Android Data Binding 系列 (二) -- Binding 与 Observer 实现原理

写在前面

上篇文章 Android Data Binding 系列(一) -- 详细介绍与使用 介绍了 Data Binding 的基础及其用法，本文接上篇，结合DataBindingDemo 来学习下 Data Binding 的实现。

绑定实现

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 Viewsprivate 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。

@Overrideprotected 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

@Overrideprotected 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

@Overrideprotected 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

@Overridepublic 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);    }}// baseLibraryprivate 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_CALLBACKpublic 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);}// WeakPropertyListenerpublic 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

@Overrideprotected 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

@Overrideprotected 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。