Android ViewModel观察者永远不会被调用

Question

我有以下代码：

public class ViewModelDrivers extends ViewModel {
    private MutableLiveData<List<DriverDetailsModel>> drivers;

    public LiveData<List<DriverDetailsModel>> getDrivers() {
        if (drivers == null) {
            drivers = new MutableLiveData<>();
            loadDrivers();
        }
        return drivers;
    }

    private void loadDrivers() {
        CloudFactory.getInstance().getAllServerDrivers(PreferenceUtils.getInstance().getDeviceId(), new IServerAllTrucksCallback() {
            @Override
            public void onAllTrucks(List<DriverDetailsModel> d) {
                drivers.setValue(d);
            }
        });
    }
}

和MainActivity的onCreate方法：

    ViewModelDrivers model = new ViewModelProvider(this, new ViewModelProvider.NewInstanceFactory()).get(ViewModelDrivers.class);

        model.getDrivers().observe(this, items -> {
//here never gets called. Why?
});

由于不推荐使用，所以我没有使用ViewModelProviders。我从未在observe（）方法中收到结果。而且drivers.setValue(d);被设置

Answer 1

也许您的drivers.setValue(d);在返回drivers;之前就已呼叫因此，从loadDrivers();方法中删除getDrivers()，如下所示：

 public LiveData<List<DriverDetailsModel>> getDrivers() {
        if (drivers == null) {
            drivers = new MutableLiveData<>();
        }
        return drivers;
    }

并在设置观察者之后从loadDrivers()调用，如下所示：

model.getDrivers().observe(this, items -> {
//here never gets called. Why?
});
loadDrivers(); //here call loadDrivers();