Android 中ViewPager重排序與更新實例詳解
最近的項目中有欄目訂閱功能，在更改欄目順序以后需要更新ViewPager。類似于網易新聞的頻道管理。
在重新排序之后調用了PagerAdapter的notifyDataSetChanged方法，發現ViewPager并沒有更新，于是我開始跟蹤源碼，在調用PagerAdapter的notifyDataSetChanged方法后，會觸發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(); } }通過源碼發現，在發生數據更新是，ViewPager會調用Adapter.getItemPosition判斷當前頁是否發生變化，如果當前頁沒有變化則返回POSITION_UNCHANGED，如果當前頁的順序發生變化則返回新的索引，如果當前頁不存在則返回POSITION_NONE將會移除當前頁并更新當前頁。
接著查看ViewPagerAdapter的getItemPosition方法
public int getItemPosition(Object object) { return POSITION_UNCHANGED; }發現默認返回POSITION_UNCHANGED，這也是為什么我們的ViewPager沒有更新的原因，網上有多種解決方案，其中一種是直接重寫getItemPosition直接返回POSITION_NONE。我也試著使用了，發現并沒有什么用，數據還是沒有更新，后來發現我的Adapter繼承的是FragmentPagerAdapter。而FragmentPagerAdapter自帶了緩存策略，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; }我們可以發現FragmentPagerAdapter通過其內部的FragmentManager管理Fragment緩存，而每一個Fragment都是通過name來分別的，而name則由makeFragmentName生成，我們查看makeFragmentName方法
private static String makeFragmentName(int viewId, long id) { return "android:switcher:" + viewId + ":" + id; }很簡單拼接的字符串，一個是Viewpager的id，一個是由getItemId方法生成，而getItemId方法更簡單直接返回position，這也就是為什么我們不能更新數據的原因。
/** * 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; }知道原因以后接著就開始改造Adapter，首先為每一個頻道生成唯一的ID我的做法是使用一個Map來保存，頻道名稱與ID的對應關系，使用一個List來保存之前的Position順序，記得在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)); } }接著重寫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; }還有getItemId
@Override public long getItemId(int position) { return IdsMap.get(getPageTitle(position)); }完整的代碼
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)); } }}感謝閱讀，希望能幫助到大家，謝謝大家對本站的支持！
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