ViewPager重排序与更新

最近的项目中有栏目订阅功能，在更改栏目顺序以后需要更新ViewPager。类似于网易新闻的频道管理。
在重新排序之后调用了PagerAdapter的notifyDataSetChanged方法，发现ViewPager并没有更新，于是我开始跟踪源码，在调用PagerAdapter的notifyDataSetChanged方法后，会触发Viewpager的dataSetChanged方法。

  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);            if (newPos == PagerAdapter.POSITION_UNCHANGED) {                continue;            }            if (newPos == PagerAdapter.POSITION_NONE) {                mItems.remove(i);                i--;                if (!isUpdating) {                    mAdapter.startUpdate(this);                    isUpdating = true;                }                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;            }        }        if (isUpdating) {            mAdapter.finishUpdate(this);        }        Collections.sort(mItems, COMPARATOR);        if (needPopulate) {            // Reset our known page widths; populate will recompute them.            final int childCount = getChildCount();            for (int i = 0; i < childCount; i++) {                final View child = getChildAt(i);                final LayoutParams lp = (LayoutParams) child.getLayoutParams();                if (!lp.isDecor) {                    lp.widthFactor = 0.f;                }            }            setCurrentItemInternal(newCurrItem, false, true);            requestLayout();        }    }

通过源码发现，在发生数据更新是，ViewPager会调用Adapter.getItemPosition判断当前页是否发生变化，如果当前页没有变化则返回POSITION_UNCHANGED，如果当前页的顺序发生变化则返回新的索引，如果当前页不存在则返回POSITION_NONE将会移除当前页并更新当前页。
接着查看ViewPagerAdapter的getItemPosition方法

    public int getItemPosition(Object object) {        return POSITION_UNCHANGED;    }

发现默认返回POSITION_UNCHANGED，这也是为什么我们的ViewPager没有更新的原因，网上有多种解决方案，其中一种是直接重写getItemPosition直接返回POSITION_NONE。我也试着使用了，发现并没有什么用，数据还是没有更新，后来发现我的Adapter继承的是FragmentPagerAdapter。而FragmentPagerAdapter自带了缓存策略，查看其instantiateItem方法。

 @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);        }        return fragment;    }

我们可以发现FragmentPagerAdapter通过其内部的FragmentManager管理Fragment缓存，而每一个Fragment都是通过name来分别的，而name则由makeFragmentName生成，我们查看makeFragmentName方法

 private static String makeFragmentName(int viewId, long id) {        return "android:switcher:" + viewId + ":" + id;    }

很简单拼接的字符串，一个是Viewpager的id，一个是由getItemId方法生成，而getItemId方法更简单直接返回position，这也就是为什么我们不能更新数据的原因。

    /**     * 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;    }

知道原因以后接着就开始改造Adapter，首先为每一个频道生成唯一的ID我的做法是使用一个Map来保存，频道名称与ID的对应关系，使用一个List来保存之前的Position顺序，记得在notifyDataSetChanged中初始化，由于List保存的是之前的Position所以需要在完成更新后，再添加。

    int id=1;    Map<String,Integer> IdsMap=new HashMap<>();    List<String> preIds=new ArrayList<>();  @Override    public void notifyDataSetChanged() {        for(MenuInfo info:data){            if(!IdsMap.containsKey(info.getTitle())){                IdsMap.put(info.getTitle(),id++);            }        }        super.notifyDataSetChanged();        preIds.clear();        int size=getCount();        for(int i=0;i<size;i++){            preIds.add((String) getPageTitle(i));        }    }

接着重写getItemPosition

 @Override    public int getItemPosition(Object object) {        ItemFragment fragment= (ItemFragment) object;        String title=fragment.getTitle();        int preId = preIds.indexOf(fragment.getTitle());        int newId=-1;        int i=0;        int size=getCount();        for(;i<size;i++){            if(getPageTitle(i).equals(fragment.getTitle())){                newId=i;                break;            }        }        if(newId!=-1&&newId==preId){            Log.i("zgh","title="+title+" POSITION_UNCHANGED");            return POSITION_UNCHANGED;        }        if(newId!=-1){            Log.i("zgh","title="+title+" newId="+newId);            return newId;        }        Log.i("zgh","title="+title+" POSITION_NONE");        return POSITION_NONE;    }

还有getItemId

 @Override    public long getItemId(int position) {        return IdsMap.get(getPageTitle(position));    }

完整的代码

package com.trs.xizang.gov.adapter;import android.os.Bundle;import android.support.v4.app.Fragment;import android.support.v4.app.FragmentManager;import android.support.v4.app.FragmentPagerAdapter;import android.util.Log;import android.view.ViewGroup;import com.trs.lib.base.TRSUrlFragment;import com.trs.lib.bean.TRSMenu;import com.trs.lib.fragment.base.SimpleTitleFragment;import com.trs.xizang.gov.bean.MenuInfo;import com.trs.xizang.gov.fragment.ItemFragment;import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;/** * Created by zhuguohui on 2016/5/12. */public class MenuInfoPageAdapter extends FragmentPagerAdapter {    List<MenuInfo> data;    int id=1;    Map<String,Integer> IdsMap=new HashMap<>();    List<String> preIds=new ArrayList<>();    public MenuInfoPageAdapter(FragmentManager manager, List<MenuInfo> data){        super(manager);        this.data= data==null?  new ArrayList<MenuInfo>() :data;    }    @Override    public int getCount() {        return data.size();    }    @Override    public Fragment getItem(int position) {        ItemFragment fragment=new ItemFragment();        Bundle bundle=new Bundle();        bundle.putString(TRSUrlFragment.KEY_URL,data.get(position).getUrl());        bundle.putString(TRSUrlFragment.KEY_TITLE, data.get(position).getTitle());        fragment.setArguments(bundle);        return fragment;    }    @Override    public CharSequence getPageTitle(int position) {        return data.get(position).getTitle();    }    @Override    public Object instantiateItem(ViewGroup container, int position) {        return super.instantiateItem(container, position);    }    @Override    public long getItemId(int position) {        return IdsMap.get(getPageTitle(position));    }    @Override    public int getItemPosition(Object object) {        ItemFragment fragment= (ItemFragment) object;        String title=fragment.getTitle();        int preId = preIds.indexOf(fragment.getTitle());        int newId=-1;        int i=0;        int size=getCount();        for(;i<size;i++){            if(getPageTitle(i).equals(fragment.getTitle())){                newId=i;                break;            }        }        if(newId!=-1&&newId==preId){            Log.i("zgh","title="+title+" POSITION_UNCHANGED");            return POSITION_UNCHANGED;        }        if(newId!=-1){            Log.i("zgh","title="+title+" newId="+newId);            return newId;        }        Log.i("zgh","title="+title+" POSITION_NONE");        return POSITION_NONE;    }    @Override    public void notifyDataSetChanged() {        for(MenuInfo info:data){            if(!IdsMap.containsKey(info.getTitle())){                IdsMap.put(info.getTitle(),id++);            }        }        super.notifyDataSetChanged();        preIds.clear();        int size=getCount();        for(int i=0;i<size;i++){            preIds.add((String) getPageTitle(i));        }    }}

好了，终于可以正常重排序了，越来越佩服自己了。今天就这样了。