RecyclerView 未在布局中显示

Question

我想要实现 Android Studio 预览（左）中所示的布局。但是如果在模拟器中执行，只有按钮可见，RecyclerView 不是 visible/populated（右）。

XML代码：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
style="@style/AppTheme">

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/list"
    android:name="com.example.app.ItemFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_marginLeft="16dp"
    android:layout_marginRight="16dp"
    app:layoutManager="LinearLayoutManager"
    tools:context=".ItemFragment"
    tools:listitem="@layout/fragment_item"
    android:scrollbars="vertical"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/floatingActionButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|center_horizontal"
        android:layout_margin="@dimen/fab_margin"
        android:clickable="true"
        android:focusable="true"
        android:src="@android:drawable/ic_input_add"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

但是，如果 RecyclerView 单独存在于片段中，则会填充列表（但当然不会显示操作按钮）。代码见下。是的，我应该显示的列表是 not 空的。

<?xml version="1.0" encoding="utf-8"?>
<androidx.recyclerview.widget.RecyclerView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/list"
    android:name="com.example.app.ItemFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_marginLeft="16dp"
    android:layout_marginRight="16dp"
    app:layoutManager="LinearLayoutManager"
    tools:context=".ItemFragment"
    tools:listitem="@layout/fragment_item" >
</androidx.recyclerview.widget.RecyclerView>

我已经尝试使用 RelativeLayout 和 FrameLayout，但我仍然得到相同的结果。如果我使用例如，会发生相同的行为一个 TextView 而不是操作按钮。

--- 请求的附加信息 ---

适配器class（由Android Studio自动生成，模板）：

class MyItemRecyclerViewAdapter(
    private val mValues: List<DummyItem>,
    private val mListener: OnListFragmentInteractionListener?
) : RecyclerView.Adapter<MyItemRecyclerViewAdapter.ViewHolder>() {

    private val mOnClickListener: View.OnClickListener

    init {
        mOnClickListener = View.OnClickListener { v ->
            val item = v.tag as DummyItem
            // Notify the active callbacks interface (the activity, if the fragment is attached to
            // one) that an item has been selected.
            mListener?.onListFragmentInteraction(item)
        }
    }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
        val view = LayoutInflater.from(parent.context)
            .inflate(R.layout.fragment_item, parent, false)
        return ViewHolder(view)
    }

    override fun onBindViewHolder(holder: ViewHolder, position: Int) {
        val item = mValues[position]
        holder.mIdView.text = item.id
        holder.mContentView.text = item.content

        with(holder.mView) {
            tag = item
            setOnClickListener(mOnClickListener)
        }
    }

    override fun getItemCount(): Int = mValues.size

    inner class ViewHolder(val mView: View) : RecyclerView.ViewHolder(mView) {
        val mIdView: TextView = mView.item_number
        val mContentView: TextView = mView.content

        override fun toString(): String {
            return super.toString() + " '" + mContentView.text + "'"
        }
    }
}

列表片段（由Android Studio自动生成，模板）：

class ItemFragment : Fragment() {

    // TODO: Customize parameters
    private var columnCount = 1

    private var listener: OnListFragmentInteractionListener? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        arguments?.let {
            columnCount = it.getInt(ARG_COLUMN_COUNT)
        }
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        val view = inflater.inflate(R.layout.fragment_item_list, container, false)

        // Set the adapter
        if (view is RecyclerView) {
            with(view) {
                layoutManager = when {
                    columnCount <= 1 -> LinearLayoutManager(context)
                    else -> GridLayoutManager(context, columnCount)
                }
                adapter = MyItemRecyclerViewAdapter(DummyContent.ITEMS, listener)
            }
        }
        return view
    }

    override fun onAttach(context: Context) {
        super.onAttach(context)
        if (context is OnListFragmentInteractionListener) {
            listener = context
        } else {
            throw RuntimeException(context.toString() + " must implement OnListFragmentInteractionListener")
        }
    }

    override fun onDetach() {
        super.onDetach()
        listener = null
    }

    /**
     * This interface must be implemented by activities that contain this
     * fragment to allow an interaction in this fragment to be communicated
     * to the activity and potentially other fragments contained in that
     * activity.
     *
     *
     * See the Android Training lesson
     * [Communicating with Other Fragments](http://developer.android.com/training/basics/fragments/communicating.html)
     * for more information.
     */
    interface OnListFragmentInteractionListener {
        // TODO: Update argument type and name
        fun onListFragmentInteraction(item: DummyItem?)
    }

    companion object {

        // TODO: Customize parameter argument names
        const val ARG_COLUMN_COUNT = "column-count"

        // TODO: Customize parameter initialization
        @JvmStatic
        fun newInstance(columnCount: Int) =
            ItemFragment().apply {
                arguments = Bundle().apply {
                    putInt(ARG_COLUMN_COUNT, columnCount)
                }
            }
    }
}

Answer 1

试试这个布局

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    style="@style/AppTheme"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/list"
        android:name="com.example.app.ItemFragment"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:scrollbars="vertical"
        app:layoutManager="LinearLayoutManager"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:context=".ItemFragment"
        tools:listitem="@layout/fragment_item" />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/floatingActionButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|center_horizontal"
        android:layout_margin="@dimen/fab_margin"
        android:clickable="true"
        android:focusable="true"
        android:src="@android:drawable/ic_input_add"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />
</androidx.constraintlayout.widget.ConstraintLayout>

在 ItemFragment.kt 中将 onCreateView 替换为

override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {

return inflater.inflate(R.layout.fragment_item_list, container, false)
}

之后继续实施 onViewCreated

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
    with(list) {
        layoutManager = when {
            columnCount <= 1 -> LinearLayoutManager(requireContext())
            else -> GridLayoutManager(requireContext(), columnCount)
        }
        adapter = MyItemRecyclerViewAdapter(DummyContent.ITEMS)
    }
}

这应该可以解决您的问题，因为 AS 模板假定根视图将是一个 RecyclerView 并且正在这样处理整个布局。

RecyclerView 未在布局中显示

RecyclerView not showing in Layout

xml

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