ViewPager和片段 - 存储片段状态的正确方法是什么？ [英] ViewPager and fragments — what's the right way to store fragment's state?

问题描述

片段似乎非常适合将UI逻辑分离成某些模块。但是随着 ViewPager ，它的生命周期对我来说依然是迷人的。所以大师的想法是非常需要的！

Fragments seem to be very nice for separation of UI logic into some modules. But along with ViewPager its lifecycle is still misty to me. So Guru thoughts are badly needed!

请参阅下面的哑软解决方案; - ）

See dumb solution below ;-)

主要活动有一个 ViewPager 带有碎片。这些片段可以为其他（子主题）活动实现一点点不同的逻辑，因此片段的数据通过活动内的回调接口填充。一切都可以在第一次启动时正常运行，但是... ...

Main activity has a ViewPager with fragments. Those fragments could implement a little bit different logic for other (submain) activities, so the fragments' data is filled via a callback interface inside the activity. And everything works fine on first launch, but!...

当活动重新创建时例如在方向改变），所以 ViewPager 的片段。代码（您将在下面找到）说，每次创建活动时，我尝试创建一个新的 ViewPager 片段适配器与片段相同（也许这是问题）但是FragmentManager已经将所有这些片段存储在某个地方（哪里？），并启动那些片段的娱乐机制。因此，娱乐机制通过我的回调接口调用，通过Activity的实现方法来启动数据，调用旧片段的onAttach，onCreateView等。但是这种方法指向通过Activity的onCreate方法创建的新创建的片段。

When the activity gets recreated (e.g. on orientation change) so do the ViewPager's fragments. The code (you'll find below) says that every time the activity is created I try to create a new ViewPager fragments adapter the same as fragments (maybe this is the problem) but FragmentManager already has all these fragments stored somewhere (where?) and starts the recreation mechanism for those. So the recreation mechanism calls the "old" fragment's onAttach, onCreateView, etc. with my callback interface call for initiating data via the Activity's implemented method. But this method points to the newly created fragment which is created via the Activity's onCreate method.

也许我使用错误的模式，但即使Android 3 Pro书也没有太多。所以，请，给我一个二冲，指出如何做到正确的方式。非常感谢！

Maybe I'm using wrong patterns but even Android 3 Pro book doesn't have much about it. So, please, give me one-two punch and point out how to do it the right way. Many thanks!

主要活动

public class DashboardActivity extends BasePagerActivity implements OnMessageListActionListener {

private MessagesFragment mMessagesFragment;

@Override
protected void onCreate(Bundle savedInstanceState) {
    Logger.d("Dash onCreate");
    super.onCreate(savedInstanceState);

    setContentView(R.layout.viewpager_container);
    new DefaultToolbar(this);

    // create fragments to use
    mMessagesFragment = new MessagesFragment();
    mStreamsFragment = new StreamsFragment();

    // set titles and fragments for view pager
    Map<String, Fragment> screens = new LinkedHashMap<String, Fragment>();
    screens.put(getApplicationContext().getString(R.string.dashboard_title_dumb), new DumbFragment());
    screens.put(getApplicationContext().getString(R.string.dashboard_title_messages), mMessagesFragment);

    // instantiate view pager via adapter
    mPager = (ViewPager) findViewById(R.id.viewpager_pager);
    mPagerAdapter = new BasePagerAdapter(screens, getSupportFragmentManager());
    mPager.setAdapter(mPagerAdapter);

    // set title indicator
    TitlePageIndicator indicator = (TitlePageIndicator) findViewById(R.id.viewpager_titles);
    indicator.setViewPager(mPager, 1);

}

/* set of fragments callback interface implementations */

@Override
public void onMessageInitialisation() {

    Logger.d("Dash onMessageInitialisation");
    if (mMessagesFragment != null)
        mMessagesFragment.loadLastMessages();
}

@Override
public void onMessageSelected(Message selectedMessage) {

    Intent intent = new Intent(this, StreamActivity.class);
    intent.putExtra(Message.class.getName(), selectedMessage);
    startActivity(intent);
}

BasePagerActivity aka helper

public class BasePagerActivity extends FragmentActivity {

BasePagerAdapter mPagerAdapter;
ViewPager mPager;
}

适配器

public class BasePagerAdapter extends FragmentPagerAdapter implements TitleProvider {

private Map<String, Fragment> mScreens;

public BasePagerAdapter(Map<String, Fragment> screenMap, FragmentManager fm) {

    super(fm);
    this.mScreens = screenMap;
}

@Override
public Fragment getItem(int position) {

    return mScreens.values().toArray(new Fragment[mScreens.size()])[position];
}

@Override
public int getCount() {

    return mScreens.size();
}

@Override
public String getTitle(int position) {

    return mScreens.keySet().toArray(new String[mScreens.size()])[position];
}

// hack. we don't want to destroy our fragments and re-initiate them after
@Override
public void destroyItem(View container, int position, Object object) {

    // TODO Auto-generated method stub
}

}

Fragment

public class MessagesFragment extends ListFragment {

private boolean mIsLastMessages;

private List<Message> mMessagesList;
private MessageArrayAdapter mAdapter;

private LoadMessagesTask mLoadMessagesTask;
private OnMessageListActionListener mListener;

// define callback interface
public interface OnMessageListActionListener {
    public void onMessageInitialisation();
    public void onMessageSelected(Message selectedMessage);
}

@Override
public void onAttach(Activity activity) {
    super.onAttach(activity);
    // setting callback
    mListener = (OnMessageListActionListener) activity;
    mIsLastMessages = activity instanceof DashboardActivity;

}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
    inflater.inflate(R.layout.fragment_listview, container);
    mProgressView = inflater.inflate(R.layout.listrow_progress, null);
    mEmptyView = inflater.inflate(R.layout.fragment_nodata, null);
    return super.onCreateView(inflater, container, savedInstanceState);
}

@Override
public void onActivityCreated(Bundle savedInstanceState) {
    super.onActivityCreated(savedInstanceState);

    // instantiate loading task
    mLoadMessagesTask = new LoadMessagesTask();

    // instantiate list of messages
    mMessagesList = new ArrayList<Message>();
    mAdapter = new MessageArrayAdapter(getActivity(), mMessagesList);
    setListAdapter(mAdapter);
}

@Override
public void onResume() {
    mListener.onMessageInitialisation();
    super.onResume();
}

public void onListItemClick(ListView l, View v, int position, long id) {
    Message selectedMessage = (Message) getListAdapter().getItem(position);
    mListener.onMessageSelected(selectedMessage);
    super.onListItemClick(l, v, position, id);
}

/* public methods to load messages from host acitivity, etc... */
}

解决方案

愚蠢的解决方案是使用putFragment将片段保存在onSaveInstanceState（主机活动）中，并获得它们通过getFragment内部onCreate。但是我仍然有一种奇怪的感觉，事情不应该像这样工作...见下面的代码：

Solution

The dumb solution is to save the fragments inside onSaveInstanceState (of host Activity) with putFragment and get them inside onCreate via getFragment. But I still have a strange feeling that things shouldn't work like that... See code below:

    @Override
protected void onSaveInstanceState(Bundle outState) {

    super.onSaveInstanceState(outState);
    getSupportFragmentManager()
            .putFragment(outState, MessagesFragment.class.getName(), mMessagesFragment);
}

protected void onCreate(Bundle savedInstanceState) {
    Logger.d("Dash onCreate");
    super.onCreate(savedInstanceState);

    ...
    // create fragments to use
    if (savedInstanceState != null) {
        mMessagesFragment = (MessagesFragment) getSupportFragmentManager().getFragment(
                savedInstanceState, MessagesFragment.class.getName());
                StreamsFragment.class.getName());
    }
    if (mMessagesFragment == null)
        mMessagesFragment = new MessagesFragment();
    ...
}

推荐答案

当 FragmentPagerAdapter 向FragmentManager添加一个片段时，它会根据片段所在的特定位置使用特殊的标签。 FragmentPagerAdapter.getItem（int position）仅在该位置的片段不存在时才调用。旋转后，Android会注意到它已经为这个特定的位置创建/保存了一个片段，所以它只是试图用 FragmentManager.findFragmentByTag（）重新连接，而不是创建一个新的。所有这些都在使用 FragmentPagerAdapter 时是免费的，并且是为什么通常将片段初始化代码放在 getItem（int）方法。

When the FragmentPagerAdapter adds a fragment to the FragmentManager, it uses a special tag based on the particular position that the fragment will be placed. FragmentPagerAdapter.getItem(int position) is only called when a fragment for that position does not exist. After rotating, Android will notice that it already created/saved a fragment for this particular position and so it simply tries to reconnect with it with FragmentManager.findFragmentByTag(), instead of creating a new one. All of this comes free when using the FragmentPagerAdapter and is why it is usual to have your fragment initialisation code inside the getItem(int) method.

即使我们没有使用 FragmentPagerAdapter ，创建不是一个好主意 Activity.onCreate（Bundle）中的每一次新片段。您已经注意到，当片段添加到FragmentManager时，它将在旋转后重新创建，并且不需要再次添加。这样做是使用碎片时常见的错误原因。

Even if we were not using a FragmentPagerAdapter, it is not a good idea to create a new fragment every single time in Activity.onCreate(Bundle). As you have noticed, when a fragment is added to the FragmentManager, it will be recreated for you after rotating and there is no need to add it again. Doing so is a common cause of errors when working with fragments.

使用片段时，通常的方法是：

A usual approach when working with fragments is this:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    ...

    CustomFragment fragment;
    if (savedInstanceState != null) {
        fragment = (CustomFragment) getSupportFragmentManager().findFragmentByTag("customtag");
    } else {
        fragment = new CustomFragment();
        getSupportFragmentManager().beginTransaction().add(R.id.container, fragment, "customtag").commit(); 
    }

    ...

}

当使用 FragmentPagerAdapter 时，我们放弃适配器的片段管理，而不必执行上述步骤。默认情况下，它只会在当前位置的前面和后面预加载一个Fragment（尽管它不会破坏它们，除非您使用 FragmentStatePagerAdapter ）。这由 ViewPager.setOffscreenPageLimit（int ）。因此，直接调用适配器之外的片段调用方法不能保证是有效的，因为它们甚至不会生效。

When using a FragmentPagerAdapter, we relinquish fragment management to the adapter, and do not have to perform the above steps. By default, it will only preload one Fragment in front and behind the current position (although it does not destroy them unless you are using FragmentStatePagerAdapter). This is controlled by ViewPager.setOffscreenPageLimit(int). Because of this, directly calling methods on the fragments outside of the adapter is not guaranteed to be valid, because they may not even be alive.

为了缩短长篇小说，您的解决方案使用 putFragment 以后才能获得参考如此疯狂，而不像正常使用碎片的方法（上图）。很难获得参考，否则是因为片段是由适配器添加的，而不是你个人的。只需确保 offscreenPageLimit 足够高，以便随时加载所需的片段，因为您依靠它存在。这忽略了ViewPager的延迟加载功能，但似乎是您对应用程序的期望。

To cut a long story short, your solution to use putFragment to be able to get a reference afterwards is not so crazy, and not so unlike the normal way to use fragments anyway (above). It is difficult to obtain a reference otherwise because the fragment is added by the adapter, and not you personally. Just make sure that the offscreenPageLimit is high enough to load your desired fragments at all times, since you rely on it being present. This bypasses lazy loading capabilities of the ViewPager, but seems to be what you desire for your application.

另一种方法是覆盖 FragmentPageAdapter.instantiateItem View，int）并保存对返回之前从超级调用返回的片段的引用（如果已存在，则具有查找片段的逻辑）。

Another approach is to override FragmentPageAdapter.instantiateItem(View, int) and save a reference to the fragment returned from the super call before returning it (it has the logic to find the fragment, if already present).

有关更全面的图片，请查看 FragmentPagerAdapter （简称）和 ViewPager （长）。

For a fuller picture, have a look at some of the source of FragmentPagerAdapter (short) and ViewPager (long).

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