如何使用 LiveData 和 ViewModel 类将数据从活动发送到片段

Question

现在，LiveData非常流行，如何使用ViewModel和LiveData将数据从活动发送到片段，反之亦然？请用编码示例进行解释。

Answer 1

in Activity

public class MyActivity extends AppCompatActivity {
MyViewModel myViewModel;
Handler handler;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.my_activity);
handler = new Handler();
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.replace(R.id.container, MyFragment.newInstance())
.commitNow();
}
//Make View Holder Object
myViewModel=ViewModelProviders.of(this).get(MyViewModel.class);
myViewModel.init();
myViewModel.sendData("Hello kamy");
// Make thread to send data again
new Thread(new Runnable() {
@Override
public void run() {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
handler.post(new Runnable() {
@Override
public void run() {
myViewModel.sendData("How are You ?");
}
});
}
}).start();
}
}

在视图模型类中

public class MyViewModel extends ViewModel {
// TODO: Implement the ViewModel
private MutableLiveData<String> stringMutableLiveData;
public void init()
{
stringMutableLiveData=new MutableLiveData<>();
}
public void sendData(String msg)
{
stringMutableLiveData.setValue(msg);
}
public LiveData<String> getMessage()
{
return stringMutableLiveData;

}
}

在片段中

public class MyFragment extends Fragment {
private MyViewModel mViewModel;
private TextView textView;
public static MyFragment newInstance() {
return new MyFragment();
}
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
@Nullable Bundle savedInstanceState) {
View view= inflater.inflate(R.layout.my_fragment, container, false);
textView=(TextView)view.findViewById(R.id.message);
return  view;
}
@Override
public void onActivityCreated(@Nullable Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
mViewModel = ViewModelProviders.of(this).get(MyViewModel.class);
ViewModelProviders.of(getActivity()).get(MyViewModel.class).getMessage().observe(this, new Observer<String>() {
@Override
public void onChanged(@Nullable String message) {
textView.setText(message);
Toast.makeText(getContext(),message,Toast.LENGTH_SHORT).show();
}
});
}
}

片段 XML

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/my"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".ui.my.MyFragment">
<TextView
android:id="@+id/message"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="MyFragment"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>