Android的DataBinding原理介绍
Activity在inflate layout时，通过DataBindingUtil来生成绑定，从代码看，是遍历contentView得到View数组对象，然后通过数据绑定library生成对应的Binding类，含Views、变量、listeners等。生成类位于
build/intermediates/classes/debug/…package…/databinding/xxx.Java 下，具体如何生成这里暂不作深入。
绑定过程
- 首先，会在父类（ViewDataBinding）中实例化回调或Handler，用于之后的绑定操作；
private static final boolean USE_CHOREOGRAPHER = SDK_INT >= 16;
if (USE_CHOREOGRAPHER) {
mChoreographer = Choreographer.getInstance();
mFrameCallback = new Choreographer.FrameCallback() {
@Override
public void doFrame(long frameTimeNanos) {
mRebindRunnable.run();
}
};
} else {
mFrameCallback = null;
mUIThreadHandler = new Handler(Looper.myLooper());
}
- 接着，通过调用 mapBindings(…) 遍历布局以获得包含bound、includes、ID Views的数组对象，再依次赋给对应View
final Object[] bindings = mapBindings(bindingComponent, root, 3, sIncludes, sViewsWithIds);
this.mboundView0 = (Android.widget.LinearLayout) bindings[0];
this.mboundView0.setTag(null);
- 然后，调用 invalidateAll() -> requestRebind() -> … -> mRebindRunnable.run() – 执行 Runnable
// 用于动态重新绑定 Views
private final Runnable mRebindRunnable = new Runnable() {
@Override
public void run() {
synchronized (this) {
mPendingRebind = false;
}
.....
executePendingBindings();
}
};
- 最后，通过该Runnable会执行到 executePendingBindings() -> … -> executeBindings()，在这里会执行绑定相关操作。
@Override
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags; // mDirtyFlags 变量更新的标志
mDirtyFlags = 0;
}
.....
}
设置变量(数据对象)
普通 Java bean 对象
- 首先，通过mDirtyFlags标识变量(所有变量共用)
synchronized(this) {
mDirtyFlags |= 0x1L;
}
- 然后，调用 notifyPropertyChanged(…) 来通知更新（若有回调）
public void notifyPropertyChanged(int fieldId) {
if (mCallbacks != null) {
mCallbacks.notifyCallbacks(this, fieldId, null);
}
}
- 最后，调用 requestRebind() -> … -> executeBindings() 再次执行绑定操作，将数据更新到Views上
@Override
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
.....
}
Observable 对象
- 在设置变量时，会先调用 updateRegistration(..) 注册一个Observable对象的监听
public void setContact(com.connorlin.databinding.model.ObservableContact contact) {
updateRegistration(0, contact);
this.mContact = contact;
synchronized(this) {
mDirtyFlags |= 0x1L;
}
notifyPropertyChanged(BR.contact);
super.requestRebind();
}
- 其他步骤同普通 Java bean 对象
ObservableFields 对象
- 前期步骤同普通 Java Bean 对象
- 与 Observable 对象不同的是，Observable对象的监听是在 executeBindings() 中注册的
@Override
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
...
if ((dirtyFlags & 0xfL) != 0) {
if ((dirtyFlags & 0xdL) != 0) {
if (contact != null) {
// read contact.mName
mNameContact = contact.mName;
}
updateRegistration(0, mNameContact);
if (mNameContact != null) {
// read contact.mName.get()
mNameContact1 = mNameContact.get();
}
}
...
}
...
}
注册Observable对象监听
- 入口 updateRegistration(0, contact) ：
protected boolean updateRegistration(int localFieldId, Observable observable) {
return updateRegistration(localFieldId, observable, CREATE_PROPERTY_LISTENER);
}
private boolean updateRegistration(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
...
// 确保不重复监听，先移除再添加观察监听
unregisterFrom(localFieldId);
registerTo(localFieldId, observable, listenerCreator);
return true;
}
protected void registerTo(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
if (observable == null) {
return;
}
// 创建对象监听并存到mLocalFieldObservers中
WeakListener listener = mLocalFieldObservers[localFieldId];
if (listener == null) {
// CREATE_PROPERTY_LISTENER -> create(...)
listener = listenerCreator.create(this, localFieldId);
mLocalFieldObservers[localFieldId] = listener;
}
// 将监听绑定到Observable对象上
listener.setTarget(observable);
}
每个Observable对象都会添加一个观察监听，保存在数组 mLocalFieldObservers 中，并以 localFieldId 索引。
- CREATE_PROPERTY_LISTENER 为何物？
private static final CreateWeakListener CREATE_PROPERTY_LISTENER = new CreateWeakListener() {
@Override
public WeakListener create(ViewDataBinding viewDataBinding, int localFieldId) {
// 返回从WeakPropertyListener实例中获取的监听器(WeakListener)
return new WeakPropertyListener(viewDataBinding, localFieldId).getListener();
}
}
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) {
// WeakPropertyListener 继承于 Observable.OnPropertyChangedCallback，
// 所以 this 其实就是 Observable对象的属性监听器
target.addOnPropertyChangedCallback(this);
}
...
}
private static class WeakListener<T> extends WeakReference<ViewDataBinding> {
private final ObservableReference<T> mObservable;
protected final int mLocalFieldId;
private T mTarget;
...
public void setTarget(T object) {
unregister();
mTarget = object;
if (mTarget != null) {
// mObservable 是上面的 WeakPropertyListener对象
// mTarget 是绑定到listener上得Observable对象
mObservable.addListener(mTarget);
}
}
...
}
CREATE_PROPERTY_LISTENER 实际上只是一个接口实例，注册时会调用它的create()方法创建一个弱引用listener，它的作用是将listener绑定到Observable对象上，
绑定时，会调用 listener.setTarget(…) 将Observable对象传给 WeakPropertyListener实例，然后，WeakPropertyListener 会为 Observable对象添加OnPropertyChangedCallback。
- addOnPropertyChangedCallback实现
addOnPropertyChangedCallback 在 BaseObservable中实现，首先会实例化一个PropertyChangeRegistry对象，同时创建一个用来通知Observable对象重新绑定更新的回调CallbackRegistry.NotifierCallback。然后将 OnPropertyChangedCallback 添加到PropertyChangeRegistry的回调列表中
@Override
public synchronized void addOnPropertyChangedCallback(OnPropertyChangedCallback callback) {
if (mCallbacks == null) {
mCallbacks = new PropertyChangeRegistry();
}
mCallbacks.add(callback);
}
这样，注册Observable对象的监听就完毕了。
更新(重新绑定)Observable对象
设置或更新Observable对象时都会调用notifyPropertyChanged()或notifyChange()来通知更新，那到底是如何更新的呢？
- 回调过程
public void notifyPropertyChanged(int fieldId) {
// mCallbacks 是 PropertyChangeRegistry对象，在 addOnPropertyChangedCallback 时实例化
// 如果注册了Observable对象监听，那么mCallbacks不为null
if (mCallbacks != null) {
mCallbacks.notifyCallbacks(this, fieldId, null);
}
}
// baseLibrary
private void notifyCallbacks(T sender, int arg, A arg2, int startIndex, int endIndex, long bits) {
long bitMask = 1L;
for(int i = startIndex; i < endIndex; ++i) {
if((bits & bitMask) == 0L) {
// mNotifier 是实例化PropertyChangeRegistry时创建的
// mNotifier 即 CallbackRegistry.NotifierCallback
this.mNotifier.onNotifyCallback(this.mCallbacks.get(i), sender, arg, arg2);
}
bitMask <<= 1;
}
}
// PropertyChangeRegistry.NOTIFIER_CALLBACK
public void onNotifyCallback(Observable.OnPropertyChangedCallback callback, Observable sender,
int arg, Void notUsed) {
// callback 是为Observable对象添加的OnPropertyChangedCallback，即WeakPropertyListener
callback.onPropertyChanged(sender, arg);
}
// WeakPropertyListener
public void onPropertyChanged(Observable sender, int propertyId) {
// binder 即生成的Binding类对象
ViewDataBinding binder = mListener.getBinder();
...
binder.handleFieldChange(mListener.mLocalFieldId, sender, propertyId);
}
private void handleFieldChange(int mLocalFieldId, Object object, int fieldId) {
// onFieldChange 实现在生成的Binding类中
boolean result = onFieldChange(mLocalFieldId, object, fieldId);
if (result) {
// 如果对象属性变化，将重新绑定
requestRebind();
}
}
通过 notifyPropertyChanged 调用到 mNotifier 回调， mNotifier 通知OnPropertyChangedCallback Observable对象属性发生变化，然后在onPropertyChanged中又转给ViewDataBinding对象(生成的Binding类)处理。
- 判断是否需要重新绑定并执行，在生成的Binding类中实现
// 生成的Binding类中得方法
protected boolean onFieldChange(int localFieldId, Object object, int fieldId) {
// 如果变量不是Observable类型或没有添加 Bindable注解，就不会判断，直接返回false
switch (localFieldId) {
case 0 :
return onChangeContact((com.connorlin.databinding.model.ObservableContact) object, fieldId);
}
return false;
}
private boolean onChangeContact(com.connorlin.databinding.model.ObservableContact contact, int fieldId) {
switch (fieldId) {
case BR.name: {
synchronized(this) {
mDirtyFlags |= 0x4L;// 通过mDirtyFlags判断对象是否变化
}
return true;
}
...
}
return false;
}
至此，更新过程完毕。
整个注册与更新过程可以用一张流程图来概括：
事件处理
事件处理的原理很简单，在生成Binding类中会实现View事件的监听，在构造时实例化View的事件监听，然后在绑定时将事件监听对象赋值给对应View，这样，点击时就会触发相应的监听。
这里以 DataBindingDemo 中 EventActivity部分为例：
- 生成的Binding类并实现View的事件监听
public class ActivityEventBinding extends Android.databinding.ViewDataBinding
implements Android.databinding.generated.callback.OnCheckedChangeListener.Listener,
Android.databinding.generated.callback.OnClickListener.Listener {
// Checkbox check监听
private final Android.widget.CompoundButton.OnCheckedChangeListener mCallback3;
private final Android.view.View.OnClickListener mCallback2;
private final Android.view.View.OnClickListener mCallback1;
// listeners
private OnClickListenerImpl mAndroidViewViewOnCl;
...
// Listener Stub Implementations
public static class OnClickListenerImpl implements Android.view.View.OnClickListener{
private com.connorlin.databinding.handler.EventHandler value;
public OnClickListenerImpl setValue(com.connorlin.databinding.handler.EventHandler value) {
this.value = value;
return value == null ? null : this;
}
@Override
public void onClick(Android.view.View arg0) {
this.value.onClickFriend(arg0);
}
}
...
}
- 实例化View的事件监听
public ActivityEventBinding(Android.databinding.DataBindingComponent bindingComponent, View root) {
super(bindingComponent, root, 0);
...
// listeners
mCallback3 = new Android.databinding.generated.callback.OnCheckedChangeListener(this, 3);
mCallback2 = new Android.databinding.generated.callback.OnClickListener(this, 2);
mCallback1 = new Android.databinding.generated.callback.OnClickListener(this, 1);
invalidateAll();
}
- 在执行绑定中绑定View事件监听
@Override
protected void executeBindings() {
...
if ((dirtyFlags & 0x6L) != 0) {
if (handler != null) {
// read handler::onClickFriend
AndroidViewViewOnCli = (((mAndroidViewViewOnCl == null)
? (mAndroidViewViewOnCl = new OnClickListenerImpl()) : mAndroidViewViewOnCl).setValue(handler));
}
}
// batch finished
if ((dirtyFlags & 0x6L) != 0) {
this.mboundView1.setOnClickListener(AndroidViewViewOnCli);
}
if ((dirtyFlags & 0x4L) != 0) {
this.mboundView2.setOnClickListener(mCallback1);
this.mboundView3.setOnClickListener(mCallback2);
Android.databinding.adapters.CompoundButtonBindingAdapter.setListeners(
this.mboundView4, mCallback3, (Android.databinding.InverseBindingListener)null);
}
}
- 触发事件并执行
ViewStub
原理类似，只是利用 ViewStubProxy 来延迟绑定。
- 使用layout中的ViewStub实例化一个ViewStubProxy对象赋给viewstub变量，并与Bingding关联
public ActivityViewStubBinding(Android.databinding.DataBindingComponent bindingComponent, View root) {
super(bindingComponent, root, 0);
final Object[] bindings = mapBindings(bindingComponent, root, 2, sIncludes, sViewsWithIds);
...
this.viewStub = new Android.databinding.ViewStubProxy((Android.view.ViewStub) bindings[1]);
this.viewStub.setContainingBinding(this);
...
}
- 实例化ViewStubProxy的同时会注册inflate监听
private OnInflateListener mProxyListener = new OnInflateListener() {
@Override
public void onInflate(ViewStub stub, View inflated) {
mRoot = inflated;
mViewDataBinding = DataBindingUtil.bind(mContainingBinding.mBindingComponent,
inflated, stub.getLayoutResource());
mViewStub = null;
if (mOnInflateListener != null) {
mOnInflateListener.onInflate(stub, inflated);
mOnInflateListener = null;
}
mContainingBinding.invalidateAll();
mContainingBinding.forceExecuteBindings();
}
};
public ViewStubProxy(ViewStub viewStub) {
mViewStub = viewStub;
mViewStub.setOnInflateListener(mProxyListener);
}
- inflate ViewStub
if (!mActivityViewStubBinding.viewStub.isInflated()) {
mActivityViewStubBinding.viewStub.getViewStub().inflate();
}
当ViewStub infate时，执行mProxyListener，其中会生成ViewStub的Binding，并强制执行主Binding重绑
- 绑定ViewStub
@Override
protected void executeBindings() {
long dirtyFlags = 0;
synchronized(this) {
dirtyFlags = mDirtyFlags;
mDirtyFlags = 0;
}
// batch finished
if (viewStub.getBinding() != null) {
viewStub.getBinding().executePendingBindings();
}
}
这样，ViewStub绑定就结束了。
