Android LiveData 在 fragment 中创建多个观察者

Question

在搜索了许多其他 SO 问题并阅读了文章之后，我仍然不明白为什么我的实现似乎不像其他示例那样执行。

我读过建议使用 getViewLifecycleOwner()在 fragment 内注册任何 LiveData 观察者以正确处理 fragment 的生命周期时。

但是，在我的 fragment 中 OnActivityCreated方法(当 fragment 显示在屏幕上时会被调用两次)我似乎有所不同 LifecycleOwner来自对 getViewLifecycleOwner() 的调用的对象我相信这是我将多个观察者注册到我的 LiveData 对象的原因。

我如何防止这种情况发生/我做错了什么？

我在代码中添加了大量日志记录，如下所示:

主 Activity

   public class MainActivity extends AppCompatActivity {

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);

            getSupportFragmentManager().beginTransaction()
                    .replace(R.id.fragment, new LiveDataTestFragment())
                    .commit();

            Log.d("debugger2", "Activity: " + this.hashCode());
        }
    }

LiveDataTestFragment

public class LiveDataTestFragment extends Fragment {

    private LiveDataViewModel viewModel;

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_live_data_test, container, false);
    }

    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);

        viewModel = ViewModelProviders.of(getActivity()).get(LiveDataViewModel.class);

        Log.d("debugger2", "ViewModel: " + viewModel.hashCode());

        LiveData<String> liveData = viewModel.getLiveData();
        Log.d("debugger2", "LiveData: " + liveData.hashCode());

        LifecycleOwner lifecycleOwner = getViewLifecycleOwner();
        Log.d("debugger2", "LifeCycleOwner: " + lifecycleOwner.hashCode());

        liveData.observe(lifecycleOwner, new Observer<String>() {
            @Override
            public void onChanged(String s) {
                Log.d("debugger2", "observer hash: " + this.hashCode());
            }
        });
    }
}

日志猫:

D: Activity: 242098714
D: ViewModel: 149122972
D: LiveData: 58736037
D: LifeCycleOwner: 50378106
D: ViewModel: 149122972
D: LiveData: 58736037
D: LifeCycleOwner: 245135826
D: observer hash: 204470220
D: observer hash: 226765595

Answer 1

new Observer<String>() {
            @Override
            public void onChanged(String s) {
                Log.d("debugger2", "observer hash: " + this.hashCode());
            }
        }

可能是你观察者的这部分负责创建多个 hashCode()您可以创建此 observer如果您需要使用以下代码，只需一次并多次调用它:

private Observer<String> singleObserver = Observer<String>() {
                @Override
                public void onChanged(String s) {
                    Log.d("debugger2", "observer hash: " + this.hashCode());
                }
}

并像这样使用它:

// your previous code
LiveData<String> liveData = viewModel.getLiveData();
liveData.observe(lifecycleOwner, singleObserver);
// rest of your code

希望它会起作用。