从活动获取ViewPager片段

Question

我有一个Activity包含一些Fragment S IN一个ViewPager 。 这是适配器（ mFooFragment是Activity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;
    }
}

然后我以后做类似的事情：

mSellingFragment.refresh();

不幸的是，成员片段可以为null 。 我可以接受并可以检查的一种情况-页面被延迟加载，因此直到显示它们后才可用。 但这没关系，我只希望refresh()在可见页面上起作用。

但有时即使在可见时它们也为null 。 我认为这是怎么回事：

我的片段具有setRetainInstance(true)所以当我离开活动时，它被销毁了，但片段被保留了，然后当我返回它时，活动以某种方式用旧片段重新创建，并且从未调用过getItem() ，因此从未设置mFooFragment 。

那正确吗？ 应该如何处理呢？ 我能想到的一种方法是添加setFooFragment(FooFragment f) { mFooFragment = f; } setFooFragment(FooFragment f) { mFooFragment = f; }函数，然后从FooFragment.onAttach()调用它们，但是必须有更好的方法！

编辑

我查找了FragmentPagerAdapter的源代码（通常是在Android平台上获取答案的唯一方法！），确实可以保存片段：

@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;
}

不幸的是，它没有使那些重新连接的Fragment可供子类使用！

Answer 1

我当前略微棘手的解决方案是制作FragmentPagerAdapter的副本，并为添加片段的时间添加一个处理程序： void onItemCreated(Fragment fragment, int position) ，然后可以在自定义寻呼机适配器中覆盖该片段，以将片段另存为成员变量。 完整的代码如下（尚未测试！）

/*
 * 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;
    }
}