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In this video we'll do some debugging and learn about how Fragments are saved and restored!
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We've just discovered that
when we rotate the app,
0:00
we end up with two list fragments.
0:03
And what's worse is that if
we keep rotating the app,
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we get another new list
fragment on each rotation.
0:08
So why is this happening?
0:12
Well, it's actually just
Android trying to be
0:13
helpful by restoring our
fragments automatically.
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When we launch the app,
we create our list fragment and
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we add it to our UI by
using the fragment manager.
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Then, when we rotate the app,
our fragment is automatically restored and
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we create a new fragment.
0:30
To see this a little better,
let's do some debugging.
0:33
Let's add a break point to our
activities super.onCreate method,
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and then hit the Debug button.
0:45
After a few seconds,
we should be stopped at our break point.
0:51
Nice.
0:56
Notice that we have a null
savedInstanceState.
1:00
Now, since we're mostly interested in
what happens after we rotate the app,
1:03
let's hit the resume program button and
then rotate the app.
1:08
And now we're back at our
breakpoint again except
1:17
this time savedInstanceState isn't null.
1:19
Let's step into this super.OnCreate
method by using this button over here.
1:23
Then let's step over this first log call.
1:30
And then let's take a look at the log.
1:35
So far so good,
just the one onCreate message.
1:42
Back on the debug tab,
let's step over this super call as well
1:47
and once that's done,
let's check the log again.
1:53
This is what I mean when I say that the
fragment will be automatically restored.
1:57
We didn't do anything and yet here we have
onAttach and onCreate for our fragment.
2:02
So going back to the debug tab,
if we step out of this method
2:08
to go back to main activity it becomes
pretty clear where the problem is.
2:13
We shouldn't be making a new list
fragment if we already have one.
2:24
Okay, let's go ahead and
end this debugging session and
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see how we can fix this.
2:32
Remember that when we first launched
the app, savedInstantState was null.
2:35
But when we rotated it,
saveInstanceState got a value.
2:41
So really all we need to do is say if
2:45
savedInstanceState is null.
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Then add our fragment.
2:59
Let's quickly run this.
3:03
And if we rotate the app it looks
like it worked and it did work.
3:10
This is a perfectly acceptable
solution to our problem.
3:18
But depending on your app,
it might not be the best solution.
3:22
What if we needed access to our list
fragment instance from another method and
3:26
main activity?
3:30
We could refactor our list
fragment to be a field, but
3:32
still this field is only being set
when savedInstanceState is null.
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To fix this above the if statement
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let's create a new list fragment
variable named savedFragment.
3:45
And set it equal to
getFragmentManager.findFrangmentById(R.id-
3:55
.placeholder).
4:02
Then let's use Alt Enter to cast
the returned fragment as a ListFragment.
4:04
The findFragmentById method takes in
a resource ID and returns a fragment.
4:13
If the fragment can't be
found it returns null.
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The resource ID we pass in should
either be the ID of the fragment
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if it was added directly to the XML or
4:26
the ID of the fragment's placeholder
if it was added dynamically.
4:28
Now instead of checking if
savedInstanceState is null we can instead
4:35
check if savedFragment is null.
4:39
Which for now makes the code
a little bit easier to read.
4:44
We look for a ListFragment and if we don't
find it, we create and add a new one.
4:49
Now, let's do one last test to make sure
everything is in good working order.
4:57
So if I rotate and
rotate and scroll a bunch.
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Nice, no double fragments.
5:17
And if we look over to the log.
5:19
We've only got one fragment on
resume call, just like it should be.
5:23
Now that we've got our life cycle
under control, let's go ahead and
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un-extend our logging classes.
5:32
Change logging activity
back to AppCompatActivity,
5:34
and over in ListFragment
change LoggingFragment
5:40
back to Fragment, and remove the log call.
5:44
Then I'm going to delete
the two logging classes.
5:53
But you should feel free
to keep them around.
5:57
In the next video we'll start work on
launching a new fragment when we receive
6:03
a tap on our ListFragment.
6:07
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