我有一个Activity在Fragment中包含一些ViewPager个。这是适配器（mFooFragment是Activity的成员。
private class HomePagerAdapter extends FragmentPagerAdapter
{
public HomePagerAdapter(FragmentManager fm)
{
super(fm);
}
@Override
public Fragment getItem(int position)
{
switch (position)
{
case 0:
mSellingFragment = new SellingFragment();
return mSellingFragment;
case 1:
mSearchFragment = new SearchFragment();
return mSearchFragment;
case 2:
mProfileFragment = new ProfileFragment();
return mProfileFragment;
}
return null;
}
@Override
public int getCount()
{
return 3;
}
}
然后我做了类似的事情：
mSellingFragment.refresh();
不幸的是，成员片段可以是null。一个案例我很好，可以检查 - 页面是延迟加载的，所以它们在显示之前是不可用的。但是没关系，我只希望refresh()在可见页面上行动。
但有时即使可见，它们也是null。我认为正在发生的事情是这样的：
我的片段有setRetainInstance(true)所以当我离开活动时它会被破坏但片段会被保留，然后当我回到它时，活动会以某种方式用旧片段重新创建，{{{永远不会调用1}}，因此永远不会设置getItem()。
是吗？一个人应该怎么处理这个？我能想到的一种方法是向Activity添加一大堆mFooFragment函数并从setFooFragment(FooFragment f) { mFooFragment = f; }调用它们，但必须有更好的方法！
我查找了FooFragment.onAttach()的源代码（通常是在Android-land中获得答案的唯一方法！）确实是它保存片段的情况：
FragmentPagerAdapter不幸的是，它不会使那些重新连接的@Override
public Object instantiateItem(View container, int position) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
// Do we already have this fragment?
String name = makeFragmentName(container.getId(), position);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
if (DEBUG) Log.v(TAG, "Attaching item #" + position + ": f=" + fragment);
mCurTransaction.attach(fragment);
} else {
fragment = getItem(position);
if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
mCurTransaction.add(container.getId(), fragment,
makeFragmentName(container.getId(), position));
}
if (fragment != mCurrentPrimaryItem) {
fragment.setMenuVisibility(false);
}
return fragment;
}
可用于子类！
答案 0 :(得分：0)
我目前稍微讨厌的解决方案是制作FragmentPagerAdapter的副本，并为添加片段添加处理程序：void onItemCreated(Fragment fragment, int position)然后您可以在自定义寻呼机适配器中覆盖该片段以将片段保存为成员变量。完整的代码如下（未经测试！）
/*
* Copyright (C) 2011 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.your.package.name;
import android.os.Parcelable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentStatePagerAdapter;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.view.PagerAdapter;
import android.util.Log;
import android.view.View;
import android.view.ViewGroup;
/**
* Implementation of {@link android.support.v4.view.PagerAdapter} that
* represents each page as a {@link Fragment} that is persistently
* kept in the fragment manager as long as the user can return to the page.
*
* <p>This version of the pager is best for use when there are a handful of
* typically more static fragments to be paged through, such as a set of tabs.
* The fragment of each page the user visits will be kept in memory, though its
* view hierarchy may be destroyed when not visible. This can result in using
* a significant amount of memory since fragment instances can hold on to an
* arbitrary amount of state. For larger sets of pages, consider
* {@link FragmentStatePagerAdapter}.
*
* <p>When using FragmentPagerAdapter the host ViewPager must have a
* valid ID set.</p>
*
* <p>Subclasses only need to implement {@link #getItem(int)}
* and {@link #getCount()} to have a working adapter.
*
* <p>Here is an example implementation of a pager containing fragments of
* lists:
*
* {@sample development/samples/Support4Demos/src/com/example/android/supportv4/app/FragmentPagerSupport.java
* complete}
*
* <p>The <code>R.layout.fragment_pager</code> resource of the top-level fragment is:
*
* {@sample development/samples/Support4Demos/res/layout/fragment_pager.xml
* complete}
*
* <p>The <code>R.layout.fragment_pager_list</code> resource containing each
* individual fragment's layout is:
*
* {@sample development/samples/Support4Demos/res/layout/fragment_pager_list.xml
* complete}
*/
public abstract class FragmentPagerAdapter extends PagerAdapter {
private static final String TAG = "FragmentPagerAdapter";
private static final boolean DEBUG = false;
private final FragmentManager mFragmentManager;
private FragmentTransaction mCurTransaction = null;
private Fragment mCurrentPrimaryItem = null;
public FragmentPagerAdapter(FragmentManager fm) {
mFragmentManager = fm;
}
/**
* Return the Fragment associated with a specified position.
*/
public abstract Fragment getItem(int position);
@Override
public void startUpdate(ViewGroup container) {
}
@Override
public Object instantiateItem(ViewGroup container, int position) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
final long itemId = getItemId(position);
// Do we already have this fragment?
String name = makeFragmentName(container.getId(), itemId);
Fragment fragment = mFragmentManager.findFragmentByTag(name);
if (fragment != null) {
if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment);
mCurTransaction.attach(fragment);
} else {
fragment = getItem(position);
if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment);
mCurTransaction.add(container.getId(), fragment,
makeFragmentName(container.getId(), itemId));
}
if (fragment != mCurrentPrimaryItem) {
fragment.setMenuVisibility(false);
fragment.setUserVisibleHint(false);
}
onItemCreated(fragment, position);
return fragment;
}
/**
* Called when a fragment has been created/attached.
*/
public abstract void onItemCreated(Fragment fragment, int position);
@Override
public void destroyItem(ViewGroup container, int position, Object object) {
if (mCurTransaction == null) {
mCurTransaction = mFragmentManager.beginTransaction();
}
if (DEBUG) Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object
+ " v=" + ((Fragment)object).getView());
mCurTransaction.detach((Fragment)object);
}
@Override
public void setPrimaryItem(ViewGroup container, int position, Object object) {
Fragment fragment = (Fragment)object;
if (fragment != mCurrentPrimaryItem) {
if (mCurrentPrimaryItem != null) {
mCurrentPrimaryItem.setMenuVisibility(false);
mCurrentPrimaryItem.setUserVisibleHint(false);
}
if (fragment != null) {
fragment.setMenuVisibility(true);
fragment.setUserVisibleHint(true);
}
mCurrentPrimaryItem = fragment;
}
}
@Override
public void finishUpdate(ViewGroup container) {
if (mCurTransaction != null) {
mCurTransaction.commitAllowingStateLoss();
mCurTransaction = null;
mFragmentManager.executePendingTransactions();
}
}
@Override
public boolean isViewFromObject(View view, Object object) {
return ((Fragment)object).getView() == view;
}
@Override
public Parcelable saveState() {
return null;
}
@Override
public void restoreState(Parcelable state, ClassLoader loader) {
}
/**
* Return a unique identifier for the item at the given position.
*
* <p>The default implementation returns the given position.
* Subclasses should override this method if the positions of items can change.</p>
*
* @param position Position within this adapter
* @return Unique identifier for the item at position
*/
public long getItemId(int position) {
return position;
}
private static String makeFragmentName(int viewId, long id) {
return "android:switcher:" + viewId + ":" + id;
}
}