android DataBind LiveData ViewModel 使用详解
1.导入android x ,升级 版本到28
implementation 'androidx.appcompat:appcompat:1.0.0' def room_version = "2.2.5" implementation "androidx.room:room-runtime:$room_version" kapt "androidx.room:room-compiler:$room_version" def lifecycle_version = "2.2.0" def arch_version = "2.1.0" implementation "androidx.recyclerview:recyclerview:1.1.0" // ViewModel implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version" // LiveData implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version" // Saved state module for ViewModel implementation "androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version" // Annotation processor annotationProcessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version" // alternately - if using Java8, use the following instead of lifecycle-compiler implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version" // optional - helpers for implementing LifecycleOwner in a Service implementation "androidx.lifecycle:lifecycle-service:$lifecycle_version" // optional - ProcessLifecycleOwner provides a lifecycle for the whole application process implementation "androidx.lifecycle:lifecycle-process:$lifecycle_version"
没什么好说的,这些基本上是迁移androidx 需要的库,你要把项目中的appcomt 库的引用去掉。
compileSdkVersion 28
提升编译版本,因为有些androidx 的依赖库只能在28上。
2.替换所有的support 包 为android X
androidx.fragment.app.Fragment
fragment 只是举例的一个类而已,有挺多类的,比如AppCompatActivty…
只要是替换成androidX 找不到的类,基本上都要换路径。
3.写一个ViewModel,里面包含LiveData
package com.example.viewModel;import com.example.database.UserDBManager;import com.example.rom.UseDao;import com.example.rom.User;import java.util.ArrayList;import java.util.List;import java.util.concurrent.Executors;import androidx.lifecycle.LiveData;import androidx.lifecycle.MutableLiveData;import androidx.lifecycle.ViewModel;/** * ======================================================================================= * 作 者:caoxinyu * 创建日期:2020/3/20. * 类的作用: * 修订历史: * ======================================================================================= */public class UserViewModel extends ViewModel { private MutableLiveData> users; public LiveData> getUsers() { if (users == null) { users = new MutableLiveData<>(new ArrayList<>()); } final List value = users.getValue(); Thread thread = Executors.defaultThreadFactory().newThread(new Runnable() { @Override public void run() { UseDao useDao = UserDBManager.getInstance().getUseDao(); List usersAll = useDao.queryAll(); if (value != null) { value.addAll(usersAll); } users.postValue(value); } }); thread.start(); return users; }}
ViewModel 是和activity 生命明周期绑定的一个类,里面一般有个字段,是LiveData,就像我上面写的一样。
一般获取方式为:
UserModel userModel = new ViewModelProvider(requireActivity()).get(UserModel.class);
具体可以看上面贴出来的类。
4.使用DataBing
首先在xml 最外加 标签,里面的 可以对应你写的ViewModel
databing 在xml 的具体写法,可以百度看下,这里先不贴了。挺多种的,或者有时间我写一篇。
6.在activity 或者 fragment 里面 使用Databing数据
// Inflate the layout for this fragment FragmentRoomTestBinding fragmentRoomTestBinding = FragmentRoomTestBinding.inflate(inflater, container, false); UserViewModel userViewModel = new ViewModelProvider(this).get(UserViewModel.class); fragmentRoomTestBinding.setAddUser(new View.OnClickListener() { @Override public void onClick(View v) { UserDBManager.getInstance().insertTestUser(); userViewModel.getUsers(); } }); LiveData> users = userViewModel.getUsers(); mUserList = new ArrayList<>(); List userList = users.getValue(); if (userList != null) { mUserList.addAll(userList); } RecyclerView recycleView = fragmentRoomTestBinding.recycleView; recycleView.setLayoutManager(new LinearLayoutManager(getContext())); UserAdapter adapter = new UserAdapter(mUserList, BR.itemInfo); fragmentRoomTestBinding.setUserIsEmpty(mUserList.isEmpty()); users.observe(this, new Observer>() { @Override public void onChanged(List users) { mUserList.clear(); mUserList.addAll(users); adapter.notifyDataSetChanged(); fragmentRoomTestBinding.setUserIsEmpty(users.isEmpty()); } }); recycleView.setAdapter(adapter);
原理:
1.线程切换问题
当我的liveData 更新之后,如果是在子线程更新的,那么回调的时候,框架是怎么切换到主线程的?
ViewDataBing 里面有两种方法: ViewDataBing 的时候,生成一个handler 保存一下。 mUIThreadHandler = new Handler(Looper.myLooper()); 需要用的时候用handler postRunable 就可以解决
第二种是 Choreographer,这个是databing 默认的方式:
mChoreographer = Choreographer.getInstance(); mFrameCallback = new Choreographer.FrameCallback() { @Override public void doFrame(long frameTimeNanos) { mRebindRunnable.run(); } }; mChoreographer.postFrameCallback(mFrameCallback);
2. 为什么可以监听到viewMode 的 变化?
androidx.databinding.ViewDataBinding.LiveDataListener
/** * Method object extracted out to attach a listener to a bound LiveData object. */ private static final CreateWeakListener CREATE_LIVE_DATA_LISTENER = new CreateWeakListener() { @Override public WeakListener create(ViewDataBinding viewDataBinding, int localFieldId) { return new LiveDataListener(viewDataBinding, localFieldId).getListener(); } };
你会看到,androidx.databinding.ViewDataBinding
这个类里面,CREATE_LIVE_DATA_LISTENER 会给所有的ViewMode 里面的LiveData 对象 设置一个监听,当ViewModel 里面的LiveData 对象的值,变化了,那么databinding 就会更新UI.
3.根据xml 生成的dataBing在哪里?
会有两个,一个是基础类,叫做bind,一个叫做impl.基本上更新UI的代码,都在impl里面。executeBindings 方法会真正的调用绑定,比如TextView. setText
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