java.lang.NullPointerException\nandroid.support.v7.widget.StaggeredGridLayoutManager$Span.access$300

写瀑布流的时候，遇到了如下crash：

Attempt to read from field 'java.util.ArrayList android.support.v7.widget.StaggeredGridLayoutManager$Span.mViews' on a null object reference
10-20 21:04:17.826 5662-5662/com.jingdong.app.mall W/System.err:     at android.support.v7.widget.StaggeredGridLayoutManager$Span.access$300(StaggeredGridLayoutManager.java:2094)
10-20 21:04:17.826 5662-5662/com.jingdong.app.mall W/System.err:     at android.support.v7.widget.StaggeredGridLayoutManager.recycleFromEnd(StaggeredGridLayoutManager.java:1761)
10-20 21:04:17.826 5662-5662/com.jingdong.app.mall W/System.err:     at android.support.v7.widget.StaggeredGridLayoutManager.recycle(StaggeredGridLayoutManager.java:1585)
10-20 21:04:17.826 5662-5662/com.jingdong.app.mall W/System.err:     at android.support.v7.widget.StaggeredGridLayoutManager.fill(StaggeredGridLayoutManager.java:1514)
10-20 21:04:17.826 5662-5662/com.jingdong.app.mall W/System.err:     at android.support.v7.widget.StaggeredGridLayoutManager.scrollBy(StaggeredGridLayoutManager.java:1926)
10-20 21:04:17.826 5662-5662/com.jingdong.app.mall W/System.err:     at android.support.v7.widget.StaggeredGridLayoutManager.scrollVerticallyBy(StaggeredGridLayoutManager.java:1846)

折腾了好久才弄好，现记录一下：

当ViewPager、RecyclerView和StaggeredGridLayoutManager配合用的时候，左右滑动，会出现这个crash

原因是当来回切换的时候，假设切换页面顺序是0->1->2->3->4->5->4->3->2->1->0，当重新切换到0的时候，viewpager可能已经把页面0的view给回收了，而recyclerview和StaggeredGridLayoutManager认为页面0的view还在，双重回收机制造成了上述空指针错误。

解决办法就是在onCreateView里判断rootView如果已存在，就重新new一个StaggeredGridLayoutManager

@Overridepublic View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {    if (rootView == null) {        rootView = ImageUtil.inflate(getContext(), R.layout.main__fragment, null);        initView(rootView);    }else{        staggeredGridLayoutManager = new JShowStaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL);        staggeredGridLayoutManager.setGapStrategy(StaggeredGridLayoutManager.GAP_HANDLING_NONE);        pullToRefreshRecyclerView.getRefreshableView().setLayoutManager(staggeredGridLayoutManager);    }    return rootView;}

这样，来回切换时不会崩溃，但是不能保存上次的滑动状态

解决办法：

重写recyclerview，在离开屏幕时保存一下状态，并加一个恢复状态的方法供fragment调用

@Overrideprotected void onDetachedFromWindow() {    LayoutManager layoutManager = getLayoutManager();    if(StaggeredGridLayoutManager.class.isInstance(layoutManager)){        saveStates = ((StaggeredGridLayoutManager)layoutManager).onSaveInstanceState();    }    super.onDetachedFromWindow();}

public void restoreSaveStateIfPossible(){    if(saveStates != null){        LayoutManager layoutManager = getLayoutManager();        if(StaggeredGridLayoutManager.class.isInstance(layoutManager)){            if(saveStates != null) {                layoutManager.onRestoreInstanceState(saveStates);            }        }    }}

在fragment里：

@Overridepublic View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {    Log.d(TAG,"onCreateView"+",cateId:"+cateId);    if (rootView == null) {        rootView = ImageUtil.inflate(...);        //初始化view    }else{        staggeredGridLayoutManager = new JShowStaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL);        staggeredGridLayoutManager.setGapStrategy(StaggeredGridLayoutManager.GAP_HANDLING_NONE);        pullToRefreshRecyclerView.getRefreshableView().setLayoutManager(staggeredGridLayoutManager);        recyclerview.restoreSaveStateIfPossible();    }    return rootView;}

好，问题解决！撒花~