由于空引用上的 RecyclerView Button setOnClickListener 而崩溃

Question

我使用this 解决方案作为指南，将页脚按钮添加到我的 RecyclerView（最后一个 ViewHolder 项目是一个按钮），但我在适配器类的 onBindViewHolder 当我尝试在按钮上设置 onClickListener 时。

我觉得问题很简单，我想我只是在调试后忽略了它。

当我设置 on click 侦听器时，您能发现导致 Button mAddChordButton 为空的原因吗？

适配器类

    public class UserChordsAdapter extends RecyclerView.Adapter<UserChordsAdapter.ChordViewHolder> {

    final private ListItemClickListener mOnClickListener;
    private static int viewHolderCount;
    private int mNumberItems;

    public interface ListItemClickListener {
        void onListItemClick(int clickedItemIndex);
    }


    public UserChordsAdapter(int numberOfItems, ListItemClickListener listener) {
        ...
    }

    @Override
    public ChordViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        ...
        View listItem;
        ChordViewHolder viewHolder;

        if (viewType == R.layout.user_chord_list_item) {    //item isn't last in list
            listItem = inflater.inflate(layoutIdForListItem, parent, shouldAttachToParentImmediately);
            viewHolder = new ChordViewHolder(listItem);
            viewHolder.viewHolderIndex.setText("Chord " + viewHolderCount);
        }
        else{   //inflate "+ Add new" button for last list item
            listItem = inflater.inflate(layoutIdForListItem, parent, shouldAttachToParentImmediately);
            viewHolder = new ChordViewHolder(listItem);
        }

        viewHolderCount++;
        return viewHolder;
    }

    @Override
    public void onBindViewHolder(ChordViewHolder holder, int position) {
        if (position == mNumberItems){
            holder.mAddChordButton.setOnClickListener(new View.OnClickListener(){   //ERROR HERE
                @Override
                public void onClick(View v) {
                    //TODO: Add New Chord into RecyclerView
                }
            });
        }
    }

    @Override
    public int getItemCount() {
        return mNumberItems + 1;// +1 is for footer button (add new)
    }

    class ChordViewHolder extends RecyclerView.ViewHolder
            implements View.OnClickListener {

        // Will display which ViewHolder is displaying this data
        TextView viewHolderIndex;
        public Button mAddChordButton;

        /**
         * Constructor for our ViewHolder. Within this constructor, we get a reference to our
         * TextViews and set an onClickListener to listen for clicks. Those will be handled in the
         * onClick method below.
         */
        public ChordViewHolder(View itemView) {
            super(itemView);

            mAddChordButton = (Button) itemView.findViewById(R.id.button_add_new);      //ATTEMPT TO INITIALIZE BUTTON HERE
            viewHolderIndex = (TextView) itemView.findViewById(R.id.tv_view_holder_instance);
            itemView.setOnClickListener(this);
        }

        /**
         * Called whenever a user clicks on an item in the list.
         * @param v The View that was clicked
         */
        @Override
        public void onClick(View v) {
            int clickedPosition = getAdapterPosition();
            mOnClickListener.onListItemClick(clickedPosition);
        }
    }

    /**
     * Distinguishes if view is a Chord list item or the last item in the list (add new chord)
     * @param position
     * @return
     */
    @Override
    public int getItemViewType(int position) {
        return (position == mNumberItems) ? R.layout.user_chord_add_new : R.layout.user_chord_list_item;
    }
}

Logcat

05-24 22:31:26.284 3083-7540/com.example.foo E/AndroidRuntime: FATAL EXCEPTION: main
                                                                 Process: com.example.foo, PID: 3083
                                                                 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                     at com.example.foo.UserChordsAdapter.onBindViewHolder(UserChordsAdapter.java:68)
                                                                     at com.example.foo.UserChordsAdapter.onBindViewHolder(UserChordsAdapter.java:15)
                                                                     at android.support.v7.widget.RecyclerView$Adapter.onBindViewHolder(RecyclerView.java:6067)
                                                                     at android.support.v7.widget.RecyclerView$Adapter.bindViewHolder(RecyclerView.java:6100)
                                                                     at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:5282)
                                                                     at android.support.v7.widget.RecyclerView$Recycler.getViewForPosition(RecyclerView.java:5158)
                                                                     at android.support.v7.widget.LinearLayoutManager$LayoutState.next(LinearLayoutManager.java:2061)
                                                                     at android.support.v7.widget.LinearLayoutManager.layoutChunk(LinearLayoutManager.java:1445)
                                                                     at android.support.v7.widget.LinearLayoutManager.fill(LinearLayoutManager.java:1408)
                                                                     at android.support.v7.widget.LinearLayoutManager.onLayoutChildren(LinearLayoutManager.java:580)
                                                                     at android.support.v7.widget.RecyclerView.dispatchLayoutStep2(RecyclerView.java:3379)
                                                                     at android.support.v7.widget.RecyclerView.onMeasure(RecyclerView.java:2906)
                                                                     at android.view.View.measure(View.java:19734)
                                                                     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
                                                                     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
                                                                     at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1117)
                                                                     at android.widget.LinearLayout.onMeasure(LinearLayout.java:642)
                                                                     at android.view.View.measure(View.java:19734)
                                                                     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
                                                                     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1464)
                                                                     at android.widget.LinearLayout.measureVertical(LinearLayout.java:758)
                                                                     at android.widget.LinearLayout.onMeasure(LinearLayout.java:640)
                                                                     at android.view.View.measure(View.java:19734)
                                                                     at android.widget.LinearLayout.measureHorizontal(LinearLayout.java:1098)
                                                                     at android.widget.LinearLayout.onMeasure(LinearLayout.java:642)
                                                                     at android.view.View.measure(View.java:19734)
                                                                     at android.widget.LinearLayout.measureVertical(LinearLayout.java:911)
                                                                     at android.widget.LinearLayout.onMeasure(LinearLayout.java:640)
                                                                     at android.view.View.measure(View.java:19734)
                                                                     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6120)
                                                                     at android.widget.FrameLayout.onMeasure(FrameLayout.java:185)
                                                                     at android.view.View.measure(View.java:19734)
                                                                     at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2271)
                                                                     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1358)
                                                                     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1607)
                                                                     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1246)
                                                                     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6301)
                                                                     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:871)
                                                                     at android.view.Choreographer.doCallbacks(Choreographer.java:683)
                                                                     at android.view.Choreographer.doFrame(Choreographer.java:619)
                                                                     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:857)
                                                                     at android.os.Handler.handleCallback(Handler.java:751)
                                                                     at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                     at android.os.Looper.loop(Looper.java:154)
                                                                     at android.app.ActivityThread.main(ActivityThread.java:6077)
                                                                     at java.lang.reflect.Method.invoke(Native Method)
                                                                     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                                                                     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

Answer 1

我解决了这个问题。我只是在 onCreateViewHolder 中夸大了错误的布局。在为页脚视图声明正确的布局 ID 并在我的 else 语句中对其进行膨胀之后，它就起作用了。

正确的代码是：

@Override
    public ChordViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        Context context = parent.getContext();
        int layoutIdForListItem = R.layout.user_chord_list_item;
        int layoutIdForFooterItem = R.layout.user_chord_add_new; //SOLUTION
        LayoutInflater inflater = LayoutInflater.from(context);
        boolean shouldAttachToParentImmediately = false;

        View listItem;
        ChordViewHolder viewHolder;

        if (viewType == R.layout.user_chord_list_item) {    //inflate normal list item
            listItem = inflater.inflate(layoutIdForListItem, parent, shouldAttachToParentImmediately);
            viewHolder = new ChordViewHolder(listItem);
            viewHolder.viewHolderIndex.setText("Chord " + viewHolderCount);
        }
        else{   //inflate "+ Add new" button for last list item
            listItem = inflater.inflate(layoutIdForFooterItem, parent, shouldAttachToParentImmediately);
            viewHolder = new ChordViewHolder(listItem);
        }

        viewHolderCount++;
        return viewHolder;
    }