Android LifeCycle學習

LifeCycle提供了一套對生命週期進行管理的方法。使用方式如下：

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MainActivity";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getLifecycle().addObserver(new DefaultLifecycleObserver() {
            @Override
            public void onCreate(@NonNull LifecycleOwner owner) {
                Log.i(TAG, "onCreate");
            }

            @Override
            public void onStart(@NonNull LifecycleOwner owner) {
                Log.i(TAG, "onStart");
            }

            @Override
            public void onResume(@NonNull LifecycleOwner owner) {
                Log.i(TAG, "onResume");
            }

            @Override
            public void onPause(@NonNull LifecycleOwner owner) {
                Log.i(TAG, "onPause");
            }

            @Override
            public void onStop(@NonNull LifecycleOwner owner) {
                Log.i(TAG, "onStop");
            }

            @Override
            public void onDestroy(@NonNull LifecycleOwner owner) {
                Log.i(TAG, "onDestroy");
            }
        });
    }
}

當執行MainActivity時，相應生命週期方法回調會打印對應的方法。

getLifecycle().addObserver從代碼上看是添加一個觀察者，等生命週期回調的時候調用對應方法。

• getLifecycle()
  先來看下這個方法，點進去可以看到：

public class ComponentActivity extends androidx.core.app.ComponentActivity ....
	//...省略其它代碼
    private final LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
	//...省略其它代碼
    @NonNull
    @Override
    public Lifecycle getLifecycle() {
        return mLifecycleRegistry;
    }

• LifecycleRegister
  這個類裏面有處理的方法:

   public void handleLifecycleEvent(@NonNull Lifecycle.Event event) {
        State next = getStateAfter(event);
        moveToState(next);
    }

我們只要找到哪調用了handleLifecycleEvent，大體邏輯就理清楚，
直接看代碼找比較難找，我們通過斷點看下，

從斷點的調用棧中看，可以看到有個ReportFragment，它是在handleLifecycleEvent之前調用的。我們找到這個ReportFragment看下

---

• ReportFragment

public class ReportFragment extends Fragment {
    private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
            + ".LifecycleDispatcher.report_fragment_tag";

    public static void injectIfNeededIn(Activity activity) {
        if (Build.VERSION.SDK_INT >= 29) {
            // On API 29+, we can register for the correct Lifecycle callbacks directly
            activity.registerActivityLifecycleCallbacks(
                    new LifecycleCallbacks());
        }
        // Prior to API 29 and to maintain compatibility with older versions of
        // ProcessLifecycleOwner (which may not be updated when lifecycle-runtime is updated and
        // need to support activities that don't extend from FragmentActivity from support lib),
        // use a framework fragment to get the correct timing of Lifecycle events
        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();
        }
    }

    @SuppressWarnings("deprecation")
    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);
            }
        }
    }

    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(@NonNull Lifecycle.Event event) {
        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);
        }
    }

    void setProcessListener(ActivityInitializationListener processListener) {
        mProcessListener = processListener;
    }

    interface ActivityInitializationListener {
        void onCreate();

        void onStart();

        void onResume();
    }

    // this class isn't inlined only because we need to add a proguard rule for it. (b/142778206)
    static class LifecycleCallbacks implements Application.ActivityLifecycleCallbacks {
        @Override
        public void onActivityCreated(@NonNull Activity activity,
                @Nullable Bundle bundle) {
        }

        @Override
        public void onActivityPostCreated(@NonNull Activity activity,
                @Nullable Bundle savedInstanceState) {
            dispatch(activity, Lifecycle.Event.ON_CREATE);
        }

        @Override
        public void onActivityStarted(@NonNull Activity activity) {
        }

        @Override
        public void onActivityPostStarted(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_START);
        }

        @Override
        public void onActivityResumed(@NonNull Activity activity) {
        }

        @Override
        public void onActivityPostResumed(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_RESUME);
        }

        @Override
        public void onActivityPrePaused(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_PAUSE);
        }

        @Override
        public void onActivityPaused(@NonNull Activity activity) {
        }

        @Override
        public void onActivityPreStopped(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_STOP);
        }

        @Override
        public void onActivityStopped(@NonNull Activity activity) {
        }

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

        @Override
        public void onActivityPreDestroyed(@NonNull Activity activity) {
            dispatch(activity, Lifecycle.Event.ON_DESTROY);
        }

        @Override
        public void onActivityDestroyed(@NonNull Activity activity) {
        }
    }
}

injectIfNeededIn這個注入的方法，我們在ComponentActivity的onCreate方法中可以找到

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

那我們就明白了，在ComponentActivity創建的時候創建了一個沒有UI的Fragment，AppCompatActivity繼承ComponentActivity，所以，我們創建Activity的時同時創建了一個沒有UI的Fragment，並和Activity關聯在一起了。

從 ReportFragment中可以看到每個生命週期方法都調用了dispatch方法:

    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);
            }
        }
    }

這裏又調用了 handleLifecycleEvent方法。
到這裏，分析就結束了，具體細節這裏就貼代碼說明了，有興趣可以打斷點看下。