Android 中高级面试题:Activity是如何实现LifecycleOwner的?
作者:互联网
…
private LifecycleRegistry mLifecycleRegistry = new LifecycleRegistry(this);
…
@Override
protected void onSaveInstanceState(Bundle outState) {
mLifecycleRegistry.markState(Lifecycle.State.CREATED);
super.onSaveInstanceState(outState);
}
…
@Override
public Lifecycle getLifecycle() {
return mLifecycleRegistry;
}
}
SupportActivity声明了mLifecycleRegistry对象,但是没有直接使用其进行生命周期的分发,而是被ReportFragment通过activity.getLifecycle()获取使用。
2、ReportFragment
SupportActivity在onCreate为自己添加了ReportFragment:
@RestrictTo(LIBRARY_GROUP)
public class SupportActivity extends Activity implements LifecycleOwner {
// …
@Override
@SuppressWarnings(“RestrictedApi”)
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ReportFragment.injectIfNeededIn(this);
}
// …
}
injectIfNeededIn是ReportFragment的静态方法:
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();
}
}
3、低版本Activity兼容Lifecycle
SupportActivity是伴随Lifecycle才出现的,android.arch.lifecycle:extensions为早期还没有继
承SupportActivity的Activity也提供了支持,通过LifecycleDispatcher实现ReportFragment的注入:
class LifecycleDispatcher {
static void init(Context context) {
if (sInitialized.getAndSet(true)) {
return;
}
((Application) context.getApplicationContext())
.registerActivityLifecycleCallbacks(new DispatcherActivityCallback());
}
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);
}
}
}
之前还疑惑为什么ReportFragment的实现不写到SupportActivity中去,看到这里终于理解了其存在的意义了吧。
LifecycleDispatcher并不需要在Application中调用,他通过ContentProvider实现初始化。
public class ProcessLifecycleOwnerInitializer extends ContentProvider {
@Override
public boolean onCreate() {
LifecycleDispatcher.init(getContext());
ProcessLifecycleOwner.init(getContext());
return true;
}
}
在android.arch.lifecycle:extensionsaar的AndroidManifest中注册:
<manifest xmlns:android=“http://schemas.android.com/apk/res/android”
package=“android.arch.lifecycle.extensions” >
<provider
android:name=“android.arch.lifecycle.ProcessLifecycleOwnerInitializer”
android:authorities="${applicationId}.lifecycle-trojan"
android:exported=“false”
android:multiprocess=“true” />
${applicationId}占位符,避免authroities冲突。
可见在无侵这件事情上做到了极致,这种无侵的初始化方法非常值得我们借鉴和使用。
4、两种Fragment
通过上面分析,我们知道Activity是通过ReportFragment代理了LifecycleOwner的实现。那么在Activity中添加的LifecycleOwner与Activity的Fragment的生命周期是否一致呢?答案是否定的。
Android中存在两种Fragment有两种:
1、SDK自带的android.app.Fragment。
2、Support包中的android.support.v4.app.Fragment(AndroidX也归为此类)。
由于前者已经被@Deprecated
,所以现在普遍使用的是后者,也就是Support或者AndroidX的Fragment。而出于低版本兼容性的考虑,ReportFragment是前者。
leOwner与Activity的Fragment的生命周期是否一致呢?答案是否定的。
Android中存在两种Fragment有两种:
1、SDK自带的android.app.Fragment。
2、Support包中的android.support.v4.app.Fragment(AndroidX也归为此类)。
由于前者已经被@Deprecated
,所以现在普遍使用的是后者,也就是Support或者AndroidX的Fragment。而出于低版本兼容性的考虑,ReportFragment是前者。
标签:面试题,LifecycleOwner,Fragment,activity,ReportFragment,Activity,android,public 来源: https://blog.csdn.net/m0_66144992/article/details/122680393