如果Activity重写的是 onCreate(@Nullable Bundle savedInstanceState, @Nullable PersistableBundle persistentState) 而不是 onCreate(@Nullable Bundle savedInstanceState)
那么Activity不会加载布局
因为 onCreate(@Nullable Bundle savedInstanceState, @Nullable PersistableBundle persistentState) （API 21） 实际上也是调用的onCreate(@Nullable Bundle savedInstanceState)
源码：
public void onCreate(@Nullable Bundle savedInstanceState,
@Nullable PersistableBundle persistentState) {
onCreate(savedInstanceState);
}
而了解activity启动流程的就知道，真正启动Activity的实现都在ActivityThread，前面的调用过程略过
ActivityThread的方法performLaunchActivity中调用了Instrumentation类中的方法callActivityOnCreate方法，继而调用了TargetActivity中的onCreate方法。
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
......
Activity activity = null;
activity = mInstrumentation.newActivity( cl, component.getClassName(), r.intent);
......
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
......
}Instrumentation类中的方法callActivityOnCreate方法源码如下：
public void callActivityOnCreate(Activity activity, Bundle icicle) {
prePerformCreate(activity);
activity.performCreate(icicle);
postPerformCreate(activity);
}
调用的Activity.performCreate如下，当persistentState不为空才会触发 onCreate(icicle, persistentState);
/** @hide */
@VisibleForTesting
public final ActivityThread getActivityThread() {
return mMainThread;
} final void performCreate(Bundle icicle) {
performCreate(icicle, null);
} final void performCreate(Bundle icicle, PersistableBundle persistentState) {
mCanEnterPictureInPicture = true;
restoreHasCurrentPermissionRequest(icicle);
if (persistentState != null) {
onCreate(icicle, persistentState);
} else {
onCreate(icicle);
}
writeEventLog(LOG_AM_ON_CREATE_CALLED, "performCreate");
mActivityTransitionState.readState(icicle); mVisibleFromClient = !mWindow.getWindowStyle().getBoolean(
com.android.internal.R.styleable.Window_windowNoDisplay, false);
mFragments.dispatchActivityCreated();
mActivityTransitionState.setEnterActivityOptions(this, getActivityOptions());
}
ps:persistentState 注解如下，大意是，如果Activity之前被关闭或者断电，可以通过onSaveInstanceState保存数据到PersistableBundle
，当Activity再次被重新创建的时候的时候通过onCreate(@Nullable Bundle savedInstanceState, @Nullable PersistableBundle persistentState) 恢复现场。
public void onSaveInstanceState(Bundle outState, PersistableBundle outPersistentState) {
onSaveInstanceState(outState);
}if the activity is being re-initialized after
* previously being shut down or powered off then this Bundle contains the data it most
* recently supplied to outPersistentState in {@link #onSaveInstanceState}.
* <b><i>Note: Otherwise it is null.</i></b>