我们平时都是用的Activity或者Fragment封装好的。对应的也就是在activity或者fragment的生命周期里监听。

getLifecycle().addObserver();

如果我们要全局监听app的状态，app在前台还是后台，以前用的就是application的如下方法，计算在前台的activity的数量来判断

app.registerActivityLifecycleCallbacks

实际上系统已经为我们封装好了类似的东西了。

ProcessLifecycleOwner

public class ProcessLifecycleOwner implements LifecycleOwner {

    @VisibleForTesting
    static final long TIMEOUT_MS = 700; //mls

先看下如何使用，单例模式，放心使用

ProcessLifecycleOwner.get().getLifecycle().addObserver(new LifecycleEventObserver() {
    @Override
    public void onStateChanged(@NonNull LifecycleOwner source, @NonNull Lifecycle.Event event) {
        System.out.println("state change==="+event);
    }
});

看下这个类的注解
You can consider this LifecycleOwner as the composite of all of your Activities, except that Lifecycle.Event.ON_CREATE will be dispatched once and Lifecycle.Event.ON_DESTROY will never be dispatched
oncreate只会发送一次，ondestory不会发送，看下图，进入app之后切入后台再切回前台来回切换的state

源码分析

单列模式

private static final ProcessLifecycleOwner sInstance = new ProcessLifecycleOwner();

/**
 * The LifecycleOwner for the whole application process. Note that if your application
 * has multiple processes, this provider does not know about other processes.
 *
 * @return {@link LifecycleOwner} for the whole application.
 */
@NonNull
public static LifecycleOwner get() {
    return sInstance;
}

static void init(Context context) {
    sInstance.attach(context);
}

init方法是通过ContentProvider来初始化的

public class ProcessLifecycleOwnerInitializer extends ContentProvider {
    @Override
    public boolean onCreate() {
        LifecycleDispatcher.init(getContext());
        ProcessLifecycleOwner.init(getContext());
        return true;
    }

看下attach（context）里做啥了，就是registerActivityLifecycleCallbacks来监听activity的状态变化了

void attach(Context context) {
    mHandler = new Handler();
    mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
    Application app = (Application) context.getApplicationContext();
    app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {

我们来看下监听到activity变化以后咋处理的，这里就看下resume和pause就行了，

里边是延迟700ms判断resume个数的，从0到1才会发送resume状态，从1到0才会发送pause状态

void activityResumed() {
    mResumedCounter++;
    if (mResumedCounter == 1) {
        if (mPauseSent) {
            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
            mPauseSent = false;
        } else {
            mHandler.removeCallbacks(mDelayedPauseRunnable);
        }
    }
}

void activityPaused() {
    mResumedCounter--;
    if (mResumedCounter == 0) {
        mHandler.postDelayed(mDelayedPauseRunnable, TIMEOUT_MS);
    }
}

end...