java - Android 5.0 - 向 RecyclerView 添加页眉/页脚

Question

我花了一点时间试图找出一种将标题添加到 RecyclerView 的方法，但没有成功。

这是我目前得到的:

@Override
protected void onCreate(Bundle savedInstanceState) {
    ...

    layouManager = new LinearLayoutManager(getActivity());
    recyclerView.setLayoutManager(layouManager);

    LayoutInflater inflater = (LayoutInflater) getActivity().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    headerPlaceHolder = inflater.inflate(R.layout.view_header_holder_medium, null, false);
    layouManager.addView(headerPlaceHolder, 0);

   ...
}

LayoutManager 似乎是处理 RecyclerView 项的处置的对象。由于找不到任何 addHeaderView(View view) 方法，我决定使用 LayoutManager 的 addView(View view, int position) 方法并将我的标题 View 添加到第一个位置以充当标题。

这就是事情变得更丑陋的地方:

java.lang.NullPointerException: Attempt to read from field 'android.support.v7.widget.RecyclerView$ViewHolder android.support.v7.widget.RecyclerView$LayoutParams.mViewHolder' on a null object reference
    at android.support.v7.widget.RecyclerView.getChildViewHolderInt(RecyclerView.java:2497)
    at android.support.v7.widget.RecyclerView$LayoutManager.addViewInt(RecyclerView.java:4807)
    at android.support.v7.widget.RecyclerView$LayoutManager.addView(RecyclerView.java:4803)
    at com.mathieumaree.showz.fragments.CategoryFragment.setRecyclerView(CategoryFragment.java:231)
    at com.mathieumaree.showz.fragments.CategoryFragment.access$200(CategoryFragment.java:47)
    at com.mathieumaree.showz.fragments.CategoryFragment$2.success(CategoryFragment.java:201)
    at com.mathieumaree.showz.fragments.CategoryFragment$2.success(CategoryFragment.java:196)
    at retrofit.CallbackRunnable$1.run(CallbackRunnable.java:41)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5221)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

在获得多个 NullPointerExceptions 尝试在 Activity 创建的不同时刻调用 addView(View view) 之后(也尝试在一切设置好后添加 View ，甚至适配器的数据)，我意识到我不知道这是否是正确的方法(而且看起来不是)。

PS:另外，非常感谢除了 LinearLayoutManager 之外还可以处理 GridLayoutManager 的解决方案!

Answer 1

我必须在我的 RecyclerView 中添加一个页脚，在这里我分享我的代码 fragment ，因为我认为它可能有用。请查看代码中的注释，以便更好地理解整个流程。

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import java.util.ArrayList;

public class RecyclerViewWithFooterAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> {

    private static final int FOOTER_VIEW = 1;
    private ArrayList<String> data; // Take any list that matches your requirement.
    private Context context;

    // Define a constructor
    public RecyclerViewWithFooterAdapter(Context context, ArrayList<String> data) {
        this.context = context;
        this.data = data;
    }

    // Define a ViewHolder for Footer view
    public class FooterViewHolder extends ViewHolder {
        public FooterViewHolder(View itemView) {
            super(itemView);
            itemView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    // Do whatever you want on clicking the item
                }
            });
        }
    }

    // Now define the ViewHolder for Normal list item
    public class NormalViewHolder extends ViewHolder {
        public NormalViewHolder(View itemView) {
            super(itemView);

            itemView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    // Do whatever you want on clicking the normal items
                }
            });
        }
    }

    // And now in onCreateViewHolder you have to pass the correct view
    // while populating the list item.

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {

        View v;

        if (viewType == FOOTER_VIEW) {
            v = LayoutInflater.from(context).inflate(R.layout.list_item_footer, parent, false);
            FooterViewHolder vh = new FooterViewHolder(v);
            return vh;
        }

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

        NormalViewHolder vh = new NormalViewHolder(v);

        return vh;
    }

    // Now bind the ViewHolder in onBindViewHolder
    @Override
    public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {

        try {
            if (holder instanceof NormalViewHolder) {
                NormalViewHolder vh = (NormalViewHolder) holder;

                vh.bindView(position);
            } else if (holder instanceof FooterViewHolder) {
                FooterViewHolder vh = (FooterViewHolder) holder;
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    // Now the critical part. You have return the exact item count of your list
    // I've only one footer. So I returned data.size() + 1
    // If you've multiple headers and footers, you've to return total count
    // like, headers.size() + data.size() + footers.size()

    @Override
    public int getItemCount() {
        if (data == null) {
            return 0;
        }

        if (data.size() == 0) {
            //Return 1 here to show nothing
            return 1;
        }

        // Add extra view to show the footer view
        return data.size() + 1;
    }

    // Now define getItemViewType of your own.

    @Override
    public int getItemViewType(int position) {
        if (position == data.size()) {
            // This is where we'll add footer.
            return FOOTER_VIEW;
        }

        return super.getItemViewType(position);
    }

    // So you're done with adding a footer and its action on onClick.
    // Now set the default ViewHolder for NormalViewHolder

    public class ViewHolder extends RecyclerView.ViewHolder {
        // Define elements of a row here
        public ViewHolder(View itemView) {
            super(itemView);
            // Find view by ID and initialize here
        }

        public void bindView(int position) {
            // bindView() method to implement actions
        }
    }
}

上面的代码 fragment 为 RecyclerView 添加了一个页脚。您可以查看this GitHub repository用于检查添加页眉和页脚的实现。