先写一个demo:通过ViewMOdel实现fragment之间的通信
布局:
【activity_main】
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity">
<fragment
android:id="@+id/master_fragment"
android:name="com.example.myapplication.MasterFragment"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1" />
<fragment
android:id="@+id/detail_fragment"
android:name="com.example.myapplication.DetailFragment"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1" />
</LinearLayout>
【fragment_detail】
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/text"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</RelativeLayout>
【fragment_master】
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="按钮" />
</RelativeLayout>
代码:
package com.example.myapplication;
import android.content.ClipData.Item;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.MutableLiveData;
import androidx.lifecycle.ViewModel;
public class SharedViewModel extends ViewModel {
private final MutableLiveData<String> selected = new MutableLiveData<String>();
public void select(String item) {
selected.setValue(item);
}
public LiveData<String> getSelected() {
return selected;
}
}
package com.example.myapplication;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}
package com.example.myapplication;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.ViewModelProviders;
public class MasterFragment extends Fragment {
private SharedViewModel model;
private TextView button;
int num = 0;
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_master, null);
button = view.findViewById(R.id.button);
return view;
}
@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
model.select("数字:" + (num++));
}
});
}
}
package com.example.myapplication;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProviders;
public class DetailFragment extends Fragment {
private SharedViewModel model;
private TextView text;
@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_detail, null);
text = view.findViewById(R.id.text);
return view;
}
@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
model = ViewModelProviders.of(getActivity()).get(SharedViewModel.class);
model.getSelected().observe(getViewLifecycleOwner(), new Observer<String>() {
@Override
public void onChanged(String str) {
text.setText(str);
}
});
}
}
这样通过点击MasterFragment的按钮就能控制DetailFragment的文本了。其实SharedViewModel就是一个中转站,一个仓库,一个存一个取。因为很多通信其实都是通过底层存储来实现的
ViewModel大部分都用来实现MVVM模型,M层用来操作数据,V层负责展示界面,VM层用来逻辑处理。
下面我们来看源码:
ViewModelProviders.of(getActivity())
of方法
@MainThread
public static ViewModelProvider of(@NonNull FragmentActivity activity,
@Nullable Factory factory) {
Application application = checkApplication(activity);
if (factory == null) {
factory = ViewModelProvider.AndroidViewModelFactory.getInstance(application);
}
return new ViewModelProvider(activity.getViewModelStore(), factory);
}
当factory为null,会根据application创建一个factory,这样保证了一个应用使用的是同一个factory
public ViewModelStore getViewModelStore() {
if (getApplication() == null) {
throw new IllegalStateException("Your activity is not yet attached to the "
+ "Application instance. You can't request ViewModel before onCreate call.");
}
if (mViewModelStore == null) {
NonConfigurationInstances nc =
(NonConfigurationInstances) getLastNonConfigurationInstance();
if (nc != null) {
// Restore the ViewModelStore from NonConfigurationInstances
mViewModelStore = nc.viewModelStore;
}
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
return mViewModelStore;
}
getViewModelStore()方法新建了一个ViewModelStore,ViewModelStore类很简单,底层用的是HashMap,这时我们就可以知道了,ViewModel就是根据activity的名字存取Map
public class ViewModelStore {
private final HashMap<String, ViewModel> mMap = new HashMap<>();
final void put(String key, ViewModel viewModel) {
ViewModel oldViewModel = mMap.put(key, viewModel);
if (oldViewModel != null) {
oldViewModel.onCleared();
}
}
final ViewModel get(String key) {
return mMap.get(key);
}
Set<String> keys() {
return new HashSet<>(mMap.keySet());
}
/**
* Clears internal storage and notifies ViewModels that they are no longer used.
*/
public final void clear() {
for (ViewModel vm : mMap.values()) {
vm.clear();
}
mMap.clear();
}
}
这里有put有get,一开始的ViewModelProviders.of(getActivity()).get(SharedViewModel.class)的get先是跳到ViewModelProvider的get方法,然后果不其然跳到了ViewModelStore的get方法
接下来我们来看是怎么触发数据变化的,先看接收方
model.getSelected().observe(getViewLifecycleOwner(), new Observer<String>() {
@Override
public void onChanged(String str) {
text.setText(str);
}
});
observe实际上就是增加一个监听,那我们只要找到什么时候调用了onChanged方法就行了
model.select("数字:" + (num++));
到
selected.setValue(item);
进到LiveData里
protected void setValue(T value) {
assertMainThread("setValue");
mVersion++;
mData = value;
dispatchingValue(null);
}
dispatchingValue方法里调用了一个considerNotify方法
private void considerNotify(ObserverWrapper observer) {
if (!observer.mActive) {
return;
}
// Check latest state b4 dispatch. Maybe it changed state but we didn't get the event yet.
//
// we still first check observer.active to keep it as the entrance for events. So even if
// the observer moved to an active state, if we've not received that event, we better not
// notify for a more predictable notification order.
if (!observer.shouldBeActive()) {
observer.activeStateChanged(false);
return;
}
if (observer.mLastVersion >= mVersion) {
return;
}
observer.mLastVersion = mVersion;
//noinspection unchecked
observer.mObserver.onChanged((T) mData);
}
这里调用了onChanged,终于完美闭环。
ViewModel基本都会跟LiveData结合使用,LiveData里面有个ObserverWrapper类,监听就是通过它实现
其实代码功能底层基本都是通过存储来实现的,无论是线程间通信还是进程间通信,一存一取,就通信了。还有监听,观察者模式的实时动态变化基本离不开监听。