语境：

我有以下视图模型：

public class DrawerPageHeaderViewModelImpl extends BaseObservable implements DrawerPageHeaderViewModel {

@Nullable

private Location currentLocation;

public DrawerPageHeaderViewModelImpl(@Nullable final Location currentLocation) {

this.currentLocation = currentLocation;

}

@Bindable

@Nullable

@Override

public String getDistanceDisplayString() {

if (currentLocation == null) {

return null;

}

float[] results = new float[1];

Location.distanceBetween(landmark.getLatitude(), landmark.getLongitude(), currentLocation.getLatitude(), currentLocation.getLongitude(), results);

final float metersToTargetLocation = results[0];

final float feetToTargetLocation = DistanceUtil.convertMetersToFeet(metersToTargetLocation);

return DistanceUtil.convertFeetToFeetOrMilesString(feetToTargetLocation);

}

@Override

public void setCurrentLocation(@Nullable final Location currentLocation) {

this.currentLocation = currentLocation;

notifyPropertyChanged(BR.distanceDisplayString);

}

}

此视图模型将传递给Fragment并存储在实例变量中.然后视图模型绑定到Fragment的onCreateView回调中的布局(此处headerView是一个空的FrameLayout)：

@Nullable

@Override

public View onCreateView(final LayoutInflater inflater, final ViewGroup container, final Bundle savedInstanceState) {

final View v = inflater.inflate(R.layout.fragment_drawer_page, container, false);

headerView = (ViewGroup) v.findViewById(R.id.headerView);

final ViewDrawerPageHeaderBinding binding = DataBindingUtil.inflate(inflater, R.layout.view_drawer_page_header, headerView, true);

binding.setViewModel(viewModel);

return v;

}

定期调用viewModel.setCurrentLocation并传递用户的当前位置：

@Override

public void update(final Observable observable, Object data) {

new Handler(Looper.getMainLooper()).post(() -> {

if (isAdded()) {

viewModel.setCurrentLocation(locationController.getCachedUserLocation());

}

});

}

目前的行为：

在首次创建每个片段时,UI会正确显示距离String.每次重新创建片段时,UI都会正确显示距离String(这些片段位于ViewPager中.

使用新位置调用viewModel.setCurrentLocation时,UI不会更新.

期望的行为：

每次使用新位置调用viewModel.setCurrentLocation时,UI都会更新.

到目前为止我看过/想过的东西：

据我所知,让视图模型实现Observable(在这种情况下,通过扩展BaseObservable)应该在调用notifyPropertyChanged时自动进行UI更新.至少,当我看到the Android documentation for data binding时,这就是我带走的信息.

BaseObservable类维护一个OnPropertyChangedCallbacks的私有列表.如果我在BaseObservable.notifyPropertyChanged方法上设置调试断点：

public void notifyPropertyChanged(int fieldId) {

if(this.mCallbacks != null) {

this.mCallbacks.notifyCallbacks(this, fieldId, (Object)null);

}

}

我看到mCallbacks在运行时始终为null.因此,假设生成的数据绑定内容不会调用BaseObservable.addOnPropertyChangedCallback来提供自动连接组件的OnPropertyChangedCallback.这是否意味着我需要手动完成？这似乎打败了数据绑定库的许多重点.