Android 中ViewPager重排序与更新实例详解
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)); } }}感谢阅读，希望能帮助到大家，谢谢大家对本站的支持！
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