在《Android开发之深度项目设计探索（一）》 这篇文章中，主要描述了深度项目架构设计会涉及到的一些知识点，那么这篇文章主要介绍的是 RxLifecycle 使用及源码分析。
RxLifecycle ：
在Android进行面试的时候，经常会被问到的就是Android的内存泄漏、泄漏场景、泄漏原因以及解决办法。如果你提到了Rxjava2，面试官可能会接连发问Rxjava2在使用过程中会有一些什么问题？这时就可以谈到Rxjava如果在一些特定场景没有及时解绑订阅可能会导致内存泄漏。
泄漏的原因是：当RxJava发布一个订阅后，此时页面执行了finish的生命周期，但订阅逻辑还未完成。如果没有及时取消订阅，就会导致Activity/Fragment无法被回收，从而引发内存泄漏。但是相应的，Rxjava系列也提供了解决办法，那就是使用RxLifecycle 。
通过官方文档了解发现，RxLifecycle库的使用和集成也是简单和快捷的
A：RxLifecycle对应的依赖（最新版本）
// 这是RxLifecycle最主要的依赖
implementation 'com.trello.rxlifecycle2:rxlifecycle:2.2.2'
// If you want to bind to Android-specific lifecycles
//如果你想绑定Android的生命周期
implementation 'com.trello.rxlifecycle2:rxlifecycle-android:2.2.2'
// If you want pre-written Activities and Fragments you can subclass as providers
//如果想在预先写好的Activities and Fragments，你可以作为其父类的子类
implementation 'com.trello.rxlifecycle2:rxlifecycle-components:2.2.2'
// If you want pre-written support preference Fragments you can subclass as providers
//如果想在预先支持的 Fragments，你可以作为其父类的子类
implementation 'com.trello.rxlifecycle2:rxlifecycle-components-preference:2.2.2'
// If you want to use Navi for providers
// Android使用的库，继承NaviActivity使用
implementation 'com.trello.rxlifecycle2:rxlifecycle-navi:2.2.2'
// If you want to use Android Lifecycle for providers
//如果你想为其提供Android生命周期
implementation 'com.trello.rxlifecycle2:rxlifecycle-android-lifecycle:2.2.2'
// If you want to use Kotlin syntax
//如果你想使用Kotlin语法
implementation 'com.trello.rxlifecycle2:rxlifecycle-kotlin:2.2.2'
// If you want to use Kotlin syntax with Android Lifecycle
//如果你想使用Kotlin语法作用在Android生命周期上
implementation 'com.trello.rxlifecycle2:rxlifecycle-android-lifecycle-kotlin:2.2.2'
下面是一些基本的使用和说明：
首先是 绑定生命周期：
myObservable.compose(RxLifecycle.bind(lifecycle)).subscribe();
接着是 绑定特殊的生命周期（onStart onStop等等）:
myObservable.compose(RxLifecycle.bindUntilEvent(lifecycle, ActivityEvent.DESTROY))
.subscribe();
然后是 通过RxLifecycle在适当的时间来结束：
myObservable.compose(RxLifecycleAndroid.bindActivity(lifecycle)).subscribe();
通过文档我们可以知道：如果在onStart( )这个生命周期里面绑定，它就会在onStop（）这个终止绑定；如果我们在onPause( )之后订阅， 那么它就会在其下一个生命周期终止绑定，（在onPause( )之后订阅，因为onStop（）是其下一个生命周期，如果Activity执行到了这里，就会解绑）
下面上一段代码加深RxLifecycle的理解和使用：
public class LifeActivity extends RxAppCompatActivity{
@Override
protected void onStart() {
super.onStart();
Observable.interval(2, TimeUnit.SECONDS)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.compose(this.bindToLifecycle())
.subscribe();
}
}
理论上我们需要使用compose操作符，然后加上bindToLifecycle即可完成简单的使用（注意：这里的 LifeActivity继承的是RxAppCompatActivity）看到了这里，可能疑惑的是，RxLifecycle具体的绑定该如何理解？里面的生命周期又是如何操作的？
是这样，这里的生命周期是由 LifecycleProvider<T> 来提供的，实现的方式有以下四种：
1：继承父类，RxActivity, RxFragment
2：使用 Navi 以及 Rxlifecycle-navi
3：使用 Android's lifecycle
4：自己编写实现
首先看第一种：也就是上面的代码，直接继承RxAppCompatActivity，然后点开RxAppCompatActivity源码会发现很多事情，这个后面说。
第二种：继承NaviActivity（需要导入上面提供的Navi对应的依赖）下面是参考代码
public class NaviLifeActivity extends NaviActivity{
private final LifecycleProvider<ActivityEvent> provider
= NaviLifecycle.createActivityLifecycleProvider(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
initLifecycle();
}
private void initLifecycle(){
Observable.interval(2, TimeUnit.SECONDS)
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.compose(provider.bindToLifecycle())
.subscribe();
}
}
第三种和第四种由于涉及到另外的内容这里就不描述了。
值得一提的是：compose方法需在subscribeOn方法之后使用。
RxLifecycle源码分析：
说到源码，我们首先进入RxAppCompatActivity看看，因为compose(this.bindToLifecycle()) 这句API里面的this，代表的就是当前继承了RxAppCompatActivity的子类，奔着研究分析的精神，下面是
RxAppCompatActivity 的源码：
public abstract class RxAppCompatActivity extends AppCompatActivity implements LifecycleProvider<ActivityEvent> {
private final BehaviorSubject<ActivityEvent> lifecycleSubject = BehaviorSubject.create();
@Override
@NonNull
@CheckResult
public final Observable<ActivityEvent> lifecycle() {
return lifecycleSubject.hide();
}
@Override
@NonNull
@CheckResult
public final <T> LifecycleTransformer<T> bindUntilEvent(@NonNull ActivityEvent event) {
return RxLifecycle.bindUntilEvent(lifecycleSubject, event);
}
@Override
@NonNull
@CheckResult
public final <T> LifecycleTransformer<T> bindToLifecycle() {
return RxLifecycleAndroid.bindActivity(lifecycleSubject);
}
@Override
@CallSuper
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
lifecycleSubject.onNext(ActivityEvent.CREATE);
}
@Override
@CallSuper
protected void onStart() {
super.onStart();
lifecycleSubject.onNext(ActivityEvent.START);
}
@Override
@CallSuper
protected void onResume() {
super.onResume();
lifecycleSubject.onNext(ActivityEvent.RESUME);
}
@Override
@CallSuper
protected void onPause() {
lifecycleSubject.onNext(ActivityEvent.PAUSE);
super.onPause();
}
@Override
@CallSuper
protected void onStop() {
lifecycleSubject.onNext(ActivityEvent.STOP);
super.onStop();
}
@Override
@CallSuper
protected void onDestroy() {
lifecycleSubject.onNext(ActivityEvent.DESTROY);
super.onDestroy();
}
}
可以看到在Activity具体生命周期里面进行了对应的事件发送！
1：BehaviorSubject与ActivityEvent
RxAppCompatActivity源码中内部第一行代码是创建了一个BehaviorSubject实例对象，里面的泛型是ActivityEvent，首先看下 ActivityEvent ，
/**
* Lifecycle events that can be emitted by Activities.
*/
public enum ActivityEvent {
CREATE,
START,
RESUME,
PAUSE,
STOP,
DESTROY
}
哦，细心的你一眼就看到了这是个枚举，里面的value对应的就是Activity生命周期的那些生命周期（因为在生命周期里面进行了发射嘛）；那么，什么是BehaviorSubject？BehaviorSubject的由于源码较多这里就不贴出来了，这个类它继承了Subject<T>，那 Subject<T> 又是什么？源码如下：
public abstract class Subject<T> extends Observable<T> implements Observer<T> {
/**
* Returns true if the subject has any Observers.
* <p>The method is thread-safe.
* @return true if the subject has any Observers
*/
public abstract boolean hasObservers();
/**
* Returns true if the subject has reached a terminal state through an error event.
* <p>The method is thread-safe.
* @return true if the subject has reached a terminal state through an error event
* @see #getThrowable()
* @see #hasComplete()
*/
public abstract boolean hasThrowable();
/**
* Returns true if the subject has reached a terminal state through a complete event.
* <p>The method is thread-safe.
* @return true if the subject has reached a terminal state through a complete event
* @see #hasThrowable()
*/
public abstract boolean hasComplete();
/**
* Returns the error that caused the Subject to terminate or null if the Subject
* hasn't terminated yet.
* <p>The method is thread-safe.
* @return the error that caused the Subject to terminate or null if the Subject
* hasn't terminated yet
*/
@Nullable
public abstract Throwable getThrowable();
/**
* Wraps this Subject and serializes the calls to the onSubscribe, onNext, onError and
* onComplete methods, making them thread-safe.
* <p>The method is thread-safe.
* @return the wrapped and serialized subject
*/
@NonNull
public final Subject<T> toSerialized() {
if (this instanceof SerializedSubject) {
return this;
}
return new SerializedSubject<T>(this);
}
}
哦，上帝，Subject继承了Observable、又实现了Observer接口，这也就说明Subject即可作为被观察者，也可以作为观察者。同理，他的子类BehaviorSubject也具备同样的功能。
下面是关于BehaviorSubject操作符的一张图：
BehaviorSubject
BehaviorSubject的简单理解就是，发送离订阅最近的上一个值，没有上一个值的时候会发送默认值，接下来（如果有数据）则继续发射原始Observable的数据。
2：bindUntilEvent( ActivityEvent event) 、LifecycleTransformer、takeUntil
在RxAppCompatActivity 源码中我们看到了bindUntilEvent（）这个方法，最终的的返回值类型是 LifecycleTransformer 那这个LifecycleTransformer又是什么？
public final class LifecycleTransformer<T> implements ObservableTransformer<T, T>,
FlowableTransformer<T, T>,
SingleTransformer<T, T>,
MaybeTransformer<T, T>,
CompletableTransformer
{
final Observable<?> observable;
LifecycleTransformer(Observable<?> observable) {
checkNotNull(observable, "observable == null");
this.observable = observable;
}
@Override
public ObservableSource<T> apply(Observable<T> upstream) {
return upstream.takeUntil(observable);
}
//......
}
经过源码可以发现，这个LifecycleTransformer实现了大量的XXXTransformer，内部的方法大都使用了apply方法，这个方法实则是调用了takeUntil操作符，那么这个takeUntil操作符是什么意思？
takeUntil
takeUntil操作符简单理解就是，当第二个Observable发射了一项数据或者终止时，丢弃原Observable发射的任何数据。注意：这里是满足条件丢弃任何发送的数据（该操作符的应用场景是不是，在onDestroy( )里面取消订阅，解决内存泄漏的隐患）
3：bindToLifecycle（）
我们知道，绑定的API是bindToLifecycle（）这一行代码，那么我们就抽丝剥茧，根据RxAppCompatActivity源码得知，bindToLifecycle（）这行代码最终返回了 RxLifecycleAndroid.bindActivity(lifecycleSubject)，源码跟进，进入到了RxLifecycleAndroid，下面是 RxLifecycleAndroid 源码：
public class RxLifecycleAndroid {
private RxLifecycleAndroid() {
throw new AssertionError("No instances");
}
/**
* Binds the given source to an Activity lifecycle.
* <p>
* This helper automatically determines (based on the lifecycle sequence itself) when the source
* should stop emitting items. In the case that the lifecycle sequence is in the
* creation phase (CREATE, START, etc) it will choose the equivalent destructive phase (DESTROY,
* STOP, etc). If used in the destructive phase, the notifications will cease at the next event;
* for example, if used in PAUSE, it will unsubscribe in STOP.
* <p>
* Due to the differences between the Activity and Fragment lifecycles, this method should only
* be used for an Activity lifecycle.
*
* @param lifecycle the lifecycle sequence of an Activity
* @return a reusable {@link LifecycleTransformer} that unsubscribes the source during the Activity lifecycle
*/
@NonNull
@CheckResult
public static <T> LifecycleTransformer<T> bindActivity(@NonNull final Observable<ActivityEvent> lifecycle) {
return bind(lifecycle, ACTIVITY_LIFECYCLE);
}
/**
* Binds the given source to a Fragment lifecycle.
* <p>
* This helper automatically determines (based on the lifecycle sequence itself) when the source
* should stop emitting items. In the case that the lifecycle sequence is in the
* creation phase (CREATE, START, etc) it will choose the equivalent destructive phase (DESTROY,
* STOP, etc). If used in the destructive phase, the notifications will cease at the next event;
* for example, if used in PAUSE, it will unsubscribe in STOP.
* <p>
* Due to the differences between the Activity and Fragment lifecycles, this method should only
* be used for a Fragment lifecycle.
*
* @param lifecycle the lifecycle sequence of a Fragment
* @return a reusable {@link LifecycleTransformer} that unsubscribes the source during the Fragment lifecycle
*/
@NonNull
@CheckResult
public static <T> LifecycleTransformer<T> bindFragment(@NonNull final Observable<FragmentEvent> lifecycle) {
return bind(lifecycle, FRAGMENT_LIFECYCLE);
}
/**
* Binds the given source to a View lifecycle.
* <p>
* Specifically, when the View detaches from the window, the sequence will be completed.
* <p>
* Warning: you should make sure to use the returned Transformer on the main thread,
* since we're binding to a View (which only allows binding on the main thread).
*
* @param view the view to bind the source sequence to
* @return a reusable {@link LifecycleTransformer} that unsubscribes the source during the View lifecycle
*/
@NonNull
@CheckResult
public static <T> LifecycleTransformer<T> bindView(@NonNull final View view) {
checkNotNull(view, "view == null");
return bind(Observable.create(new ViewDetachesOnSubscribe(view)));
}
// Figures out which corresponding next lifecycle event in which to unsubscribe, for Activities
private static final Function<ActivityEvent, ActivityEvent> ACTIVITY_LIFECYCLE =
new Function<ActivityEvent, ActivityEvent>() {
@Override
public ActivityEvent apply(ActivityEvent lastEvent) throws Exception {
switch (lastEvent) {
case CREATE:
return ActivityEvent.DESTROY;
case START:
return ActivityEvent.STOP;
case RESUME:
return ActivityEvent.PAUSE;
case PAUSE:
return ActivityEvent.STOP;
case STOP:
return ActivityEvent.DESTROY;
case DESTROY:
throw new OutsideLifecycleException("Cannot bind to Activity lifecycle when outside of it.");
default:
throw new UnsupportedOperationException("Binding to " + lastEvent + " not yet implemented");
}
}
};
// Figures out which corresponding next lifecycle event in which to unsubscribe, for Fragments
private static final Function<FragmentEvent, FragmentEvent> FRAGMENT_LIFECYCLE =
new Function<FragmentEvent, FragmentEvent>() {
@Override
public FragmentEvent apply(FragmentEvent lastEvent) throws Exception {
switch (lastEvent) {
case ATTACH:
return FragmentEvent.DETACH;
case CREATE:
return FragmentEvent.DESTROY;
case CREATE_VIEW:
return FragmentEvent.DESTROY_VIEW;
case START:
return FragmentEvent.STOP;
case RESUME:
return FragmentEvent.PAUSE;
case PAUSE:
return FragmentEvent.STOP;
case STOP:
return FragmentEvent.DESTROY_VIEW;
case DESTROY_VIEW:
return FragmentEvent.DESTROY;
case DESTROY:
return FragmentEvent.DETACH;
case DETACH:
throw new OutsideLifecycleException("Cannot bind to Fragment lifecycle when outside of it.");
default:
throw new UnsupportedOperationException("Binding to " + lastEvent + " not yet implemented");
}
}
};
}
嗯，首先看到bindActivity(final Observable<ActivityEvent> lifecycle)这个方法，这个方法最终返回了bind(lifecycle, ACTIVITY_LIFECYCLE);
请注意，前方高能： ACTIVITY_LIFECYCLE，这个ACTIVITY_LIFECYCLE的源码也是上面的截图，可以看到通过switch语句进行条件筛选最终返回ActivityEvent这个枚举里面的属性值（仔细观看的话源码下面还有Fragment对应的生命周期），可能你会问，为什么这是对应的生命周期？让我们首先看回bind(lifecycle, ACTIVITY_LIFECYCLE); 这个方法，点进去看，进入到了 RxLifecycle 这个类，源码如下：
public class RxLifecycle {
private RxLifecycle() {
throw new AssertionError("No instances");
}
/**
* Binds the given source to a lifecycle.
* <p>
* When the lifecycle event occurs, the source will cease to emit any notifications.
*
* @param lifecycle the lifecycle sequence
* @param event the event which should conclude notifications from the source
* @return a reusable {@link LifecycleTransformer} that unsubscribes the source at the specified event
*/
@Nonnull
@CheckReturnValue
public static <T, R> LifecycleTransformer<T> bindUntilEvent(@Nonnull final Observable<R> lifecycle,
@Nonnull final R event) {
checkNotNull(lifecycle, "lifecycle == null");
checkNotNull(event, "event == null");
return bind(takeUntilEvent(lifecycle, event));
}
private static <R> Observable<R> takeUntilEvent(final Observable<R> lifecycle, final R event) {
return lifecycle.filter(new Predicate<R>() {
@Override
public boolean test(R lifecycleEvent) throws Exception {
return lifecycleEvent.equals(event);
}
});
}
/**
* Binds the given source to a lifecycle.
* <p>
* This helper automatically determines (based on the lifecycle sequence itself) when the source
* should stop emitting items. Note that for this method, it assumes <em>any</em> event
* emitted by the given lifecycle indicates that the lifecycle is over.
*
* @param lifecycle the lifecycle sequence
* @return a reusable {@link LifecycleTransformer} that unsubscribes the source whenever the lifecycle emits
*/
@Nonnull
@CheckReturnValue
public static <T, R> LifecycleTransformer<T> bind(@Nonnull final Observable<R> lifecycle) {
return new LifecycleTransformer<>(lifecycle);
}
/**
* Binds the given source to a lifecycle.
* <p>
* This method determines (based on the lifecycle sequence itself) when the source
* should stop emitting items. It uses the provided correspondingEvents function to determine
* when to unsubscribe.
* <p>
* Note that this is an advanced usage of the library and should generally be used only if you
* really know what you're doing with a given lifecycle.
*
* @param lifecycle the lifecycle sequence
* @param correspondingEvents a function which tells the source when to unsubscribe
* @return a reusable {@link LifecycleTransformer} that unsubscribes the source during the Fragment lifecycle
*/
@Nonnull
@CheckReturnValue
public static <T, R> LifecycleTransformer<T> bind(@Nonnull Observable<R> lifecycle,
@Nonnull final Function<R, R> correspondingEvents) {
checkNotNull(lifecycle, "lifecycle == null");
checkNotNull(correspondingEvents, "correspondingEvents == null");
return bind(takeUntilCorrespondingEvent(lifecycle.share(), correspondingEvents));
}
private static <R> Observable<Boolean> takeUntilCorrespondingEvent(final Observable<R> lifecycle,
final Function<R, R> correspondingEvents) {
return Observable.combineLatest(
lifecycle.take(1).map(correspondingEvents),
lifecycle.skip(1),
new BiFunction<R, R, Boolean>() {
@Override
public Boolean apply(R bindUntilEvent, R lifecycleEvent) throws Exception {
return lifecycleEvent.equals(bindUntilEvent);
}
})
.onErrorReturn(Functions.RESUME_FUNCTION)
.filter(Functions.SHOULD_COMPLETE);
}
}
最终执行到了 RxLifecycle 这个类源码里面的最后的两个方法！
4：takeUntilCorrespondingEvent（）
takeUntilCorrespondingEvent这个方法内容较多，首先分析第一个，combineLatest操作符，这个操作符是指 将传进来的BehaviorSubject的事件进行了一次分割；假设我们现在在onCreate（）方法里面进行RxLifecycle的绑定，那么这里的lifecycle.take(1).map(correspondingEvents)简单理解其对应的代码就是 RxLifecycleAndroid源码 里面的ACTIVITY_LIFECYCLE 中的，
case CREATE:
return ActivityEvent.DESTROY;
另外，lifecycle.skip(1)意味着除去第一个（ActivityEvent.CREATE），保留剩下的ActivityEvent枚举值
new BiFunction<R, R, Boolean>这行代码是对属性值进行判断：对比结果是
false,false,fasle,false,true
最后，onErrorReturn和filter这两行代码分别是对异常的处理以及判断是否应该结束订阅、
final class Functions {
static final Function<Throwable, Boolean> RESUME_FUNCTION = new Function<Throwable, Boolean>() {
@Override
public Boolean apply(Throwable throwable) throws Exception {
if (throwable instanceof OutsideLifecycleException) {
return true;
}
//noinspection ThrowableResultOfMethodCallIgnored
Exceptions.propagate(throwable);
return false;
}
};
static final Predicate<Boolean> SHOULD_COMPLETE = new Predicate<Boolean>() {
@Override
public boolean test(Boolean shouldComplete) throws Exception {
return shouldComplete;
}
};
static final Function<Object, Completable> CANCEL_COMPLETABLE = new Function<Object, Completable>() {
@Override
public Completable apply(Object ignore) throws Exception {
return Completable.error(new CancellationException());
}
};
private Functions() {
throw new AssertionError("No instances!");
}
}
综上：本次订阅，当Activity走到onStart生命周期时，为false,订阅不会取消，直到onDestroy，为true，订阅取消
5：最后调用的方法
根据逻辑，上面最终调用了下面的代码：
public static <T, R> LifecycleTransformer<T> bind(@Nonnull final Observable<R> lifecycle) {
return new LifecycleTransformer<>(lifecycle);
}
可以看到，这里最终生成了一个LifecycleTransformer。
源码逻辑总结：
A：继承了RxAppCompatActivity的子类Activity,在每个生命的周期里，BehaviorSubject发射相对应的事件
B：bind方法主要是做赋值、匹配、判断
C：LifecycleTransformer内部进行takeUntil操作符，如果是true，就终止订阅，反之
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