使用片段在多個視圖持有者回收器視圖上投射異常

Question

我正在嘗試從我的應用程序中的谷歌示例中實現此代碼。 它在 recyclerview Adview 和 MenuItem 中有兩個視圖。 如果它在一個活動中實現，它工作得很好。 但就我而言，我使用的是底部導航欄的片段。 它在第一次通過底部欄打開片段時起作用，但之后它給出了強制轉換異常。

java.lang.ClassCastException: com.google.android.gms.ads.AdView cannot be cast to com.neupanedinesh.models.MenuItem
        at com.neupanedinesh.adapters.RecyclerViewAdapter.onBindViewHolder

我已將片段的 onResume、onPause 和 onDestroy 的訪問類型從受保護更改為公開。

 @Override
    public void onResume() {
        for (Object item : recyclerViewItems) {
            if (item instanceof AdView) {
                AdView adView = (AdView) item;
                adView.resume();
            }
        }
        super.onResume();
    }

    @Override
    public void onPause() {
        for (Object item : recyclerViewItems) {
            if (item instanceof AdView) {
                AdView adView = (AdView) item;
                adView.pause();
            }
        }
        super.onPause();
    }

    @Override
    public void onDestroy() {
        for (Object item : recyclerViewItems) {
            if (item instanceof AdView) {
                AdView adView = (AdView) item;
                adView.destroy();
            }
        }
        super.onDestroy();
    }

這是我的回收視圖適配器：

package com.neupanedinesh.adapters;

import android.content.Context;
import androidx.recyclerview.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.google.android.gms.ads.AdView;
import com.neupanedinesh.coolcaptions.MenuItemFragment;
import com.neupanedinesh.coolcaptions.R;
import com.neupanedinesh.models.MenuItem;

import java.util.List;

/**
 * The {@link RecyclerViewAdapter} class.
 * <p>The adapter provides access to the items in the {@link MenuItemViewHolder}
 * or the {@link AdViewHolder}.</p>
 */
public class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> {
    // A menu item view type.
    private static final int MENU_ITEM_VIEW_TYPE = 0;

    // The banner ad view type.
    private static final int BANNER_AD_VIEW_TYPE = 1;

    // An Activity's Context.
    private final Context context;

    // The list of banner ads and menu items.
    private final List<Object> recyclerViewItems;

    /**
     * For this example app, the recyclerViewItems list contains only
     * {@link MenuItem} and {@link AdView} types.
     */
    public RecyclerViewAdapter(Context context, List<Object> recyclerViewItems) {
        this.context = context;
        this.recyclerViewItems = recyclerViewItems;
    }

    /**
     * The {@link MenuItemViewHolder} class.
     * Provides a reference to each view in the menu item view.
     */
    public class MenuItemViewHolder extends RecyclerView.ViewHolder {
        private TextView menuItemName;

        MenuItemViewHolder(View view) {
            super(view);
            menuItemName = view.findViewById(R.id.menu_item_name);
        }
    }

    /**
     * The {@link AdViewHolder} class.
     */
    public class AdViewHolder extends RecyclerView.ViewHolder {

        AdViewHolder(View view) {
            super(view);
        }
    }

    @Override
    public int getItemCount() {
        return recyclerViewItems.size();
    }

    /**
     * Determines the view type for the given position.
     */
    @Override
    public int getItemViewType(int position) {
        return (position % MenuItemFragment.ITEMS_PER_AD == 0) ? BANNER_AD_VIEW_TYPE
                : MENU_ITEM_VIEW_TYPE;
    }

    /**
     * Creates a new view for a menu item view or a banner ad view
     * based on the viewType. This method is invoked by the layout manager.
     */
    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup viewGroup, int viewType) {
        switch (viewType) {
            case MENU_ITEM_VIEW_TYPE:
                View menuItemLayoutView = LayoutInflater.from(viewGroup.getContext()).inflate(
                        R.layout.menu_item_container, viewGroup, false);
                return new MenuItemViewHolder(menuItemLayoutView);
            case BANNER_AD_VIEW_TYPE:
                // fall through
            default:
                View bannerLayoutView = LayoutInflater.from(
                        viewGroup.getContext()).inflate(R.layout.banner_ad_container,
                        viewGroup, false);
                return new AdViewHolder(bannerLayoutView);
        }
    }

    /**
     * Replaces the content in the views that make up the menu item view and the
     * banner ad view. This method is invoked by the layout manager.
     */
    @Override
    public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
        int viewType = getItemViewType(position);
        switch (viewType) {
            case MENU_ITEM_VIEW_TYPE:
                MenuItemViewHolder menuItemHolder = (MenuItemViewHolder) holder;
                MenuItem menuItem = (MenuItem) recyclerViewItems.get(position);
                menuItemHolder.menuItemName.setText(menuItem.getName());
                break;
            case BANNER_AD_VIEW_TYPE:
                // fall through
            default:
                AdViewHolder bannerHolder = (AdViewHolder) holder;
                AdView adView = (AdView) recyclerViewItems.get(position);
                ViewGroup adCardView = (ViewGroup) bannerHolder.itemView;
                // The AdViewHolder recycled by the RecyclerView may be a different
                // instance than the one used previously for this position. Clear the
                // AdViewHolder of any subviews in case it has a different
                // AdView associated with it, and make sure the AdView for this position doesn't
                // already have a parent of a different recycled AdViewHolder.
                if (adCardView.getChildCount() > 0) {
                    adCardView.removeAllViews();
                }
                if (adView.getParent() != null) {
                    ((ViewGroup) adView.getParent()).removeView(adView);
                }

                // Add the banner ad to the ad view.
                adCardView.addView(adView);

        }
    }

}

這是我的片段 class：

package com.neupanedinesh.coolcaptions;
import android.os.Bundle;
import androidx.fragment.app.Fragment;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.google.android.gms.ads.AdListener;
import com.google.android.gms.ads.AdRequest;
import com.google.android.gms.ads.AdSize;
import com.google.android.gms.ads.AdView;

import java.util.ArrayList;
import java.util.List;

import com.neupanedinesh.adapters.RecyclerViewAdapter;
import com.neupanedinesh.models.MenuItem;


/**
 * A simple {@link Fragment} subclass.
 */
public class MenuItemFragment extends Fragment {

    // A banner ad is placed in every 8th position in the RecyclerView.
    public static final int ITEMS_PER_AD = 8;

    private static final String AD_UNIT_ID = "ca-app-pub-3940256099942544/4177191030";

    // The RecyclerView that holds and displays banner ads and menu items.
    private RecyclerView recyclerView;

    // List of banner ads and MenuItems that populate the RecyclerView.
    private List<Object> recyclerViewItems = new ArrayList<>();


    public MenuItemFragment() {
        // Required empty public constructor
    }


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_menu_item, container, false);
        recyclerView = view.findViewById(R.id.recycler_view);

        // Use this setting to improve performance if you know that changes
        // in content do not change the layout size of the RecyclerView.
        recyclerView.setHasFixedSize(true);

        // Specify a linear layout manager.
        RecyclerView.LayoutManager layoutManager = new LinearLayoutManager(getActivity());
        recyclerView.setLayoutManager(layoutManager);

        // Update the RecyclerView item's list with menu items and banner ads.
        addMenuItemsFromJson();
        addBannerAds();
        loadBannerAds();

        // Specify an adapter.
        RecyclerView.Adapter<RecyclerView.ViewHolder> adapter = new RecyclerViewAdapter(getActivity(),
                recyclerViewItems);
        recyclerView.setAdapter(adapter);



        return view;
    }

    @Override
    public void onResume() {
        for (Object item : recyclerViewItems) {
            if (item instanceof AdView) {
                AdView adView = (AdView) item;
                adView.resume();
            }
        }
        super.onResume();
    }

    @Override
    public void onPause() {
        for (Object item : recyclerViewItems) {
            if (item instanceof AdView) {
                AdView adView = (AdView) item;
                adView.pause();
            }
        }
        super.onPause();
    }

    @Override
    public void onDestroy() {
        for (Object item : recyclerViewItems) {
            if (item instanceof AdView) {
                AdView adView = (AdView) item;
                adView.destroy();
            }
        }
        super.onDestroy();
    }


    /**
     * Adds banner ads to the items list.
     */
    private void addBannerAds() {
        // Loop through the items array and place a new banner ad in every ith position in
        // the items List.
        for (int i = 0; i <= recyclerViewItems.size(); i += ITEMS_PER_AD) {
            final AdView adView = new AdView(getActivity());
            adView.setAdSize(AdSize.BANNER);
            adView.setAdUnitId(AD_UNIT_ID);
            recyclerViewItems.add(i, adView);
        }
    }

    /**
     * Sets up and loads the banner ads.
     */
    private void loadBannerAds() {
        // Load the first banner ad in the items list (subsequent ads will be loaded automatically
        // in sequence).
        loadBannerAd(0);
    }

    /**
     * Loads the banner ads in the items list.
     */
    private void loadBannerAd(final int index) {

        if (index >= recyclerViewItems.size()) {
            return;
        }

        Object item = recyclerViewItems.get(index);
        if (!(item instanceof AdView)) {
            throw new ClassCastException("Expected item at index " + index + " to be a banner ad"
                    + " ad.");
        }

        final AdView adView = (AdView) item;

        // Set an AdListener on the AdView to wait for the previous banner ad
        // to finish loading before loading the next ad in the items list.
        adView.setAdListener(new AdListener() {
            @Override
            public void onAdLoaded() {
                super.onAdLoaded();
                // The previous banner ad loaded successfully, call this method again to
                // load the next ad in the items list.
                loadBannerAd(index + ITEMS_PER_AD);
            }

            @Override
            public void onAdFailedToLoad(int errorCode) {
                // The previous banner ad failed to load. Call this method again to load
                // the next ad in the items list.
               /* Log.e("MainActivity", "The previous banner ad failed to load. Attempting to"
                        + " load the next banner ad in the items list.");*/
                loadBannerAd(index + ITEMS_PER_AD);
            }
        });

        // Load the banner ad.
        adView.loadAd(new AdRequest.Builder().build());
    }


    private void addMenuItemsFromJson() {
        String menuItemName="item";
        MenuItem menuItem = new MenuItem(menuItemName);
        recyclerViewItems.add(menuItem);
    }


}

Answer 1

由於這部分代碼，您會發生崩潰：

@Override
public int getItemViewType(int position) {
    return (position % MenuItemFragment.ITEMS_PER_AD == 0) 
          ? BANNER_AD_VIEW_TYPE
          : MENU_ITEM_VIEW_TYPE;
}

在你的情況下它肯定會崩潰，因為你沒有檢查實際的 class 類型 - 這取決於你的代碼 - 在視圖類型上。 這意味着如果 recyclerViewItem 實際上是 AdView 或 MenuItem，您還應該檢查onBindViewHolder()內部：

Object item = recyclerViewItems.get(position);
if (item instanceOf AdView) {
    AdView adView = (AdView) recyclerViewItems.get(position);
} else if (item instanceOf MenuItem) {
    MenuItem menuItem = (MenuItem) recyclerViewItems.get(position);
}

或者您可以將此檢查放在 getItemViewType(int position) 中，如下所示：

@Override
public int getItemViewType(int position) {
    return recyclerViewItems.get(position) instanceOf AdView 
          ? BANNER_AD_VIEW_TYPE
          : MENU_ITEM_VIEW_TYPE;
}

這樣你就可以確定你會在 onBindViewHolder 中得到正確的onBindViewHolder並且 class 鑄件會適合。