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Android Fragments

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Keeping Track of Instance State

6:09 with Ben Deitch

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, 0:05
    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. 0:16
    When we launch the app, we create our list fragment and 0:19
    we add it to our UI by using the fragment manager. 0:22
    Then, when we rotate the app, our fragment is automatically restored and 0:25
    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, 0:38
    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 2:30
    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. 2:50
    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. 3:35
    To fix this above the if statement 3:40
    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. 4:18
    The resource ID we pass in should either be the ID of the fragment 4:22
    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. 5:06
    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 5:29
    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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