为什么arraylist没有显示在recyclerview中？答案

【问题标题】：Why is the arraylist not being shown in the recyclerview?为什么arraylist没有显示在recyclerview中？
【发布时间】：2019-11-16 20:28:20
【问题描述】：

我使用 for 循环创建了一个 arraylist，该循环在我的模型类中创建了 100 个项目。然后，我尝试使用适配器将此数据绑定到我的 recyclerview，但由于某种奇怪的原因，这些项目没有显示在 recyclerview 中。

我首先调查了我的适配器和 recyclerview 代码，以确保它们得到正确实施，我没有发现任何异常，这使我得出结论，数据可能实际上并没有被创建。

代码如下：

    package com.demoapp.moester96.petheart.Models;

    import android.content.Context;

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

    public class PetRecord {
        private static PetRecord sPetRecord;

        private List<Pet> mPets;

        public static PetRecord get(Context context)
        {
            if(sPetRecord == null)
            {
                sPetRecord = new PetRecord(context);
            }

            return sPetRecord;
        }

        private PetRecord(Context context)
        {
            mPets = new ArrayList<>();

            for(int i=0; i<100; ++i)
            {
                Pet pet = new Pet();
                pet.setmTitle("Pet #" + i);
                pet.setFaved(i % 2 == 0);
                mPets.add(pet);
            }
        }

        public List<Pet> getPets()
        {
            return mPets;
        }

        public Pet getPet(UUID id)
        {
            for(Pet pet : mPets)
            {
                if(pet.getmId().equals(id))
                {
                    return pet;
                }
            }

            return null;
        }

    }

    package com.demoapp.moester96.petheart;

    import android.os.Bundle;
    import android.support.annotation.NonNull;
    import android.support.annotation.Nullable;
    import android.support.v4.app.Fragment;
    import android.support.v7.widget.LinearLayoutManager;
    import android.support.v7.widget.RecyclerView;
    import android.view.LayoutInflater;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.LinearLayout;
    import android.widget.TextView;
    import com.demoapp.moester96.petheart.Models.Pet;
    import com.demoapp.moester96.petheart.Models.PetRecord;

    import java.util.List;

    public class PetListFragment extends Fragment {

        private class PetHolder extends RecyclerView.ViewHolder
        {
            private Pet mPet;
            private TextView mTitleTextView;
            private TextView mDateTextView;

            public PetHolder(LayoutInflater inflater, ViewGroup parent)
            {
                super(inflater.inflate(R.layout.list_item_pet,parent,false));

                mTitleTextView = (TextView) itemView.findViewById(R.id.pet_list_title);
                mDateTextView = (TextView) itemView.findViewById(R.id.pet_list_date);
            }

            public void bind(Pet pet)
            {
                mPet = pet;
                mTitleTextView.setText(mPet.getmTitle());
                mDateTextView.setText(mPet.getmDate().toString());
            }
        }

        private class PetAdapter extends RecyclerView.Adapter<PetHolder>
        {
            private List<Pet> mPets;

            public PetAdapter(List<Pet> pets)
            {
                mPets = pets;
            }

            @NonNull
            @Override
            public PetHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
                LayoutInflater layoutInflater = LayoutInflater.from(getActivity());
                return new PetHolder(layoutInflater,parent);
            }

            @Override
            public void onBindViewHolder(@NonNull PetHolder petHolder, int i) {
                Pet pet = mPets.get(i);
                petHolder.bind(pet);
            }

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

        private RecyclerView mPetRecyclerView;
        private PetAdapter mAdapter;

        @Override
        public void onCreate(Bundle savedInstanceState)
        {
            super.onCreate(savedInstanceState);
        }

        @Nullable
        @Override
        public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
            View view = inflater.inflate(R.layout.activity_pet_list,container,false);
            mPetRecyclerView = (RecyclerView) view.findViewById(R.id.pet_recycler_view);
            mPetRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
            updateUI();
            return view;
        }

        private void updateUI()
        {
            PetRecord petRecord = PetRecord.get(getActivity());
            List<Pet> pets = petRecord.getPets();

            mAdapter = new PetAdapter(pets);
            mPetRecyclerView.setAdapter(mAdapter);
        }
    }

我希望在 recyclerview 中有一个包含宠物编号、日期和时间的 100 个项目的列表，但我得到了一个 recyclerview 中完全没有显示任何内容。

【问题讨论】：

我不确定是否是这种情况，但您能否更新您将布局管理器设置为此的行 mPetRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity(), RecyclerView.VERTICAL);
是的，不幸的是它没有工作它给了我一个错误，说无法解析构造函数 LinearLayoutManager
以防万一，您能否也发布您的项目布局和片段布局？
是的，我的错误 new LinearLayoutManager(getActivity(), RecyclerView.VERTICAL, false) 是正确的构造函数
是的，即使将其放入构造函数中它仍然无法正常工作，是的，我会为您发布项目和片段布局

标签： java android arraylist android-recyclerview android-arrayadapter

【解决方案1】：

视图应该在onCreateViewHolder 中创建，并且应该传递给视图持有者类。然后在你的ViewHolder 类中你还没有创建视图但是你已经使用itemView 来初始化布局的视图。

public class PetListFragment extends Fragment {
   Context context;

   @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        this.context = context; 
        //this is one of the best way to get context of the activity to which the particular activity is associated with
      }
    private class PetHolder extends RecyclerView.ViewHolder
    {
        private Pet mPet;
        private TextView mTitleTextView;
        private TextView mDateTextView;

        public PetHolder(@NonNull View itemView)
        {
             super(itemView);
            mTitleTextView = (TextView) itemView.findViewById(R.id.pet_list_title);
            mDateTextView = (TextView) itemView.findViewById(R.id.pet_list_date);
        }

        public void bind(Pet pet)
        {
            mPet = pet;
            mTitleTextView.setText(mPet.getmTitle());
            mDateTextView.setText(mPet.getmDate().toString());
        }
    }

    private class PetAdapter extends RecyclerView.Adapter<PetHolder>
    {
        private List<Pet> mPets;
        private Context context;
        public PetAdapter(List<Pet> pets,Context context) 
        {
            this.context=context;
            mPets = pets;
        }

        @NonNull
        @Override
        public PetHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
            LayoutInflater layoutInflater = LayoutInflater.from(context);
            View view=layoutInflater.inflate(R.layout.list_item_pet,parent,false);
            return new PetHolder(view);
        }

        @Override
        public void onBindViewHolder(@NonNull PetHolder petHolder, int i) {
            Pet pet = mPets.get(i);
            petHolder.bind(pet);
        }

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

    private RecyclerView mPetRecyclerView;
    private PetAdapter mAdapter;

    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.activity_pet_list,container,false);
        mPetRecyclerView = (RecyclerView) view.findViewById(R.id.pet_recycler_view);
        mPetRecyclerView.setLayoutManager(new LinearLayoutManager(context));
        updateUI();
        return view;
    }

    private void updateUI()
    {
        PetRecord petRecord = PetRecord.get(context);
        List<Pet> pets = petRecord.getPets();

        mAdapter = new PetAdapter(pets,context);
        mPetRecyclerView.setAdapter(mAdapter);
    }
}

【讨论】：

【解决方案2】：

首先，你应该调用：

mPetRecyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));

设置适配器之后，而不是之前。这就是它的工作原理。另外，您是否有用于项目设计的 .xml 文件？应该在你说的适配器上添加它：

@NonNull
@Override
public PetHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
    LayoutInflater layoutInflater = LayoutInflater.from(getContext();
    layoutInflater.inflate(R.layout.insert_your_design_element, parent,false);
    return new PetHolder(layoutInflater,parent);
}

【讨论】：

我把xml代码放在评论区，如果你想看，谢谢你的帮助
LayoutInflater 行出现不兼容的类型错误
将layoutinflater创建（返回LayoutInflater）和inflate方法调用自身（返回View）分开。 LayoutInflater layoutInflater = LayoutInflater.from(getContext(); layoutInflater.inflate(R.layout.insert_your_design_element, parent,false);
我像你说的那样分离了代码，不幸的是这些项目仍然没有显示在 recyclerview 中

【解决方案3】：

setAdapter 之后应该调用 notifyDataSetChanged

    recyclerView = layout.findViewById(R.id.list_recyclerView);
    recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
    recyclerView.addItemDecoration(new DividerItemDecoration(getActivity(), DividerItemDecoration.VERTICAL));
    ListAdapter adapter = new ListAdapter(getActivity());
    recyclerView.setAdapter(adapter);
    adapter.setList(searchList);
    adapter.notifyDataSetChanged();

【讨论】：