Lifecycle启动源码分析

首先从Lifecycle的manifests文件说起：

<manifest xmlns:android="http://schemas.android.com/apk/res/android"    package="android.arch.lifecycle.extensions"    android:versionCode="1"    android:versionName="1.0" >    <uses-sdk        android:minSdkVersion="14"        android:targetSdkVersion="26" />    <application>        <provider            android:name="android.arch.lifecycle.LifecycleRuntimeTrojanProvider"            android:authorities="${applicationId}.lifecycle-trojan"            android:exported="false"            android:multiprocess="true" />    </application></manifest>

Lifecycle框架会在应用的manifest文件中添加一个provider（LifecycleRuntimeTrojanProvider）。

<manifest xmlns:android="http://schemas.android.com/apk/res/android"          package="com.example.android.persistence">    <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION" />    <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />    <uses-feature android:name="android.hardware.location.gps" />    <application        android:name=".viewmodel.MyApplication"        android:allowBackup="false"        android:icon="@mipmap/ic_launcher"        android:label="@string/app_name"        android:supportsRtl="true"        android:theme="@style/AppTheme">        <activity android:name="com.example.android.persistence.MainActivity">            <intent-filter>                <action android:name="android.intent.action.MAIN"/>                <category android:name="android.intent.category.LAUNCHER" />            </intent-filter>        </activity>    </application></manifest>

通过apktool拿到的最终apk中的manifest：

<?xml version="1.0" encoding="utf-8" standalone="no"?>        <manifest xmlns:android="http://schemas.android.com/apk/res/android"             package="com.example.android.persistence" platformBuildVersionCode="26" platformBuildVersionName="8.0.0">            <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/>            <uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION"/>            <uses-feature android:name="android.hardware.location.gps"/>            <application android:allowBackup="false" android:debuggable="true" android:icon="@mipmap/ic_launcher" android:label="@string/app_name" android:supportsRtl="true" android:theme="@style/AppTheme">                <activity android:name="com.example.android.persistence.MainActivity">                    <intent-filter>                        <action android:name="android.intent.action.MAIN"/>                        <category android:name="android.intent.category.LAUNCHER"/>                    </intent-filter>                </activity>                <meta-data android:name="android.support.VERSION" android:value="26.0.2"/>                <provider android:authorities="com.example.android.persistence.lifecycle-trojan" android:exported="false" android:multiprocess="true" android:name="android.arch.lifecycle.LifecycleRuntimeTrojanProvider"/>            </application>        </manifest>

可以看到lifecycle框架会向应用中添加一个LifecycleRuntimeTrojanProvider，而这个provider只是为了启动lifecycle框架。接下来我们看一下LifecycleRuntimeTrojanProvider。

public class LifecycleRuntimeTrojanProvider extends ContentProvider {    @Override    public boolean onCreate() {        LifecycleDispatcher.init(getContext());        ProcessLifecycleOwner.init(getContext());        return true;    }    @Nullable    @Override    public Cursor query(@NonNull Uri uri, String[] strings, String s, String[] strings1,            String s1) {        return null;    }    @Nullable    @Override    public String getType(@NonNull Uri uri) {        return null;    }    @Nullable    @Override    public Uri insert(@NonNull Uri uri, ContentValues contentValues) {        return null;    }    @Override    public int delete(@NonNull Uri uri, String s, String[] strings) {        return 0;    }    @Override    public int update(@NonNull Uri uri, ContentValues contentValues, String s, String[] strings) {        return 0;    }}

可以看到除了onCreate方法，其他都是返回null或0。初始化方法中
一个是初始化lifecycle框架，一个是创建了ProcessLifecycleOwner来监听进程的lifecycle。

LifecycleDispatcher.init(getContext());ProcessLifecycleOwner.init(getContext());

先看LifecycleDispatcher.init(getContext());

class LifecycleDispatcher {    private static final String REPORT_FRAGMENT_TAG = "android.arch.lifecycle"            + ".LifecycleDispatcher.report_fragment_tag";    private static AtomicBoolean sInitialized = new AtomicBoolean(false);    static void init(Context context) {        if (sInitialized.getAndSet(true)) {            return;        }        ((Application) context.getApplicationContext())                .registerActivityLifecycleCallbacks(new DispatcherActivityCallback());    }    ...}

在init中注册了activitylifecyclecallback,在onActivityCreated中又注册了fragmentLifecycleCallback。onActivityStopped和onActivitySaveInstanceState中发送CREATED状态。（可以参考lifecycle状态图）。

static class DispatcherActivityCallback extends EmptyActivityLifecycleCallbacks {        private final FragmentCallback mFragmentCallback;        DispatcherActivityCallback() {            mFragmentCallback = new FragmentCallback();        }        @Override        public void onActivityCreated(Activity activity, Bundle savedInstanceState) {            if (activity instanceof FragmentActivity) {                ((FragmentActivity) activity).getSupportFragmentManager()                        .registerFragmentLifecycleCallbacks(mFragmentCallback, true);            }            ReportFragment.injectIfNeededIn(activity);        }        @Override        public void onActivityStopped(Activity activity) {            if (activity instanceof FragmentActivity) {                markState((FragmentActivity) activity, CREATED);            }        }        @Override        public void onActivitySaveInstanceState(Activity activity, Bundle outState) {            if (activity instanceof FragmentActivity) {                markState((FragmentActivity) activity, CREATED);            }        }    }

在fragmentLifecycleCallback的onFragmentCreated回调中又添加了一个空的DestructionReportFragment。fragmentLifecycleCallback用来监听onCreate、onStart、onResume事件。

static class FragmentCallback extends FragmentManager.FragmentLifecycleCallbacks {        @Override        public void onFragmentCreated(FragmentManager fm, Fragment f, Bundle savedInstanceState) {            dispatchIfLifecycleOwner(f, ON_CREATE);            if (!(f instanceof LifecycleRegistryOwner)) {                return;            }            if (f.getChildFragmentManager().findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {                f.getChildFragmentManager().beginTransaction().add(new DestructionReportFragment(),                        REPORT_FRAGMENT_TAG).commit();            }        }        @Override        public void onFragmentStarted(FragmentManager fm, Fragment f) {            dispatchIfLifecycleOwner(f, ON_START);        }        @Override        public void onFragmentResumed(FragmentManager fm, Fragment f) {            dispatchIfLifecycleOwner(f, ON_RESUME);        }    }

空的DestructionReportFragment用来监听onPause、onStop、onDestroy事件。

public static class DestructionReportFragment extends Fragment {        @Override        public void onPause() {            super.onPause();            dispatch(ON_PAUSE);        }        @Override        public void onStop() {            super.onStop();            dispatch(ON_STOP);        }        @Override        public void onDestroy() {            super.onDestroy();            dispatch(ON_DESTROY);        }        protected void dispatch(Lifecycle.Event event) {            dispatchIfLifecycleOwner(getParentFragment(), event);        }    }

这块总结一下
1. 注册registerActivityLifecycleCallbacks，onActivityCreated方法中注册registerFragmentLifecycleCallbacks。onActivityStopped和onActivitySaveInstanceState中发送CREATED状态。
2. FragmentCallback在onFragmentCreated发送ON_CREATE事件；onFragmentStarted发送ON_START;onFragmentResumed发送ON_RESUME事件。同时onFragmentCreated中添加DestructionReportFragment（用来监听onPause、onStop、onDestroy）。

可以看到lifecycle用FragmentCallback和DestructionReportFragment结合的方式来监听生命周期。这么做的目的应该是为了onCreate、onStart、onResume事件发生在activity相应回调之后，onPause、onStop、onDestroy事件发生在相应回调之前。（调试发现FragmentCallback的方法会多次回调，并且FragmentCallback的onFragmentStarted还是发生在了activity的onStart之前）

ActivityLifecycleCallbacks的onActivityCreated方法中还有一句：
ReportFragment.injectIfNeededIn(activity);
前面的分析是在if (activity instanceof FragmentActivity) ，也就是在应用中采用的是FragmentActivity的前提下进行了，有可能应用中用的是app包中的Activity，所以这里对这种情况进行处理。

public static void injectIfNeededIn(Activity activity) {        // ProcessLifecycleOwner should always correctly work and some activities may not extend        // FragmentActivity from support lib, so we use framework fragments for activities        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();        }    }

在这种情况下会添加一个空的ReportFragment，在onActivityCreated、onStart、onResume、onPause、onStop、onDestroy中发送相应的生命周期事件。

public class ReportFragment extends Fragment {    private static final String REPORT_FRAGMENT_TAG = "android.arch.lifecycle"            + ".LifecycleDispatcher.report_fragment_tag";    public static void injectIfNeededIn(Activity activity) {        // ProcessLifecycleOwner should always correctly work and some activities may not extend        // FragmentActivity from support lib, so we use framework fragments for activities        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();        }    }    static ReportFragment get(Activity activity) {        return (ReportFragment) activity.getFragmentManager().findFragmentByTag(                REPORT_FRAGMENT_TAG);    }    private ActivityInitializationListener mProcessListener;    private void dispatchCreate(ActivityInitializationListener listener) {        if (listener != null) {            listener.onCreate();        }    }    private void dispatchStart(ActivityInitializationListener listener) {        if (listener != null) {            listener.onStart();        }    }    private void dispatchResume(ActivityInitializationListener listener) {        if (listener != null) {            listener.onResume();        }    }    @Override    public void onActivityCreated(Bundle savedInstanceState) {        super.onActivityCreated(savedInstanceState);        dispatchCreate(mProcessListener);        dispatch(Lifecycle.Event.ON_CREATE);    }    @Override    public void onStart() {        super.onStart();        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;    }    private void dispatch(Lifecycle.Event event) {        if (getActivity() instanceof LifecycleRegistryOwner) {            ((LifecycleRegistryOwner) getActivity()).getLifecycle().handleLifecycleEvent(event);        }    }    void setProcessListener(ActivityInitializationListener processListener) {        mProcessListener = processListener;    }    interface ActivityInitializationListener {        void onCreate();        void onStart();        void onResume();    }}

可以看到在onActivityCreated、onStart、onStop中除了dispatch(Lifecycle.Event.xx);还多了dispatchXX(mProcessListener);方法。这个mProcessListener从名字也可以看出，是ProcessLifecycleOwner用来监听有activity真正的进入到激活状态的。
接下来我们分析下ProcessLifecycleOwner。

public class ProcessLifecycleOwner implements LifecycleOwner {    @VisibleForTesting    static final long TIMEOUT_MS = 700; //mls    // ground truth counters    private int mStartedCounter = 0;    private int mResumedCounter = 0;    private boolean mPauseSent = true;    private boolean mStopSent = true;    private Handler mHandler;    private final LifecycleRegistry mRegistry = new LifecycleRegistry(this);    private Runnable mDelayedPauseRunnable = new Runnable() {        @Override        public void run() {            dispatchPauseIfNeeded();            dispatchStopIfNeeded();        }    };    private ActivityInitializationListener mInitializationListener =            new ActivityInitializationListener() {                @Override                public void onCreate() {                }                @Override                public void onStart() {                    activityStarted();                }                @Override                public void onResume() {                    activityResumed();                }            };    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.     */    public static LifecycleOwner get() {        return sInstance;    }    static void init(Context context) {        sInstance.attach(context);    }    void activityStarted() {        mStartedCounter++;        if (mStartedCounter == 1 && mStopSent) {            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_START);            mStopSent = false;        }    }    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);        }    }    void activityStopped() {        mStartedCounter--;        dispatchStopIfNeeded();    }    private void dispatchPauseIfNeeded() {        if (mResumedCounter == 0) {            mPauseSent = true;            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);        }    }    private void dispatchStopIfNeeded() {        if (mStartedCounter == 0 && mPauseSent) {            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_STOP);            mStopSent = true;        }    }    private ProcessLifecycleOwner() {    }    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);            }            @Override            public void onActivityPaused(Activity activity) {                activityPaused();            }            @Override            public void onActivityStopped(Activity activity) {                activityStopped();            }        });    }    @Override    public Lifecycle getLifecycle() {        return mRegistry;    }}

可以看到ProcessLifecycleOwner代码比较清晰了，onStart和onResume是由上面分析的ReportFragment的对应回调发送过来的，保证activity真正的被启动。而onPause和onStop是由自己注册的ActivityLifecycleCallbacks回调的。

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

onActivityCreated中设置了listenerReportFragment .get(activity).setProcessListener(mInitializationListener);

private ActivityInitializationListener mInitializationListener =            new ActivityInitializationListener() {                @Override                public void onCreate() {                }                @Override                public void onStart() {                    activityStarted();                }                @Override                public void onResume() {                    activityResumed();                }            };

不论是ReportFragment回调的onStart、onResume还是自己注册回调的onPause、onStop最终都调用相应的activityXX方法。
ProcessLifecycleOwner利用两个counter计算当前活着的activity数量

// ground truth counters    private int mStartedCounter = 0;    private int mResumedCounter = 0;

比较有意思的是pause和stop中加了个700ms的延时。这个延时足以保证ProcessLifecycleOwner在activities销毁或者因为配置变化重建的时候不会发送任何事件。

void activityPaused() {        mResumedCounter--;        if (mResumedCounter == 0) {            mHandler.postDelayed(mDelayedPauseRunnable, TIMEOUT_MS);        }    }    void activityStopped() {        mStartedCounter--;        dispatchStopIfNeeded();    }    private void dispatchPauseIfNeeded() {        if (mResumedCounter == 0) {            mPauseSent = true;            mRegistry.handleLifecycleEvent(Lifecycle.Event.ON_PAUSE);        }    }