源码-ComponentActivity的LifecycleOwner

Posted on | In |

原文：https://weread.qq.com/web/reader/d82322e0813ab6e2eg015684

起因：使用Dialog未dismiss时Activity关闭了，导致内存泄露
解决：给Dialog添加生命周期监听

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class TipDialog(context: Context) : Dialog(context), LifecycleObserver {
    init {
        if (context is ComponentActivity) {
            (context as ComponentActivity).lifecycle.addObserver(this)
        }
    }
    override fun onCreate(savedInstanceState: Bundle?) {        
        super.onCreate(savedInstanceState)      
        setContentView(R.layout.item_tip_dialog)   
    } 
    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    private fun onDestroy() {
        if (isShowing) {
            dismiss()  
        }
    }
}

androidx.core.app.ComponentActivity默认实现了LifecycleOwner接口，getLifecycle返回的实际是一个LifecycleRegistry对象，LifecycleRegistry是Lifecycle的唯一实现类。

Lifecycle抽象类定义了添加观察者addObserver()、移除观察者removeObserver()、获取当前状态getCurrentState()

androidx.core.app.ComponentActivity都做了些什么：

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@SupressLint("RestrictedApi")
@Override
protected void onCreate(@Nullable Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    ReportFragment.injectIfNeededIn(this);
}

其中ReportFragment是个无页面的Fragment用来兼容处理不是继承自FragmentActivity的视图，确保ProcessLifecycleOwner可以正常工作（让那些不是直接继承自FragmentActivity的页面可以使用Lifecycle）

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public static  void injectIfNeededIn(Activity activity){
    //使用Fragment兼容处理不是继承自FragmentActivity的视图，确保ProcessLifecycleOwner可以正常工作
    android.app.FragmentManager manager = activity.getFragmentManger();
    if(manager.findFragmentByTag(REPORT_FRAGMENT_TAG)==null){
        manager.beginTransaction().add(new ReportFragment(), REPORT_FRAGMENT_TAG).commint();
        //希望立即执行
        manager.executePendingTransactions();
    }
}

在ReportFragment中可以看到对应生命周期的方法中都会执行dispatch方法：

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@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);
    //赋值为空，确保不会导致内存泄露
    mProcessListener = null;
}

再来看看dispatch方法的主要代码：

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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);
        }
    }
}

dispatch方法最终都会进入handleLifecycleEvent方法中，通过handleLifecycleEvent设置状态并通知观察者，Activity便能监听到生命周期的变化了。