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1、缘由

• 在 Android 开发中，如果想限制 RecyclerView 控件显示的子项数量，一般是在布局文件中通过设置控件高度的方式来实现，这种方式无论内容项多少，都会显示固定的高度。如果内容项多于显示高度，可通过上下滚动来显示全部内容；如果内容项小于显示高度，还是会按设置高度显示，此时 RecyclerView 控件就会有出现空白区域。如下所示：

• 若想实现既限制 RecyclerView 控件显示数量，又不出现空白区域的效果，则需要通过自定义控件来实现。

2、自定义 RecyclerView 控件

package com.android.recycleview.viewimport android.content.Context
import android.util.AttributeSet
import androidx.core.content.withStyledAttributes
import androidx.recyclerview.widget.RecyclerView
import com.android.recycleview.Rclass MaxHeightRecyclerView @JvmOverloads constructor(context: Context,attrs: AttributeSet? = null,defStyle: Int = 0,
): RecyclerView(context, attrs, defStyle) {private var maxHeight = 0init {attrs?.let {context.withStyledAttributes(it, R.styleable.MaxHeightRecyclerView) {val height = getDimension(R.styleable.MaxHeightRecyclerView_maxHeight, 0f)if (height > 0) {maxHeight = height.toInt()}}}}override fun onMeasure(widthSpec: Int, heightSpec: Int) {val height = MeasureSpec.getSize(heightSpec)var tmpHeight = heightSpecif (maxHeight in 1..<height) {tmpHeight = MeasureSpec.makeMeasureSpec(maxHeight, MeasureSpec.AT_MOST)}super.onMeasure(widthSpec, tmpHeight)}
}

3、完整示例

（1）子项布局文件

• item_recyclerview.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="match_parent"android:layout_height="wrap_content"xmlns:app="http://schemas.android.com/apk/res-auto"><TextViewandroid:id="@+id/text_view"android:layout_width="wrap_content"android:layout_height="wrap_content"android:textSize="@dimen/sp_16"app:layout_constraintLeft_toLeftOf="parent"app:layout_constraintTop_toTopOf="parent"/></androidx.constraintlayout.widget.ConstraintLayout>

（2）适配器

• CustomRecyclerViewAdapter.kt

package com.android.recycleview.adapterimport android.annotation.SuppressLint
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView
import androidx.recyclerview.widget.RecyclerView
import com.android.recycleview.Rclass CustomRecyclerViewAdapter(private var items: List<String>
): RecyclerView.Adapter<CustomRecyclerViewAdapter.ViewHolder>() {inner class ViewHolder(itemView: View): RecyclerView.ViewHolder(itemView) {private val textView = itemView.findViewById<TextView>(R.id.text_view)fun setText(text: String) {textView.text = text}}override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {val view = LayoutInflater.from(parent.context).inflate(R.layout.item_recyclerview, parent, false)return ViewHolder(view)}override fun getItemCount(): Int {return items.size}override fun onBindViewHolder(holder: ViewHolder, position: Int) {holder.setText(items[position])}@SuppressLint("NotifyDataSetChanged")fun setData(itemList: List<String>) {items = itemListnotifyDataSetChanged()}
}

（3）activity 布局文件

• activity_recyclerview.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="match_parent"android:layout_height="match_parent"xmlns:app="http://schemas.android.com/apk/res-auto"><com.android.recycleview.view.MaxHeightRecyclerViewandroid:id="@+id/recycleView"android:layout_width="@dimen/dp_150"android:layout_height="wrap_content"android:background="@color/purple_200"android:scrollbars="vertical"android:scrollbarSize="@dimen/dp_6"android:fadeScrollbars="false"app:maxHeight="@dimen/dp_160"app:layout_constraintLeft_toLeftOf="parent"app:layout_constraintTop_toTopOf="parent"android:layout_marginStart="@dimen/dp_50"android:layout_marginTop="@dimen/dp_50"/><Buttonandroid:id="@+id/button_less"android:layout_width="wrap_content"android:layout_height="wrap_content"android:textSize="@dimen/sp_18"android:text="@string/less_max_height"app:layout_constraintLeft_toRightOf="@+id/recycleView"app:layout_constraintTop_toTopOf="@+id/recycleView"android:layout_marginStart="@dimen/dp_30"/><Buttonandroid:id="@+id/button_more"android:layout_width="wrap_content"android:layout_height="wrap_content"android:textSize="@dimen/sp_18"android:text="@string/more_max_height"app:layout_constraintLeft_toLeftOf="@+id/button_less"app:layout_constraintTop_toBottomOf="@+id/button_less"android:layout_marginTop="@dimen/dp_20"/></androidx.constraintlayout.widget.ConstraintLayout>

• MaxHeightRecyclerView 控件的 layout_height 设置为 wrap_content 按内容显示，而不是固定的高度值。

（4）activity 文件

• RecyclerViewActivity.kt

package com.android.recycleview.uiimport android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.recyclerview.widget.LinearLayoutManager
import com.android.recycleview.adapter.CustomRecyclerViewAdapter
import com.android.recycleview.databinding.ActivityRecyclerviewBindingclass RecyclerViewActivity: AppCompatActivity() {private lateinit var viewBinding: ActivityRecyclerviewBindingprivate lateinit var adapter: CustomRecyclerViewAdapteroverride fun onCreate(savedInstanceState: Bundle?) {super.onCreate(savedInstanceState)viewBinding = ActivityRecyclerviewBinding.inflate(layoutInflater)setContentView(viewBinding.root)val dataList = listOf("子项1", "子项2", "子项3", "子项4", "子项5", "子项6", "子项7", "子项8")adapter = CustomRecyclerViewAdapter(dataList)viewBinding.recycleView.adapter = adapterviewBinding.recycleView.layoutManager = LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false)viewBinding.buttonLess.setOnClickListener {val items = listOf("子项1", "子项2", "子项3")adapter.setData(items)}viewBinding.buttonMore.setOnClickListener {val items = listOf("子项1", "子项2", "子项3", "子项4", "子项5", "子项6", "子项7", "子项8")adapter.setData(items)}}
}

（5）显示效果