查看最后一个和第一个片段时如何使 back/next 按钮不可见

Question

我正在尝试使用 viewpager 和片段创建多页表单。表单需要有底部点和点的右侧和左侧的“后退”/“下一步”按钮，其中下一个按钮在最后一张幻灯片上变为“完成”。

我设法使底部的点和按钮起作用。然而，我不知道如何让后退按钮在第一个片段和最后一个按钮时不可见。

我的代码：

MainActivity.java

import androidx.appcompat.app.AppCompatActivity;
import androidx.viewpager.widget.ViewPager;
import android.os.Bundle;
import android.os.Handler;
import android.view.View;

import com.google.android.material.tabs.TabLayout;

public class MainActivity extends AppCompatActivity {

    TabLayout tabLayout;
    ViewPager viewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tabLayout = findViewById(R.id.tabLayout);
        viewPager = findViewById(R.id.viewPager);

        getTabs();
    }

    public void getTabs(){
       final ViewPagerAdapter viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());

        new Handler().post(new Runnable() {
            @Override
            public void run() {
        viewPagerAdapter.addFragment(oneFragment.getInstance(),null); // this line can cause crashes
        viewPagerAdapter.addFragment(twoFragment.getInstance(),null); // this line can cause crashes
        viewPagerAdapter.addFragment(threeFragment.getInstance(),null); // this line can cause crashes

        viewPager.setAdapter(viewPagerAdapter);
        tabLayout.setupWithViewPager(viewPager);
            }
        });
    }

    //Implement the listeners for the back and previous items
    public void goBack(View view) {
        viewPager.setCurrentItem(viewPager.getCurrentItem()-1);
    }
    public void goNext(View view) {
        viewPager.setCurrentItem(viewPager.getCurrentItem()+1);
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <androidx.viewpager.widget.ViewPager
        android:id="@+id/viewPager"
        android:layout_width="match_parent"
        android:layout_height="match_parent"/>

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true">

        <Button
            android:id="@+id/back"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/back"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            android:onClick="goBack"/>

        <Button
            android:id="@+id/next"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="@string/next"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            android:onClick="goNext"/>

        <com.google.android.material.tabs.TabLayout
            android:id="@+id/tabLayout"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:tabBackground="@drawable/tab_selector"
            app:tabIndicatorHeight="0dp" />

    </androidx.constraintlayout.widget.ConstraintLayout>
</RelativeLayout>

ViewPagerAdapter.java

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

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentStatePagerAdapter;

public class ViewPagerAdapter extends FragmentStatePagerAdapter {

    private List<Fragment> fragmentList = new ArrayList<>(); // this line can cause crashes
    private List<String> stringList = new ArrayList<>();

    public ViewPagerAdapter(@NonNull FragmentManager fm) {
        super(fm);
    }

    @NonNull
    @Override
    public Fragment getItem(int position) {
        return fragmentList.get(position);
    }

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

    @Nullable
    @Override
    public CharSequence getPageTitle(int position) {
        return stringList.get(position);
    }

    public void addFragment(Fragment fragment, String title){
        fragmentList.add(fragment); // this line can cause crashes
        stringList.add(title);
    }
}

片段示例：oneFragment.java 布局 row_one

import android.content.Context;
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;

public class OneFragment extends Fragment {

    public static OneFragment getInstance(){
        OneFragment oneFragment = new OneFragment();
        return oneFragment;
    }

    @Override
    public void onAttach(@NonNull Context context) {
        super.onAttach(context);
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.row_one, container,false);
        return view;
    }
}

Answer 1

您可以向 ViewPager 添加 addOnPageChangeListener 侦听器，并在 onPageSelected() 回调中将 selected/current position 与第一个（位置 = 0）或最后一个位置进行比较。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    tabLayout = findViewById(R.id.tabLayout);
    viewPager = findViewById(R.id.viewPager);
    final Button back = findViewById(R.id.back);
    final Button finish = findViewById(R.id.next);



    viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

        }

        @Override
        public void onPageSelected(int position) {

            back.setVisibility(View.VISIBLE);
            finish.setVisibility(View.VISIBLE);
            
            if (position == 0) {
                back.setVisibility(View.GONE);
            } else {
                PagerAdapter adapter = viewPager.getAdapter();
                if (adapter != null && position == adapter.getCount() - 1) {
                    finish.setVisibility(View.GONE);
                }
            }
        }

        @Override
        public void onPageScrollStateChanged(int state) {

        }
    });

    getTabs();
}

此外，初始化 FragmentPagerAdapter 的正确方法是 return 来自 getItem() 的新 Fragment 实例。您应该 永远不要 将实际的 Fragment 引用传递给 FragmentPagerAdapter，并且您不应该将它们保存为列表。

public class ViewPagerAdapter extends FragmentStatePagerAdapter {
    public ViewPagerAdapter(FragmentManager manager) {
        super(manager);
    }

    @Override
    public Fragment getItem(int position) {
        if(position == 0) return new SampleFragment();
        if(position == 1) return new SampleFragment2();
        if(position == 2) return new SampleFragment3();
        throw new IllegalStateException("Unexpected position " + position);
    }

    @Override
    public int getCount() {
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        if(position == 0) return "Tab 1";
        if(position == 1) return "Tab 2";
        if(position == 2) return "Tab 3";
        throw new IllegalStateException("Unexpected position " + position);
    }
}

查看最后一个和第一个片段时如何使 back/next 按钮不可见

How to make back/next buttons invisible while viewing the last and first fragment

java

android

android-fragments

android-viewpager

android-navigation