RecyclerView 无法正常显示？

Question

我正在关注Big Nerd Ranch Guide（2015 年版）。我们从片段开始。本章介绍了RecycleView。

目标是使用RecycleView 显示100 个Crimes，如下所示：

作者使用Crime 作为数据对象（字段：id、title、resolved、Date）。

我们有一个超类Activity，叫做SingleFragmentActivity：

    public abstract class SingleFragmentActivity extends FragmentActivity {

    protected abstract Fragment createFragment();
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fragment); //container for different fragments...

        FragmentManager fm = getSupportFragmentManager();
        Fragment fragment = fm.findFragmentById(R.id.fragment_container);

        if (fragment == null) {
            fragment = createFragment();
            fm.beginTransaction()
                    .add(R.id.fragment_container, fragment)
                    .commit();
        }
    }
}

伴随的布局（你可以在 SingleFragmentActivity 中看到）是fragment_container.xml：

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/fragment_container"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

然后我们有一个子类，叫做CrimeListActivity：

public class CrimeListActivity extends SingleFragmentActivity {
    @Override
    protected Fragment createFragment() {
        return new CrimeListFragment();
    }
}

CrimeListFragment 是一个 Fragment，其内部类为 ViewHolder、Adapter，RecycleView 需要它来创建必要的 Views 并回收它们。

public class CrimeListFragment extends Fragment {
    private RecyclerView mCrimeRecyclerView;
    private CrimeAdapter mAdapter;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View view = inflater.inflate(R.layout.fragment_crime_list, container, false);

        mCrimeRecyclerView = (RecyclerView) view.findViewById(R.id.crime_recycler_view);

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

        updateUI();

        return view;
    }

    private void updateUI() {
        CrimeLab crimeLab = CrimeLab.get(getActivity());
        List<Crime> crimes = crimeLab.getCrimes();
        mAdapter = new CrimeAdapter(crimes);
        mCrimeRecyclerView.setAdapter(mAdapter);
    }

    private class CrimeHolder extends RecyclerView.ViewHolder implements View.OnClickListener{
        private Crime mCrime;
        private TextView mTitleTextView;
        private TextView mDateTextView;
        private CheckBox mSolvedCheckBox;

        public CrimeHolder(View itemView) {
            super(itemView);
            itemView.setOnClickListener(this);
            mTitleTextView = (TextView)
                    itemView.findViewById(R.id.list_item_crime_title_text_view);
            mDateTextView = (TextView)
                    itemView.findViewById(R.id.list_item_crime_date_text_view);
            mSolvedCheckBox = (CheckBox)
                    itemView.findViewById(R.id.list_item_crime_solved_check_box);
        }

        public void bindCrime(Crime crime) {
            mCrime = crime;
            mTitleTextView.setText(mCrime.getTitle());
            mDateTextView.setText(mCrime.getDate().toString());
            mSolvedCheckBox.setChecked(mCrime.isSolved());
        }

        @Override
        public void onClick(View v) {
            Toast.makeText(getActivity(),mCrime.getTitle() + " clicked!", Toast.LENGTH_SHORT)
                    .show();
        }
    }

    private class CrimeAdapter extends RecyclerView.Adapter<CrimeHolder> {
        private List<Crime> mCrimes;
        public CrimeAdapter(List<Crime> crimes) {
            mCrimes = crimes;
        }
        @Override
        public CrimeHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            /*onCreateViewHolder is called by the RecyclerView when it needs a new View to display an item. In
            this method, you create the View and wrap it in a ViewHolder. The RecyclerView does not expect that
            you will hook it up to any data yet.
            For the View, you inflate a layout from the Android standard library called simple_list_item_1. This
            layout contains a single TextView, styled to look nice in a list. Later in the chapter, you will make a
            more advanced View for the list items*/
            LayoutInflater layoutInflater = LayoutInflater.from(getActivity());

            View view = layoutInflater.inflate(R.layout.list_item_crime, parent, false);

            return new CrimeHolder(view);

        }
        @Override
        public void onBindViewHolder(CrimeHolder holder, int position) {
            /*onBindViewHolder: This method will bind a ViewHolder’s View to your model object. It receives
            the ViewHolder and a position in your data set. To bind your View, you use that position to find the
            right model data. Then you update the View to reflect that model data.
            In your implementation, that position is the index of the Crime in your array. Once you pull it out, you
            bind that Crime to your View by sending its title to your ViewHolder’s TextView.*/
            Crime crime = mCrimes.get(position);
            holder.bindCrime(crime);
        }
        @Override
        public int getItemCount() {
            return mCrimes.size();
        }
    }




}

这个片段的伴随布局自然是xml格式的RecycleView，命名为fragment_crime_list.xml：

<?xml version="1.0" encoding="utf-8"?>
<androidx.recyclerview.widget.RecyclerView
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/crime_recycler_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

最后，Adapter 将通过其绑定数据的实际ViewItem 以两个TextViews 的形式给出（一个用于Crime 的日期，一个用于Crime 的标题）和一个CheckBox 标记是犯罪已解决。

它被命名为list_item_crime.xml，看起来像这样：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <CheckBox
        android:id="@+id/list_item_crime_solved_check_box"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentRight="true"
        android:padding="4dp"/>

    <TextView
        android:id="@+id/list_item_crime_title_text_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_toLeftOf="@id/list_item_crime_solved_check_box"
        android:textStyle="bold"
        android:padding="4dp"
        tools:text="Crime Title"/>

    <TextView
        android:id="@+id/list_item_crime_date_text_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_toLeftOf="@id/list_item_crime_solved_check_box"
        android:layout_below="@id/list_item_crime_title_text_view"
        android:padding="4dp"
        tools:text="Crime Date"/>

</RelativeLayout>

现在，这是书上的内容，我仔细检查了所有内容，希望没有遗漏任何内容。我的Crime 项目仅出现在屏幕顶部，我需要向下滚动到屏幕底部，只有第二个犯罪项目出现。像这样：

有人知道我在这里缺少什么吗？我在过去三天一直在寻找解决这个问题的方法。其实在这里问是我最后的希望:)

Answer 1

这个问题很难解决。仔细查看您的 list_item_crime.xml 文件。特别是在这一行：

android:layout_height="match_parent"

您将单个项目的高度设置为整个可用高度。如果您尝试滚动，您应该会看到下面的其他项目占据相同的高度。简单修复，改成：

android:layout_height="wrap_content"

当我第一次开始使用 Android 时，我讨厌 RecyclerView，因为它可能会出现很多问题。这在我身上发生了不止一次:-)

Answer 2

没有足够的时间阅读所有这些，但我认为将 recyclerview list_item_crime.xml 的视图持有者的父布局高度设置为 wrap_content

比如这样的

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <CheckBox
        android:id="@+id/list_ite.....