FragmentPagerAdapter动态移除问题解决

在做一个项目的时候，遇到了FragmentPagerAdapter 要动态移除和添加fragment的问题，开始时我用的是如下代码：

public class NewsPagerAdapter extends FragmentPagerAdapter  {
    private List<NewsFragment> fragments;
    public NewsPagerAdapter(FragmentManager fm, List<NewsFragment> fragments) {
        super(fm);
        this.fragments = fragments;
    }

    @Override
    public Fragment getItem(int i) {
        return fragments.get(i);
    }

    @Override
    public int getCount() {
        return fragments.size();
    }

    public void updateList(List<NewsFragment> fragments) {
        this.fragments = fragments;
        notifyDataSetChanged();
    }
    
}

然后调用UpdateList发现并没有更新，原本的页面还在，查了资料说FragmentStatePagerAdapter 可以完美解决

public class NewsPagerAdapter extends FragmentStatePagerAdapter {
    private List<NewsFragment> fragments;
    public NewsPagerAdapter(FragmentManager fm, List<NewsFragment> fragments) {
        super(fm);
        this.fragments = fragments;
    }

    @Override
    public Fragment getItem(int i) {
        return fragments.get(i);
    }

    @Override
    public int getCount() {
        return fragments.size();
    }

    public void updateList(List<NewsFragment> fragments) {
        this.fragments = fragments;
        notifyDataSetChanged();
    }
    /**
     * 使用这个方式，让页面不缓存，能够在清除fragment的时候对其做了删除
     *
     * @param object
     * @return
     */
    @Override
    public int getItemPosition(Object object) {
        return PagerAdapter.POSITION_NONE;
    }
}

注意要重写getItemPosition方法
跟踪原因,查看FragmentPagerAdapter内部的添加和移除方法：

@Override
    public Object instantiateItem(ViewGroup container, int position) {
        //这里获取的itemId，就是position
        final long itemId = getItemId(position);

        // Do we already have this fragment?
        //获取的tag，就是容器id和position，问题就在这里
        String name = makeFragmentName(container.getId(), itemId);
        //利用Tag获取Fragment，那么获取出来当然还是最开始添加的一个，获取出来不为null，就不是使用新的列表中创建的了
        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));
        }

        return fragment;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
                //移除的时候竟然不是remove，是detach，看来它是为了还可以继续用
        mCurTransaction.detach((Fragment)object);
    }

原来不是remove而是detach，所以没办法真的移除

而FragmentStatePagerAdapter内部销毁是采用remove

@Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        Fragment fragment = (Fragment) object;
        //它是采用的remove方式
        mCurTransaction.remove(fragment);
    }

那么为什么还要重写getItemPosition方法？

我们看源码

void dataSetChanged() {
        // This method only gets called if our observer is attached, so mAdapter is non-null.

        final int adapterCount = mAdapter.getCount();
        mExpectedAdapterCount = adapterCount;
        boolean needPopulate = mItems.size() < mOffscreenPageLimit * 2 + 1
                && mItems.size() < adapterCount;
        int newCurrItem = mCurItem;

        boolean isUpdating = false;
        for (int i = 0; i < mItems.size(); i++) {
            final ItemInfo ii = mItems.get(i);
            final int newPos = mAdapter.getItemPosition(ii.object);
            //默认的值，即不改变
            if (newPos == PagerAdapter.POSITION_UNCHANGED) {
                continue;
            }

            //如果设置这个的话，会移除item+调用destroyItem
            if (newPos == PagerAdapter.POSITION_NONE) {
                mItems.remove(i);
                i--;

                if (!isUpdating) {
                    mAdapter.startUpdate(this);
                    isUpdating = true;
                }
                //调用destroyItem，就可以remove掉了
                mAdapter.destroyItem(this, ii.position, ii.object);
                needPopulate = true;

                if (mCurItem == ii.position) {
                    // Keep the current item in the valid range
                    newCurrItem = Math.max(0, Math.min(mCurItem, adapterCount - 1));
                    needPopulate = true;
                }
                continue;
            }

            if (ii.position != newPos) {
                if (ii.position == mCurItem) {
                    // Our current item changed position. Follow it.
                    newCurrItem = newPos;
                }

                ii.position = newPos;
                needPopulate = true;
            }
        }
    }

getItemPosition使用PagerAdapter.POSITION_NONE，在adapter调用notifyDataSetChange的时候，那么ViewPager的dataSetChanged()方法会被调用。

最终效果

1566983118457.gif