前言
现在在 Google 应用市场当中，大部分 APP 已经开始使用 Android Jetpack。
为什么大家都会接入Jetpack呢？主要原因官网也进行了描述
-
遵循最佳做法
- Android Jetpack 组件采用最新的设计方法构建，具有向后兼容性，可以减少崩溃和内存泄露。
-
消除样板代码
- Android Jetpack 可以管理各种繁琐的 Activity（如后台任务、导航和生命周期管理），以便您可以专注于打造出色的应用。
-
减少不一致
- 这些库可在各种 Android 版本和设备中以一致的方式运作，助您降低复杂性。
本文为JetPack首篇入门及介绍，后续将不断学习并进行记录。
目录
一、JetPack中热门和常用的库
| 库 | 作用 |
|---|---|
| [activity.*](https://developer.android.google.cn/jetpack/androidx/releases/activity) | 访问基于 Activity 构建的可组合 API。 |
| [appcompat *](https://developer.android.google.cn/jetpack/androidx/releases/appcompat) | 允许在平台旧版 API 上访问新 API（很多使用 Material Design）。 |
| [camera *](https://developer.android.google.cn/jetpack/androidx/releases/camera) | 构建移动相机应用。 |
| [compose *](https://developer.android.google.cn/jetpack/androidx/releases/compose) | 使用描述界面形状和数据依赖项的可组合函数，以编程方式定义界面。 |
| [databinding *](https://developer.android.google.cn/jetpack/androidx/releases/databinding) | 使用声明性格式将布局中的界面组件绑定到应用中的数据源。 |
| [fragment *](https://developer.android.google.cn/jetpack/androidx/releases/fragment) | 将您的应用细分为在一个 Activity 中托管的多个独立屏幕。 |
| [hilt *](https://developer.android.google.cn/jetpack/androidx/releases/hilt) | 扩展了 Dagger Hilt 的功能，以实现 androidx 库中某些类的依赖项注入。 |
| [lifecycle *](https://developer.android.google.cn/jetpack/androidx/releases/lifecycle) | 构建生命周期感知型组件，这些组件可以根据 Activity 或 Fragment 的当前生命周期状态调整行为。 |
| [Material Design 组件*](https://material.io/develop/android) | 适用于 Android 的 Material Design 组件可帮助您在构建界面时支持 Material Design。这些组件能够实现可靠的开发工作流，构建功能强大且美观的 Android 应用。 |
| [navigation *](https://developer.android.google.cn/jetpack/androidx/releases/navigation) | 构建和组织应用内界面，处理深层链接以及在屏幕之间导航。 |
| [paging *](https://developer.android.google.cn/jetpack/androidx/releases/paging) | 在页面中加载数据，并在 RecyclerView 中呈现。 |
| [room *](https://developer.android.google.cn/jetpack/androidx/releases/room) | 创建、存储和管理由 SQLite 数据库支持的持久性数据。 |
| [test *](https://developer.android.google.cn/jetpack/androidx/releases/test) | 在 Android 中进行测试。 |
| [work *](https://developer.android.google.cn/jetpack/androidx/releases/work) | 调度和执行可延期且基于约束条件的后台任务。 |
| [ads](https://developer.android.google.cn/jetpack/androidx/releases/ads) | 获取广告 ID（无论是否通过 Play 服务）。 |
| [annotation](https://developer.android.google.cn/jetpack/androidx/releases/annotation) | 公开元数据，帮助工具开发者和其他开发者了解您的应用代码。 |
| [arch.core](https://developer.android.google.cn/jetpack/androidx/releases/arch-core) | 其他架构依赖项的帮助程序，包括可与 LiveData 配合使用的 JUnit 测试规则。 |
二、Lifecycle
相必大家都知道Lifecycle的作用，在Android开发中，Activity、Fragment的生命周期管理是不可避免的，因此有了Lifecycle.
简单用法示例如下
public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getLifecycle().addObserver(new MainPresenter());
}
@Override
protected void onResume() {
super.onResume();
}
}
public class MainPresenter implements LifecycleObserver{
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
void onResume(){
}
}
三、Lifecycle原理
MainActivity继承AppCompatActivity，AppCompatActivity继承了FragmentActivity，Android8.0下，FragmentActivity继承SupportActivity，9.0 是继承自ComponentActivity，它们都实现了LifecycleOwner接口。
Lifecycle原理相关时序图如下，细节部分没有体现
下面以ComponentActivity为例进行分析
@RestrictTo(LIBRARY_GROUP_PREFIX)
public class ComponentActivity extends Activity implements
LifecycleOwner,
KeyEventDispatcher.Component {
@SuppressWarnings("deprecation")
private SimpleArrayMap<Class<? extends ExtraData>, ExtraData> mExtraDataMap =
new SimpleArrayMap<>();
//注释1
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
//注释2
@SuppressLint("RestrictedApi")
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
//注释3
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
注释 1 创建了LifecycleRegistry，LifecycleRegistry继承自Lifecycle
注释 3 实现了LifecycleOwner接口，返回LifecycleRegistry
注释 2 Lifecycle对生命周期的管理，交给了ReportFragment
关注点转变为ReportFragment
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP_PREFIX)
public class ReportFragment extends Fragment {
public static void injectIfNeededIn(Activity activity) {
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();
}
}
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);
//注释 1 上面的先不用管，先研究生命周期管理
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;
}
//注释 2
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);
}
}
}
}
可以看到在 onActivityCreated() 方法中会调用 dispatch(Lifecycle.Event.ON_CREATE) 方法
dispatch方法中，会先判断 activity 是否是实现 LifecycleRegistryOwner接口，还是LifecycleOwner接口，然后调用不同，这两个接口方法返回值不一样，一个是Lifecycle，一个是LifecycleRegistry。
@Deprecated
public interface LifecycleRegistryOwner extends LifecycleOwner {
@NonNull
@Override
LifecycleRegistry getLifecycle();
}
public interface LifecycleOwner {
/**
* Returns the Lifecycle of the provider.
*
* @return The lifecycle of the provider.
*/
@NonNull
Lifecycle getLifecycle();
}
ComponentActivty 实现的是 LifecycleOwner 接口，所以走得是第二段代码流程
if (activity instanceof LifecycleOwner) {
//注释 1 返回的是 ComponentActivity中创建的 LifecycleRegistry
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
最终走的是 LifecycleRegistry 的 handleLifecycleEvent 方法
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
// 注释 1 得到 下一步要发生的事件 当前事件执行后，即将会处于什么状态
State next = getStateAfter(event);
// 注释 2 主要是调用 sync 方法 来管理状态
moveToState(next);
}
//注释 3
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);
}
//注释 4
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;
}
对于 Lifecycle.Event 和 Lifecycle.State 关系从 getStateAfter() 方法 中你也能看出，如下
继续分析moToState() 方法 中调用的 sync() 方法
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.");
}
// 注释 1
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); //注释 2
}
Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner); //注释 3
}
}
mNewEventOccurred = false;
}
private boolean isSynced() {
if (mObserverMap.size() == 0) {
return true;
}
State eldestObserverState = mObserverMap.eldest().getValue().mState;
State newestObserverState = mObserverMap.newest().getValue().mState;
return eldestObserverState == newestObserverState && mState == newestObserverState;
}
注释 1 处 isSynced() 方法 会判断 当前状态和mObserverMap中的eldest和newest的状态做对比，直到状态一致
注释 2 和 注释 3 可以结合之前 Event 和 State 图 来分析，主要是根据当前状态和eldset和newest状态对比，判断是向前还是向后。这里以向前为例，即 forwardPass() 方法
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
ObserverWithState observer = entry.getValue();
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
//向 mParentStates 添加该 State
pushParentState(observer.mState);
// 更新状态 通知 LifecycleObserver 更新
observer.dispatchEvent(lifecycleOwner, upEvent(observer.mState));
//向 mParentStates 删除该 State
popParentState();
}
}
}
继续看 LifecycleObserver 的 dispatchEvent() 方法
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = getStateAfter(event);
mState = min(mState, newState);
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
mLifecycleObserver 是 通过 Lifecycling.lifecycleEventObserver(observer) 构造的，
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
// false while循环执行时 不考虑一下情况分析，这个 object LifecycleObserver 在 ComponentActivity 构造函数中 调用了addObserver方法 有添加进来的，如果是 则 直接返回你添加的 LifecycleObserver 执行 onStateChanged 方法
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
// false
boolean isFullLifecycleObserver = object instanceof FullLifecycleObserver;
if (isLifecycleEventObserver && isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object,
(LifecycleEventObserver) object);
}
if (isFullLifecycleObserver) {
return new FullLifecycleObserverAdapter((FullLifecycleObserver) object, null);
}
if (isLifecycleEventObserver) {
return (LifecycleEventObserver) object;
}
final Class<?> klass = object.getClass();
// ClassesInfoCache.sInstance.hasLifecycleMethods() -> createInfo()
int type = getObserverConstructorType(klass);
if (type == GENERATED_CALLBACK) {
List<Constructor<? extends GeneratedAdapter>> constructors =
sClassToAdapters.get(klass);
if (constructors.size() == 1) {
GeneratedAdapter generatedAdapter = createGeneratedAdapter(
constructors.get(0), object);
return new SingleGeneratedAdapterObserver(generatedAdapter);
}
GeneratedAdapter[] adapters = new GeneratedAdapter[constructors.size()];
for (int i = 0; i < constructors.size(); i++) {
adapters[i] = createGeneratedAdapter(constructors.get(i), object);
}
return new CompositeGeneratedAdaptersObserver(adapters);
}
return new ReflectiveGenericLifecycleObserver(object);
}
该方法就是判断用LifecycleEventObserver的实现类 CompositeGeneratedAdaptersObserver 还是 ReflectiveGenericLifecycleObserver ， 下面主要对第二个进行分析，看其onStateChanged 是如何处理的
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);
}
}
然后关注点变到了 mInfo 是如何创建的，我们点击进入getInfo 方法，查看到是在ClassesInfoCache 的createInfo 方法里讲该值缓存了, 其内部方法 hasLifecycleMethods 中调用了createInfo() 方法，一直反推，发现 static LifecycleEventObserver lifecycleEventObserver(Object object) 方法内部会调用 createInfo() 方法。关注点变成了 createInfo() 是如何构建 CallbackInfo 类的 。
private CallbackInfo createInfo(Class klass, @Nullable Method[] declaredMethods) {
Class superclass = klass.getSuperclass();
Map<MethodReference, Lifecycle.Event> 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<MethodReference, Lifecycle.Event> 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;
}
主要就是收集注解 OnLifecycleEvent 标记的方法 和 onStateChanged 方法，如果有参数，必须是 (LifecycleOwner,Lifecycle)类型。构建相应的数据结构，将数据存放再 handlerToEvent 中，然后我们可会回过头来看 mInfo.invokeCallbacks(source, event, mWrapped) 方法了，主要是根据ReportFragment传递过来的Lifecycle.Event事件，做出匹配，反射调用相应的方法。
void invokeCallbacks(LifecycleOwner source, Lifecycle.Event event, Object target) {
invokeMethodsForEvent(mEventToHandlers.get(event), source, event, target);
invokeMethodsForEvent(mEventToHandlers.get(Lifecycle.Event.ON_ANY), source, event,
target);
}
