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));
}
}
}
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