Android Jetpack架构组件（三）一文带你了解Lifecycle（原理篇）

刘望舒 · 掘金 · android · 2019-10-25 04:23

正文

请到「今天看啥」查看全文

Activity implements LifecycleOwner { private SimpleArrayMap<Class<? extends ExtraData>, ExtraData> mExtraDataMap = new SimpleArrayMap<>(); private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry( this ); //1 @RestrictTo (LIBRARY_GROUP) public void putExtraData (ExtraData extraData) { mExtraDataMap.put(extraData.getClass(), extraData); } @Override @SuppressWarnings ( "RestrictedApi" ) protected void onCreate (@Nullable Bundle savedInstanceState) { super .onCreate(savedInstanceState); ReportFragment.injectIfNeededIn( this ); //2 } @CallSuper @Override protected void onSaveInstanceState (Bundle outState) { mLifecycleRegistry.markState(Lifecycle.State.CREATED); //3 super .onSaveInstanceState(outState); } @RestrictTo (LIBRARY_GROUP) public <T extends ExtraData> T getExtraData (Class<T> extraDataClass) { return (T) mExtraDataMap.get(extraDataClass); } @Override public Lifecycle getLifecycle () { return mLifecycleRegistry; //4 } @RestrictTo (LIBRARY_GROUP) public static class ExtraData { } } 复制代码

注释1处创建了LifecycleRegistry，它是Lifecycle的实现类。注释4处实现了LifecycleOwner接口定义的getLifecycle方法，返回了LifecycleRegistry。在注释3处，将Lifecycle的State设置为CREATED。正常来说应该在ComponentActivity的各个生命周期方法中改变Lifecycle的State，显然在ComponentActivity中没有做这些，而是将这个任务交给了ReportFragment，注释2处的将ComponentActivity注入到ReportFragment中。

frameworks/support/lifecycle/runtime/src/main/java/androidx/lifecycle/ReportFragment.java

@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
public class ReportFragment extends Fragment {
    private static final String REPORT_FRAGMENT_TAG = "androidx.lifecycle"
            + ".LifecycleDispatcher.report_fragment_tag";
    public static void injectIfNeededIn(Activity activity) {
        android.app.FragmentManager manager = activity.getFragmentManager();
        if (manager.findFragmentByTag(REPORT_FRAGMENT_TAG) == null) {
            manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commit();
            manager.executePendingTransactions();
        }
    }
    static ReportFragment get(Activity activity) {
        return (ReportFragment) activity.getFragmentManager().findFragmentByTag(
                REPORT_FRAGMENT_TAG);
    }
   ...
    @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);//1
    }

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

  ...
    private void dispatch(Lifecycle.Event event) {
        Activity activity = getActivity();
        if (activity instanceof LifecycleRegistryOwner) {//2
            ((LifecycleRegistryOwner) activity).getLifecycle().handleLifecycleEvent(event);
            return;
        }

        if