ViewModel浅析
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。
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