Android的DataBinding原理

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

](www.zhihu.com/search?q=%E… 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

](www.zhihu.com/search?q=%E…

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();
        }
    }
    ...
}
...
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```
}

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);
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```
}

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 { final WeakListener 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);
}

...
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}

private static class WeakListener extends WeakReference { private final ObservableReference 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);
    }
}

...
复制代码

}

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);
}
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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

](www.zhihu.com/search?q=vi…

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();
}
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```
};

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。