android.app.Activity阅读摘要，有时候会不会需要保持一些现场数据呢？ 想让系统帮你退出到后台或者挂掉前做些前置保持工作吗，重点参考吧：

*
* @param savedInstanceState If the activity is being re-initialized after
* previously being shut down then this Bundle contains the data it most
* recently supplied in {@link #onSaveInstanceState}. <b><i>Note: Otherwise it is null.</i></b>
*
* @see #onStart
* @see #onSaveInstanceState
* @see #onRestoreInstanceState
* @see #onPostCreate
*/
protected void onCreate(@Nullable Bundle savedInstanceState) {
if (DEBUG_LIFECYCLE) Slog.v(TAG, "onCreate " + this + ": " + savedInstanceState);
if (mLastNonConfigurationInstances != null) {
mAllLoaderManagers = mLastNonConfigurationInstances.loaders;
}
if (mActivityInfo.parentActivityName != null) {
if (mActionBar == null) {
mEnableDefaultActionBarUp = true;
} else {
mActionBar.setDefaultDisplayHomeAsUpEnabled(true);
}
}
if (savedInstanceState != null) {
Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
mFragments.restoreAllState(p, mLastNonConfigurationInstances != null
? mLastNonConfigurationInstances.fragments : null);
}
mFragments.dispatchCreate();
getApplication().dispatchActivityCreated(this, savedInstanceState);
if (mVoiceInteractor != null) {
mVoiceInteractor.attachActivity(this);
}
mCalled = true;
}

/**
* Same as {@link #onCreate(android.os.Bundle)} but called for those activities created with
* the attribute {@link android.R.attr#persistableMode} set to
* <code>persistAcrossReboots</code>.
*
* @param savedInstanceState if the activity is being re-initialized after
* previously being shut down then this Bundle contains the data it most
* recently supplied in {@link #onSaveInstanceState}.
* <b><i>Note: Otherwise it is null.</i></b>
* @param persistentState 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>
*
* @see #onCreate(android.os.Bundle)
* @see #onStart
* @see #onSaveInstanceState
* @see #onRestoreInstanceState
* @see #onPostCreate
*/
public void onCreate(@Nullable Bundle savedInstanceState,
@Nullable PersistableBundle persistentState) {
onCreate(savedInstanceState);
}

/**
* The hook for {@link ActivityThread} to restore the state of this activity.
*
* Calls {@link #onSaveInstanceState(android.os.Bundle)} and
* {@link #restoreManagedDialogs(android.os.Bundle)}.
*
* @param savedInstanceState contains the saved state
*/
final void performRestoreInstanceState(Bundle savedInstanceState) {
onRestoreInstanceState(savedInstanceState);
restoreManagedDialogs(savedInstanceState);
}

/**
* The hook for {@link ActivityThread} to restore the state of this activity.
*
* Calls {@link #onSaveInstanceState(android.os.Bundle)} and
* {@link #restoreManagedDialogs(android.os.Bundle)}.
*
* @param savedInstanceState contains the saved state
* @param persistentState contains the persistable saved state
*/
final void performRestoreInstanceState(Bundle savedInstanceState,
PersistableBundle persistentState) {
onRestoreInstanceState(savedInstanceState, persistentState);
if (savedInstanceState != null) {
restoreManagedDialogs(savedInstanceState);
}
}

/**
* This method is called after {@link #onStart} when the activity is
* being re-initialized from a previously saved state, given here in
* <var>savedInstanceState</var>. Most implementations will simply use {@link #onCreate}
* to restore their state, but it is sometimes convenient to do it here
* after all of the initialization has been done or to allow subclasses to
* decide whether to use your default implementation. The default
* implementation of this method performs a restore of any view state that
* had previously been frozen by {@link #onSaveInstanceState}.
*
* <p>This method is called between {@link #onStart} and
* {@link #onPostCreate}.
*
* @param savedInstanceState the data most recently supplied in {@link #onSaveInstanceState}.
*
* @see #onCreate
* @see #onPostCreate
* @see #onResume
* @see #onSaveInstanceState
*/
protected void onRestoreInstanceState(Bundle savedInstanceState) {
if (mWindow != null) {
Bundle windowState = savedInstanceState.getBundle(WINDOW_HIERARCHY_TAG);
if (windowState != null) {
mWindow.restoreHierarchyState(windowState);
}
}
}

/**
* This is the same as {@link #onRestoreInstanceState(Bundle)} but is called for activities
* created with the attribute {@link android.R.attr#persistableMode} set to
* <code>persistAcrossReboots</code>. The {@link android.os.PersistableBundle} passed
* came from the restored PersistableBundle first
* saved in {@link #onSaveInstanceState(Bundle, PersistableBundle)}.
*
* <p>This method is called between {@link #onStart} and
* {@link #onPostCreate}.
*
* <p>If this method is called {@link #onRestoreInstanceState(Bundle)} will not be called.
*
* @param savedInstanceState the data most recently supplied in {@link #onSaveInstanceState}.
* @param persistentState the data most recently supplied in {@link #onSaveInstanceState}.
*
* @see #onRestoreInstanceState(Bundle)
* @see #onCreate
* @see #onPostCreate
* @see #onResume
* @see #onSaveInstanceState
*/
public void onRestoreInstanceState(Bundle savedInstanceState,
PersistableBundle persistentState) {
if (savedInstanceState != null) {
onRestoreInstanceState(savedInstanceState);
}
}

/**
* Restore the state of any saved managed dialogs.
*
* @param savedInstanceState The bundle to restore from.
*/
private void restoreManagedDialogs(Bundle savedInstanceState) {
final Bundle b = savedInstanceState.getBundle(SAVED_DIALOGS_TAG);
if (b == null) {
return;
}

final int[] ids = b.getIntArray(SAVED_DIALOG_IDS_KEY);
final int numDialogs = ids.length;
mManagedDialogs = new SparseArray<ManagedDialog>(numDialogs);
for (int i = 0; i < numDialogs; i++) {
final Integer dialogId = ids[i];
Bundle dialogState = b.getBundle(savedDialogKeyFor(dialogId));
if (dialogState != null) {
// Calling onRestoreInstanceState() below will invoke dispatchOnCreate
// so tell createDialog() not to do it, otherwise we get an exception
final ManagedDialog md = new ManagedDialog();
md.mArgs = b.getBundle(savedDialogArgsKeyFor(dialogId));
md.mDialog = createDialog(dialogId, dialogState, md.mArgs);
if (md.mDialog != null) {
mManagedDialogs.put(dialogId, md);
onPrepareDialog(dialogId, md.mDialog, md.mArgs);
md.mDialog.onRestoreInstanceState(dialogState);
}
}
}
}