Navigation Drawer - what does syncState() do and why it should be called inside onPostCreate()?

Question

I am learning to create navigation drawer in Android. While reading this, I can't understand following code:

@Override protected void onPostCreate(Bundle savedInstanceState) {     super.onPostCreate(savedInstanceState);     // Sync the toggle state after onRestoreInstanceState has occurred.     mDrawerToggle.syncState(); } 

The documentation says:

Synchronize the state of the drawer indicator/affordance with the linked DrawerLayout.

This should be called from your Activity's onPostCreate method to synchronize after the DrawerLayout's instance state has been restored, and any other time when the state may have diverged in such a way that the ActionBarDrawerToggle was not notified. (For example, if you stop forwarding appropriate drawer events for a period of time.)

Further I read about onPostCreate() from sstn's answer here: OnPostCreate in Fragment

onPostCreate() is mainly intented for framework use (although you can override it). The docs say that it is called after onStart() and onRestoreInstanceState().

This might lead to the assumption that it might be called before onResume() and thus probably before the message loop is dispatching events (including AsyncTask's onPostExecute() method), meaning your onPostExecute() will only fire after onPause().

As onPostCreate() is not properly documented and not really intended for application use - I might want to say it is not a good idea to rely on any observed behaviour.

From these two I couldn't understand anything. What does syncState() exactly do and why it should be inside onPostcreate()? Can anyone explain it better?

Answer 1

Well, I think of this question as a good one. And I will collect this question and its answers. So, let's do some summaries here:

First, as to ActionBarDrawerToggle.syncState(), just as the document says,

Synchronize the state of the drawer indicator/affordance with the linked DrawerLayout.

This should be called from your Activity's onPostCreate method to synchronize after the DrawerLayout's instance state has been restored, and any other time when the state may have diverged in such a way that the ActionBarDrawerToggle was not notified. (For example, if you stop forwarding appropriate drawer events for a period of time.)

Second, as to Activity.onPostCreate(Bundle), it is called when activity start-up is complete (after onStart() and onRestoreInstanceState(Bundle) have been called). Applications will generally not implement this method; it is intended for system classes to do final initialization after application code has run.

But, it is derived classes must call through to the super class's implementation of this method. If they do not, an exception will be thrown.

So, What does syncState() exactly do?

Well, ActionBarDrawerToggle.syncState() will synchronize the changed icon's state, which deponds on actions of DrawerLayout. If you ever tried to remove the syncState(), you will realize that the icon of arrow won't rotate any more.

And why syncState() should be called inside onPostcreate()?

Well, onPostcreate() is called when activity start-up is complete (after onStart() and onRestoreInstanceState(Bundle) have been called), while at the moment, Activity needs animations to be ready to work. So, if it isn't the best moment for animations, when is it?