java - 为什么我的 CardView 不会出现在我的 fragment 中？

Question

我正在关注this有关如何使用 View 分页器创建滑动 View 的教程，并尝试将其转换为与 fragment 一起使用。

我的问题是，当我选择 fragment 时，卡片没有出现。

我认为问题出在这行代码

adapter = new Adapter(models, getActivity());

(教程中最初是 adapter = new Adapter(models, this);)，但我可能是错的。

我对 android studio 比较陌生，这是我第一次使用 Fragments，任何帮助将不胜感激!

这是我的代码:

fragment_cards.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"
    android:background="@android:color/background_light"
    >

    <androidx.viewpager.widget.ViewPager
        android:id="@+id/viewPager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:clipToPadding="false"
        android:foregroundGravity="center"
        android:overScrollMode="never"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0"></androidx.viewpager.widget.ViewPager>

    <Button
        android:id="@+id/button_shuffle"
        android:text="SHUFFLE"
        android:textColor="#fff"
        android:background="@drawable/round"
        android:paddingLeft="30dp"
        android:paddingRight="30dp"
        android:layout_marginBottom="60dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        app:layout_constraintBottom_toBottomOf="@+id/viewPager"
        app:layout_constraintEnd_toEndOf="@+id/viewPager"
        app:layout_constraintStart_toStartOf="@+id/viewPager">

    </Button>


</androidx.constraintlayout.widget.ConstraintLayout>

item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorPrimary">

    <androidx.cardview.widget.CardView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:cardCornerRadius="20dp"
        android:layout_margin="8dp">

        <RelativeLayout
            android:layout_width="match_parent"
            android:layout_height="300dp">

            <ImageView
                android:id="@+id/image"
                android:src="@drawable/brochure"
                android:scaleType="centerCrop"
                android:layout_width="match_parent"
                android:layout_height="200dp"/>

            <TextView
                android:id="@+id/title"
                android:textColor="#262626"
                android:layout_below="@id/image"
                android:layout_marginTop="10dp"
                android:layout_marginLeft="16dp"
                android:text="Brochure"
                android:textSize="16sp"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                />

            <TextView
                android:id="@+id/desc"
                android:layout_below="@id/title"
                android:layout_marginTop="3dp"
                android:layout_marginLeft="16dp"
                android:layout_marginRight="16dp"
                android:maxLines="3"
                android:drawablePadding="10dp"
                android:ellipsize="end"
                android:text="Description"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"/>

        </RelativeLayout>

    </androidx.cardview.widget.CardView>

</LinearLayout>

CardsFragment.java

package com.example.musicassistant;

import android.animation.ArgbEvaluator;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.viewpager.widget.ViewPager;

import java.util.ArrayList;
import java.util.List;
import java.util.Objects;

public class CardsFragment extends Fragment {

    ViewPager viewPager;
    Adapter adapter;
    List<Model> models;
    Integer[] colors = null;
    ArgbEvaluator argbEvaluator = new ArgbEvaluator();

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

        models = new ArrayList<>();
        models.add(new Model(R.drawable.brochure, "Brochure", "oooooo"));
        models.add(new Model(R.drawable.sticker, "Sticker", "oooooo"));
        models.add(new Model(R.drawable.poster, "Poster", "oooooo"));
        models.add(new Model(R.drawable.namecard, "Namecard", "oooooo"));

        adapter = new Adapter(models, getActivity());

        viewPager = inflater.inflate(R.layout.fragment_cards, container, false).findViewById(R.id.viewPager);
        viewPager.setAdapter(adapter);
        viewPager.setPadding(130,0,130,0);

        Integer[] colors_temp = {
                getResources().getColor(R.color.color1),
                getResources().getColor(R.color.color2),
                getResources().getColor(R.color.color3),
                getResources().getColor(R.color.color4)

        };

        colors = colors_temp;

        viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
                if (position < (adapter.getCount() -1) && position < (colors.length -1)) {
                    viewPager.setBackgroundColor(
                            (Integer) argbEvaluator.evaluate(
                                    positionOffset,
                                    colors[position],
                                    colors[position + 1]
                            )
                    );
                } else {
                    viewPager.setBackgroundColor(colors[colors.length - 1]);
                }
            }

            @Override
            public void onPageSelected(int position) {

            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });

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

Adapter.java

package com.example.musicassistant;


import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ImageView;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.viewpager.widget.PagerAdapter;

import java.util.List;

public class Adapter extends PagerAdapter {

    private List<Model> models;
    private LayoutInflater layoutInflater;
    private Context context;

    public Adapter(List<Model> models, Context context) {
        this.models = models;
        this.context = context;
    }

    @Override
    public int getCount() {
        return models.size();
    }

    @Override
    public boolean isViewFromObject(@NonNull View view, @NonNull Object object) {
        return view.equals(object);
    }

    @NonNull
    @Override
    public Object instantiateItem(@NonNull ViewGroup container, int position) {
        layoutInflater = LayoutInflater.from(context);
        View view = layoutInflater.inflate(R.layout.item, container, false);

        ImageView imageView;
        TextView title, desc;

        imageView = view.findViewById(R.id.image);
        title = view.findViewById(R.id.title);
        desc = view.findViewById(R.id.desc);

        imageView.setImageResource(models.get(position).getImage());
        title.setText(models.get(position).getTitle());
        desc.setText(models.get(position).getDesc());

        container.addView(view, 0);
        return view;
    }

    @Override
    public void destroyItem(@NonNull ViewGroup container, int position, @NonNull Object object) {
        container.removeView((View)object);
    }
}

Model.java

package com.example.musicassistant;

public class Model {

    private int image;
    private String title;
    private  String desc;

    public Model(int image, String title, String desc) {
        this.image = image;
        this.title = title;
        this.desc = desc;
    }

    public int getImage() {
        return image;
    }

    public void setImage(int image) {
        this.image = image;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getDesc() {
        return desc;
    }

    public void setDesc(String desc) {
        this.desc = desc;
    }
}

值得注意的是， fragment 本身正在工作，因为其中的按钮显示正确。唯一的问题是我的卡片根本没有显示。

任何帮助将不胜感激，因为这让我彻底发疯!

非常感谢您抽出宝贵的时间，非常感谢，

罗马

Answer 1

每次使用 Inflate 时都会创建一个新的布局对象( View 对象)，因此这是一个 View 布局

viewPager = inflater.inflate(R.layout.fragment_cards, container, false).findViewById(R.id.viewPager);

并且您将返回一个没有任何内容的新 View 对象

return inflater.inflate(R.layout.fragment_cards, container, false);

因此，您需要在膨胀后存储 View 的单个引用，然后使用它并返回它，如下所示

View root  = inflater.inflate(R.layout.fragment_cards, container, false).findViewById(R.id.viewPager);
viewPager = root.findViewById(R.id.viewPager);
// ... code
return root;