105

我正在使用 Android 4.0 ICS 和 Fragments 开发应用程序。

考虑 ICS 4.0.3（API 级别 15）API 的演示示例应用程序中的这个修改示例：

public class FragmentTabs extends Activity {

private static final String TAG = FragmentTabs.class.getSimpleName();

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

    final ActionBar bar = getActionBar();
    bar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
    bar.setDisplayOptions(0, ActionBar.DISPLAY_SHOW_TITLE);

    bar.addTab(bar.newTab()
            .setText("Simple")
            .setTabListener(new TabListener<SimpleFragment>(
                    this, "mysimple", SimpleFragment.class)));

    if (savedInstanceState != null) {
        bar.setSelectedNavigationItem(savedInstanceState.getInt("tab", 0));
        Log.d(TAG, "FragmentTabs.onCreate tab: " + savedInstanceState.getInt("tab"));
        Log.d(TAG, "FragmentTabs.onCreate number: " + savedInstanceState.getInt("number"));
    }

}

@Override
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt("tab", getActionBar().getSelectedNavigationIndex());
}

public static class TabListener<T extends Fragment> implements ActionBar.TabListener {
    private final Activity mActivity;
    private final String mTag;
    private final Class<T> mClass;
    private final Bundle mArgs;
    private Fragment mFragment;

    public TabListener(Activity activity, String tag, Class<T> clz) {
        this(activity, tag, clz, null);
    }

    public TabListener(Activity activity, String tag, Class<T> clz, Bundle args) {
        mActivity = activity;
        mTag = tag;
        mClass = clz;
        mArgs = args;

        // Check to see if we already have a fragment for this tab, probably
        // from a previously saved state.  If so, deactivate it, because our
        // initial state is that a tab isn't shown.
        mFragment = mActivity.getFragmentManager().findFragmentByTag(mTag);
        if (mFragment != null && !mFragment.isDetached()) {
            Log.d(TAG, "constructor: detaching fragment " + mTag);
            FragmentTransaction ft = mActivity.getFragmentManager().beginTransaction();
            ft.detach(mFragment);
            ft.commit();
        }
    }

    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        if (mFragment == null) {
            mFragment = Fragment.instantiate(mActivity, mClass.getName(), mArgs);
            Log.d(TAG, "onTabSelected adding fragment " + mTag);
            ft.add(android.R.id.content, mFragment, mTag);
        } else {
            Log.d(TAG, "onTabSelected attaching fragment " + mTag);
            ft.attach(mFragment);
        }
    }

    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        if (mFragment != null) {
            Log.d(TAG, "onTabUnselected detaching fragment " + mTag);
            ft.detach(mFragment);
        }
    }

    public void onTabReselected(Tab tab, FragmentTransaction ft) {
        Toast.makeText(mActivity, "Reselected!", Toast.LENGTH_SHORT).show();
    }
}

public static class SimpleFragment extends Fragment {
    TextView textView;
    int mNum;

    /**
     * When creating, retrieve this instance's number from its arguments.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Log.d(FragmentTabs.TAG, "onCreate " + (savedInstanceState != null ? ("state " + savedInstanceState.getInt("number")) : "no state"));
        if(savedInstanceState != null) {
            mNum = savedInstanceState.getInt("number");
        } else {
            mNum = 25;
        }
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        Log.d(TAG, "onActivityCreated");
        if(savedInstanceState != null) {
            Log.d(TAG, "saved variable number: " + savedInstanceState.getInt("number"));
        }
        super.onActivityCreated(savedInstanceState);
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        Log.d(TAG, "onSaveInstanceState saving: " + mNum);
        outState.putInt("number", mNum);
        super.onSaveInstanceState(outState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        Log.d(FragmentTabs.TAG, "onCreateView " + (savedInstanceState != null ? ("state: " + savedInstanceState.getInt("number")) : "no state"));
        textView = new TextView(getActivity());
        textView.setText("Hello world: " + mNum);
        textView.setBackgroundDrawable(getResources().getDrawable(android.R.drawable.gallery_thumb));
        return textView;
    }
}

}

以下是运行此示例然后旋转手机检索到的输出：

06-11 11:31:42.559: D/FragmentTabs(10726): onTabSelected adding fragment mysimple
06-11 11:31:42.559: D/FragmentTabs(10726): onCreate no state
06-11 11:31:42.559: D/FragmentTabs(10726): onCreateView no state
06-11 11:31:42.567: D/FragmentTabs(10726): onActivityCreated
06-11 11:31:45.286: D/FragmentTabs(10726): onSaveInstanceState saving: 25
06-11 11:31:45.325: D/FragmentTabs(10726): onCreate state 25
06-11 11:31:45.340: D/FragmentTabs(10726): constructor: detaching fragment mysimple
06-11 11:31:45.340: D/FragmentTabs(10726): onTabSelected attaching fragment mysimple
06-11 11:31:45.348: D/FragmentTabs(10726): FragmentTabs.onCreate tab: 0
06-11 11:31:45.348: D/FragmentTabs(10726): FragmentTabs.onCreate number: 0
06-11 11:31:45.348: D/FragmentTabs(10726): onCreateView state: 25
06-11 11:31:45.348: D/FragmentTabs(10726): onActivityCreated
06-11 11:31:45.348: D/FragmentTabs(10726): saved variable number: 25
06-11 11:31:45.348: D/FragmentTabs(10726): onCreateView no state
06-11 11:31:45.348: D/FragmentTabs(10726): onActivityCreated

我的问题是，为什么 onCreateView 和 onActivityCreated 被调用了两次？第一次使用带有已保存状态的 Bundle，第二次使用 null savedInstanceState？

这会导致在旋转时保留片段状态的问题。

4

5 回答 5

45

我也为此摸不着头脑，由于戴夫的解释有点难以理解，我将发布我的（显然是工作的）代码：

private class TabListener<T extends Fragment> implements ActionBar.TabListener {
    private Fragment mFragment;
    private Activity mActivity;
    private final String mTag;
    private final Class<T> mClass;

    public TabListener(Activity activity, String tag, Class<T> clz) {
        mActivity = activity;
        mTag = tag;
        mClass = clz;
        mFragment=mActivity.getFragmentManager().findFragmentByTag(mTag);
    }

    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        if (mFragment == null) {
            mFragment = Fragment.instantiate(mActivity, mClass.getName());
            ft.replace(android.R.id.content, mFragment, mTag);
        } else {
            if (mFragment.isDetached()) {
                ft.attach(mFragment);
            }
        }
    }

    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        if (mFragment != null) {
            ft.detach(mFragment);
        }
    }

    public void onTabReselected(Tab tab, FragmentTransaction ft) {
    }
}

如您所见，它与 Android 示例非常相似，除了在构造函数中不分离，并且使用replace 而不是 add。

经过多次头疼和反复试验后，我发现在构造函数中找到片段似乎使双重 onCreateView 问题神奇地消失了（我假设当通过 ActionBar.setSelectedNavigationItem() 路径调用 onTabSelected 时它最终为 null保存/恢复状态）。

于 2012-10-07T08:07:28.297 回答
28

我在一个只携带一个片段（有时会被替换）的简单活动中遇到了同样的问题。然后我意识到我只在片段中使用 onSaveInstanceState（和 onCreateView 来检查 savedInstanceState），而不是在活动中。

在设备上，包含片段的活动重新启动并调用 onCreated。在那里我确实附上了所需的片段（在第一次开始时是正确的）。

在设备上，Android 首先重新创建了可见的片段，然后调用了我的片段所附加的包含活动的 onCreate，从而替换了原来的可见片段。

为了避免这种情况，我只是更改了我的活动以检查 savedInstanceState：

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

if (savedInstanceState != null) {
/**making sure you are not attaching the fragments again as they have 
 been 
 *already added
 **/
 return; 
 }
 else{
  // following code to attach fragment initially
 }

 }

我什至没有覆盖活动的 onSaveInstanceState。

于 2013-09-30T17:52:33.187 回答
26

好的，这就是我发现的。

我不明白的是，当配置更改发生（手机旋转）时附加到活动的所有片段都会重新创建并添加回活动。（这是有道理的）

TabListener 构造函数中发生的情况是，如果找到该选项卡并将其附加到活动，则该选项卡将被分离。见下文：

mFragment = mActivity.getFragmentManager().findFragmentByTag(mTag);
    if (mFragment != null && !mFragment.isDetached()) {
        Log.d(TAG, "constructor: detaching fragment " + mTag);
        FragmentTransaction ft = mActivity.getFragmentManager().beginTransaction();
        ft.detach(mFragment);
        ft.commit();
    }

稍后在 onCreate 活动中，从保存的实例状态中选择了先前选择的选项卡。见下文：

if (savedInstanceState != null) {
    bar.setSelectedNavigationItem(savedInstanceState.getInt("tab", 0));
    Log.d(TAG, "FragmentTabs.onCreate tab: " + savedInstanceState.getInt("tab"));
    Log.d(TAG, "FragmentTabs.onCreate number: " + savedInstanceState.getInt("number"));
}

选择选项卡后，它将在 onTabSelected 回调中重新附加。

public void onTabSelected(Tab tab, FragmentTransaction ft) {
    if (mFragment == null) {
        mFragment = Fragment.instantiate(mActivity, mClass.getName(), mArgs);
        Log.d(TAG, "onTabSelected adding fragment " + mTag);
        ft.add(android.R.id.content, mFragment, mTag);
    } else {
        Log.d(TAG, "onTabSelected attaching fragment " + mTag);
        ft.attach(mFragment);
    }
}

附加的片段是对 onCreateView 和 onActivityCreated 方法的第二次调用。（第一次是当系统重新创建活动和所有附加的片段时） onSavedInstanceState Bundle 第一次会保存数据，但第二次不会。

解决方案是不在 TabListener 构造函数中分离片段，只需将其保持连接即可。（您仍然需要通过它的标签在 FragmentManager 中找到它）此外，在 onTabSelected 方法中，我在附加之前检查片段是否已分离。像这样的东西：

public void onTabSelected(Tab tab, FragmentTransaction ft) {
            if (mFragment == null) {
                mFragment = Fragment.instantiate(mActivity, mClass.getName(), mArgs);
                Log.d(TAG, "onTabSelected adding fragment " + mTag);
                ft.add(android.R.id.content, mFragment, mTag);
            } else {

                if(mFragment.isDetached()) {
                    Log.d(TAG, "onTabSelected attaching fragment " + mTag);
                    ft.attach(mFragment);
                } else {
                    Log.d(TAG, "onTabSelected fragment already attached " + mTag);
                }
            }
        }
于 2012-06-12T12:27:15.510 回答
12

这里的两个赞成的答案显示了具有导航模式的 Activity 的解决方案NAVIGATION_MODE_TABS，但我对NAVIGATION_MODE_LIST. 当屏幕方向改变时，它导致我的 Fragment 莫名其妙地失去了它们的状态，这真的很烦人。谢天谢地，由于他们有用的代码，我设法弄明白了。

基本上，当使用列表导航onNavigationItemSelected()时，无论您喜欢与否，都会在您的活动被创建/重新创建时自动调用。为了防止你的 FragmentonCreateView()被调用两次，这个初始的自动调用onNavigationItemSelected()应该检查 Fragment 是否已经存在于你的 Activity 中。如果是，立即返回，因为无事可做；如果不是，那么只需像往常一样构造 Fragment 并将其添加到 Activity 中。执行此检查可防止您的 Fragment 被不必要地再次创建，这就是导致onCreateView()被调用两次的原因！

请参阅onNavigationItemSelected()下面的实现。

public class MyActivity extends FragmentActivity implements ActionBar.OnNavigationListener
{
    private static final String STATE_SELECTED_NAVIGATION_ITEM = "selected_navigation_item";

    private boolean mIsUserInitiatedNavItemSelection;

    // ... constructor code, etc.

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

        if (savedInstanceState.containsKey(STATE_SELECTED_NAVIGATION_ITEM))
        {
            getActionBar().setSelectedNavigationItem(savedInstanceState.getInt(STATE_SELECTED_NAVIGATION_ITEM));
        }
    }

    @Override
    public void onSaveInstanceState(Bundle outState)
    {
        outState.putInt(STATE_SELECTED_NAVIGATION_ITEM, getActionBar().getSelectedNavigationIndex());

        super.onSaveInstanceState(outState);
    }

    @Override
    public boolean onNavigationItemSelected(int position, long id)
    {    
        Fragment fragment;
        switch (position)
        {
            // ... choose and construct fragment here
        }

        // is this the automatic (non-user initiated) call to onNavigationItemSelected()
        // that occurs when the activity is created/re-created?
        if (!mIsUserInitiatedNavItemSelection)
        {
            // all subsequent calls to onNavigationItemSelected() won't be automatic
            mIsUserInitiatedNavItemSelection = true;

            // has the same fragment already replaced the container and assumed its id?
            Fragment existingFragment = getSupportFragmentManager().findFragmentById(R.id.container);
            if (existingFragment != null && existingFragment.getClass().equals(fragment.getClass()))
            {
                return true; //nothing to do, because the fragment is already there 
            }
        }

        getSupportFragmentManager().beginTransaction().replace(R.id.container, fragment).commit();
        return true;
    }
}

我从这里借鉴了这个解决方案的灵感。

于 2013-01-12T16:44:18.300 回答
8

在我看来，这是因为您每次都在实例化您的 TabListener ......所以系统正在从 savedInstanceState 重新创建您的片段，然后您在 onCreate 中再次执行此操作。

您应该将其包装在 a 中，if(savedInstanceState == null)以便仅在没有 savedInstanceState 时才会触发。

于 2012-06-11T16:57:50.947 回答