- What are Fragments? 2:15
- My First Fragment 6:05
- Fragments? 5 questions
- Adding Fragments to Activities 4:13
- Adding a Fragment 2 objectives
- Making a List 8:56
- Checking It Twice 3:42
- Handling Clicks 5:20
- Handling Clicks 2 objectives
- Fragment Lifecycle 7:00
- Fragment Lifecycle 5 questions
- Keeping Track of Instance State 6:09
- Saving State 2 objectives
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Keeping Track of Instance State
6:09In this video we'll do some debugging and learn about how Fragments are saved and restored!
We've just discovered that
when we rotate the app,
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we end up with two list fragments.
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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.
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So why is this happening?
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Well, it's actually just
Android trying to be
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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.
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To see this a little better,
let's do some debugging.
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Let's add a break point to our
activities super.onCreate method,
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and then hit the Debug button.
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After a few seconds,
we should be stopped at our break point.
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Nice.
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Notice that we have a null
savedInstanceState.
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Now, since we're mostly interested in
what happens after we rotate the app,
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let's hit the resume program button and
then rotate the app.
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And now we're back at our
breakpoint again except
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this time savedInstanceState isn't null.
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Let's step into this super.OnCreate
method by using this button over here.
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Then let's step over this first log call.
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And then let's take a look at the log.
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So far so good,
just the one onCreate message.
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Back on the debug tab,
let's step over this super call as well
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and once that's done,
let's check the log again.
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This is what I mean when I say that the
fragment will be automatically restored.
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We didn't do anything and yet here we have
onAttach and onCreate for our fragment.
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So going back to the debug tab,
if we step out of this method
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to go back to main activity it becomes
pretty clear where the problem is.
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We shouldn't be making a new list
fragment if we already have one.
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Okay, let's go ahead and
end this debugging session and
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see how we can fix this.
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Remember that when we first launched
the app, savedInstantState was null.
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But when we rotated it,
saveInstanceState got a value.
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So really all we need to do is say if
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savedInstanceState is null.
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Then add our fragment.
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Let's quickly run this.
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And if we rotate the app it looks
like it worked and it did work.
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This is a perfectly acceptable
solution to our problem.
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But depending on your app,
it might not be the best solution.
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What if we needed access to our list
fragment instance from another method and
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main activity?
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We could refactor our list
fragment to be a field, but
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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.
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And set it equal to
getFragmentManager.findFrangmentById(R.id-
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.placeholder).
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Then let's use Alt Enter to cast
the returned fragment as a ListFragment.
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The findFragmentById method takes in
a resource ID and returns a fragment.
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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
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the ID of the fragment's placeholder
if it was added dynamically.
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Now instead of checking if
savedInstanceState is null we can instead
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check if savedFragment is null.
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Which for now makes the code
a little bit easier to read.
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We look for a ListFragment and if we don't
find it, we create and add a new one.
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Now, let's do one last test to make sure
everything is in good working order.
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So if I rotate and
rotate and scroll a bunch.
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Nice, no double fragments.
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And if we look over to the log.
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We've only got one fragment on
resume call, just like it should be.
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Now that we've got our life cycle
under control, let's go ahead and
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un-extend our logging classes.
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Change logging activity
back to AppCompatActivity,
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and over in ListFragment
change LoggingFragment
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back to Fragment, and remove the log call.
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Then I'm going to delete
the two logging classes.
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But you should feel free
to keep them around.
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In the next video we'll start work on
launching a new fragment when we receive
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a tap on our ListFragment.
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