Android中Lifecycle的原理详解

        lifecycle是android architecture components的成员，是一个生命周期感知组件，能够感知activity、fragment等组件的生命周期变化，并将变化通知到已注册的观察者。正确的使用有助于更好地组织代码，减少内存泄漏，增强稳定。下面分析他的实现原理，看看到底只怎么感知生命周期的。

一、基本使用

1、引入依赖库

dependencies {
    def lifecycle_version = "2.5.0-alpha01"
    def arch_version = "2.1.0"
 
    implementation "androidx.lifecycle:lifecycle-viewmodel:$lifecycle_version"
  
    implementation "androidx.lifecycle:lifecycle-livedata:$lifecycle_version"
 
    implementation "androidx.lifecycle:lifecycle-runtime:$lifecycle_version"
 
    //非java8使用
    annotationprocessor "androidx.lifecycle:lifecycle-compiler:$lifecycle_version" 
    //java8使用
    implementation "androidx.lifecycle:lifecycle-common-java8:$lifecycle_version"  
}

2、自定义一个类实现lifecycleobserver，在类中监听生命周期回调

public class basejsapi implements lifecycleobserver {
 
  
    @onlifecycleevent(lifecycle.event.on_create)
    void create() {
        oncreate();
    }
 
    @onlifecycleevent(lifecycle.event.on_destroy)
    void destroy() {
        ondestroy();
    }
 
 
    @onlifecycleevent(lifecycle.event.on_resume)
    void resume() {
        onresume();
    }
 
    @onlifecycleevent(lifecycle.event.on_pause)
    void pause() {
        onpause();
    }
 
    ……………………………………
 
  }

3、在activity或者fragment中注册。

 getlifecycle().addobserver(getjsbridge());

通过以上两步就可以感知当前activity的生命周期了，非常的简便实用。

二、lifecycleobserver接口和lifecycleowner接口

public interface lifecycleobserver {
 
}
 
public interface lifecycleowner {
  @nonnull
  lifecycle getlifecycle();
}

        lifecycleobserver是一个空接口，lifecycleowner提供getlifecycle返回lifecycle，该接口由componentactivity实现，提供了getlifecycle的实现。

三、getlifecycle()

        getlifecycle方法实际上是调用的componentactivity的getlifecycle方法，通过查看，activity确实也是继承的componentactivity，该方法返回mlifecycleregistry对象，该对象继承自lifecycle，具体实现了添加观察者，移出观察者的方法，还定义了一个生命周期的枚举。

public class lifecycleregistry extends lifecycle {
  
 
}
 
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();
 
    public enum event {
       
        on_create,
       
        on_start,
        
        on_resume,
        
        on_pause,
        
        on_stop,
        
        on_destroy,
       
        on_any;
        …………………………………………
      }
      ………………………………
}

        也就可以看出getlifecycle().addobserver实际上就是执行lifecycleregistry的addobserver的方法添加观察者，做一个小总结如下：

观察者实现lifecycleobserver，实现需要的生命周期。在activity、fragment中添通过getlifecycle().addobserver加观察者。实际上调用的是componentactivity的getlifecycle方法返回mlifecycleregistry。调用lifecycleregistry的addobserver加观察者。三、添加观察者的流程

        详情看注释：

@override
public void addobserver(@nonnull lifecycleobserver observer) {
    //检查是否在主线程
    enforcemainthreadifneeded("addobserver");
    //设置状态值
    state initialstate = mstate == destroyed ? destroyed : initialized;
    //将观察者和初始化的值封装到observerwithstate中
    observerwithstate statefulobserver = new observerwithstate(observer, initialstate);
    //以观察者为key，statefulobserver为value放到hashmap集合中
    observerwithstate previous = mobservermap.putifabsent(observer, statefulobserver);
    //不为null表示之前已经放进去了，否则没放进去，放进去了就添加结束
    if (previous != null) {
        return;
    }
    //获取被观察者对象，即activity，如果被销毁了，则结束
    lifecycleowner lifecycleowner = mlifecycleowner.get();
    if (lifecycleowner == null) {
        // it is null we should be destroyed. fallback quickly
        return;
    }
 
    //maddingobservercounter 表示正在添加的观察者的数量，
    //开始添加新的observe之前，对该变量进行++操作，添加完成后，进行--操作。
    //mhandlingevent表示是否正在处理
    boolean isreentrance = maddingobservercounter != 0 || mhandlingevent;
    //计算状态，进行while循环
    state targetstate = calculatetargetstate(observer);
    maddingobservercounter++;
    //新添加的lifecycleobserver ，会不断更新自己的生命周期到指定的targetstate
    while ((statefulobserver.mstate.compareto(targetstate) < 0
            && mobservermap.contains(observer))) {
        //添加状态到list，计算当前状态，用于计算状态calculatetargetstate
        pushparentstate(statefulobserver.mstate);
        //
        final event event = event.upfrom(statefulobserver.mstate);
        if (event == null) {
            throw new illegalstateexception("no event up from " + statefulobserver.mstate);
        }
        //生命周期分发：分发到mlifecycleobserver里面
        statefulobserver.dispatchevent(lifecycleowner, event);
        //移除状态到list
        popparentstate();
        // mstate / subling may have been changed recalculate
        //重新计算状态
        targetstate = calculatetargetstate(observer);
    }
    //没有事件正在处理，或者是添加了observe,就需要重新同步一次状态,下面详细介绍
    if (!isreentrance) {
        // we do sync only on the top level.
        sync();
    }
    maddingobservercounter--;
}

//计算状态规则
private state calculatetargetstate(lifecycleobserver observer) {
    //在map中获取到当前observer的前一个observer
    map.entry<lifecycleobserver, observerwithstate> previous = mobservermap.ceil(observer);
    //前一个observer的state
    state siblingstate = previous != null ? previous.getvalue().mstate : null;
    //如果是嵌套添加observer，获取父级observer的状态
    state parentstate = !mparentstates.isempty() ? mparentstates.get(mparentstates.size() - 1)
            : null;
    //获取最小状态，destroyed < initialized < created < started < resumed
    return min(min(mstate, siblingstate), parentstate);
}

四、绑定生命周期

        上面介绍了addobserver的流程，在componentactivity的oncreate方法中：

reportfragment.injectifneededin(this);

        reportfragment是一个fragment，它负责分派生命周期的事件，injectifneededin()就是在当前的activity里添加一个reportfragment，和glide的绑定生命周期比较类似。

public static void injectifneededin(activity activity) {
    //当api>=29注册registerin绑定生命周期
    if (build.version.sdk_int >= 29) {
        // on api 29+, we can register for the correct lifecycle callbacks directly
        lifecyclecallbacks.registerin(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();
    }
}

         当api>=29是通过application.activitylifecyclecallbacks的方式注册生命周期， 以onstart为例，其他onresume、onpause等类似：

@override
public void onstart() {
    super.onstart();
    dispatchstart(mprocesslistener);
    dispatch(lifecycle.event.on_start);
}

         重点看看dispatch：

private void dispatch(@nonnull lifecycle.event event) {
    //当api<29分发dispatch绑定生命周期
    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);
    }
}

        不管是pplication.activitylifecyclecallbacks还是当前fragment的生命周期方式都会执行如下dispatch：

static void dispatch(@nonnull activity activity, @nonnull lifecycle.event event) {
    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);
        }
    }
}

        调用lifecycleregistry的handlelifecycleevent将生命周期分发出去，lifecycleregistry上面已经介绍过了addobserve逻辑了，代码比较长，只显示关键代码：

handlelifecycleevent -> movetostate -> 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.");
    }
    //没有同步过，则同步
    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) {
            //如果observerwithstate的state小于当前state，那么就调用forwardpass方法，
            backwardpass(lifecycleowner);
        }
        map.entry<lifecycleobserver, observerwithstate> newest = mobservermap.newest();
        if (!mneweventoccurred && newest != null
                && mstate.compareto(newest.getvalue().mstate) > 0) {
            //如果大于当前state，那么就调用backwardpass方法。
            forwardpass(lifecycleowner);
        }
    }
    mneweventoccurred = false;
}

        backwardpass和forwardpass的代码都会调用：

observer.dispatchevent(lifecycleowner, event)

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 = event.gettargetstate();
        mstate = min(mstate, newstate);
        mlifecycleobserver.onstatechanged(owner, event);
        mstate = newstate;
    }
}

        调用了lifecycling.getcallback方法来获得mlifecycleobserver的实例，通过这个实例调用onstatechanged：

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;
    }
    //通过反射，找到lifecycle 自动生成的类
    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);
            最终使用生成的adapter，创建singlegeneratedadapterobserver
            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);
}

class singlegeneratedadapterobserver implements lifecycleeventobserver {
 
    private final generatedadapter mgeneratedadapter;
 
    singlegeneratedadapterobserver(generatedadapter generatedadapter) {
        mgeneratedadapter = generatedadapter;
    }
 
    @override
    public void onstatechanged(@nonnull lifecycleowner source, @nonnull lifecycle.event event) {
        mgeneratedadapter.callmethods(source, event, false, null);
        mgeneratedadapter.callmethods(source, event, true, null);
    }
}

        到这里会根据依赖中添加的annotationprocessor "androidx.lifecycle:lifecycle-compiler:2.2.0"根据不同生命周期注解生成不同的类，整个过程还是比较复杂的。整体过程如下图：

总结

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