
* Adds a callback to listen for changes to the Observable.
* @param callback The callback to start listening.
*/
void addOnPropertyChangedCallback(OnPropertyChangedCallback callback);
/**
* Removes a callback from those listening for changes.
* @param callback The callback that should stop listening.
*/
void removeOnPropertyChangedCallback(OnPropertyChangedCallback callback);
/**
* The callback that is called by Observable when an observable property has changed.
*/
abstract class OnPropertyChangedCallback {
/**
* Called by an Observable whenever an observable property changes.
* @param sender The Observable that is changing.
* @param propertyId The BR identifier of the property that has changed. The getter
* for this property should be annotated with {@link Bindable}.
*/
public abstract void onPropertyChanged(Observable sender, int propertyId);
}
}
可以看到Observable有回调接口OnPropertyChangedCallback，所以在DataBinding里面肯定是有对BaseObservable数据的监听设置的。那么它在哪儿呢？
在上面的executeBindings()方法内部有这样一个方法：updateRegistration(),其实就是在这里绑定，我们看其源码：
protected boolean updateRegistration(int localFieldId, Observable observable) {
return updateRegistration(localFieldId, observable, CREATE_PROPERTY_LISTENER);
}
private boolean updateRegistration(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
if (observable == null) {
return unregisterFrom(localFieldId);
}
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;
}
/**
* @hide
*/
protected void registerTo(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
if (observable == null) {
return;
}
WeakListener listener = mLocalFieldObservers[localFieldId];
if (listener == null) {
listener = listenerCreator.create(this, localFieldId);
mLocalFieldObservers[localFieldId] = listener;
if (mLifecycleOwner != null) {
listener.setLifecycleOwner(mLifecycleOwner);
}
}
listener.setTarget(observable);
}
最终到了WeakListener中，我们看下WeakListener是什么?
private static class WeakListener<T> extends WeakReference<ViewDataBinding> {
private final ObservableReference<T> mObservable;
protected final int mLocalFieldId;
private T mTarget;
public WeakListener(ViewDataBinding binder, int localFieldId,
ObservableReference<T> observable) {
super(binder, sReferenceQueue);
mLocalFieldId = localFieldId;
mObservable = observable;
}
public void setLifecycleOwner(LifecycleOwner lifecycleOwner) {
mObservable.setLifecycleOwner(lifecycleOwner);
}
public void setTarget(T object) {
unregister();
mTarget = object;
if (mTarget != null) {
mObservable.addListener(mTarget);
}
}
public boolean unregister() {
boolean unregistered = false;
if (mTarget != null) {
mObservable.removeListener(mTarget);
unregistered = true;
}
mTarget = null;
return unregistered;
}
public T getTarget() {
return mTarget;
}
protected ViewDataBinding getBinder() {
ViewDataBinding binder = get();
if (binder == null) {
unregister(); // The binder is dead
}
return binder;
}
}
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) {
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);
}
}
private static class WeakListListener extends ObservableList.OnListChangedCallback
implements ObservableReference<ObservableList> {
final WeakListener<ObservableList> mListener;
public WeakListListener(ViewDataBinding binder, int localFieldId) {
mListener = new WeakListener<ObservableList>(binder, localFieldId, this);
}
@Override
public void setLifecycleOwner(LifecycleOwner lifecycleOwner) {
}
@Override
public WeakListener<ObservableList> getListener() {
return mListener;
}
@Override
public void addListener(ObservableList target) {
target.addOnListChangedCallback(this);
}
@Override
public void removeListener(ObservableList target) {
target.removeOnListChangedCallback(this);
}
@Override
public void onChanged(ObservableList sender) {
ViewDataBinding binder = mListener.getBinder();
if (binder == null) {
return;
}
ObservableList target = mListener.getTarget();
if (target != sender) {
return; // We expect notifications only from sender
}
binder.handleFieldChange(mListener.mLocalFieldId, target, 0);
}
@Override
public void onItemRangeChanged(ObservableList sender, int positionStart, int itemCount) {
onChanged(sender);
}
@Override
public void onItemRangeInserted(ObservableList sender, int positionStart, int itemCount) {
onChanged(sender);
}
@Override
public void onItemRangeMoved(ObservableList sender, int fromPosition, int toPosition,
int itemCount) {
onChanged(sender);
}
@Override
public void onItemRangeRemoved(ObservableList sender, int positionStart, int itemCount) {
onChanged(sender);
}
}
private static class WeakMapListener extends ObservableMap.OnMapChangedCallback
implements ObservableReference<ObservableMap> {
final WeakListener<ObservableMap> mListener;
public WeakMapListener(ViewDataBinding binder, int localFieldId) {
mListener = new WeakListener<ObservableMap>(binder, localFieldId, this);
}
@Override
public void setLifecycleOwner(LifecycleOwner lifecycleOwner) {
}
@Override
public WeakListener<ObservableMap> getListener() {
return mListener;
}
@Override
public void addListener(ObservableMap target) {
target.addOnMapChangedCallback(this);
}
@Override
public void removeListener(ObservableMap target) {
target.removeOnMapChangedCallback(this);
}
@Override
public void onMapChanged(ObservableMap sender, Object key) {
ViewDataBinding binder = mListener.getBinder();
if (binder == null || sender != mListener.getTarget()) {
return;
}
binder.handleFieldChange(mListener.mLocalFieldId, sender, 0);
}
}
private static class LiveDataListener implements Observer,
ObservableReference<LiveData<?>> {
final WeakListener<LiveData<?>> mListener;
LifecycleOwner mLifecycleOwner;
public LiveDataListener(ViewDataBinding binder, int localFieldId) {
mListener = new WeakListener(binder, localFieldId, this);
}
@Override
public void setLifecycleOwner(LifecycleOwner lifecycleOwner) {
LifecycleOwner owner = (LifecycleOwner) lifecycleOwner;
LiveData<?> liveData = mListener.getTarget();
if (liveData != null) {
if (mLifecycleOwner != null) {
liveData.removeObserver(this);
}
if (lifecycleOwner != null) {
liveData.observe(owner, this);
}
}
mLifecycleOwner = owner;
}
@Override
public WeakListener<LiveData<?>> getListener() {
return mListener;
}
@Override
public void addListener(LiveData<?> target) {
if (mLifecycleOwner != null) {
target.observe(mLifecycleOwner, this);
}
}
@Override
public void removeListener(LiveData<?> target) {
target.removeObserver(this);
}
@Override
public void onChanged(@Nullable Object o) {
ViewDataBinding binder = mListener.getBinder();
if (binder != null) {
binder.handleFieldChange(mListener.mLocalFieldId, mListener.getTarget(), 0);
}
}
}
private interface CreateWeakListener {
WeakListener create(ViewDataBinding viewDataBinding, int localFieldId);
}
WeakListener有好几种实现，比如LiveDataListener、WeakMapListener、WeakListListener、WeakPropertyListener、WeakListener，我们通过源码知道，WeakListener通过CREATE_PROPERTY_LISTENER创建的，我们看下CREATE_PROPERTY_LISTENER是啥：
/**
* Method object extracted out to attach a listener to a bound Observable object.
*/
private static final CreateWeakListener CREATE_PROPERTY_LISTENER = new CreateWeakListener() {
@Override
public WeakListener create(ViewDataBinding viewDataBinding, int localFieldId) {
return new WeakPropertyListener(viewDataBinding, localFieldId).getListener();
}
};
/**
* Method object extracted out to attach a listener to a bound ObservableList object.
*/
private static final CreateWeakListener CREATE_LIST_LISTENER = new CreateWeakListener() {
@Override
public WeakListener create(ViewDataBinding viewDataBinding, int localFieldId) {
return new WeakListListener(viewDataBinding, localFieldId).getListener();
}
};
/**
* Method object extracted out to attach a listener to a bound ObservableMap object.
*/
private static final CreateWeakListener CREATE_MAP_LISTENER = new CreateWeakListener() {
@Override
public WeakListener create(ViewDataBinding viewDataBinding, int localFieldId) {
return new WeakMapListener(viewDataBinding, localFieldId).getListener();
}
};
/**
* Method object extracted out to attach a listener to a bound LiveData object.
*/
private static final CreateWeakListener CREATE_LIVE_DATA_LISTENER = new CreateWeakListener() {
@Override
public WeakListener create(ViewDataBinding viewDataBinding, int localFieldId) {
return new LiveDataListener(viewDataBinding, localFieldId).getListener();
}
};
CREATE_PROPERTY_LISTENER内部创建了WeakPropertyListener，我们定位到WeakPropertyListener：
c 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) {
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);
}
}
这个就很明显了，target.addOnPropertyChangedCallback设置监听，然后在onPropertyChanged方法内部调用handleFieldChange()方法，我们看下handleFieldChange源码：
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;
}
boolean result = onFieldChange(mLocalFieldId, object, fieldId);
if (result) {
requestRebind();
}
}
看到requestRebind()方法，继续找下去发现最终还是调用了executeBindings()刷新View的值。
这样，View和BaseObservable数据的相互绑定就完成了。
3.BindingAdapter的原理
在DataBinding中，我们知道在xml中可以直接自定义绑定数据，那么它是怎么实现的呢？
其实很简单，我们还是看下executeBindings()方法：
@Override
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
com.example.databingtest2.Person person = mPerson;
int personColor = 0;
androidx.databinding.ObservableField<java.lang.String> personUserName = null;
java.lang.String personUserNameGet = null;
if ((dirtyFlags & 0x7L) != 0) {
if ((dirtyFlags & 0x6L) != 0) {
if (person != null) {
// read person.color
personColor = person.getColor();
}
}
if (person != null) {
// read person.userName
personUserName = person.getUserName();
}
updateRegistration(0, personUserName);
if (personUserName != null) {
// read person.userName.get()
personUserNameGet = personUserName.get();
}
}
// batch finished
if ((dirtyFlags & 0x6L) != 0) {
// api target 1
com.example.databingtest2.DataBindingAdapterExtKt.setTextViewColor(this.text1, personColor); // ---->①
}
if ((dirtyFlags & 0x7L) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setText(this.text1, personUserNameGet);// ----> ②
}
if ((dirtyFlags & 0x4L) != 0) {
// api target 1
androidx.databinding.adapters.TextViewBindingAdapter.setTextWatcher(this.text1, (androidx.databinding.adapters.TextViewBindingAdapter.BeforeTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.OnTextChanged)null, (androidx.databinding.adapters.TextViewBindingAdapter.AfterTextChanged)null, text1androidTextAttrChanged);
}
}
在代码中①②中，我们看到对View设置属性时是直接调用了相对应的方法。其实DataBinding有很多默认实现，我们看下：
image.png
它们里面是静态方法，可以直接调用。
上面介绍了DataBinding的原理和作用机制，如果大家有什么问题，欢迎提出讨论！
网友评论