android - 即使指定了 "match_parent"，使用 ConstraintLayout 的 RecyclerView 项目也不会填满屏幕的整个宽度

Question

我正在尝试设置一个回收器 View ，它使用 ConstraintLayout 显示元素。我用了layout from this example by Google as a guideline.

然而，尽管一直指定 android:layout_width="match_parent"，最终显示在屏幕上的结果更类似于 wrap_content。

到目前为止我尝试了什么

1. 将 ConstraintLayout 更改为 LinearLayout
2. 此 stackoverflow question 中建议的解决方法
3. 设置 android:layout_width=0，同时使用 ConstraintLayout 并将开始、结束和顶部锚定到父级
4. 将整个东西包装在 CardView 中
5. 在多个设备上运行应用，包括模拟设备和物理设备

相关代码

如果缺少任何其他感兴趣的内容，请告诉我。

recipe_list_entry.xml

    <?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="@dimen/recipe_entry_height" >

    <TextView
        android:id="@+id/recipe_title"
        style="@style/Widget.RecipeTracker.ListItemTextView"
        android:layout_marginHorizontal="@dimen/margin_between_elements"
        android:background="@color/design_default_color_on_secondary"
        android:layout_width="match_parent"
        android:fontFamily="sans-serif"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        android:maxLines="1"
        android:ellipsize="end"
        tools:text="amazing stuff"/>
</androidx.constraintlayout.widget.ConstraintLayout>

样式.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <style name="Widget.RecipeTracker.ListItemTextView" parent="Widget.MaterialComponents.TextView">
        <item name="android:gravity">center_vertical</item>
        <item name="android:layout_height">48dp</item>
        <item name="android:textAppearance">?attr/textAppearanceBody1</item>
    </style>
</resources>

recipe_list_fragment.xml

<?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"
    tools:context=".RecipeListFragment">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/recyclerView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginHorizontal="@dimen/margin_between_elements"
        android:scrollbars="vertical"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/add_entry"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_marginEnd="@dimen/add_entry_fab_margin"
        android:layout_marginBottom="@dimen/add_entry_fab_margin"
        android:contentDescription="@string/add_new_recipe_contentDescription"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:srcCompat="@drawable/ic_add" />

</androidx.constraintlayout.widget.ConstraintLayout>

RecipeListFragment.kt

import android.os.Bundle
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import androidx.fragment.app.Fragment
import androidx.fragment.app.activityViewModels
import androidx.navigation.fragment.findNavController
import androidx.recyclerview.widget.LinearLayoutManager
import me.lindenbauer.recipetracker.databinding.RecipeListFragmentBinding

/**
 * A simple [Fragment] subclass as the default destination in the navigation.
 */
class RecipeListFragment : Fragment() {

    private val viewModel: RecipeTrackerViewModel by activityViewModels {
        RecipeTrackerViewModelFactory(
            (activity?.application as RecipeTrackerApplication).database.recipeDao()
        )
    }

    private var _binding: RecipeListFragmentBinding? = null

    // This property is only valid between onCreateView and
    // onDestroyView.
    private val binding get() = _binding!!

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

        _binding = RecipeListFragmentBinding.inflate(inflater, container, false)
        return binding.root

    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        // Set up the automatic data updates for the recyclerView using the RecipeListAdapter
        val adapter = RecipeListAdapter {
            // TODO: Define onRecipeClicked here
        }
        // This specifies how the "cards" are displayed by the recyclerView
        binding.recyclerView.layoutManager = LinearLayoutManager(this.context)
        binding.recyclerView.adapter = adapter

        // Attach an observer on the allItems list to update the UI automatically when the data
        // changes.
        viewModel.allRecipes.observe(this.viewLifecycleOwner) { recipes ->
            recipes.let{ adapter.submitList(it) }
        }

        binding.addEntry.setOnClickListener{
            findNavController().navigate(R.id.action_RecipeListFragment_to_AddEntryFragment)
        }
    }

    override fun onDestroyView() {
        super.onDestroyView()
        _binding = null
    }
}

RecipeListAdapter.kt

import android.view.LayoutInflater
import android.view.ViewGroup
import androidx.recyclerview.widget.ListAdapter
import androidx.recyclerview.widget.DiffUtil
import androidx.recyclerview.widget.RecyclerView
import me.lindenbauer.recipetracker.data.Recipe
import me.lindenbauer.recipetracker.databinding.RecipeListEntryBinding

class RecipeListAdapter(private val onRecipeClicked: (Recipe) -> Unit):
    ListAdapter<Recipe, RecipeListAdapter.RecipeViewHolder>(DiffCallback) {

    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecipeViewHolder {
        return RecipeViewHolder(
            RecipeListEntryBinding.inflate(
                LayoutInflater.from(
                    parent.context
                )
            )
        )
    }

    override fun onBindViewHolder(holder: RecipeViewHolder, position: Int) {
        // Set up anything that is supposed to happen when interacting with a single item in the recyclerView
        val current = getItem(position)
        holder.itemView.setOnClickListener {
            onRecipeClicked(current)
        }

        holder.bind(current)
    }

    class RecipeViewHolder(private var binding: RecipeListEntryBinding) :
        RecyclerView.ViewHolder(binding.root) {

        fun bind(recipe: Recipe) {
            binding.recipeTitle.text = recipe.recipeTitle
        }
    }


    companion object {
        private val DiffCallback = object : DiffUtil.ItemCallback<Recipe>() {
            override fun areItemsTheSame(oldRecipe: Recipe, newRecipe: Recipe): Boolean {
                return oldRecipe === newRecipe
            }

            override fun areContentsTheSame(oldRecipe: Recipe, newRecipe: Recipe): Boolean {
                // TODO: Might need to make this more sophisticated
                return oldRecipe.recipeTitle == newRecipe.recipeTitle
            }
        }
    }
}

设计器预览

Android Studio 设计器预览

实际结果

运行 API 30/Android 11 的模拟 Pixel 5 的实际结果

Answer 1

您的代码在这里:

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecipeViewHolder {
    return RecipeViewHolder(
        RecipeListEntryBinding.inflate(
            LayoutInflater.from(
                parent.context
            )
        )
    )
}

不会在父 View 的上下文中膨胀 View ，因此某些布局参数不会像您期望的那样被处理。在这种情况下，如果它不知道其父对象是谁，它的宽度就不能为 match_parent。将其更改为:

override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): RecipeViewHolder {
    return RecipeViewHolder(
        RecipeListEntryBinding.inflate(
            LayoutInflater.from(
                parent.context
            ),
            parent,
            false
        )
    )
}

还有一个提示。您不需要创建一个伴随对象来保存您的 DiffCallback。而且您也不需要属性来保存 object。为了更简洁的代码，而不是

companion object {
    private val DiffCallback = object : DiffUtil.ItemCallback<Recipe>() {
        //...
    }
}

你可以把

private object DiffCallback: DiffUtil.ItemCallback<Recipe>() {
    //...
}