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。