Android 自定义Livedata使用示例解析
作者：流浪汉kylin
这篇文章主要为大家介绍了Android 自定义Livedata使用示例解析，有需要的朋友可以借鉴参考下，希望能够有所帮助，祝大家多多进步，早日升职加薪
前言
我们在开发中在使用MVVM的情况下经常会配合livedata来达到快速开发的效果，但是一般都是在activity或者fragment中去使用，我今天想介绍一种自定义的方式，如果你有复杂的自定义View或者某些场景，也可以使用livedata来达到一个很不错的效果。
Livedata分析
我们平时使用livedata都会在activity或者fragment中使用，配合 Lifecycle就不用管理生命周期什么的了，所以一般以activity或fragment作为view层（当然service内也有相应的封装）。
viewmodel层，继承lifecycler的ViewModel
var data : MutableLiveData<Int> = MutableLiveData()
fun test(){
data.value = 1
}
view层
var viewmodel = ViewModelProvider(this).get(TestViewModel::class.java)
viewmodel?. data?.observe(this, Observer {
// todo
})
一般来说就这样写嘛，也不用考虑注销什么的，它自己内部帮你实现，很方便，but 也只能在activity或者fragment中能这样写
假如在view中这样写，传this的地方会报错，为什么呢，我们可以看看view层的两个this传的是什么。
创建ViewModelProvider时传
public ViewModelProvider(@NonNull ViewModelStoreOwner owner)
调用observe方法时传
public void observe(@NonNull LifecycleOwner owner, @NonNull Observer<? super T> observer)
可以看到一个是ViewModelStoreOwner，另一个是LifecycleOwner，并不是同一个东西
public interface ViewModelStoreOwner {
/**
* Returns owned {@link ViewModelStore}
*
* @return a {@code ViewModelStore}
*/
@NonNull
ViewModelStore getViewModelStore();
}
public interface LifecycleOwner {
/**
* Returns the Lifecycle of the provider.
*
* @return The lifecycle of the provider.
*/
@NonNull
Lifecycle getLifecycle();
}
我们看看Activity内部是怎么封装的
public class FragmentActivity extends ComponentActivity implements
ViewModelStoreOwner,
ActivityCompat.OnRequestPermissionsResultCallback,
ActivityCompat.RequestPermissionsRequestCodeValidator
public class ComponentActivity extends Activity
implements LifecycleOwner, KeyEventDispatcher.Component
看接口的实现
public ViewModelStore getViewModelStore() {
......
if (mViewModelStore == null) {
......
if (mViewModelStore == null) {
mViewModelStore = new ViewModelStore();
}
}
return mViewModelStore;
}
看到内部是有引用一个ViewModelStore对象
在onDestroy时
protected void onDestroy() {
super.onDestroy();
if (mViewModelStore != null && !isChangingConfigurations()) {
mViewModelStore.clear();
}
......
}
可以看出实现ViewModelStoreOwner接口就是持有ViewModelStore对象，并保证它的创建和销毁，而它的内部会持有viewmodel
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);
}
/**
* Clears internal storage and notifies ViewModels that they are no longer used.
*/
public final void clear() {
for (ViewModel vm : mMap.values()) {
vm.onCleared();
}
mMap.clear();
}
}
那这个ViewModelStore在哪里使用呢，我们看到FragmentActivity这里只做了创建和销毁，并没有执行put和get方法，我们深入去看可以发现put/get是在ViewModelProvider中调用。这也对应了我们最初的初始化ViewModel的方法
var viewmodel = ViewModelProvider(this).get(TestViewModel::class.java)
所以很容易能看出ViewModelStore就是用来管理viewmodel的。
接下来我们看LifecycleOwner，在activity的实现这个接口的方法
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
看得出返回LifecycleRegistry对象，LifecycleRegistry就是Lifecycle的实现类，在Activity中存在调用方法
// 有很多地方有调addObserver方法
getLifecycle().addObserver(new LifecycleEventObserver() {......})
// 在这里调setCurrentState方法
protected void onSaveInstanceState(@NonNull Bundle outState) {
Lifecycle lifecycle = getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).setCurrentState(Lifecycle.State.CREATED);
}
super.onSaveInstanceState(outState);
mSavedStateRegistryController.performSave(outState);
}
值得注意的是LifecycleRegistry中的setCurrentState方法和handleLifecycleEvent方法
@MainThread
public void setCurrentState(@NonNull State state) {
moveToState(state);
}
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
看得出它们最终都是调用moveToState，调用handleLifecycleEvent只是为了把 Lifecycle.Event转成State
private void moveToState(State next) {
if (mState == next) {
return;
}
mState = next;
......
}
Lifecycle的代码就不分析了，这边主要讲Livedata。
同样能看出FragmentActivity有调用handleLifecycleEvent
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
mFragments.dispatchCreate();
}
@Override
protected void onStart() {
......
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);
mFragments.dispatchStart();
}
protected void onResumeFragments() {
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
mFragments.dispatchResume();
}
@Override
protected void onStop() {
......
mFragments.dispatchStop();
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);
}
@Override
protected void onDestroy() {
super.onDestroy();
mFragments.dispatchDestroy();
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
}
再看看另一个LifecycleOwner的实现，Service，这个Service叫LifecycleService
public class LifecycleService extends Service implements LifecycleOwner
它内部引用一个ServiceLifecycleDispatcher对象，而这个对象内部引用LifecycleRegistry。
/**
* Must be a first call in {@link Service#onCreate()} method, even before super.onCreate call.
*/
public void onServicePreSuperOnCreate() {
postDispatchRunnable(Lifecycle.Event.ON_CREATE);
}
/**
* Must be a first call in {@link Service#onBind(Intent)} method, even before super.onBind
* call.
*/
public void onServicePreSuperOnBind() {
postDispatchRunnable(Lifecycle.Event.ON_START);
}
/**
* Must be a first call in {@link Service#onStart(Intent, int)} or
* {@link Service#onStartCommand(Intent, int, int)} methods, even before
* a corresponding super call.
*/
public void onServicePreSuperOnStart() {
postDispatchRunnable(Lifecycle.Event.ON_START);
}
/**
* Must be a first call in {@link Service#onDestroy()} method, even before super.OnDestroy
* call.
*/
public void onServicePreSuperOnDestroy() {
postDispatchRunnable(Lifecycle.Event.ON_STOP);
postDispatchRunnable(Lifecycle.Event.ON_DESTROY);
}
@NonNull
public Lifecycle getLifecycle() {
return mRegistry;
}
static class DispatchRunnable implements Runnable {
private final LifecycleRegistry mRegistry;
final Lifecycle.Event mEvent;
private boolean mWasExecuted = false;
DispatchRunnable(@NonNull LifecycleRegistry registry, Lifecycle.Event event) {
mRegistry = registry;
mEvent = event;
}
@Override
public void run() {
if (!mWasExecuted) {
mRegistry.handleLifecycleEvent(mEvent);
mWasExecuted = true;
}
}
}
在外层调用
@CallSuper
@Override
public void onCreate() {
mDispatcher.onServicePreSuperOnCreate();
super.onCreate();
}
@CallSuper
@Nullable
@Override
public IBinder onBind(@NonNull Intent intent) {
mDispatcher.onServicePreSuperOnBind();
return null;
}
@SuppressWarnings("deprecation")
@CallSuper
@Override
public void onStart(@Nullable Intent intent, int startId) {
mDispatcher.onServicePreSuperOnStart();
super.onStart(intent, startId);
}
// this method is added only to annotate it with @CallSuper.
// In usual service super.onStartCommand is no-op, but in LifecycleService
// it results in mDispatcher.onServicePreSuperOnStart() call, because
// super.onStartCommand calls onStart().
@CallSuper
@Override
public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
return super.onStartCommand(intent, flags, startId);
}
@CallSuper
@Override
public void onDestroy() {
mDispatcher.onServicePreSuperOnDestroy();
super.onDestroy();
}
@Override
@NonNull
public Lifecycle getLifecycle() {
return mDispatcher.getLifecycle();
}
那么我们得出一个结论，要实现LifecycleOwner，主要就是自己去使用handleLifecycleEvent方法去设置生命周期。
那么这里有个问题，如果我有个Service继承LifecycleService，它能直接快速的使用Livedata吗，当然不能，因为LifecycleService只实现了LifecycleOwner，并没有实现ViewModelStoreOwner
自定义Livedata
按照上面Activity的源码，我们知道，要实现Livedata，主要分为两个步骤：
- 1. 实现ViewModelStoreOwner并完成ViewModelStore的创建和销毁
- 2. 实现LifecycleOwner并手动设置生命周期其实现在网上也有很多人讲在自定义View上使用Livedata，我这里就做点不同的，我在window上去实现，其实原理都是一样的。
class MyWindow internal constructor(val context: Context) : AbstractWindow(), LifecycleOwner,
ViewModelStoreOwner {
private var mViewModel : MyViewModel? = null
private var mViewModelStore: ViewModelStore ?= null
private val mRegistry = LifecycleRegistry(this)
fun init(){
// todo一些初始化操作
mRegistry.currentState = Lifecycle.State.CREATED
mViewModel = ViewModelProvider(this).get(MyViewModel::class.java)
mViewModel?.data?.observe(this, Observer {
......
})
}
fun show(){
mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START)
mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME)
mWindowManager.addView(mView, getLayoutParams());
}
fun close(){
mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE)
mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP)
mWindowManager.removeViewImmediate(mView);
}
override fun getLifecycle(): Lifecycle {
return mRegistry
}
override fun getViewModelStore(): ViewModelStore {
if (mViewModelStore == null){
mViewModelStore = ViewModelStore()
}
return mViewModelStore!!
}
fun onDestroy(){
mRegistry?.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY)
mViewModelStore?.clear()
}
这样就能在非activity/fragment的view层中实现livedata功能。
总结
自定义livedata其实没有太大的难度，这是一个开发的过程，你想在一些地方去使用官方封装好的框架，可以先去看看它是怎么做的，再自己进行一个二次封装，就能实现那么一个效果，比如这里的自定义View或者Window使用Livadata。
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