1

我有一个Activity包含一些Fragments 的 a ViewPager。这是适配器（mFooFragment's 是Activity.

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;
    }
}

然后我稍后会做类似的事情：

mSellingFragment.refresh();

不幸的是，成员片段可以是null. 一种情况我可以接受并且可以检查 - 页面是延迟加载的，因此在显示之前它们不可用。不过没关系，我只想refresh()对可见页面进行操作。

但有时它们null甚至是可见的。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被破坏但片段被保留，然后当我回到它时，活动以某种方式用旧片段重新创建，并且getItem()永远不会被调用，所以mFooFragment永远不会设置。

是对的吗？究竟应该如何处理这个问题？我能想到的一种方法是向setFooFragment(FooFragment f) { mFooFragment = f; }Activity 添加大量函数并从中调用它们，FooFragment.onAttach()但必须有更好的方法！

编辑

我查找了FragmentPagerAdapter（通常是在 Android 领域获得答案的唯一方法！）的源代码，它确实保存了片段：

@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，它不会使那些重新附加Fragment的 s 可用于子类！

4

1 回答 1

0

我目前的，有点老套的解决方案是制作 FragmentPagerAdapter 的副本，并在添加片段时添加一个处理程序：void onItemCreated(Fragment fragment, int position)然后您可以在自定义寻呼机适配器中覆盖它以将片段保存为成员变量。下面的完整代码（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;
    }
}
于 2012-11-07T17:36:55.057 回答