片段 onCreateView 和 onActivityCreated 调用了两次 [英] Fragment onCreateView and onActivityCreated called twice
问题描述
我正在使用 Android 4.0 ICS 和片段开发应用.
I'm developing an app using Android 4.0 ICS and fragments.
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示例:
Consider this modified example from the ICS 4.0.3 (API level 15) API's demo example app:
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;
}
}
}
这是通过运行此示例然后旋转手机检索到的输出:
Here is the output retrieved from running this example and then rotating the phone:
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?第一次使用具有保存状态的捆绑包，第二次使用空的savedInstanceState?
My question is, why is the onCreateView and onActivityCreated called twice? The first time with a Bundle with the saved state and the second time with a null savedInstanceState?
这会导致在旋转时保留片段状态的问题.
This is causing problems with retaining the state of the fragment on rotation.
推荐答案
好的，这是我发现的.
我不明白的是，当发生配置更改(手机旋转)时，所有附加到 Activity 的片段都会重新创建并添加回 Activity.(这是有道理的)
What I didn't understand is that all fragments that are attached to an activity when a config change happens (phone rotates) are recreated and added back to the activity. (which makes sense)
在 TabListener 构造函数中发生的事情是，如果找到并附加到活动，则该选项卡将被分离.见下文:
What was happening in the TabListener constructor was the tab was detached if it was found and attached to the activity. See below:
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:
Later in the activity onCreate the previously selected tab was selected from the saved instance state. See below:
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"));
}
When the tab was selected it would be reattached in the onTabSelected callback.
When the tab was selected it would be reattached in the onTabSelected callback.
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);
}
}
被附加的片段是对 onCreateView 和 onActivityCreated 方法的第二次调用.(第一次是在系统重新创建活动和所有附加片段时)第一次 onSavedInstanceState Bundle 会保存数据，但不会第二次.
The fragment being attached is the second call to the onCreateView and onActivityCreated methods. (The first being when the system is recreating the acitivity and all attached fragments) The first time the onSavedInstanceState Bundle would have saved data but not the second time.
解决方案是不要在 TabListener 构造函数中分离片段，只需将其附加即可.(您仍然需要通过它的标签在 FragmentManager 中找到它)此外，在 onTabSelected 方法中，我会在附加之前检查该片段是否已分离.像这样:
The solution is to not detach the fragment in the TabListener constructor, just leave it attached. (You still need to find it in the FragmentManager by it's tag) Also, in the onTabSelected method I check to see if the fragment is detached before I attach it. Something like this:
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);
}
}
}
这篇关于片段 onCreateView 和 onActivityCreated 调用了两次的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
