RecyclerView中CardView自定义参数运行时失效问题求助

问题情况 #

使用RecyclerView展示课程列表时，在lesson_item.xml中为CardView设置的cardBackgroundColor、cardCornerRadius等样式参数完全没有生效，实际显示的CardView没有圆角和自定义背景色。

预期效果：

实际运行效果：

相关代码 #

lesson_item.xml #

<?xml version="1.0" encoding="utf-8"?>
<androidx.cardview.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:card_view="http://schemas.android.com/tools"
    android:id="@+id/lesson_card"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="20dp"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    card_view:cardBackgroundColor="#E8DFDF"
    card_view:cardCornerRadius="30dp">

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:padding="4dp">

        <TextView
            android:id="@+id/lesson_item_title"
            style="@style/TextAppearance.MaterialComponents.Headline4"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:gravity="center_horizontal"
            android:text="@string/lesson_name"
            android:textColor="#000000"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

        <TextView
            android:id="@+id/lesson_item_desc"
            style="@style/TextAppearance.MaterialComponents.Body1"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:gravity="center_horizontal"
            android:layout_marginTop="4dp"
            android:padding="10dp"
            android:text="@string/lesson_text"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toBottomOf="@id/lesson_item_title" />
    </androidx.constraintlayout.widget.ConstraintLayout>
</androidx.cardview.widget.CardView>

fragment_lessons.xml #

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
    android:background="#FFFFFF"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    tools:context=".presentation.fragments.LessonsFragment">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <ImageButton
            android:id="@+id/button_back_from_lessons_to_menu"
            android:layout_width="38dp"
            android:layout_height="48dp"
            android:background="#ED8C3E"
            android:contentDescription="button to back"
            android:src="@drawable/ic_back_24"
            android:textSize="24sp"
            android:textStyle="bold"/>

        <TextView
            android:id="@+id/hello_text_view"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:paddingTop="8dp"
            android:paddingBottom="8dp"
            android:background="#ED8C3E"
            android:paddingStart="120dp"
            android:text="@string/lessons"
            android:textColor="#000000"
            android:textSize="24sp"
            android:textStyle="bold"/>
    </LinearLayout>

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/lessons_recycler_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
</LinearLayout>

LessonAdapter.kt #

package com.example.englishease.presentation.adapter

import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import android.widget.TextView
import androidx.cardview.widget.CardView
import androidx.recyclerview.widget.RecyclerView
import com.example.englishease.R

class LessonAdapter(val list: List<String>, val lessonDescs: List<String>, val listener: LessonAdapter.OnItemClickListener) :
    RecyclerView.Adapter<LessonAdapter.MyLesson>(){

        class MyLesson(item: View) : RecyclerView.ViewHolder(item) {
            val cardView: CardView = item.findViewById(R.id.lesson_card)
        }

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): LessonAdapter.MyLesson {
        val myItem = LayoutInflater.from(parent.context).inflate(R.layout.lesson_item, parent, false)
        return LessonAdapter.MyLesson(myItem)
    }

    override fun onBindViewHolder(holder: LessonAdapter.MyLesson, position: Int) {
        val lessonName = list[position]
        val lessonDescs = lessonDescs[position]
        holder.cardView.findViewById<TextView>(R.id.lesson_item_title).text = lessonName
        holder.cardView.findViewById<TextView>(R.id.lesson_item_desc).text = lessonDescs
        holder.itemView.setOnClickListener {
            listener.onItemClickListener(list[position])
        }
    }

    override fun getItemCount() = list.size

    interface OnItemClickListener {
        fun onItemClickListener(lessonName: String)
    }
}

解决方法 #

核心问题：命名空间错误 #

你给CardView属性用的card_view命名空间指向了http://schemas.android.com/tools，这个是Android Studio工具的命名空间，不是CardView控件的属性命名空间，导致系统无法识别这些样式参数。

修改方案 #

直接使用已经声明好的app命名空间（xmlns:app="http://schemas.android.com/apk/res-auto"）来设置CardView的属性，把card_view:前缀替换成app:，同时删除无效的card_view命名空间声明：

修改后的lesson_item.xml头部及属性部分：

<?xml version="1.0" encoding="utf-8"?>
<androidx.cardview.widget.CardView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/lesson_card"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="20dp"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:cardBackgroundColor="#E8DFDF"
    app:cardCornerRadius="30dp">

额外优化建议 #

把fragment_lessons.xml中RecyclerView的宽高改为match_parent，避免内容被截断：

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/lessons_recycler_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

内容的提问来源于stack exchange，提问作者StevM23