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 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) {
        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) {
        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> {
        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) {
        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 {
    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。

本篇完，敬请期待下篇…

---

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