Android 中ViewPager重排序与更新实例详解

发布时间：2020-10-15 18:27:09 作者：lqh
来源：脚本之家阅读：236

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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Android 中ViewPager重排序与更新实例详解

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