首先分析下jetpack框架的入口，ide我使用的是as（3.5）正式版。
添加依賴庫

    implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0-rc03'
    implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0-alpha02'

先看應(yīng)用構(gòu)建完成之后的完整manifest文件，app/build/intermediates/instant_app_manifest/debug/AndroidManifest.xml
發(fā)現(xiàn)有部分我們沒有自己寫的配置的文件

       <provider
           android:name="androidx.lifecycle.ProcessLifecycleOwnerInitializer"
           android:authorities="com.pinshi.testjetpack.lifecycle-process"
           android:exported="false"
           android:multiprocess="true" />

ProcessLifecycleOwnerInitializer就是Lifecycle組件初始化的地方。這是一個(gè)ContentProvider
會(huì)自動(dòng)調(diào)用其中的

   @Override
    public boolean onCreate() {
        LifecycleDispatcher.init(getContext());  //1
        ProcessLifecycleOwner.init(getContext()); //2
        return true;
    }
``
注釋1 的部分是將生命周期時(shí)間感知,具體的實(shí)現(xiàn)是

static void init(Context context) {
if (sInitialized.getAndSet(true)) {
return;
}
((Application) context.getApplicationContext())
.registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
}

@SuppressWarnings("WeakerAccess")
@VisibleForTesting
static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {

    @Override
    public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
        ReportFragment.injectIfNeededIn(activity);
    }

    @Override
    public void onActivityStopped(Activity activity) {
    }

    @Override
    public void onActivitySaveInstanceState(Activity activity, Bundle outState) {
    }
}

這個(gè)地方的主要作用有2個(gè)，一個(gè)是通過`registerActivityLifecycleCallbacks()`方法感知Activity或者Fragment 的生命周期變化，另一個(gè)是當(dāng)一些異常情況發(fā)生的時(shí)候，將會(huì)停止所有的關(guān)于生命周期的標(biāo)記。
注釋2的作用是分發(fā)生命周期的變化的。

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

直接調(diào)用`attach()` 方法。

void attach(Context context) {
mHandler = new Handler();
mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE); //1
Application app = (Application) context.getApplicationContext();
app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
@Override
public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
ReportFragment.get(activity).setProcessListener(mInitializationListener);
}

        @Override
        public void onActivityPaused(Activity activity) {
            activityPaused();
        }

        @Override
        public void onActivityStopped(Activity activity) {
            activityStopped();
        }
    });
}

注釋1
`mRegistry`是`LifecycleRegistry` 繼承自`Lifecycle`。主要用于處理生命周期事件。
在26.0.1的以及更高版本的`support`的`Activity`和`Fragment`中，已經(jīng)實(shí)現(xiàn)了`LifecycleOwer`接口。
`LifecycleOwer`其實(shí)可以理解為被觀察者。
我用`MainActivity`中繼承的是`AppCompatActivity`,它繼承的是`FragmentActivity`,再上一級(jí)繼承的是`ComponentActivity`，在其中的`onCreate`方法中，

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mSavedStateRegistryController.performRestore(savedInstanceState);
ReportFragment.injectIfNeededIn(this); //1
if (mContentLayoutId != 0) {
setContentView(mContentLayoutId);
}
}

1處的`ReportFragment`,綁定它的生命周期

@Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
dispatchCreate(mProcessListener);
dispatch(Lifecycle.Event.ON_CREATE);
}

@Override
public void onStart() {
    super.onStart(); //1
    dispatchStart(mProcessListener);
    dispatch(Lifecycle.Event.ON_START);
}

@Override
public void onResume() {
    super.onResume();
    dispatchResume(mProcessListener);
    dispatch(Lifecycle.Event.ON_RESUME);
}

@Override
public void onPause() {
    super.onPause();
    dispatch(Lifecycle.Event.ON_PAUSE);
}

@Override
public void onStop() {
    super.onStop();
    dispatch(Lifecycle.Event.ON_STOP);
}

@Override
public void onDestroy() {
    super.onDestroy();
    dispatch(Lifecycle.Event.ON_DESTROY);
    // just want to be sure that we won't leak reference to an activity
    mProcessListener = null;
}

再看下1處的`dispatchStart ()`方法

private void dispatchStart(ActivityInitializationListener listener) {
if (listener != null) {
listener.onStart();
}
}

這里的listener是程序初始化的時(shí)候就配置了的 在，`ProcessLifecycleOwner`的`attach()`方法中

        void attach(Context context) {
    mHandler = new Handler();
    mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);
    Application app = (Application) context.getApplicationContext();
    app.registerActivityLifecycleCallbacks(new EmptyActivityLifecycleCallbacks() {
        @Override
        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {
            ReportFragment.get(activity).setProcessListener(mInitializationListener);//1
        }

        @Override
        public void onActivityPaused(Activity activity) {
            activityPaused();
        }

        @Override
        public void onActivityStopped(Activity activity) {
            activityStopped();
        }
    });
}

主要的作用是監(jiān)聽`onstart()`和`onResume`方法。設(shè)置當(dāng)前狀態(tài)的變化和通知當(dāng)前的被觀察者。

private void dispatch(Lifecycle.Event event) {
    Activity activity = getActivity();
    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);
        }
    }
}