片段似乎非常适合将UI逻辑分离到一些模块中。但是ViewPager的生命周期对我来说仍然是模糊的。所以我们非常需要大师的思想!

Edit

参见下面的哑巴解决方案;-)

范围

主活动有一个带有片段的ViewPager。这些片段可以为其他(子主)活动实现稍微不同的逻辑,因此片段的数据是通过活动内部的回调接口填充的。第一次发射时一切正常,但是……

问题

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.

问题

也许我使用了错误的模式,但即使是Android 3 Pro书也没有太多关于它的内容。所以,请给我一记重拳告诉我怎么做才是正确的。很多谢谢!

Code

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又名助手

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
}

}

片段

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

愚蠢的解决方案是将碎片保存在onSaveInstanceState(主机活动)与putFragment和获得他们在onCreate通过getFragment。但我仍然有一种奇怪的感觉,事情不应该这样发展……参见下面的代码:

    @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();
    ...
}

当前回答

BasePagerAdapter是什么?你应该使用一个标准的寻页适配器——FragmentPagerAdapter或FragmentStatePagerAdapter,这取决于你是否想要ViewPager不再需要的片段被保留(前者)或保存它们的状态(后者)并在再次需要时重新创建。

使用ViewPager的示例代码可以在这里找到

的确,跨活动实例管理视图寻呼机中的片段有点复杂,因为框架中的FragmentManager负责保存状态并恢复寻呼机所生成的任何活动片段。这实际上意味着,适配器在初始化时需要确保它与任何恢复的片段重新连接。您可以查看FragmentPagerAdapter或FragmentStatePagerAdapter的代码,以了解这是如何实现的。

其他回答

对于你的问题,我找到了另一个相对简单的解决方法。

正如你从FragmentPagerAdapter源代码中看到的,由FragmentPagerAdapter管理的片段存储在FragmentManager的标签下,使用:

String tag="android:switcher:" + viewId + ":" + index;

viewId是container. getid(),容器是你的ViewPager实例。索引是片段的位置。因此,你可以保存对象id到outState:

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt("viewpagerid" , mViewPager.getId() );
}

@Override
    protected void onCreate(Bundle savedInstanceState) {
    setContentView(R.layout.activity_main);
    if (savedInstanceState != null)
        viewpagerid=savedInstanceState.getInt("viewpagerid", -1 );  

    MyFragmentPagerAdapter titleAdapter = new MyFragmentPagerAdapter (getSupportFragmentManager() , this);        
    mViewPager = (ViewPager) findViewById(R.id.pager);
    if (viewpagerid != -1 ){
        mViewPager.setId(viewpagerid);
    }else{
        viewpagerid=mViewPager.getId();
    }
    mViewPager.setAdapter(titleAdapter);

如果你想与这个片段通信,你可以从FragmentManager中获取,比如:

getSupportFragmentManager().findFragmentByTag("android:switcher:" + viewpagerid + ":0")

我想为一个可能略有不同的情况提供一个替代解决方案,因为我对答案的许多搜索一直将我引向这个主题。

我的情况 -我动态创建/添加页面,并将它们滑动到ViewPager,但当旋转(onConfigurationChange),我最终会有一个新页面,因为当然OnCreate再次被调用。但是我想保持对旋转之前创建的所有页面的引用。

问题 -我没有为我创建的每个片段提供唯一的标识符,所以引用的唯一方法是以某种方式将引用存储在数组中,以便在旋转/配置更改后恢复。

解决方案 -关键的概念是让Activity(显示片段)也管理现有片段的引用数组,因为这个Activity可以利用onSaveInstanceState中的Bundles

public class MainActivity extends FragmentActivity

因此,在这个Activity中,我声明了一个私有成员来跟踪打开的页面

private List<Fragment> retainedPages = new ArrayList<Fragment>();

每次onSaveInstanceState被调用时都会更新,并在onCreate中恢复

@Override
protected void onSaveInstanceState(Bundle outState) {
    retainedPages = _adapter.exportList();
    outState.putSerializable("retainedPages", (Serializable) retainedPages);
    super.onSaveInstanceState(outState);
}

...所以一旦它被存储,它就可以被检索…

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if (savedInstanceState != null) {
        retainedPages = (List<Fragment>) savedInstanceState.getSerializable("retainedPages");
    }
    _mViewPager = (CustomViewPager) findViewById(R.id.viewPager);
    _adapter = new ViewPagerAdapter(getApplicationContext(), getSupportFragmentManager());
    if (retainedPages.size() > 0) {
        _adapter.importList(retainedPages);
    }
    _mViewPager.setAdapter(_adapter);
    _mViewPager.setCurrentItem(_adapter.getCount()-1);
}

这些是对主活动的必要更改,因此我需要FragmentPagerAdapter中的成员和方法来工作,因此在

public class ViewPagerAdapter extends FragmentPagerAdapter

一个相同的构造(如上面的MainActivity中所示)

private List<Fragment> _pages = new ArrayList<Fragment>();

这种同步(如上面在onSaveInstanceState中使用的那样)是由方法特别支持的

public List<Fragment> exportList() {
    return _pages;
}

public void importList(List<Fragment> savedPages) {
    _pages = savedPages;
}

最后,在fragment类中

public class CustomFragment extends Fragment

为了使这一切顺利进行,有两个改变,首先

public class CustomFragment extends Fragment implements Serializable

然后将其添加到onCreate,这样Fragments就不会被破坏

setRetainInstance(true);

我仍然在思考Fragments和Android生命周期,所以这里需要注意的是,这种方法可能存在冗余/低效率。但它对我有用,我希望它能对其他有类似情况的人有所帮助。

add:

   @SuppressLint("ValidFragment")

在你上课之前。

如果它不起作用,可以这样做:

@SuppressLint({ "ValidFragment", "HandlerLeak" })

要获得方向改变后的片段,必须使用. gettag()。

    getSupportFragmentManager().findFragmentByTag("android:switcher:" + viewPagerId + ":" + positionOfItemInViewPager)

为了更多的处理,我为我的PageAdapter写了我自己的数组列表,通过viewPagerId和FragmentClass在任何位置获得片段:

public class MyPageAdapter extends FragmentPagerAdapter implements Serializable {
private final String logTAG = MyPageAdapter.class.getName() + ".";

private ArrayList<MyPageBuilder> fragmentPages;

public MyPageAdapter(FragmentManager fm, ArrayList<MyPageBuilder> fragments) {
    super(fm);
    fragmentPages = fragments;
}

@Override
public Fragment getItem(int position) {
    return this.fragmentPages.get(position).getFragment();
}

@Override
public CharSequence getPageTitle(int position) {
    return this.fragmentPages.get(position).getPageTitle();
}

@Override
public int getCount() {
    return this.fragmentPages.size();
}


public int getItemPosition(Object object) {
    //benötigt, damit bei notifyDataSetChanged alle Fragemnts refrehsed werden

    Log.d(logTAG, object.getClass().getName());
    return POSITION_NONE;
}

public Fragment getFragment(int position) {
    return getItem(position);
}

public String getTag(int position, int viewPagerId) {
    //getSupportFragmentManager().findFragmentByTag("android:switcher:" + R.id.shares_detail_activity_viewpager + ":" + myViewPager.getCurrentItem())

    return "android:switcher:" + viewPagerId + ":" + position;
}

public MyPageBuilder getPageBuilder(String pageTitle, int icon, int selectedIcon, Fragment frag) {
    return new MyPageBuilder(pageTitle, icon, selectedIcon, frag);
}


public static class MyPageBuilder {

    private Fragment fragment;

    public Fragment getFragment() {
        return fragment;
    }

    public void setFragment(Fragment fragment) {
        this.fragment = fragment;
    }

    private String pageTitle;

    public String getPageTitle() {
        return pageTitle;
    }

    public void setPageTitle(String pageTitle) {
        this.pageTitle = pageTitle;
    }

    private int icon;

    public int getIconUnselected() {
        return icon;
    }

    public void setIconUnselected(int iconUnselected) {
        this.icon = iconUnselected;
    }

    private int iconSelected;

    public int getIconSelected() {
        return iconSelected;
    }

    public void setIconSelected(int iconSelected) {
        this.iconSelected = iconSelected;
    }

    public MyPageBuilder(String pageTitle, int icon, int selectedIcon, Fragment frag) {
        this.pageTitle = pageTitle;
        this.icon = icon;
        this.iconSelected = selectedIcon;
        this.fragment = frag;
    }
}

public static class MyPageArrayList extends ArrayList<MyPageBuilder> {
    private final String logTAG = MyPageArrayList.class.getName() + ".";

    public MyPageBuilder get(Class cls) {
        // Fragment über FragmentClass holen
        for (MyPageBuilder item : this) {
            if (item.fragment.getClass().getName().equalsIgnoreCase(cls.getName())) {
                return super.get(indexOf(item));
            }
        }
        return null;
    }

    public String getTag(int viewPagerId, Class cls) {
        // Tag des Fragment unabhängig vom State z.B. nach bei Orientation change
        for (MyPageBuilder item : this) {
            if (item.fragment.getClass().getName().equalsIgnoreCase(cls.getName())) {
                return "android:switcher:" + viewPagerId + ":" + indexOf(item);
            }
        }
        return null;
    }
}

所以只要用这些片段创建一个MyPageArrayList:

    myFragPages = new MyPageAdapter.MyPageArrayList();

    myFragPages.add(new MyPageAdapter.MyPageBuilder(
            getString(R.string.widget_config_data_frag),
            R.drawable.ic_sd_storage_24dp,
            R.drawable.ic_sd_storage_selected_24dp,
            new WidgetDataFrag()));

    myFragPages.add(new MyPageAdapter.MyPageBuilder(
            getString(R.string.widget_config_color_frag),
            R.drawable.ic_color_24dp,
            R.drawable.ic_color_selected_24dp,
            new WidgetColorFrag()));

    myFragPages.add(new MyPageAdapter.MyPageBuilder(
            getString(R.string.widget_config_textsize_frag),
            R.drawable.ic_settings_widget_24dp,
            R.drawable.ic_settings_selected_24dp,
            new WidgetTextSizeFrag()));

并将它们添加到viewPager:

    mAdapter = new MyPageAdapter(getSupportFragmentManager(), myFragPages);
    myViewPager.setAdapter(mAdapter);

在此之后,你可以通过使用它的类来获得正确的方向改变片段:

        WidgetDataFrag dataFragment = (WidgetDataFrag) getSupportFragmentManager()
            .findFragmentByTag(myFragPages.getTag(myViewPager.getId(), WidgetDataFrag.class));

有一点不同的意见,不要自己存储碎片,而是把它留给FragmentManager,当你需要对碎片做一些事情时,在FragmentManager中查找它们:

//make sure you have the right FragmentManager 
//getSupportFragmentManager or getChildFragmentManager depending on what you are using to manage this stack of fragments
List<Fragment> fragments = fragmentManager.getFragments();
if(fragments != null) {
   int count = fragments.size();
   for (int x = 0; x < count; x++) {
       Fragment fragment = fragments.get(x);
       //check if this is the fragment we want, 
       //it may be some other inspection, tag etc.
       if (fragment instanceof MyFragment) {
           //do whatever we need to do with it
       }
   }
}

如果你有很多fragment, instanceof check的成本可能不是你想要的,但是记住FragmentManager已经记录了Fragments,这是一件好事。