使用新的架构组件 ViewModel 在片段之间共享数据 [英] Sharing data between fragments using new architecture component ViewModel
问题描述
在 Last Google IO 上，Google 发布了一些新架构组件的预览，其中之一是 ViewModel.
On Last Google IO, Google released a preview of some new arch components, one of which, ViewModel.
在 docs 中，谷歌显示了一种可能的用途对于这个组件:
In the docs google shows one of the possible uses for this component:
一个活动中的两个或多个片段需要互相交流.这从来都不是微不足道的，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将两者绑定在一起.而且，两个片段都必须处理这种情况另一个片段尚未创建或不可见.
It is very common that two or more fragments in an activity need to communicate with each other. This is never trivial as both fragments need to define some interface description, and the owner activity must bind the two together. Moreover, both fragments must handle the case where the other fragment is not yet created or not visible.
这个常见的痛点可以通过使用 ViewModel 对象来解决.想象一下主从片段的常见情况，其中我们有一个用户从列表中选择一个项目和另一个项目的片段显示所选项目内容的片段.
This common pain point can be addressed by using ViewModel objects. Imagine a common case of master-detail fragments, where we have a fragment in which the user selects an item from a list and another fragment that displays the contents of the selected item.
这些片段可以使用它们的活动范围共享一个 ViewModel处理此通信.
These fragments can share a ViewModel using their activity scope to handle this communication.
并展示了一个实现示例:
And shows a implementation example:
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
});
}
}
对于不需要那些用于片段的接口通过活动进行通信的可能性，我感到非常兴奋.
I was quite excited about the possibility of not needing those interfaces used for fragments to communicate through the activity.
但 Google 的示例并未确切显示我将如何从 master 调用 detail 片段.
But Google's example does not show exactly how would I call the detail fragment from master.
我仍然需要使用一个界面由活动实现，它将调用 fragmentManager.replace(...)，或者有另一种使用新架构的方法吗?
I'd still have to use an interface that will be implemented by the activity, which will call fragmentManager.replace(...), or there is another way to do that using the new architecture?
推荐答案
于 6/12/2017 更新，
Android 官方提供了一个简单、精确的例子来举例说明 ViewModel 如何在 Master-Detail 模板上工作，你应该先看看它.在片段之间共享数据
正如@CommonWare、@Quang Nguyen 所说，Yigit 的目的不是从主到细节的调用，而是更好地使用中间人模式.但是如果要进行一些分片交易，则应该在活动中完成.此时ViewModel类应该是Activity中的静态类，可能包含一些Ugly Callback来回调Activity进行fragment事务.
As @CommonWare, @Quang Nguyen methioned, it is not the purpose for Yigit to make the call from master to detail but be better to use the Middle man pattern. But if you want to make some fragment transaction, it should be done in the activity. At that moment, the ViewModel class should be as static class in Activity and may contain some Ugly Callback to call back the activity to make the fragment transaction.
我已经尝试实现这一点，并为此做了一个简单的项目.你可以看看它.大部分代码引用自Google IO 2017，还有结构.https://github.com/charlesng/SampleAppArch
I have tried to implement this and make a simple project about this. You can take a look it. Most of the code is referenced from Google IO 2017, also the structure. https://github.com/charlesng/SampleAppArch
我没有使用Master Detail Fragment来实现组件，而是旧的(ViewPager中Fragment之间的通信.)逻辑应该是一样的.
I do not use Master Detail Fragment to implement the component, but the old one ( communication between fragment in ViewPager.) The logic should be the same.
但我发现使用这些组件很重要
But I found something is important using these components
- 你想在中间人发送和接收的东西，它们应该只在视图模型中发送和接收
- fragment 类中的修改似乎并不多.因为它只改变了接口回调"的实现到聆听和响应 ViewModel"
- View Model initialize 似乎很重要，并且可能会在 Activity 中调用.
- 使用 MutableLiveData 使源仅在活动中同步.
1.Pager Activity
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)
2.PagerAgentViewModel (It deserved a better name rather than this)
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;
}
}
3.BlankFragmentA
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;
}
}
4.BlankFragmentB
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;
}
}
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