Android RecyclerView - clearOldPositions - NullPointerExceptions

Question

我在使用支持包中的RecyclerView 时遇到了一个非常奇怪的问题。

我在ViewPager 中有一个RecyclerView。在ViewPager 显示RecyclerView 的那一刻，应用程序崩溃并显示以下堆栈跟踪：

java.lang.NullPointerException
            at android.support.v7.widget.RecyclerView.clearOldPositions(RecyclerView.java:2378)
            at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:1968)
            at android.support.v7.widget.RecyclerView.onLayout(RecyclerView.java:2237)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
            at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
            at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
            at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
            at android.widget.ScrollView.onLayout(ScrollView.java:1468)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
            at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
            at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
            at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.support.v4.view.ViewPager.onLayout(ViewPager.java:1594)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1671)
            at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1525)
            at android.widget.LinearLayout.onLayout(LinearLayout.java:1434)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1055)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.widget.RelativeLayout.onLayout(RelativeLayout.java:1055)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.support.v4.widget.DrawerLayout.onLayout(DrawerLayout.java:890)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.widget.FrameLayout.layoutChildren(FrameLayout.java:453)
            at android.widget.FrameLayout.onLayout(FrameLayout.java:388)
            at android.view.View.layout(View.java:14817)
            at android.view.ViewGroup.layout(ViewGroup.java:4631)
            at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:1987)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1744)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5670)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:761)
            at android.view.Choreographer.doCallbacks(Choreographer.java:574)
            at android.view.Choreographer.doFrame(Choreographer.java:544)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:747)
            at android.os.Handler.handleCallback(Handler.java:733)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)

我在互联网上找不到任何关于此的资源，因此，如果有人之前遇到过此问题，我们将不胜感激。

一些背景：

我在 build.gradle 中导入 RecyclerView，如下所示：

compile 'com.android.support:recyclerview-v7:+'

我在 RecyclerView 的构造函数中将 Adapter 和 LayoutManager 设置为 RecyclerView，如下所示：

    public CategoryParallaxView(Context context)
    {
        super(context);
        init();
    }

    public CategoryParallaxView(Context context, AttributeSet attrs)
    {
        super(context, attrs);
        init();
    }

    public CategoryParallaxView(Context context, AttributeSet attrs, int defStyle)
    {
        super(context, attrs, defStyle);
        init();
    }

    private void init()
    {
        mLayoutManager = new CategoryParallaxLayoutManager();
        setLayoutManager(mLayoutManager);

        mCategoryList = new ArrayList<CartCategory>();
        mAdapter = new CategoryParallaxAdapter();
        setAdapter(mAdapter);
    }

显示RecyclerView时，数据源（mCategoryList）的大小为0，但列表本身不为空。

RecyclerView 嵌套在 LinearLayout 中（它本身嵌套在 ScrollView 中），并在宽度和高度中定义了 MATCH_PARENT。从一些调试来看，为RecyclerView 计算的宽度和高度似乎是正确的。

不确定这是否相关，但在我的实现中，Adapter、ViewHolder 和 LayoutManager 子类都是我的 RecyclerView 子类的私有内部类。

Answer 1

问题是RecyclerView 在RecyclerView 在ViewPager 中可见的那一刻清除了我在init 方法中设置的LayoutManager。

我通过在 RecyclerView 变得可见后立即分配 LayoutManager 来解决此问题。感觉有点hacky，但我找不到任何其他工作。如果有人能找到更好的解决方案，请发帖。