如何在回收器適配器中初始化偵聽器？

Question

我需要能夠通過偵聽器將點擊的回收器項目的 position 發送到父片段，但每次點擊它時，它都會說偵聽器是 null。

java.lang.NullPointerException: Attempt to invoke interface method 'void com.eataway.partner.adapters.MenuAdapter$SectionAdapterListener.onSectionSend(int)' on a null object reference
        at com.rest.partner.adapters.MenuAdapter.lambda$onBindViewHolder$0$MenuAdapter(MenuAdapter.java:82)
        at com.rest.partner.adapters.-$$Lambda$MenuAdapter$TzEQ_dT_4Yb5iXO0LwjA0DjCNEI.onClick(Unknown Source:4)

這是相關的適配器代碼。 頂部是構造函數 class 后跟監聽器定義和設置。 然后在 onBindViewHolder 中，我觸發了該項目的點擊操作，即該項目的整個視圖都是可點擊的。 底部是相關viewHolder的內部class。

// Constructor class
    public MenuAdapter(Context adapterContext, ArrayList<DishItem> restaurantDishItemArrayList) {
        this.adapterContext = adapterContext;
        this.restaurantDishItemArrayList = restaurantDishItemArrayList;
    }

    // Define listener to send chosen section
    public interface SectionAdapterListener {
        void onSectionSend(int position);
    }

    // Set the listener. Must be called from the fragment
    public void setSectionAdapterListener(SectionAdapterListener sectionAdapterListener) {
        this.sectionAdapterListener = sectionAdapterListener;
    }

@Override
    public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, final int position) {
        if (adapterContext instanceof SectionAdapterListener) sectionAdapterListener = (SectionAdapterListener) adapterContext;
        if (holder instanceof SectionViewHolder) {
            ((SectionViewHolder)holder).tvHeaderName.setText(restaurantDishItemArrayList.get(position).getHeaderName());
            if (sectionAdapterListener == null ) Log.d(TAG, "listener is null");
            ((SectionViewHolder)holder).viewForeground.setOnClickListener(v -> sectionAdapterListener.onSectionSend(position));

        } else if (holder instanceof DishViewHolder) {
            ((DishViewHolder)holder).tvMenuItemName.setText(restaurantDishItemArrayList.get(position).getMenuItemName());
            ((DishViewHolder)holder).tvMenuItemDescription.setText(restaurantDishItemArrayList.get(position).getMenuItemDescription());
            }
    }

public class SectionViewHolder extends RecyclerView.ViewHolder {

        private TextView tvHeaderName;
        private RelativeLayout viewForeground;

        public SectionViewHolder(@NonNull View itemView) {
            super(itemView);

            tvHeaderName = itemView.findViewById(R.id.menu_header_name);
            viewForeground = itemView.findViewById(R.id.foreground_view);
        }
    }

Answer 1

創建接口 class。 然后將接口添加到您的 RecyclerAdapter 構造函數。

 YourInterface listener;

 public MenuAdapter(YourInterface listener, Context adapterContext, ArrayList<DishItem> restaurantDishItemArrayList) {
        this.listener = listener;
        this.adapterContext = adapterContext;
        this.restaurantDishItemArrayList = restaurantDishItemArrayList;
    }

然后在你的活動中

MenuAdapter menuAdapter = new MenuAdapter(listener, context, arraylist);

Answer 2

為避免NullPointerException在使用其方法之前檢查偵聽器是否為 null，因此如果sectionAdapterListener為null ，請更改onBindViewHolder()以返回

@Override
    public void onBindViewHolder(@NonNull RecyclerView.ViewHolder holder, final int position) {
        if (adapterContext instanceof SectionAdapterListener) sectionAdapterListener = (SectionAdapterListener) adapterContext;
        if (holder instanceof SectionViewHolder) {
            ((SectionViewHolder)holder).tvHeaderName.setText(restaurantDishItemArrayList.get(position).getHeaderName());
            if (sectionAdapterListener == null ) {
                Log.d(TAG, "listener is null");
                return;
            }
            ((SectionViewHolder)holder).viewForeground.setOnClickListener(v -> sectionAdapterListener.onSectionSend(position));

        } else if (holder instanceof DishViewHolder) {
            ((DishViewHolder)holder).tvMenuItemName.setText(restaurantDishItemArrayList.get(position).getMenuItemName());
            ((DishViewHolder)holder).tvMenuItemDescription.setText(restaurantDishItemArrayList.get(position).getMenuItemDescription());
        }
    }

否則，您需要從創建此適配器的活動/片段中調用setSectionAdapterListener()

Answer 3

您可以將以下代碼添加到適配器 class 的 onBindViewHolder 方法中。

holder.itemView.setonClickListener{
 //add your code here....
}