Android 使用新的体系结构组件ViewModel在片段之间共享数据

在上一次谷歌IO上，谷歌发布了一些新arch组件的预览，其中之一是ViewModel

在中，google展示了此组件的一种可能用途：

一个活动中的两个或多个片段需要修改是很常见的 相互交流。这从来都不是小事，因为这两个片段 需要定义一些接口描述，并且所有者活动必须 把两者绑在一起。此外，两个片段都必须处理这个案例 另一个片段尚未创建或不可见

使用ViewModel对象可以解决这个常见的难点。 想象一个主细节片段的常见情况，我们有一个 用户从一个列表和另一个列表中选择一个项目的片段 显示所选项目内容的片段

这些片段可以使用其活动范围共享一个ViewModel 处理这个通讯

并显示了一个实现示例：

public class SharedViewModel extends ViewModel {
    private final SavedStateHandle state;

    public SharedViewModel(SavedStateHandle state) {
        this.state = state;
    }

    private final MutableLiveData<Item> selected = state.getLiveData("selected");

    public void select(Item item) {
        selected.setValue(item);
    }

    public LiveData<Item> getSelected() {
        return selected;
    }
}

public class MasterFragment extends Fragment {
    private SharedViewModel model;

    @Override
    protected void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        model = new ViewModelProvider(getActivity()).get(SharedViewModel.class);
        itemSelector.setOnClickListener(item -> {
            model.select(item);
        });
    }
}

public class DetailFragment extends Fragment {
    @Override
    protected void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);
        SharedViewModel model = new ViewModelProvider(getActivity()).get(SharedViewModel.class);
        model.getSelected().observe(this, { item ->
           // update UI
        });
    }
}

公共类SharedViewModel扩展了ViewModel{
私有最终SavedStateHandle状态；
public SharedViewModel（SavedStateHandle状态）{
this.state=状态；
}
private final MutableLiveData selected=state.getLiveData（“selected”）；
公共作废选择（项目）{
已选择。设置值（项目）；
}
公共LiveData getSelected（）{
返回选中的；
}
}
公共类主片段扩展片段{
私有共享模型；
@凌驾
已创建受保护的空视图（视图、捆绑包savedInstanceState）{
super.onViewCreated（视图，savedInstanceState）；
model=newviewmodelprovider（getActivity（））.get（SharedViewModel.class）；
itemSelector.setOnClickListener（项->{
型号。选择（项目）；
});
}
}
公共类DetailFragment扩展了片段{
@凌驾
已创建受保护的空视图（视图、捆绑包savedInstanceState）{
super.onViewCreated（视图，savedInstanceState）；
SharedViewModel model=newviewmodelprovider（getActivity（））.get（SharedViewModel.class）；
model.getSelected（）.observe（此，{item->
//更新用户界面
});
}
}

对于不需要那些用于片段的接口就可以通过活动进行通信，我感到非常兴奋

但谷歌的例子并没有确切地显示我如何从master调用细节片段

---

我仍然必须使用将由活动实现的方法，该方法将调用fragmentManager.replace（…），或者有另一种方法使用新的体系结构来实现这一点？

在使用附加到被视为容器的活动的回调之前。
那个回调是两个片段之间的中间人。 以前的解决方案的缺点是：

• 活动必须进行回调，这意味着需要进行大量的工作 活动
• 两个片段紧密耦合，以后很难更新或更改逻辑

有了新的ViewModel（支持LiveData），您就有了一个优雅的解决方案。它现在扮演中间人的角色，您可以将其生命周期附加到活动中

• 两个片段之间的逻辑和数据现在显示在ViewModel中
• 两个片段从ViewModel获取数据/状态，因此它们不需要相互了解
• 此外，借助LiveData的强大功能，您可以根据主片段的更改以反应式方式更改细节片段，而不是以前的回调方式

您现在完全摆脱了与活动和相关片段紧密耦合的回调。

---

我强烈推荐你通过。在步骤5中，您可以找到一个很好的例子。

您可以像这样设置从细节片段到主片段的值

model.selected.setValue(item)

---

于2017年6月12日更新，

Android Official提供了一个简单、精确的示例，说明ViewModel如何在主详细信息模板上工作，您应该先看看它。

作为@CommonWare，@Quang-Nguyen method，Yigit的目的不是从大师到细节，而是更好地使用中间人模式。但是，如果您想进行一些片段事务，则应该在活动中完成。此时，ViewModel类应该是活动中的静态类，并且可能包含一些丑陋的回调来回调活动以生成片段事务

我已经试着实现了这一点，并为此做了一个简单的项目。你可以看看。大部分代码引用自Google IO 2017，也是结构。

我没有使用Master-Detail片段来实现该组件，但是使用旧的（ViewPager中片段之间的通信）逻辑应该是相同的

但我发现使用这些组件很重要

你想在中间人发送和接收什么，他们应该只在视图模型中发送和接收。

在fragment类中，修改似乎不太多。因为它只将实现从“接口回调”更改为“侦听和响应ViewModel”

视图模型初始化似乎很重要，可能会在活动中调用

使用MutableLiveData使源仅在活动中同步

1.寻呼机活动

public class PagerActivity extends AppCompatActivity {
    /**
     * The pager widget, which handles animation and allows swiping horizontally to access previous
     * and next wizard steps.
     */
    private ViewPager mPager;
    private PagerAgentViewModel pagerAgentViewModel;
    /**
     * The pager adapter, which provides the pages to the view pager widget.
     */
    private PagerAdapter mPagerAdapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_pager);
        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
        mPager = (ViewPager) findViewById(R.id.pager);
        mPagerAdapter = new ScreenSlidePagerAdapter(getSupportFragmentManager());
        mPager.setAdapter(mPagerAdapter);
        pagerAgentViewModel = new ViewModelProvider(this).get(PagerAgentViewModel.class);
        pagerAgentViewModel.init();
    }

    /**
     * A simple pager adapter that represents 5 ScreenSlidePageFragment objects, in
     * sequence.
     */
    private class ScreenSlidePagerAdapter extends FragmentStatePagerAdapter {
       ...Pager Implementation
    }

}

2.PagerAgentViewModel（它应该有一个更好的名字，而不是这个）

公共类PagerAgentViewModel扩展了ViewModel{
私有最终SavedStateHandle状态；
私有最终可变LiveData messageContainerA；
私有最终可变LiveData messageContainerB；
公共页面RagentViewModel（SavedStateHandle状态）{
this.state=状态；
messageContainerA=state.getLiveData（“默认消息”）；
messageContainerB=state.getLiveData（“默认消息”）；
}
public void sendMessageToB（字符串消息）
{
messageContainerB.setValue（msg）；
}
公共无效sendMessageToA
public class PagerAgentViewModel extends ViewModel {
    private final SavedStateHandle state;
    private final MutableLiveData<String> messageContainerA;
    private final MutableLiveData<String> messageContainerB;

    public PagerAgentViewModel(SavedStateHandle state) {
        this.state = state;

        messageContainerA = state.getLiveData("Default Message");
        messageContainerB = state.getLiveData("Default Message");
    }

    public void sendMessageToB(String msg)
    {
        messageContainerB.setValue(msg);
    }
    public void sendMessageToA(String msg)
    {
        messageContainerA.setValue(msg);

    }
    public LiveData<String> getMessageContainerA() {
        return messageContainerA;
    }

    public LiveData<String> getMessageContainerB() {
        return messageContainerB;
    }
}

public class BlankFragmentA extends Fragment {

    private PagerAgentViewModel viewModel;

    public BlankFragmentA() {
        // Required empty public constructor
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        viewModel = new ViewModelProvider(getActivity()).get(PagerAgentViewModel.class);


        textView = (TextView) view.findViewById(R.id.fragment_textA);
        // set the onclick listener
        Button button = (Button) view.findViewById(R.id.btnA);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                viewModel.sendMessageToB("Hello B");
            }
        });

        //setup the listener for the fragment A
        viewModel.getMessageContainerA().observe(getViewLifecycleOwner(), new Observer<String>() {
            @Override
            public void onChanged(@Nullable String msg) {
                textView.setText(msg);
            }
        });

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_blank_a, container, false);
        return view;
    }

}

public class BlankFragmentB extends Fragment {
 
    public BlankFragmentB() {
        // Required empty public constructor
    }

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        viewModel = new ViewModelProvider(getActivity()).get(PagerAgentViewModel.class);

        textView = (TextView) view.findViewById(R.id.fragment_textB);
        //set the on click listener
        Button button = (Button) view.findViewById(R.id.btnB);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                viewModel.sendMessageToA("Hello A");
            }
        });

        //setup the listener for the fragment B
        viewModel.getMessageContainerB().observe(getViewLifecycleOwner(), new Observer<String>() {
            @Override
            public void onChanged(@Nullable String msg) {
                textView.setText(msg);

            }
        });
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_blank_b, container, false);
        return view;
    }

}

public class SharedViewModel<T> extends ViewModel {

    private final MutableLiveData<T> selected = new MutableLiveData<>();
    private OnSelectListener<T> listener = item -> {};

    public interface OnSelectListener <T> {
        void selected (T item);
    }


    public void setListener(OnSelectListener<T> listener) {
        this.listener = listener;
    }

    public void select(T item) {
        selected.setValue(item);
        listener.selected(item);
    }

    public LiveData<T> getSelected() {
        return selected;
    }

}

SharedViewModel stepViewModel = ViewModelProviders.of(this).get("step", SharedViewModel.class);
stepViewModel.setListener(this);

@Override
public void selected(Step item) {
    Log.d(TAG, "selected: "+item);
}

stepViewModel = ViewModelProviders.of(getActivity()).get("step", SharedViewModel.class);

stepViewModel.select(step);

public class TestViewModel extends ViewModel {
    private MutableLiveData<Enums.state> mState;

    public TestViewModel() {
        mState=new MutableLiveData<>();
        mState.setValue(Enums.state.Master);
    }

    public void onDetail() {
        mState.setValue(Enums.state.Detail);
    }

    public void onMaster() {
        mState.setValue(Enums.state.Master);
    }

    public LiveData<Enums.state> getState() {

        return mState;
    }
}

public class Enums {
    public enum state {
        Master,
        Detail
    }
}

public class TestActivity extends LifecycleActivity {
    private ActivityTestBinding mBinding;
    private TestViewModel mViewModel;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mBinding=DataBindingUtil.setContentView(this, R.layout.activity_test);
        mViewModel=ViewModelProviders.of(this).get(TestViewModel.class);
        mViewModel.getState().observe(this, new Observer<Enums.state>() {
            @Override
            public void onChanged(@Nullable Enums.state state) {
                switch(state) {
                    case Master:
                        setMasterFragment();
                        break;
                    case Detail:
                        setDetailFragment();
                        break;
                }
            }
        });
    }

    private void setMasterFragment() {
        MasterFragment masterFragment=MasterFragment.newInstance();
        getSupportFragmentManager().beginTransaction().replace(R.id.frame_layout, masterFragment,"MasterTag").commit();
    }

    private void setDetailFragment() {
        DetailFragment detailFragment=DetailFragment.newInstance();
        getSupportFragmentManager().beginTransaction().replace(R.id.frame_layout, detailFragment,"DetailTag").commit();
    }

    @Override
    public void onBackPressed() {
        switch(mViewModel.getState().getValue()) {
            case Master:
                super.onBackPressed();
                break;
            case Detail:
                mViewModel.onMaster();
                break;
        }
    }
}

public class MasterFragment extends Fragment {
    private FragmentMasterBinding mBinding;


    public static MasterFragment newInstance() {
        MasterFragment fragment=new MasterFragment();
        return fragment;
    }

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

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        mBinding=DataBindingUtil.inflate(inflater,R.layout.fragment_master, container, false);
        mBinding.btnDetail.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final TestViewModel viewModel=ViewModelProviders.of(getActivity()).get(TestViewModel.class);
                viewModel.onDetail();
            }
        });

        return mBinding.getRoot();
    }
}

public class DetailFragment extends Fragment {
    private FragmentDetailBinding mBinding;

    public static DetailFragment newInstance() {
        DetailFragment fragment=new DetailFragment();
        return fragment;
    }

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

    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        mBinding=DataBindingUtil.inflate(inflater,R.layout.fragment_detail, container, false);
        mBinding.btnMaster.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                final TestViewModel viewModel=ViewModelProviders.of(getActivity()).get(TestViewModel.class);
                viewModel.onMaster();
            }
        });
        return mBinding.getRoot();
    }
}

import android.arch.lifecycle.MutableLiveData;
import android.arch.lifecycle.ViewModel;
import yourPackage.YourObjectModel;

public class SharedViewModel extends ViewModel {

   public MutableLiveData<YourObjectModel> item = new MutableLiveData<>();

   public YourObjectModel getItem() {
      return item.getValue();
   }

   public void setItem(YourObjectModel item) {
      this.item.setValue(item);
   }

}

public class ListenerFragment extends Fragment{
   private SharedViewModel model;
  @Override
  public void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);

    model.item.observe(getActivity(), new Observer<YourObjectModel>(){

        @Override
        public void onChanged(@Nullable YourObjectModel updatedObject) {
            Log.i(TAG, "onChanged: recieved freshObject");
            if (updatedObject != null) {
                // Do what you want with your updated object here. 
            }
        }
    });
}
}

public class UpdaterFragment extends DialogFragment{
    private SharedViewModel model;
    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
   }
   // Call this method where it is necessary
   private void updateViewModel(YourObjectModel yourItem){
      model.setItem(yourItem);
   }
}

dependencies {
  def lifecycle_version = "1.1.1"
  implementation "android.arch.lifecycle:extensions:$lifecycle_version"
}

// Use the 'by activityViewModels()' Kotlin property delegate
// from the fragment-ktx artifact
private val model: SharedViewModel by activityViewModels()

  class MainFragment : Fragment() {

      private lateinit var viewModel: ViewModel

      override fun onActivityCreated(savedInstanceState: Bundle?) {
          super.onActivityCreated(savedInstanceState)

          // kotlin does not have a getActivity() built in method instead we use activity, which is null-safe
          activity?.let {
              viemModel = ViewModelProvider(it).get(SharedViewModel::class.java)
          }
      }
  }