在AndroidJetpack应用指南学习笔记5中LiveData与ViewModel可以结合使用,ViewModel能够将数据从Activity中剥离出来。只要Activity不被销毁,ViewModel会一直存在,并且独立于Activity的配置变化,即旋转屏幕导致的Activity重建,不会影响到ViewModel。
Fragment可以看作是Activity的子页面,即,一个Activity中可以包含多个Fragment,这些Fragment彼此独立,但是又都属于同一个Activity。
基于这些组件的特性,我们可以巧妙地利用ViewModel和LiveData,实现同一个Activity中的不同Fragment间通信。
实现步骤如下:
1.定义ViewModel及LiveData。
/**
* @author: njb
* @date: 2020/9/2 0002 0:43
* @desc:
*/
public class MyDataViewModel extends ViewModel {
private MutableLiveData progress;
public LiveData getProgress() {
if (progress == null) {
progress = new MutableLiveData<>();
}
return progress;
}
@Override
protected void onCleared() {
super.onCleared();
progress = null;
}
}
2.fragment_one.xml布局代码:
2.1.fragment_two.xml代码:
3.OneFragment和TwoFragment实现Fragment间的通信
package com.example.livedataandfragment.fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.SeekBar;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;
import com.example.livedataandfragment.R;
import com.example.livedataandfragment.viewmodel.MyDataViewModel;
/**
* @author: njb
* @date: 2020/9/2 0002 0:45
* @desc:
*/
public class OneFragment extends Fragment {
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState)
{
View parentView = inflater.inflate(R.layout.fragment_one, container, false);
final SeekBar seekBar = parentView.findViewById(R.id.seekBar);
//注意:这里ViewModelProviders.of(getActivity())这里的参数需要是Activity,而不能是Fragment,否则收不到监听
final MyDataViewModel myDataViewModel = ViewModelProviders.of(getActivity()).get(MyDataViewModel.class);
final MutableLiveData liveData = (MutableLiveData) myDataViewModel.getProgress();
//通过observe方法观察ViewModel中字段数据的变化,并在变化时,得到通知
liveData.observe(getActivity(), new Observer()
{
@Override
public void onChanged(@Nullable Integer progress)
{
seekBar.setProgress(progress);
}
});
seekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener()
{
@Override
public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser)
{
//用户操作SeekBar时,更新ViewModel中的数据
liveData.setValue(progress);
}
@Override
public void onStartTrackingTouch(SeekBar seekBar)
{
}
@Override
public void onStopTrackingTouch(SeekBar seekBar)
{
}
});
return parentView;
}
}
package com.example.livedataandfragment.fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.SeekBar;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;
import com.example.livedataandfragment.R;
import com.example.livedataandfragment.viewmodel.MyDataViewModel;
/**
* @author: njb
* @date: 2020/9/4 0004 1:08
* @desc:
*/
public class TwoFragment extends Fragment {
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View parentView = inflater.inflate(R.layout.fragment_two, container, false);
final SeekBar seekBar = parentView.findViewById(R.id.seekBar);
//注意:这里ViewModelProviders.of(getActivity())这里的参数需要是Activity,而不能是Fragment,否则收不到监听
final MyDataViewModel myDataViewModel = ViewModelProviders.of(getActivity()).get(MyDataViewModel.class);
final MutableLiveData liveData = (MutableLiveData) myDataViewModel.getProgress();
//通过observe方法观察ViewModel中字段数据的变化,并在变化时,得到通知
liveData.observe(getActivity(), new Observer() {
@Override
public void onChanged(@Nullable Integer progress) {
seekBar.setProgress(progress);
}
});
seekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
@Override
public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
//用户操作SeekBar时,更新ViewModel中的数据
liveData.setValue(progress);
}
@Override
public void onStartTrackingTouch(SeekBar seekBar) {
}
@Override
public void onStopTrackingTouch(SeekBar seekBar) {
}
});
return parentView;
}
}
4.activity_main.xml代码:
5.实现的效果图如下:
6.小结:可以看到,无论是滑动OneFragment还是TwoFragment中的SeekBar,另外一个Fragment中的SeekBar也会跟着滑动。滑动SeekBar时,通过LiveData.setValue(),修改了ViewModel中LiveData包装的数据(progress字段)。由于Fragment通过LiveData.observe()方法,监听了数据的变化,所以progress字段被修改后,Fragment能够第一事件收到通知,进而更新UI。这就是利用ViewMode和LiveData实现Fragment间通信的原理。另外,从演示图中,我们还能看到,屏幕旋转后SeekBar的进度与旋转前保持一直,数据并未丢失,这也是ViewModel带来的好处之一。