为什么要用Lifecyle

Android开发中,咱们经常需求在功用组件中感知到对应宿主(Activity,Fragment)的生命周期的改变,例如当Activity可见时,在presenter目标中改写数据,毁掉时释放某些资源等等

Lifecycle出来之前,咱们一般都用最粗暴的办法,在Activity中手动把在每个生命周期回调分发到功用组件(例如逻辑层presenter)中

但是这种办法存在一些问题,首先代码会显得很臃肿不高雅,每个地方都需求手动处理,多人维护不免出现遗失

别的还存在一个逻辑上的隐患,假定在onCreate中去敞开运用某个资源,对应在onStop中咱们需求去释放这个资源,但是onCreate中的操作是个耗时操作,就会存在onStop现已回调完成后,onCreate中的办法却又去敞开了这个资源,这种状况这个资源就得不到释放了,会引起内存走漏

基于这些问题,google为咱们供给了Lifecyle,能够简化功用组件感知生命周期的进程,经过观察者形式,主动分发对应生命周期事情

Lifecycle的运用

增加依靠


// 非Androidx 项目
implementation "android.arch.lifecycle:extensions:1.1.1" 
// Androidx 项目
implementation 'androidx.appcompat:appcompat:1.4.1'

假如项目还没迁移到AndroidX,建议尽早迁移

运用

分两大步骤

  1. 把功用组件包装为生命周期观察者
  2. 把观察者与生命周期组件绑定

界说观察者

运用Lifecycle供给的两个接口DefaultLifecycleObserverLifecycleEventObserver,让功用组件具有感知生命周期的才能

DefaultLifecycleObserver供给了onCreate~OnDestroy各个生命周期的回调办法,并供给了默许完成,按需重写

LifecycleEventObserver只供给了一个onStateChanged接口,可根据传入的参数判断当时所在的生命周期状况

旧版本的Lifecycle运用的是LifecycleObserver接口,然后在功用组件中经过注解的办法去增加各个生命周期的回调逻辑,这种运用起来很费事,目前现已声明为抛弃了

class MyLifecycleObserver : DefaultLifecycleObserver {
    override fun onCreate(owner: LifecycleOwner) {
        Log.i(TAG, "onCreate")
    }
    override fun onResume(owner: LifecycleOwner) {
        Log.i(TAG, "onResume")
    }
    override fun onPause(owner: LifecycleOwner) {
        Log.i(TAG, "onPause")
    }
    override fun onDestroy(owner: LifecycleOwner) {
        Log.i(TAG, "onDestroy")
    }
    companion object {
        const val TAG = "MyLifecycleObserver"
    }
}

这样这个MyLifecycleObserver就具有了感知生命周期的才能,当他跟对应生命周期宿主绑定之后,就能在对应接口中收到回调

观察者和生命周期绑定

绑定的进程可了解为给生命周期宿主(Activity等)增加观察者的进程,运用LifecycleOwner接口的getLifecycle函数,获取到当时宿主的Lifecycle目标,经过这个Lifecycle目标的addObserver办法,就能把生命周期组件和功用组件相关起来,树立观察关系,AppCompatActivity已完成了LifecycleOwner(父类ComponentActivity完成的)

class LifecycleActivity : AppCompatActivity() {
    private val mBinding by lazy {
        ActivityLifecycleBinding.inflate(layoutInflater)
    }
    override fun onCreate(savedInstanceState: Bundle/?) {
        super.onCreate(savedInstanceState)
        Log.i(TAG, "activity onCreate")
        setContentView(mBinding.root)
        lifecycle.addObserver(MyLifecycleObserver())
    }
    override fun onResume() {
        super.onResume()
        Log.i(TAG, "activity onResume")
    }
    override fun onPause() {
        super.onPause()
        Log.i(TAG, "activity onResume")
    }
    override fun onDestroy() {
        super.onDestroy()
        Log.i(TAG, "activity onDestroy")
    }
    companion object {
        const val TAG = "LifecycleActivity"
    }
}

运行结果

I/LifecycleActivity: activity onCreate
I/MyLifecycleObserver: onCreate
I/LifecycleActivity: activity onResume
I/MyLifecycleObserver: onResume
I/MyLifecycleObserver: onPause
I/LifecycleActivity: activity onPause
I/MyLifecycleObserver: onDestroy
I/LifecycleActivity: activity onDestroy

能够看到MyLifecycleObserver和Activity的生命周期回调保持了同步,不过仔细的童鞋或许现已注意到这儿有一个点很有意思,在OnResume之前,都是Activity中的日志先打印,然后才打印Observer中的日志,而在OnResume之后,却是observer中的先履行,这是为啥勒?看图(这个图很关键

原来你是这样的AAC——Lifecycle的使用及原理

图中states是Lifecycle为生命周期界说的5中状况INITIALIZED,DESTROYED,CREATED,STARTED,RESUMED

以RESUMED状况为例,ON_RESUME即代表onResume办法的回调履行,这个办法履行完成之后,Activity才进入到了RESUMED状况,而关于ON_PAUSE,也便是onPause刚开始履行的时分还是在Resumed态,履行后就进入了STARTED状况。

所以对应便是onResumed履行之后,lifecycle才会分发对应的ON_RESUME事情到observer中,而onPause刚开始履行,就会把ON_PAUSE事情分发到observer中,这样做的好处是当observer收到对应的事情时,对应Activity所在的生命周期状况是准确的

Lifecyle原理

从两个角度进行剖析

  1. 观察者跟生命周期组件怎么树立绑定
  2. 生命周期事情怎么回调到观察者

注册观察者

观察者跟生命周期组件怎么树立绑定,也便是看看addObserver的背面都干了什么,上面的比如中,咱们在addObserver的时分,是先调用了getLifecycle函数,跟一下这个函数,发现是一个接口LifecycleOwner供给的,而Activity完成了这个接口

LifecyclerOwner

一个非常简略的接口,只供给了一个函数,getLifecycle,回来一个Lifecycle目标,而Lifecycle类是Lifecycle库的核心,用于把生命周期分发出去,这儿接口和类的姓名就很好了解,LifecycleOwner表明生命周期一切者

public interface LifecycleOwner {
    /**
     * @return The lifecycle of the provider.
     */
    @NonNull
    Lifecycle getLifecycle();
}

所以具有生命周期的类都能够去完成这个接口,在AndroidX内已有三个类完成了LifecycleOwner接口,分别是Activity,Fragment,以及ProcessLifecycleOwnerProcessLifecycleOwner用于监听应用的生命周期,包含前后台切换,不过需求额外引入依靠

implementation "androidx.lifecycle:lifecycle-process:$lifecycle"

Lifecycle Lifecycle是一个抽象类,界说了一个具有生命周期的目标

原来你是这样的AAC——Lifecycle的使用及原理

这个类主要界说了两个枚举类,EventState

public enum Event {
    ON_CREATE,
    ON_START,
    ON_RESUME,
    ON_PAUSE,
    ON_STOP,
    ON_DESTROY,
    ON_ANY;
}
public enum State {
    DESTROYED,
    INITIALIZED,
    CREATED,
    STARTED,
    RESUMED;
}

Event表明会分发给其他组件的生命周期事情,ON_ANY表明恣意事情都会触发

State表明当时生命周期一切者所在的状况,跟上面提到的那张图是对应上的

不知道大家会不会有疑问,为什么要搞出这两个值,直接分发详细的event事情不就够了?不香么?这儿留个疑问,后边会讲到

别的还有两个抽象办法addObserver和removeObserver

// 增加观察者
@MainThread
public abstract void addObserver(@NonNull LifecycleObserver observer);
// 移除观察者
@MainThread
public abstract void removeObserver(@NonNull LifecycleObserver observer);

LifecycleRegistry Lifecycle的完成类,负责增加观察者,分发生命周期事情给观察者,看看addObserver的逻辑

@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);
    if (previous != null) {
        return;
    }
    LifecycleOwner lifecycleOwner = mLifecycleOwner.get();
    if (lifecycleOwner == null) {
        // it is null we should be destroyed. Fallback quickly
        return;
    }
    // 省掉部分代码
}

这儿省掉了一些逻辑,看要点,很显然把observer放到了一个mObserverMap傍边,实际上这个不是真正意义上的Map,而是一个自界说的可在遍历进程中增删observer的数据结构,此处先不深凿了,注意这儿把增加进来的Observer目标包装到了一个ObserverWithState目标中,注意这个目标,后边会用到

事情怎么分发

那么到底是怎么感知到Activity等组件的生命周期的改变,并分发给一切observer的了?

直接看看ComponentActivity的源码

private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
public Lifecycle getLifecycle() {
    return mLifecycleRegistry;
}

getLifecycle回来了一个LifecycleRegistry目标,再看看onCreate回调

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    mSavedStateRegistryController.performRestore(savedInstanceState);
    ReportFragment.injectIfNeededIn(this);
    if (mContentLayoutId != 0) {
        setContentView(mContentLayoutId);
    }
}

但是并没有看到LifecycleRegistry目标,不过这个ReportFragment是lifecycle库下的类,那跟他必定有点儿关系了,点进去看看

public class ReportFragment extends android.app.Fragment {
    public static void injectIfNeededIn(Activity activity) {
        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();
        }
    }
    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        dispatchCreate(mProcessListener);
        dispatch(Lifecycle.Event.ON_CREATE);
    }
    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);
            }
        }
    }
}

公然,这不就跟glide感知生命周期的做法一样,用了一个没有界面的Fragment,经过fragment感知宿主Activity的生命周期改变,然后分发出去

感知到生命周期的改变之后,会把对应生命周期事情的event经过LifecycleRegistry的handleLifecycleEvent分发出去,再跟下这块儿代码

public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
    enforceMainThreadIfNeeded("handleLifecycleEvent");
    moveToState(event.getTargetState());
}
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;
}
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);
        }
        Map.Entry<LifecycleObserver, ObserverWithState> newest = mObserverMap.newest();
        if (!mNewEventOccurred && newest != null
                && mState.compareTo(newest.getValue().mState) > 0) {
            forwardPass(lifecycleOwner);
        }
    }
    mNewEventOccurred = false;
}

经过Event的getTargetState()获取到到当时的生命周期状况State值,然后就预备向observer分发当时的生命周期状况,这儿会把当时所拿到的最新State状况和Observer目前的State状况进行比较,假如最新状况比Observer的状况要小,这儿小需求看枚举的比较,界说在前面的比界说在后边的小,这儿要对照state状况和那张图一起看就很好了解

    public enum State {

        DESTROYED,

        INITIALIZED,

        CREATED,

        STARTED,

        RESUMED;
    }

假定旧的状况是RESUMED,而最新的是STARTED,这时就表明新的状况比旧的状况要小,就会走到backwardPass函数里边,反之则会走到forwardPass


private void backwardPass(LifecycleOwner lifecycleOwner) {
    Iterator<Map.Entry<LifecycleObserver, ObserverWithState>> descendingIterator =
            mObserverMap.descendingIterator();
    while (descendingIterator.hasNext() && !mNewEventOccurred) {
        Map.Entry<LifecycleObserver, ObserverWithState> entry = descendingIterator.next();
        ObserverWithState observer = entry.getValue();
        while ((observer.mState.compareTo(mState) > 0 && !mNewEventOccurred
                && mObserverMap.contains(entry.getKey()))) {
            Event event = Event.downFrom(observer.mState);
            if (event == null) {
                throw new IllegalStateException("no event down from " + observer.mState);
            }
            pushParentState(event.getTargetState());
            observer.dispatchEvent(lifecycleOwner, event);
            popParentState();
        }
    }
}

直接看看backwardPass,这儿对mObserverMap进行了遍历,把当时遍历到的observer对应state取出来,经过Event.downFrom计算出需求分发的事情,最终经过dispatchEvent分发出去

所以前面抛出的那个问题,为什么需求state和event,这儿看到他的妙处了吧,因为不是每一个observer都是处于同一状况的,因为有或许observer的状况还没更新完,新的事情又来了,这样就会重新遍历,所以会出现有的observer的状况更最新的状况之间不是连续的,这样经过最新的State值和当时的State值,就能够把这之间的一切生命周期事情都回调一遍,不然有的生命周期事情就或许错过了,那写在里边的逻辑永久得不到履行,当然addObserver的时分,也是运用这个,让刚新注册的observer,能把注册之前走过的一切生命周期事情都走一遍,所以这是一种粘性事情

再看看addObserver的这部分代码,经过while循环,把之前的一切事情都分发了一遍

public void addObserver(@NonNull LifecycleObserver observer) {
    //省掉部分代码
    mAddingObserverCounter++;
    while ((statefulObserver.mState.compareTo(targetState) < 0
            && mObserverMap.contains(observer))) {
        pushParentState(statefulObserver.mState);
        statefulObserver.dispatchEvent(lifecycleOwner, upEvent(statefulObserver.mState));
        popParentState();
        // mState / subling may have been changed recalculate
        targetState = calculateTargetState(observer);
    }
    if (!isReentrance) {
        // we do sync only on the top level.
        sync();
    }
    mAddingObserverCounter--;
}

再来看看dispatchEvent

void dispatchEvent(LifecycleOwner owner, Event event) {
    State newState = event.getTargetState();
    mState = min(mState, newState);
    mLifecycleObserver.onStateChanged(owner, event);
    mState = newState;
}

很简略,调用了LifecycleEventObserver的onStateChanged办法分发了出去,不过咱们前面说过,咱们一般还会运用完成DefaultLifecycleObserver的办法,这种是怎么回调到的?

咱们能够看到DefaultLifecycleObserver是承继FullLifecycleObserver的,而LifecycleEventObserver有一个完成类FullLifecycleObserverAdapter

@Override
public void onStateChanged(LifecycleOwner source, Lifecycle.Event event) {
    switch (event) {
        case ON_CREATE:
            mFullLifecycleObserver.onCreate(source);
            break;
        case ON_START:
            mFullLifecycleObserver.onStart(source);
            break;
        case ON_RESUME:
            mFullLifecycleObserver.onResume(source);
            break;
        case ON_PAUSE:
            mFullLifecycleObserver.onPause(source);
            break;
        case ON_STOP:
            mFullLifecycleObserver.onStop(source);
            break;
        case ON_DESTROY:
            mFullLifecycleObserver.onDestroy(source);
            break;
        case ON_ANY:
            throw new IllegalArgumentException("ON_ANY must not been send by anybody");
    }
    if (mLifecycleEventObserver != null) {
        mLifecycleEventObserver.onStateChanged(source, event);
    }
}

看到这很容易猜想到,DefaultLifecycleObserver的完成类,会被包装到这儿面,然后分发

还记得之前提到的,在addObserver的时分,会把增加进来的observer目标包装到ObserverWithState中么,看看这个类的结构函数

ObserverWithState(LifecycleObserver observer, State initialState) {
    mLifecycleObserver = Lifecycling.lifecycleEventObserver(observer);
    mState = initialState;
}
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);
    }
    // 省掉部分代码
}

公然 ,在这儿,经过Lifecycling,把add进来的observer包装到了FullLifecycleObserverAdapter中,并把FullLifecycleObserverAdapter目标回来赋值给了ObserverWithState目标的mLifecycleObserver属性,所以最终dispatchEvent中调用的的mLifecycleObserver.onStateChanged(owner, event)就经过Adapter回调给了对应的observer的函数

到这儿,从绑定observer到回调生命周期的流程就走通了