写在前面：Jetpack的更新速度非常快，可能你一个月前看WorkManager是这样用的，下个月这个使用方法就有可能被废弃了（我看源码的时候是遇到过的，而且源码也变了，但核心原理是不变的），所以我们这一系列文章偏重讲原理，使用就一带而过（因为讲了也没用啊，会变的。。。。。，读者使用最好看官方文档官方文档
，当然我这里讲的也是截止到目前的最新用法）。
Jetpack使用（一）Lifecycles核心原理
Jetpack使用（二）LiveData核心原理
Jetpack使用（三）DataBinding核心原理
Jetpack使用（四）ViewModel核心原理
Jetpack使用（五）Navigation核心原理
Jetpack使用（六） WorkManager的4种用法
首先，我们要知道Jetpack是什么？引用谷歌官方的一段话：Jetpack 是一套库、工具和指南，可帮助开发者更轻松地编写优质应用。这些组件可帮助您遵循最佳做法、让您摆脱编写样板代码的工作并简化复杂任务，以便您将精力集中放在所需的代码上。
Jetpack 包含与平台 API 解除捆绑的 androidx.* 软件包库。这意味着，它可以提供向后兼容性，且比 Android 平台的更新频率更高，以此确保您始终可以获取最新且最好的 Jetpack 组件版本。jetpack组件国内官网地址
所以简单一句话来概括：Jetpack组件是很多库和组件的集合（其中包含了androidx），可以加速我们开发app的速度，并且设计稳健减少崩溃和内存泄漏。
首先读者需要注意的是：我们这一系列的文章着重点是源码分析其中的原理，所以只会讲基本的使用，后面最后会出一篇文章统一来讲实际项目中的使用。
谷歌推出Jetpack框架就是想统一架构，就像web开发现在基本都统一使用mvc架构了，谷歌也想把移动端的架构统一起来,估计以后2年市场上的项目都会用Jetpack这一套架构来写了。也就是说你不学Jetpack，以后的安卓代码你可能会看不懂了
从这篇文章开始，到往后的几篇文章，我们会来介绍Jetpack一些常用的组件和库的核心原理，下图给出了Jetpack所有的组件和库
Jetpack所有的组件和库
这篇文章我们先来介绍一下Lifecycles的使用及其内部首先的源码解析。
Lifecycles介绍
Lifecycles中文意思就是生命周期感知型组件，用来感知activity和fragment的生命周期的组件，主要运用观察者模式，Jetpack里很多组件都是基于Lifecycles来实现的，可以毫不夸张的说，Lifecycles是Jetpack的基础、是Jetpack组件库的重中之重。
Lifecycles基本使用
- 被观察者：主要是activity和fragment，实现LifecycleOwner接口（Support Library 26.1.0 及更高版本中的 Fragment 和 Activity 已实现 LifecycleOwner接口），并且在初始化方法里添加观察者就行了
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
public class MainActivity extends BaseActivity implements IGoodsView {
ListView listView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
listView=findViewById(R.id.listView);
presenter.fetch();
}
@Override
protected GoodsPresenter createPresenter() {
return new GoodsPresenter();
}
@Override
public void showGoodsView(List goods) {
listView.setAdapter(new GoodsAdapter(this,goods));
}
@Override
public void showErrorMessage(String msg) {
}
@Override
protected void init() {
super.init();
getLifecycle().addObserver(presenter);
}
}
- 观察者：实现LifecycleObserver接口，就能观测到被观察者的生命周期了，并且想观测哪些生命周期就运用哪个注解
public class BasePresenter implements LifecycleObserver {
WeakReference iGoodsView;
/**
* 绑定view
*/
public void attachView(T view){
iGoodsView=new WeakReference<>(view);
}
/**
* 解绑
*/
public void detachView(){
if(iGoodsView!=null){
iGoodsView.clear();
iGoodsView=null;
}
}
@OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
void onCreateX(LifecycleOwner owner) {
}
@OnLifecycleEvent(Lifecycle.Event.ON_START)
void onStartX(LifecycleOwner owner) {
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
void onStop(LifecycleOwner owner) {
}
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
void onResume(LifecycleOwner owner) {
}
@OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
void onPause(LifecycleOwner owner) {
}
@OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
void onDestory(LifecycleOwner owner) {
}
@OnLifecycleEvent(Lifecycle.Event.ON_ANY)
void onAny(LifecycleOwner owner) {
}
}
我们现在只监听create和destroy，然后运行下代码看看结果：
public class GoodsPresenter extends BasePresenter{
IGoodsModel iGoodsModel=new GoodsModel();
/**
* 执行业务逻辑
*/
public void fetch(){
if(iGoodsView !=null && iGoodsModel!=null){
iGoodsModel.loadGoodsData(new IGoodsModel.OnLoadListener() {
@Override
public void onComplete(List goods) {
((IGoodsView)iGoodsView.get()).showGoodsView(goods);
}
@Override
public void onError(String msg) {
}
});
}
}
@Override
void onCreateX(LifecycleOwner owner) {
super.onCreateX(owner);
Log.i("xiaosanye","执行了create");
}
@Override
void onDestory(LifecycleOwner owner) {
super.onDestory(owner);
Log.i("xiaosanye","执行了destroy");
}
}
结果打印
2020-03-28 13:50:49.034 16007-16007/com.example.mvp/xiaosanye: 执行了create
2020-03-28 13:50:51.550 16007-16007/com.example.mvp/xiaosanye: 执行了destroy
注意：本文用的是mvp的架构，对mvp不了解的小伙伴请自行谷歌
打印结果和我们预期的也是一样的，到这里，我们Lifecycles的基本用法讲完了，那么Lifecycles是怎么实现监听生命周期变化的呢？其实内部和glide的生命周期监听也是类似的，其实就是在当前Activity或者fragment中绑定一个空的fragment来实现监听的。
Lifecycles源码分析
我们上面说过Support Library 26.1.0 及更高版本中的 Activity 已经实现 LifecycleOwner接口，所以先找实现LifecycleOwner接口的Activity的父类
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner,
ViewModelStoreOwner,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
我们先进入接口LifecycleOwner看看
/**
* A class that has an Android lifecycle. These events can be used by custom components to
* handle lifecycle changes without implementing any code inside the Activity or the Fragment.
*
* @see Lifecycle
*/
@SuppressWarnings({"WeakerAccess", "unused"})
public interface LifecycleOwner {
/**
* Returns the Lifecycle of the provider.
*
* @return The lifecycle of the provider.
*/
@NonNull
Lifecycle getLifecycle();
}
我们发现里面就一个Lifecycle变量声明的方法getLifecycle，所有我们直接进入Lifecycle类，Lifecycle是个抽象类，我们查看里面的方法和变量
image.png
我们看到有Event，它里面包含了生命周期的一些事件的枚举，然后是State，它里面是一些状态的枚举。
构成 Android Activity 生命周期的状态和事件
您可以将状态State看作图中的节点，将事件Event看作这些节点之间的边，比如你执行ON_CREATE事件的时候，你的状态就会从INITIALIZED变为CREATED。
还有addObserver添加和removeObserver删除观察者的方法，还有一个getCurrentState获取当前状态的方法，我们现进入addObserver方法看下
@Override
public void addObserver(@NonNull LifecycleObserver observer) {
State initialState = mState == DESTROYED ? DESTROYED : INITIALIZED;
ObserverWithState statefulObserver = new ObserverWithState(observer, initialState);
ObserverWithState previous = mObserverMap.putIfAbsent(observer, statefulObserver);
首先会获得初始状态值，如果是第一次进入肯定不等于DESTROYED，所以初始状态会是INITIALIZED，再把这个状态initialState和observer观察者放入ObserverWithState类型的变量statefulObserver里，最后再把statefulObserver和observer放进一个Map数组里mObserverMap，addObserver我们先看到这一步，因为再看下去我怕你们会懵逼，所以我们先直接到ComponentActivity的onCreate方法看
/**
* {@inheritDoc}
*
* If your ComponentActivity is annotated with {@link ContentView}, this will
* call {@link #setContentView(int)} for you.
*/
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
我看进入ReportFragment.injectIfNeededIn(this)的方法里看下
public static void injectIfNeededIn(Activity activity) {
// ProcessLifecycleOwner should always correctly work and some activities may not extend
// FragmentActivity from support lib, so we use framework fragments for activities
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
// Hopefully, we are the first to make a transaction.
manager.executePendingTransactions();
}
}
我们看到injectIfNeededIn的作用就是把ReportFragment通过beginTransaction().add添加到Fragment管理器manager里面，我们再进入ReportFragment里看看
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
public class ReportFragment extends Fragment {
private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
+ ".LifecycleDispatcher.report_fragment_tag";
public static void injectIfNeededIn(Activity activity) {
// ProcessLifecycleOwner should always correctly work and some activities may not extend
// FragmentActivity from support lib, so we use framework fragments for activities
android.app.FragmentManager manager = activity.getFragmentManager();
if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
// Hopefully, we are the first to make a transaction.
manager.executePendingTransactions();
}
}
static ReportFragment get(Activity activity) {
return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
REPORT_FRAGMENT_TAG);
}
private ActivityInitializationListener mProcessListener;
private void dispatchCreate(ActivityInitializationListener listener) {
if (listener != null) {
listener.onCreate();
}
}
private void dispatchStart(ActivityInitializationListener listener) {
if (listener != null) {
listener.onStart();
}
}
private void dispatchResume(ActivityInitializationListener listener) {
if (listener != null) {
listener.onResume();
}
}
@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
dispatchCreate(mProcessListener);
dispatch(Lifecycle.Event.ON_CREATE);
}
@Override
public void onStart() {
super.onStart();
dispatchStart(mProcessListener);
dispatch(Lifecycle.Event.ON_START);
}
@Override
public void onResume() {
super.onResume();
dispatchResume(mProcessListener);
dispatch(Lifecycle.Event.ON_RESUME);
}
@Override
public void onPause() {
super.onPause();
dispatch(Lifecycle.Event.ON_PAUSE);
}
@Override
public void onStop() {
super.onStop();
dispatch(Lifecycle.Event.ON_STOP);
}
@Override
public void onDestroy() {
super.onDestroy();
dispatch(Lifecycle.Event.ON_DESTROY);
// just want to be sure that we won't leak reference to an activity
mProcessListener = null;
}
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
if (activity instanceof LifecycleRegistryOwner) {
((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
return;
}
if (activity instanceof LifecycleOwner) {
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
void setProcessListener(ActivityInitializationListener processListener) {
mProcessListener = processListener;
}
interface ActivityInitializationListener {
void onCreate();
void onStart();
void onResume();
}
}
我们看到ReportFragment的生命周期里对应分发了Lifecycle里的那些枚举事件，所以activity生命周期发生变化的时候就会走ReportFragment的生命周期里的一些方法，所以我们再进入dispatch()方法
private void dispatch(Lifecycle.Event event) {
Activity activity = getActivity();
if (activity instanceof LifecycleRegistryOwner) {
((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
return;
}
if (activity instanceof LifecycleOwner) {
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
dispatch方法里前面就是判断了activity是否有实现LifecycleRegistryOwner接口或者是LifecycleOwner接口（LifecycleRegistryOwner其实也是派生于LifecycleOwner的），如果是的话就进入执行handleLifecycleEvent方法，
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
State next = getStateAfter(event);
moveToState(next);
}
一开始先通过getStateAfter获取当前状态的下一个状态
static State getStateAfter(Event event) {
switch (event) {
case ON_CREATE:
case ON_STOP:
return CREATED;
case ON_START:
case ON_PAUSE:
return STARTED;
case ON_RESUME:
return RESUMED;
case ON_DESTROY:
return DESTROYED;
case ON_ANY:
break;
}
throw new IllegalArgumentException("Unexpected event value " + event);
}
看过前面那张状态和事件的枚举图现在来看这个getStateAfter方法就很好理解了把，例如如果你当前正在ON_CREATE或者ON_STOP，下一个状态就会走到CREATED状态。我们再把获得的下一个状态传到moveToState方法里，去真正移动到下一个状态
private void moveToState(State next) {
if (mState == next) {
return;
}
mState = next;
if (mHandlingEvent || mAddingObserverCounter != 0) {
mNewEventOccurred = true;
// we will figure out what to do on upper level.
return;
}
mHandlingEvent = true;
sync();
mHandlingEvent = false;
}
如果下一个状态和当前状态相同就直接返回，否则执行sync()同步操作，我们再进入sync
private void sync() {
LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
if (lifecycleOwner == null) {
throw new IllegalStateException("LifecycleOwner of this LifecycleRegistry is already"
+ "garbage collected. It is too late to change lifecycle state.");
}
while (!isSynced()) {
mNewEventOccurred = false;
// no need to check eldest for nullability, because isSynced does it for us.
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Entry newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
在看这个同步方法之前，我们再来理解下那张状态图，
构成 Android Activity 生命周期的状态和事件
我们知道我们的activity随着用户的操作，状态会不断的改变，状态改变就会遵循上面这张图，比如onCreate跑完，状态就会变成created，如果执行完onStart，状态就会从created变成started，执行完onResune状态就会从started变成resumed以此类推。我们在前面代码知道，我们的观察者刚刚进来的初始换状态是INITIALIZED状态，所以观察者的状态可能和我们的activity的状态不一样，所以sync方法就是来让观察者和activity的状态同步的，我们再看到代码，里面主要是两个方法backwardPass和forwardPass，一个是向前移动，一个是向后移动，
如果activity的状态是started，观察者的状态是created，那就要把观察者往后推；如果activity的状态是started，观察者的状态是resumed，那就要把观察者往前推。我们进入其中一个方法看看，我们进入backwardPass
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator> descendingIterator =
mObserverMap.descendingIterator();
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Entry entry = descendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
Event event = downEvent(observer.mState);
pushParentState(getStateAfter(event));
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
一开始就是拿到mObserverMap（所有观察者的map）的迭代器，然后循环每一个观察者，然后把被观察者lifecycleOwner和观察者的执行event 传入到dispatchEvent执行，我们进入dispatchEvent方法
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
我们发现dispatchEvent是静态内部类ObserverWithState其中的一个方法，ObserverWithState类还记得么？就是前面我们把所有观察者和状态存起来的类，
所以我们先看ObserverWithState的构造方法里的lifecycleEventObserver，我们发现lifecycleEventObserver最后都 return new ReflectiveGenericLifecycleObserver(object)到这里，所以我们直接进入
/**
* An internal implementation of {@link LifecycleObserver} that relies on reflection.
*/
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
@Override
public void onStateChanged(LifecycleOwner source, Event event) {
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
CallbackInfo getInfo(Class klass) {
CallbackInfo existing = mCallbackMap.get(klass);
if (existing != null) {
return existing;
}
existing = createInfo(klass, null);
return existing;
}
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
Class superclass = klass.getSuperclass();
Map handlerToEvent = new HashMap<>();
if (superclass != null) {
CallbackInfo superInfo = getInfo(superclass);
if (superInfo != null) {
handlerToEvent.putAll(superInfo.mHandlerToEvent);
}
}
Class[] interfaces = klass.getInterfaces();
for (Class intrfc : interfaces) {
for (Map.Entry entry : getInfo(
intrfc).mHandlerToEvent.entrySet()) {
verifyAndPutHandler(handlerToEvent, entry.getKey(), entry.getValue(), klass);
}
}
Method[] methods = declaredMethods != null ? declaredMethods : getDeclaredMethods(klass);
boolean hasLifecycleMethods = false;
for (Method method : methods) {
OnLifecycleEvent annotation = method.getAnnotation(OnLifecycleEvent.class);
if (annotation == null) {
continue;
}
hasLifecycleMethods = true;
Class[] params = method.getParameterTypes();
int callType = CALL_TYPE_NO_ARG;
if (params.length > 0) {
callType = CALL_TYPE_PROVIDER;
if (!params[0].isAssignableFrom(LifecycleOwner.class)) {
throw new IllegalArgumentException(
"invalid parameter type. Must be one and instanceof LifecycleOwner");
}
}
Lifecycle.Event event = annotation.value();
if (params.length > 1) {
callType = CALL_TYPE_PROVIDER_WITH_EVENT;
if (!params[1].isAssignableFrom(Lifecycle.Event.class)) {
throw new IllegalArgumentException(
"invalid parameter type. second arg must be an event");
}
if (event != Lifecycle.Event.ON_ANY) {
throw new IllegalArgumentException(
"Second arg is supported only for ON_ANY value");
}
}
if (params.length > 2) {
throw new IllegalArgumentException("cannot have more than 2 params");
}
MethodReference methodReference = new MethodReference(callType, method);
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
其实一步步追踪进去ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());就是通过反射拿到带有那些注解的方法， mInfo.invokeCallbacks(source, event, mWrapped);就是最后真正执行那些注解方法的地方。
至此，我们已经分析完Lifecycles全部源码的，了解了Lifecycles的实际具体执行过程。