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引言
- Lifecycle 是一个类,用于存储有关组件(如 Activity 或 Fragment)的生命周期状态的信息,并允许其他对象观察此状态。
- Lifecycle 使用两种主要枚举跟踪其关联组件的生命周期状态。
- Lifecycle.Event 从框架和 Lifecycle 类分派的生命周期事件。这些事件映射到 Activity 和 Fragment 中的回调事件。
- Lifecycle.State 由 Lifecycle 对象跟踪的组件的当前状态。
预览图
1. 诞生
在应用开发中,处理生命周期相关业务,一种常见的模式是在 Activity 和 Fragment 的生命周期方法中实现依赖组件的操作。我们就拿大家习惯的 MVP 举例:
class LifecycleActivity : AppCompatActivity() {
private lateinit var commonPresenter: CommonPresenter
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_lifecycle)
commonPresenter = CommonPresenter()
}
override fun onResume() {
super.onResume()
commonPresenter.onResume()
}
override fun onStop() {
super.onStop()
commonPresenter.onStop()
}
}
private const val TAG = "CommonPresenter"
class CommonPresenter {
fun onResume() {
Log.i(TAG, "onResume: ")
checkConfig(object : CheckResultCallback {
override fun success() {
Log.i(TAG, "onResume: success to work")
work()
}
})
}
fun onStop() {
Log.i(TAG, "onStop: ")
}
private fun checkConfig(callback: CheckResultCallback) {
Log.i(TAG, "checkConfig: ")
// TODO: check username and password, need 500ms
callback.success()
}
private fun work() {
Log.i(TAG, "work: ")
}
}
从上面的代码中,会有诸多问题:
- 会在 Activity 的生命周期中(如
onResume
和onStop
)中放置大量的代码,难以维护。 - 在我们需要执行长时间运行的操作(如
onResume()
中的checkConfig()
),onStop()
方法会在onResume()
之前结束,这使得组件留存的时间比所需的时间要长。
诞生日记 :于是它来了,androidx.lifecycle
软件包提供的类和接口可帮助以弹性和隔离的方式解决这些问题。
2. 使用
2.1 依赖添加
官方指导文档
dependencies {
val lifecycle_version = "2.4.0-alpha02"
val arch_version = "2.1.0"
// ViewModel
implementation("androidx.lifecycle:lifecycle-viewmodel-ktx:$lifecycle_version")
// LiveData
implementation("androidx.lifecycle:lifecycle-livedata-ktx:$lifecycle_version")
// Lifecycles only (without ViewModel or LiveData)
implementation("androidx.lifecycle:lifecycle-runtime-ktx:$lifecycle_version")
// Saved state module for ViewModel
implementation("androidx.lifecycle:lifecycle-viewmodel-savedstate:$lifecycle_version")
// Annotation processor
kapt("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")
// optional - ReactiveStreams support for LiveData
implementation("androidx.lifecycle:lifecycle-reactivestreams-ktx:$lifecycle_version")
// optional - Test helpers for LiveData
testImplementation("androidx.arch.core:core-testing:$arch_version")
}
2.2 使用方法
- 添加观察者:
getLifecycle()
获取 Lifecycle 实例,然后调用addObserve()
添加观察者。 - 观察者注解:观察者实现 LifecycleObserver,使用 OnLifecycleEvent 注解添加对应生命周期的方法。
我们依然优化上面的 诞生 例子。
class LifecycleActivity : AppCompatActivity() {
private lateinit var lifePresenter: LifePresenter
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_lifecycle)
lifePresenter = LifePresenter()
lifecycle.addObserver(lifePresenter)
}
}
private const val TAG = "LifePresenter"
class LifePresenter : LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
fun onResume(lifecycleOwner: LifecycleOwner) {
Log.i(TAG, "onResume() called with: lifecycleOwner = $lifecycleOwner")
checkConfig(object : CheckResultCallback {
override fun success() {
if (lifecycleOwner.lifecycle.currentState.isAtLeast(Lifecycle.State.STARTED)) {
Log.i(TAG, "onResume: success to work")
work()
}
}
})
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
fun onStop() {
Log.i(TAG, "onStop: ")
}
private fun checkConfig(callback: CheckResultCallback) {
Log.i(TAG, "checkConfig: ")
// TODO: check username and password, need 500ms
callback.success()
}
private fun work() {
Log.i(TAG, "work: ")
}
}
LifePresenter 实现了接口 LifecycleObserver,LifecycleObserver 用于标记一个类是生命周期的观察者。然后在具体的方法上面,添加上 @OnLifecycleEvent注解,便可以 follow 生命周期拥有者(Activity)的生命周期回调。ps: 方法的参数可以选配一个 LifecycleOwner 类型的参数; 如果 Event 是 ON_ANY, 还可以接受一个 Event 类型的参数。
打开应用并退出,观察 log 输出:
LifePresenter: onResume()
LifePresenter: checkConfig:
LifePresenter: onResume: success to work
LifePresenter: work:
LifePresenter: onStop:
2.3 自定义 LifecycleOwner
- 支持库 26.1.0 及更高版本中的 Fragment 和 Activity 已实现
LifecycleOwner
接口。 - 如果有一个自定义类并希望使其成为
LifecycleOwner
,可以使用LifecycleRegistry
类,但需要将事件转发到该类,如以下代码示例中所示:
class UserDefineActivity : Activity(), LifecycleOwner {
private lateinit var lifecycleRegistry: LifecycleRegistry
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_user_define)
lifecycleRegistry = LifecycleRegistry(this)
lifecycleRegistry.currentState = Lifecycle.State.CREATED
}
override fun onStart() {
super.onStart()
lifecycleRegistry.currentState = Lifecycle.State.STARTED
}
override fun getLifecycle(): Lifecycle {
return lifecycleRegistry
}
}
需要调用 setCurrentState()
来设置生命周期状态。
2.4 Application 生命周期
- Lifecycle 也为 Application 提供了生命周期的拥有者:ProcessLifecycleOwner。
- 需要引入独立依赖:
lifecycle-process
- ProcessLifecycleOwner 依然是通过计数器来维护 Application 的生命周期,获取前后台切换状态。
- 使用方式如下:
private const val TAG = "MyApplication"
class MyApplication : Application() {
override fun onCreate() {
super.onCreate()
ProcessLifecycleOwner.get().lifecycle.addObserver(ApplicationLifecycleObserver)
}
object ApplicationLifecycleObserver : LifecycleObserver {
@OnLifecycleEvent(Lifecycle.Event.ON_START)
private fun onFront() {
Log.i(TAG, "onFront: ")
}
@OnLifecycleEvent(Lifecycle.Event.ON_STOP)
private fun onBack() {
Log.i(TAG, "onBack: ")
}
}
}
3. 关系
3.1 类关系
3.2 State & Event 对应关系
官网对照图:状态,事件,流向
私房图:状态,事件,流向,升级,降级(利于辅助阅读源码)
3.3 调用时序关系
4. 原理
4.1 角色扮演
- 观察者:LifecycleObserver 接口。通过 Lifecycle 的
addObserver()
注册监听。实现该接口的类使用注解方式,会被反射生成对应类,在生命周期变化时,调用对应类的方法,从而使得实现了该接口的类可以收到 生命周期事件的回调。 - 持有者:LifecycleOwner 接口。关联了 Lifecycle(核心实现在 LifecycleRegistry 中),LifecycleOwner 可以添加观察者(LifecycleObserver),在生命周期变更时,通过到观察者。
- 分发者:ReportFragment 。接受 AMS 侧的生命周期事件,并分发到 LifecycleRegistry。
- 定义者:Lifecycle 。生命状态和生命周期事件集合定义。
- 同步者:LifecycleRegistry 。接受新的生命周期事件, 与之前的留存状态对比,进行同步,然后分发到 观察者(LifecycleObserver)。
4.2 持有者 LifecycleOwner
AppCompatActivity 的基类 ComponentActivity 实现了 LifecycleOwner,关联了 LifecycleRegistry。
public class ComponentActivity extends androidx.core.app.ComponentActivity implements
LifecycleOwner, //持有者
ViewModelStoreOwner,
SavedStateRegistryOwner,
OnBackPressedDispatcherOwner {
//同步者
private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
@NonNull
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
//分发者
ReportFragment.injectIfNeededIn(this);
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}
}
4.3 分发者 ReportFragment
ReportFragment 统筹 AMS 侧的生命周期分发,最终会调用到 dispatch()
,调用到同步者(LifecycleRegistry)。
public class ReportFragment extends android.app.Fragment {
public static void injectIfNeededIn(Activity activity) {
//版本 不低于 29, 使用 ActivityLifecycleCallbacks 是进行分发
if (Build.VERSION.SDK_INT >= 29) {
// On API 29+, we can register for the correct Lifecycle callbacks directly
LifecycleCallbacks.registerIn(activity);
}
// 添加空的 Fragment(ReportFragment)
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();
}
}
//分发到 同步者(LifecycleRegistry)
@SuppressWarnings("deprecation")
static void dispatch(@NonNull Activity activity, @NonNull Lifecycle.Event event) {
//向前兼容,LifecycleRegistryOwner已不在使用
if (activity instanceof LifecycleRegistryOwner) {
((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
return;
}
//进行分发
if (activity instanceof LifecycleOwner) {
Lifecycle lifecycle = ((LifecycleOwner) activity).getLifecycle();
if (lifecycle instanceof LifecycleRegistry) {
((LifecycleRegistry) lifecycle).handleLifecycleEvent(event);
}
}
}
//Fragment 生命周期事件分发
@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);
}
//省略其他生命周期分发代码
//Fragment 生命周期分发,版本低于 29 才会执行
private void dispatch(@NonNull Lifecycle.Event event) {
if (Build.VERSION.SDK_INT < 29) {
// Only dispatch events from ReportFragment on API levels prior
// to API 29. On API 29+, this is handled by the ActivityLifecycleCallbacks
// added in ReportFragment.injectIfNeededIn
dispatch(getActivity(), event);
}
}
@RequiresApi(29)
static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {
//LifecycleCallbacks 注册监听
static void registerIn(Activity activity) {
activity.registerActivityLifecycleCallbacks(new LifecycleCallbacks());
}
//LifecycleCallbacks 生命周期事件分发
@Override
public void onActivityPostCreated(@NonNull Activity activity,
@Nullable Bundle savedInstanceState) {
dispatch(activity, Lifecycle.Event.ON_CREATE);
}
@Override
public void onActivityPostStarted(@NonNull Activity activity) {
dispatch(activity, Lifecycle.Event.ON_START);
}
//省略其他生命周期分发代码
}
}
4.4 定义者 Lifecycle
Lifecycle 包含生命周期事件和状态。
public abstract class Lifecycle {
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);
@MainThread
@NonNull
public abstract State getCurrentState();
//生命周期事件
public enum Event {
ON_CREATE,
ON_START,
ON_RESUME,
ON_PAUSE,
ON_STOP,
ON_DESTROY,
ON_ANY;
//降级使用
@Nullable
public static Event downFrom(@NonNull State state) {
switch (state) {
case CREATED:
return ON_DESTROY;
case STARTED:
return ON_STOP;
case RESUMED:
return ON_PAUSE;
default:
return null;
}
}
//可以忽略
@Nullable
public static Event downTo(@NonNull State state) {
switch (state) {
case DESTROYED:
return ON_DESTROY;
case CREATED:
return ON_STOP;
case STARTED:
return ON_PAUSE;
default:
return null;
}
}
//升级使用
@Nullable
public static Event upFrom(@NonNull State state) {
switch (state) {
case INITIALIZED:
return ON_CREATE;
case CREATED:
return ON_START;
case STARTED:
return ON_RESUME;
default:
return null;
}
}
//可以忽略
@Nullable
public static Event upTo(@NonNull State state) {
switch (state) {
case CREATED:
return ON_CREATE;
case STARTED:
return ON_START;
case RESUMED:
return ON_RESUME;
default:
return null;
}
}
//获取事件对应状态
@NonNull
public State getTargetState() {
switch (this) {
case ON_CREATE:
case ON_STOP:
return State.CREATED;
case ON_START:
case ON_PAUSE:
return State.STARTED;
case ON_RESUME:
return State.RESUMED;
case ON_DESTROY:
return State.DESTROYED;
case ON_ANY:
break;
}
throw new IllegalArgumentException(this + " has no target state");
}
}
//状态(状态越来越大)
@SuppressWarnings("WeakerAccess")
public enum State {
DESTROYED,
INITIALIZED,
CREATED,
STARTED,
RESUMED;
//状态不低于指定状态
public boolean isAtLeast(@NonNull State state) {
return compareTo(state) >= 0;
}
}
}
还是针对 Event & State 的关系图来说明。
事件:这个大家很好理解,就是我们常见的生命周期。
状态:聚合了生命周期的事件,给出的优先级状态。比如我们只会在 RESUMED 状态处理一些显示,在 CREATE 状态就要关闭一些后台处理。
状态升级:对应方法 upFrom()
,计算从当前状态向上升一级,对应什么事件,然后进行分发。比如:当前状态是 STARTED,状态升级,调用 upFrom()
计算出来的事件是 ON_RESUME。
状态降级:对应方法 downFrom()
,计算从当前状态向下降一级,对应什么事件,然后进行分发。比如:当前状态是 STARTED,状态降级,调用 downFrom()
计算出来的事件是 ON_STOP。
前提: 事件,状态,升级,降级。理解了这些,才能真正理解下面的状态同步处理。
4.5 同步者 LifecycleRegistry
继承自 Lifecycle。接收了分发者(ReportFragment)的事件分发。进行状态的同步和对应的生命周期事件分发给观察者(继承了 LifecycleObserver 的类)。
public class LifecycleRegistry extends Lifecycle {
//接受了分发过来的生命周期事件
public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
enforceMainThreadIfNeeded("handleLifecycleEvent");
moveToState(event.getTargetState());
}
//状态跳转判断,以便安全同步
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;
}
//是否已经同步完成
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;
}
//同步处理
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.
//如果目标状态比历史状态底,那么就要状态降级,调用 backwardPass
if (mState.compareTo(mObserverMap.eldest().getValue().mState) < 0) {
backwardPass(lifecycleOwner);
}
Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
//如果目标状态比历史状态高,那么就要状态升级,调用 forwardPass
if (!mNewEventOccurred && newest != null
&& mState.compareTo(newest.getValue().mState) > 0) {
forwardPass(lifecycleOwner);
}
}
mNewEventOccurred = false;
}
//状态升级
private void forwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> ascendingIterator =
mObserverMap.iteratorWithAdditions();
//遍历所有所有观察者,进行状态升级
while (ascendingIterator.hasNext() && !mNewEventOccurred) {
Map.Entry<LifecycleObserver, ObserverWithState> entry = ascendingIterator.next();
ObserverWithState observer = entry.getValue();
//如果状态升级需要跨级,那么就需要一层一层去回调
while ((observer.mState.compareTo(mState) < 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
pushParentState(observer.mState);
//状态升级计算
final Event event = Event.upFrom(observer.mState);
if (event == null) {
throw new IllegalStateException("no event up from " + observer.mState);
}
//每调整一层状态,就需要对观察者分发一次
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
//状态降级
private void backwardPass(LifecycleOwner lifecycleOwner) {
Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
mObserverMap.descendingIterator();
//遍历所有所有观察者,进行状态降级
while (descendingIterator.hasNext() && !mNewEventOccurred) {
Map.Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
ObserverWithState observer = entry.getValue();
//如果状态降级需要跨级,那么就需要一层一层去回调
while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
&& mObserverMap.contains(entry.getKey()))) {
//状态降级计算
Event event = Event.downFrom(observer.mState);
if (event == null) {
throw new IllegalStateException("no event down from " + observer.mState);
}
pushParentState(event.getTargetState());
//每调整一层状态,就需要对观察者分发一次
observer.dispatchEvent(lifecycleOwner, event);
popParentState();
}
}
}
//这个类的处理,会在后面单独说明
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
ObserverWithState(LifecycleObserver observer, State initialState) {
//反射获取 LifecycleEventObserver 的实例
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = event.getTargetState();
mState = min(mState, newState);
//调用观察者回调方法
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
}
4.6 观察者 LifecycleObserver
继承了 LifecycleObserver 接口的实现类(添加了 @OnLifecycleEvent 的注解)。通过 Lifecycle.addObserver()
的添加,会调用到 LifecycleRegistry.addObserver()
方法,会创建 ObserverWithState 类,也会进行状态校准,进行同步分发。当然,我们本次主要来分析 ObserverWithState。
4.6.1 ObserverWithState 生成
LifecycleRegistry.java
static class ObserverWithState {
State mState;
LifecycleEventObserver mLifecycleObserver;
//反射获取 LifecycleEventObserver 的实例
ObserverWithState(LifecycleObserver observer, State initialState) {
mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
mState = initialState;
}
void dispatchEvent(LifecycleOwner owner, Event event) {
State newState = event.getTargetState();
mState = min(mState, newState);
//调用观察者回调方法
mLifecycleObserver.onStateChanged(owner, event);
mState = newState;
}
}
4.6.2 LifecycleEventObserver 生成
Lifecycling.java
@NonNull
static LifecycleEventObserver lifecycleEventObserver(Object object) {
boolean isLifecycleEventObserver = object instanceof LifecycleEventObserver;
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();
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);
}
//上面处理流程忽略,直接到要生成的具体类
return new ReflectiveGenericLifecycleObserver(object);
}
4.6.3 ReflectiveGenericLifecycleObserver 生成
ReflectiveGenericLifecycleObserver.java
class ReflectiveGenericLifecycleObserver implements LifecycleEventObserver {
private final Object mWrapped;
private final CallbackInfo mInfo;
ReflectiveGenericLifecycleObserver(Object wrapped) {
mWrapped = wrapped;
// CallbackInfo 生成
mInfo = ClassesInfoCache.sInstance.getInfo(mWrapped.getClass());
}
//事件分发调用
@Override
public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Event event) {
//回调方法调用
mInfo.invokeCallbacks(source, event, mWrapped);
}
}
4.6.4 CallbackInfo 生成 & 事件分发调用
ClassesInfoCache.java
CallbackInfo getInfo(Class<?> klass) {
CallbackInfo existing = mCallbackMap.get(klass);
if (existing != null) {
return existing;
}
//创建 CallbackInfo
existing = createInfo(klass, null);
return existing;
}
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);
}
}
//注解方法
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;
//注解方法参数
Class<?>[] params = method.getParameterTypes();
int callType = CALL_TYPE_NO_ARG;
//如果有参数,第一个参数的类型不是 LifecycleOwner,那么报非法异常
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();
//如果存在第二个参数的话
if (params.length > 1) {
callType = CALL_TYPE_PROVIDER_WITH_EVENT;
//第二个参数,如果不是 Lifecycle.Event 类型,那么报异常
if (!params[1].isAssignableFrom(Lifecycle.Event.class)) {
throw new IllegalArgumentException(
"invalid parameter type. second arg must be an event");
}
//第二个参数,如果是 Lifecycle.Event 类型,当方法注解不是 ON_ANY 时,报异常
if (event != Lifecycle.Event.ON_ANY) {
throw new IllegalArgumentException(
"Second arg is supported only for ON_ANY value");
}
}
//多余两个参数,直接报异常
if (params.length > 2) {
throw new IllegalArgumentException("cannot have more than 2 params");
}
//创建类型和方法
MethodReference methodReference = new MethodReference(callType, method);
//缓存校验与设置
verifyAndPutHandler(handlerToEvent, methodReference, event, klass);
}
//创建 CallbackInfo
CallbackInfo info = new CallbackInfo(handlerToEvent);
mCallbackMap.put(klass, info);
mHasLifecycleMethods.put(klass, hasLifecycleMethods);
return info;
}
@SuppressWarnings("WeakerAccess")
static class CallbackInfo {
final Map<Lifecycle.Event, List<MethodReference>> mEventToHandlers;
final Map<MethodReference, Lifecycle.Event> mHandlerToEvent;
CallbackInfo(Map<MethodReference, Lifecycle.Event> handlerToEvent) {
mHandlerToEvent = handlerToEvent;
mEventToHandlers = new HashMap<>();
for (Map.Entry<MethodReference, Lifecycle.Event> entry : handlerToEvent.entrySet()) {
Lifecycle.Event event = entry.getValue();
List<MethodReference> methodReferences = mEventToHandlers.get(event);
if (methodReferences == null) {
methodReferences = new ArrayList<>();
mEventToHandlers.put(event, methodReferences);
}
methodReferences.add(entry.getKey());
}
}
//调用事件
@SuppressWarnings("ConstantConditions")
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);
}
//调用事件
private static void invokeMethodsForEvent(List<MethodReference> handlers,
LifecycleOwner source, Lifecycle.Event event, Object mWrapped) {
if (handlers != null) {
for (int i = handlers.size() - 1; i >= 0; i--) {
//方法调用
handlers.get(i).invokeCallback(source, event, mWrapped);
}
}
}
}
@SuppressWarnings("WeakerAccess")
static final class MethodReference {
final int mCallType;
final Method mMethod;
MethodReference(int callType, Method method) {
mCallType = callType;
mMethod = method;
mMethod.setAccessible(true);
}
//方法调用
void invokeCallback(LifecycleOwner source, Lifecycle.Event event, Object target) {
//noinspection TryWithIdenticalCatches
try {
//方法的参数调用类型,总共三种
switch (mCallType) {
case CALL_TYPE_NO_ARG:
mMethod.invoke(target);
break;
case CALL_TYPE_PROVIDER:
mMethod.invoke(target, source);
break;
case CALL_TYPE_PROVIDER_WITH_EVENT:
mMethod.invoke(target, source, event);
break;
}
} catch (InvocationTargetException e) {
throw new RuntimeException("Failed to call observer method", e.getCause());
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
}
}
}
5.小结
- 使界面控制器(Activity 和 Fragment)尽可能保持精简。它们不应试图获取自己的数据,而应使用
ViewModel
执行此操作,并观察LiveData
对象以将更改体现到视图中。 - 设法编写数据驱动型界面,对于此类界面,界面控制器的责任是随着数据更改而更新视图,或者将用户操作通知给
ViewModel
。 - 将数据逻辑放在
ViewModel
类中。ViewModel
应充当界面控制器与应用其余部分之间的连接器。不过要注意,ViewModel
不负责获取数据(例如,从网络获取)。但是,ViewModel
应调用相应的组件来获取数据,然后将结果提供给界面控制器。 - 使用
DataBinding
在视图与界面控制器之间维持干净的接口。这样一来,可以使视图更具声明性,并尽量减少需要在 Activity 和 Fragment 中编写的更新代码。 - 如果界面很复杂,不妨考虑创建 presenter 类来处理界面的修改。这可能是一项艰巨的任务,但这样做可使界面组件更易于测试。
- 避免在
ViewModel
中引用View
或Activity
上下文。如果ViewModel
存在的时间比 Activity 更长(在配置更改的情况下),Activity 将泄漏并且不会获得垃圾回收器的妥善处置。