FragmentPagerAdapter 页面类型、数量、内容更新问题
场景 存在一种需求，当用户系统中，属于某一组织的用户登录之后（或者账户切换），要求主页面显示不同的ViewPager + Fragment组合，并且要求app无需退出就能刷新组合以及组合中的页面。 此外，为了保证Fragment和Fragment中View不必要的inflate和渲染，要求尽可能重用已存在的Fragment和View。显然FragmentPagerAdapter是首选。但是存在三个问题： 1、FragmentPagerAdapter默认无法更新，需要重写getItemPosition，返回值为PagerAdapter.POSITION_NONE才可以更新 2、重用的Fragment设置参数无法重新初始化 3、重用的Fragment类型和新的Fragment类型存在不匹配问题，如旧的UserFragment页面，但是新的要求是ListFragment，所以类型存在问题。 解决方案 我们需要重写FragmentPagerAdapter，但问题是存在各种不方便的因素，因此，我们需要自定义FragmentPagerAdapter。 public abstract class CustomFragmentPagerAdapter extends PagerAdapter { private static final String TAG = "FragmentPagerAdapter"; private static final boolean DEBUG = false; private final FragmentManager mFragmentManager; private FragmentTransaction mCurTransaction = null; private Fragment mCurrentPrimaryItem = null; private final LongSparseArray<String> fragmentViewTypeManager = new LongSparseArray<String>(); public CustomFragmentPagerAdapter(FragmentManager fm) { mFragmentManager = fm; } @Override public void startUpdate(ViewGroup container) { if (container.getId() == View.NO_ID) { //viewPager必须赋值ID，否则无法添加fragment throw new IllegalStateException("ViewPager with adapter " + this + " requires a view id"); } } @SuppressWarnings("ReferenceEquality") @Override public Object instantiateItem(ViewGroup container, int position) { mCurTransaction = beginTransaction(); final long itemId = getItemId(position); final int count = this.getFragmentTypeCount(); int fragmentType = 0; if(count>0){ fragmentType = getItemFragmentType(position); } if(fragmentType>0 && fragmentType>=count){ throw new IllegalArgumentException("{fragmentType's number >= fragmentTypeCount's number} is illegal"); } // 生成tag，用于保存和标记每个位置的fragment final String name = makeFragmentName(container.getId(), itemId); //生成tag final String oldFragmentName = fragmentViewTypeManager.get(fragmentType); Fragment fragment = mFragmentManager.findFragmentByTag(name); if (fragment != null) { final String fragmentClassName = fragment.getClass().getName(); if(!fragmentClassName.equals(oldFragmentName)) { //如果发现新旧类型不一致，移除旧类型 if (DEBUG) Log.v(TAG, "Removeing item #" + itemId + ": f=" + fragment); mCurTransaction.remove(fragment); //获取新类型 fragment = getItem(null,position); if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment); mCurTransaction.add(container.getId(), fragment,name); }else { Fragment newFragment = getItem(fragment,position); //获取newFragment ，如果2次fragment不一致，移除旧的fragment if(newFragment!=fragment){ if (DEBUG) Log.v(TAG, "Removeing item #" + itemId + ": f=" + fragment); mCurTransaction.remove(fragment); fragment = newFragment; if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment); mCurTransaction.add(container.getId(), fragment,name); }else { //如果获取到fragment与原来的是同一个，attach即可 if (DEBUG) Log.v(TAG, "Attaching item #" + itemId + ": f=" + fragment); mCurTransaction.attach(fragment); } } } else { fragment = getItem(fragment,position); if (DEBUG) Log.v(TAG, "Adding item #" + itemId + ": f=" + fragment); mCurTransaction.add(container.getId(), fragment,name); } if(fragment!=null){ //保存类型，用来校验缓存的正确性 fragmentViewTypeManager.put(fragmentType,fragment.getClass().getName()); } if (fragment != mCurrentPrimaryItem) { fragment.setMenuVisibility(false); fragment.setUserVisibleHint(false); } return fragment; } @Override public void destroyItem(ViewGroup container, int position, Object object) { mCurTransaction = beginTransaction(); if (DEBUG) Log.v(TAG, "Detaching item #" + getItemId(position) + ": f=" + object + " v=" + ((Fragment)object).getView()); mCurTransaction.detach((Fragment)object); //dattach fragment } @SuppressWarnings("ReferenceEquality") @Override public void setPrimaryItem(ViewGroup container, int position, Object object) { Fragment fragment = (Fragment)object; if (fragment != mCurrentPrimaryItem) { if (mCurrentPrimaryItem != null) { mCurrentPrimaryItem.setMenuVisibility(false); mCurrentPrimaryItem.setUserVisibleHint(false); } if (fragment != null) { fragment.setMenuVisibility(true); fragment.setUserVisibleHint(true); } mCurrentPrimaryItem = fragment; //设置当前的fragment } } @Override public void finishUpdate(ViewGroup container) { if (mCurTransaction != null) { mCurTransaction.commitNowAllowingStateLoss(); //提交，注意该方法将任务加入到mainLooper中，可能产生延迟 mCurTransaction = null; } } @Override public boolean isViewFromObject(View view, Object object) { return ((Fragment)object).getView() == view; } @Override public Parcelable saveState() { return null; } @Override public void restoreState(Parcelable state, ClassLoader loader) { } /** * * 获取每个fragment的id，注意保证唯一性 */ public long getItemId(int position) { return position; } //生成tag public static String makeFragmentName(int viewId, long id) { return "android:switcher:" + viewId + ":" + id; } public FragmentTransaction beginTransaction(){ if (mCurTransaction == null) { mCurTransaction = mFragmentManager.beginTransaction(); } return mCurTransaction; } public FragmentManager getFragmentManager(){ return mFragmentManager; } /** * 获取当前位置的fragment */ public abstract Fragment getItem(Fragment contentFragment,int position); /** * 获取当前位置的type FragmentType */ public abstract int getItemFragmentType(int position); /** * 获取当前类型的数量 FragmentCount */ public abstract int getFragmentTypeCount(); /** * 在ViewPager中调用，告诉ViewPager该位置的Fragment是可以被替换和更新的 * 这里人可以继续优化，由于ViewPager.LayoutParams中的position是非public的，因此要优化可以在该类的基类中完成 */ @Override public int getItemPosition(Object object) { if(!(object instanceOf Fragment)) { return PagerAdapter.POSITION_NONE; } return PagerAdapter.POSITION_UNCHANGED; } public boolean isEmpty(){ return getCount()==0; } } 到这里，我们便可以实现他的子类 static class MyPagerAdapter extends CustomFragmentPagerAdapter{ private ArrayList<FragmentTabEntity> dataEntities; private final String TAG_NAME = "MyPagerAdapter "; public MyPagerAdapter(FragmentManager fm,List<FragmentTabEntity> dataEntities) { super(fm); this.dataEntities = new ArrayList<>(); this.dataEntities.addAll(dataEntities); } @SuppressWarnings("unchecked") public void updateDataEntities(List<FragmentTabEntity> dataEntities) { this.dataEntities.clear(); if(dataEntities!=null && dataEntities.size()>0){ this.dataEntities.addAll(dataEntities); } this.notifyDataSetChanged(); } @Override public CharSequence getPageTitle(int position) { final FragmentTabEntity entity = dataEntities.get(position); return entity.getTitle(); } @Override public int getItemFragmentType(int position) { final FragmentTabEntity dataEntity = dataEntities.get(position); return dataEntity.getType(position); //获取类型，注意，最大值不能大于getFragmentTypeCount() } @Override public int getFragmentTypeCount() { return FragmentTabEntity.getTotalTypeCount(); //获取所有fragment的类型数量，一般是固定值，主要看程序实现方式了 } @Override public Fragment getItem(Fragment contentFragment,int position) { final int fragmentType = getItemFragmentType(position); final FragmentTabEntity dataEntity = dataEntities.get(position); BaseFragment fragment = null; if(contentFragment==null) { if (fragmentType == 0) { fragment = new IndexFragment(); } else if(fragmentType ==1){ fragment = new UserFragment(); } else if(fragmentType ==2){ fragment = new WebFragment(); }else if(fragmentType ==3){ fragment = new ListFragment(); } }else{ fragment = (BaseFragment) contentFragment; } fragment.setPosition(position); if(fragment!=null) { Bundle fb = new Bundle(); fb.putString(BaseFragment.KEY_TYPE, dataEntity.getType()); fb.putString(BaseFragment.KEY_TITLE, dataEntity.getTitle()); fragment.setNoneStateArguments(fb); //使用非状态参数传递方法 } return fragment; } @Override public int getCount() { return dataEntities.size(); } } 使用方法 private MyPagerAdapter mTabPagerAdapter; public void updatePager(List<FragmentTabEntity> data) { if((pager.getAdapter() instanceof MyPagerAdapter)){ mTabPagerAdapter = (MyPagerAdapter) pager.getAdapter(); } if(mTabPagerAdapter==null){ mTabPagerAdapter = new MyPagerAdapter(getChildFragmentManager(),data); pager.setAdapter(mTabPagerAdapter); }else{ mTabPagerAdapter.updateDataEntities(data); } } Fragment ViewCache问题 & 生命周期问题 到这一步事实上我们的自定义FragmentPagerAdapter已经完成了，但是这里还存在不完美的问题，那就是Fragment中添加了View Cache的情况，此外，对于生命周期的控制，可能或多或少出现旧页面向新页面过渡时闪烁问题。 1、View Cache 问题 先来看看这种Fragment的定义方式 public class BaseFragment extends Fragment{ private SoftReference<View> mRootViewCache = null; //实现viewcache private boolean isFinishedInflated = false; private Bundle mNoneStateArguments; private int position = -1; public void setPosition(int position){ this.position = position; } public int getPosition(){ return this.position; } @Nullable @Override public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState){ View root = null; if(!cacheIsEmpty()){ root = mRootViewCache.get(); } if(root==null){ root = inflater.inflate(R.layout.base_view_layout, container, false); root.findViewById(R.id.toolbar).setVisibility(View.GONE); mRootViewCache = new SoftReference<View>(root); } return root; } @Override public void onViewCreated(View view, @Nullable Bundle savedInstanceState) { super.onViewCreated(view, savedInstanceState); try { isFinishedInflated = true; renderFragmentView(view); } catch (Exception e) { e.printStackTrace(); } } private boolean cacheIsEmpty(){ return mRootViewCache==null || mRootViewCache.get()==null; } @Override public void onResume() { super.onResume(); if(getUserVisibleHint() ){ onFragmetShow(); } } @Override public void setUserVisibleHint(boolean isVisibleToUser) { super.setUserVisibleHint(isVisibleToUser); if(!isFinishedInflated) return; if( getUserVisibleHint()){ onFragmetShow(); }else if(isResumed()){ onFragmetHide(); } } @Override public void onStop() { super.onStop(); if(getUserVisibleHint()){ onFragmetHide(); } } public void onFragmetShow(){ if(getView()==null) return ; getView().post(new Runnable(){ public void run(){ //这里可以用来获取Fragment的参数，然后更新 } }); } public void onFragmetHide(){ } public void setNoneStateArguments(Bundle bundle){ //解决已初始化状态的参数刷新问题 this.mNoneStateArguments = bundle; } public void getNoneStateArguments(){ return this.mNoneStateArguments!=null? this.mNoneStateArguments:new Bundle(); } } 对于原生页面，新旧页面闪烁并不是很明显，但是对于Webview页面，这种闪烁很明显，导致该问题的原因是View Cache，因此，我们需要在Fragment中添加clearView方法来清空一下Cache public void clearView() { if(mRootViewCache!=null){ mRootViewCache.clear(); } } 在MyPagerAdapter的getItem方法中，我们有必要植入一个flag @Override public Fragment getItem(Fragment contentFragment,int position) { final int fragmentType = getItemFragmentType(position); final FragmentTabEntity dataEntity = dataEntities.get(position); BaseFragment fragment = null; if(contentFragment==null) { if (viewType == 0) { fragment = new IndexFragment(); } else if(fragmentType ==1){ fragment = new UserFragment(); } else if(fragmentType ==2){ fragment = new WebFragment(); }else if(fragmentType ==3){ fragment = new ListFragment(); } }else{ fragment = (BaseFragment) contentFragment; } final Bundle fa = fragment.getArguments(); if(fa!=null) { final String oldTag = fa.getString("md5", ""); if (!TextUtils.isEmpty(oldTag) && !oldUrl.oldTag(dataEntity.getMd5())) { fragment.clearView(); //如果tag不一致，清空一下view cache } } if(fragment!=null) { Bundle fb = new Bundle(); fb.putString("md5",dataEntity.getMd5()); //植入新的md5 tag fb.putString(BaseFragment.KEY_TYPE, dataEntity.getType()); fb.putString(BaseFragment.KEY_TITLE, dataEntity.getTitle()); fragment.setArguments(fb); } return fragment; } 2、生命周期问题 关于onFragmentShow与onFragmentHide的生命周期用法，请参考《Fragment页面切换》，这里我们主要说一下mainLooper问题 public void onFragmetShow(){ if(getView()==null) return ; getView().post(new Runnable(){ public void run(){ //这里可以用来获取Fragment的参数，然后更新 } }); } 如果要更新UI，我们建议这里使用post将消息发送到mainLooper，为什么要这样呢？ 主要原因是FragmentPagerAdapter的finishUpdate中使用了commit方法，这个方法是将任务发送到mainLooper的队列中，而不是立即执行。 @Override public void finishUpdate(ViewGroup container) { if (mCurTransaction != null) { mCurTransaction.commitNowAllowingStateLoss(); mCurTransaction = null; } } 基于队列的先进先出，FragmentTransaction将更新消息加入到Fragment add/attach消息之后，我们如果直接获取argument可能出现数据不一致的问题，因此我们需要将我们的方法作为任务同样放入到mainLooper中。如果不这么做，可能导致获取到的argument是旧的，导致我们更新时使用了旧的参数。当然，可以参考《Android Fragment重复添加问题解决方法》，原理基本相同。 以上是一般常见的问题，至于其他问题，可以留言。
