错误日志如下

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.support.v7.widget.RecyclerView$ViewHolder.shouldIgnore()' on a null object reference

    at android.support.v7.widget.RecyclerView.dispatchLayout(RecyclerView.java:2913)

    at android.support.v7.widget.RecyclerView.consumePendingUpdateOperations(RecyclerView.java:1445)

    at android.support.v7.widget.RecyclerView.access$400(RecyclerView.java:144)

    at android.support.v7.widget.RecyclerView$1.run(RecyclerView.java:282)

    at android.view.Choreographer$CallbackRecord.run(Choreographer.java:821)

    at android.view.Choreographer.doCallbacks(Choreographer.java:606)

    at android.view.Choreographer.doFrame(Choreographer.java:575)

    at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:807)

    at android.os.Handler.handleCallback(Handler.java:739)

    at android.os.Handler.dispatchMessage(Handler.java:95)

    at android.os.Looper.loop(Looper.java:145)

    at android.app.ActivityThread.main(ActivityThread.java:6895)

    at java.lang.reflect.Method.invoke(Native Method)

    at java.lang.reflect.Method.invoke(Method.java:372)

    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1404)

    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)

原因

在onBindViewHolder中将item的rootview设置了新的RecyclerView.LayoutParams。RecyclerView视图实际上将ViewHolder存储在一个自定义的LayoutParams类中， 所以重置LayoutParams时，ViewHolder引用消失了， 之后会导致NullPointerException。出错做法如下：

    @Override

    public void onBindViewHolder(Holder holder, final int position) {

        holder.iv.setImageDrawable(apps.get(position).getIcon());

        holder.tv.setText(apps.get(position).getLabel());

        RecyclerView.LayoutParams param = new RecyclerView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, recyclerView.getHeight() / ROW_NUM);

        holder.layout.setLayoutParams(param);

    }

正确的做法

在onCreateViewHolder中设置宽高

    @Override

    public Holder onCreateViewHolder(ViewGroup parent, int viewType) {

        View v = LayoutInflater.from(context).inflate(R.layout.app_item, parent, false);

        v.getLayoutParams().height = recyclerView.getHeight() / ROW_NUM;

        return new Holder(v);

    }