介绍
本篇介绍当model更新数据的时候，View如何自动更新的。
注意
源码对这一块的处理非常的复杂，梳理的时候非常烧脑。
我们接着ActivityMainBindingImpl.executeBindings()方法,这个方法中没有一个view都会调用updateRegistration更新注册表。
我们就从updateRegistration着手
注册回调
protected boolean updateRegistration(int localFieldId, Observable observable) {
return updateRegistration(localFieldId, observable, CREATE_PROPERTY_LISTENER);
}
这个方法传进来的是一个Observable ，而我们的ObservableField是Observable的一个子类，这里一定要记住这一点，因为后边都是根据这个类处理逻辑的。
CREATE_PROPERTY_LISTENER这个对象里边很复杂，这个就像一个门面模式中的门面，所有的逻辑都在内部实现，这就是一个对外的门面，供外部使用。
private static final CreateWeakListener CREATE_PROPERTY_LISTENER = new CreateWeakListener() {
@Override
public WeakListener create(ViewDataBinding viewDataBinding, int localFieldId) {
return new WeakPropertyListener(viewDataBinding, localFieldId).getListener();
}
};
如果你正在看源码，看到这里请你向下看看，其他几个静态对象 CREATE_LIST_LISTENER,CREATE_MAP_LISTENER,等等这几个对应着其他的可观察对象ObservableList等等。
我们看看WeakPropertyListener的内容
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);//1
}
@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);
}
}
注意这个类继承Observable.OnPropertyChangedCallback并且实现了ObservableReference。
我们看看注释1的实现
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) {//setTarget
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;
}
}
通过new出来的对象，传递的是WeakPropertyListener实例。
现在我们回到updateRegistration方法中
private boolean updateRegistration(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
...
WeakListener listener = mLocalFieldObservers[localFieldId];
if (listener == null) {
registerTo(localFieldId, observable, listenerCreator);
return true;
}
....
}
这时候的mLocalFieldObservers数组是没有内容的。我们看一下registerTo
protected void registerTo(int localFieldId, Object observable,
CreateWeakListener listenerCreator) {
....
WeakListener listener = mLocalFieldObservers[localFieldId];
if (listener == null) {
listener = listenerCreator.create(this, localFieldId);
mLocalFieldObservers[localFieldId] = listener;
if (mLifecycleOwner != null) {
listener.setLifecycleOwner(mLifecycleOwner);
}
}
listener.setTarget(observable);
}
这里通过刚才的CREATE_PROPERTY_LISTENER的create创建一个WeakListener实例，紧接着调用了setTarget将我们传递的BaseObservableField实例传递进去了。
我们回到WeakListener.setTarget的方法中
public void setTarget(T object) {
unregister();
mTarget = object;
if (mTarget != null) {
mObservable.addListener(mTarget);
}
}
还记得前边说的mObservable是new WeakListener传递的WeakPropertyListener实例吗？
@Override
public void addListener(Observable target) {
target.addOnPropertyChangedCallback(this);
}
这里的target是我们的BaseObservableField实例name或者nickName
我们去看看BaseObservableField的addOnPropertyChangedCallback，实际是父类BaseObservable的实现
@Override
public void addOnPropertyChangedCallback(@NonNull OnPropertyChangedCallback callback) {
synchronized (this) {
if (mCallbacks == null) {
mCallbacks = new PropertyChangeRegistry();
}
}
mCallbacks.add(callback);
}
mCallbacks 是null所以new 一个PropertyChangeRegistry 对象
public class PropertyChangeRegistry extends
CallbackRegistry<Observable.OnPropertyChangedCallback, Observable, Void> {
private static final CallbackRegistry.NotifierCallback<Observable.OnPropertyChangedCallback, Observable, Void> NOTIFIER_CALLBACK = new CallbackRegistry.NotifierCallback<Observable.OnPropertyChangedCallback, Observable, Void>() {
@Override
public void onNotifyCallback(Observable.OnPropertyChangedCallback callback, Observable sender,
int arg, Void notUsed) {
callback.onPropertyChanged(sender, arg);
}
};
public PropertyChangeRegistry() {
super(NOTIFIER_CALLBACK);
}
}
PropertyChangeRegistry 内部一个常量NOTIFIER_CALLBACK实现了NotifierCallback接口。空构造方法中调用了父类CallbackRegistry的构造。
public CallbackRegistry(NotifierCallback<C, T, A> notifier) {
mNotifier = notifier;
}
分析到这里，我们发现了监听事件的notifier注册到了CallbackRegistry（回调注册器）中了。
通知更新
我们从userInfo.setNickName("延迟三秒");入手
调用到ObservableField的set方法
public void set(T value) {
if (value != mValue) {
mValue = value;
notifyChange();
}
}
BaseObservable.notifyChange方法
public void notifyChange() {
synchronized (this) {
if (mCallbacks == null) {
return;
}
}
mCallbacks.notifyCallbacks(this, 0, null);
}
前边已经把回调注册进来了，所以mCallbacks不在是null了。
public synchronized void notifyCallbacks(T sender, int arg, A arg2) {
mNotificationLevel++;
notifyRecurse(sender, arg, arg2);
mNotificationLevel--;
...
}
notifyChange是通知页面内所有的View都更新数据，所以这里是递归通知回调。
private void notifyRecurse(T sender, int arg, A arg2) {
final int callbackCount = mCallbacks.size();
final int remainderIndex = mRemainderRemoved == null ? -1 : mRemainderRemoved.length - 1;
.
notifyRemainder(sender, arg, arg2, remainderIndex);
final int startCallbackIndex = (remainderIndex + 2) * Long.SIZE;
notifyCallbacks(sender, arg, arg2, startCallbackIndex, callbackCount, 0);
}
private void notifyCallbacks(T sender, int arg, A arg2, final int startIndex,
final int endIndex, final long bits) {
long bitMask = 1;
for (int i = startIndex; i < endIndex; i++) {
if ((bits & bitMask) == 0) {
mNotifier.onNotifyCallback(mCallbacks.get(i), sender, arg, arg2);
}
bitMask <<= 1;
}
}
我们回到PropertyChangeRegistry#NOTIFIER_CALLBACK中
@Override
public void onNotifyCallback(Observable.OnPropertyChangedCallback callback, Observable sender,
int arg, Void notUsed) {
callback.onPropertyChanged(sender, arg);
}
callback调用onPropertyChanged方法，这个callback是WeakPropertyListener实例。
@Override
public void onPropertyChanged(Observable sender, int propertyId) {
ViewDataBinding binder = mListener.getBinder();
...
binder.handleFieldChange(mListener.mLocalFieldId, sender, propertyId);
}
private void handleFieldChange(int mLocalFieldId, Object object, int fieldId) {
...
boolean result = onFieldChange(mLocalFieldId, object, fieldId);
if (result) {
requestRebind();
}
}
onFieldChange的实现在ActivityMainBindingImpl中
@Override
protected boolean onFieldChange(int localFieldId, Object object, int fieldId) {
switch (localFieldId) {
case 0 :
return onChangeUserInfoNickName((androidx.databinding.ObservableField<java.lang.String>) object, fieldId);
case 1 :
return onChangeUserInfoName((androidx.databinding.ObservableField<java.lang.String>) object, fieldId);
}
return false;
}
我前边调用的是setNickName 函数，所以localFieldId是0.
private boolean onChangeUserInfoNickName(androidx.databinding.ObservableField<java.lang.String> UserInfoNickName, int fieldId) {
if (fieldId == BR._all) {
synchronized(this) {
mDirtyFlags |= 0x1L;
}
return true;
}
return false;
}
notifyChange()传递的是0 即fieldId ==BR._all 是true。
回到handleFieldChange函数中，继续执行requestRebind，后边的流程我们在上一篇文章分析过了。
总结
model驱动view更新的过程，注册观察者回调是一个很烧脑的过程，需要慢慢分析。
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