两次调用片段onCreateView和onActivityCreated
两次调用片段onCreateView和onActivityCreated
提问于 2012-06-11 23:44:51
我正在开发一个使用Android 4.0 ICS和fragments的应用程序。
考虑来自ICS 4.0.3 (API级别15) API的演示示例应用程序的修改后的示例：
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电话时检索到的输出：
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？第一次使用包含已保存状态的包，第二次使用空savedInstanceState？
这会导致在旋转时保留碎片状态的问题。
回答 4
Stack Overflow用户
回答已采纳
发布于 2012-06-12 20:27:16
好吧，这是我发现的。
我不理解的是，当配置发生变化(手机轮换)时，附加到活动的所有片段都会重新创建并添加回活动中。(这是有道理的)
在TabListener构造函数中发生的情况是，如果找到选项卡并将其附加到活动，则会将其分离。如下所示：
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activity onCreate中，会从保存的实例状态中选择先前选定的选项卡。如下所示：
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"));
}选择该选项卡后，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连接的片段时)第一次onSavedInstanceState捆绑包将保存数据，而不是第二次。
解决方案是不要分离TabListener构造函数中的片段，只需将其附加。(您仍然需要通过它的标签在FragmentManager中找到它)另外，在onTabSelected方法中，我在附加它之前检查片段是否被分离。如下所示：
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);
}
}
}Stack Overflow用户
发布于 2013-10-01 01:52:33
我也遇到过同样的问题，一个简单的活动只携带一个片段(有时会被替换)。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。
在设备打开时，包含片段的活动将重新启动，并调用onCreated。在那里我确实附加了所需的片段(在第一次开始时是正确的)。
在设备上，安卓首先重新创建可见的片段，然后调用包含活动的onCreate，其中附加了我的片段，从而替换了原始的可见片段。
为了避免这种情况，我简单地将我的活动更改为检查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
}
}我甚至没有覆盖活动的onSaveInstanceState。
Stack Overflow用户
发布于 2013-01-13 00:44:18
这里的两个高分答案显示了导航模式为NAVIGATION_MODE_TABS的活动的解决方案，但我在NAVIGATION_MODE_LIST上也遇到了同样的问题。当屏幕方向改变时，它导致我的片段莫名其妙地丢失了它们的状态，这真的很烦人。谢天谢地，由于他们的有用代码，我设法弄清楚了。
基本上，在使用列表导航时，无论您是否喜欢，在创建/重新创建活动时都会自动调用onNavigationItemSelected()。为了防止片段的onCreateView()被调用两次，这个对onNavigationItemSelected()的初始自动调用应该检查片段是否已经存在于您的活动中。如果是，立即返回，因为没有什么可做的；如果不是，那么只需构造片段并像往常一样将其添加到活动中。执行此检查可以防止不必要的片段被再次创建，这就是导致onCreateView()被调用两次的原因！
请参阅下面我的onNavigationItemSelected()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here那里得到了这个解决方案的灵感。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/10983396
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