java如何在查看最后和第一个片段时使后退/下一步按钮不可见

Question

我正在尝试使用viewpager和fragments创建一个多页表单。表单需要有底部的点和点左右两侧的“后退”/“下一步”按钮，下一步按钮在最后一张幻灯片上更改为“完成”

我设法让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片段的下一个按钮时不可见

我的代码：

主要活动。爪哇

import 安卓x.appcompat.app.AppCompatActivity;
import 安卓x.viewpager.widget.ViewPager;
import 安卓.os.Bundle;
import 安卓.os.Handler;
import 安卓.view.View;

import com.google.安卓.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);
    }
}

主要活动。xml

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

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

    <安卓x.constraintlayout.widget.ConstraintLayout
        安卓:layout_width="match_parent"
        安卓:layout_height="wrap_content"
        安卓:layout_alignParentBottom="true">

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

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

        <com.google.安卓.material.tabs.TabLayout
            安卓:id="@+id/tabLayout"
            安卓:layout_width="wrap_content"
            安卓: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" />

    </安卓x.constraintlayout.widget.ConstraintLayout>
</RelativeLayout>

ViewPagerAdapter。爪哇

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

import 安卓x.annotation.NonNull;
import 安卓x.annotation.Nullable;
import 安卓x.fragment.app.Fragment;
import 安卓x.fragment.app.FragmentManager;
import 安卓x.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。布局为第一行的java

import 安卓.content.Context;
import 安卓.os.Bundle;
import 安卓.view.LayoutInflater;
import 安卓.view.View;
import 安卓.view.ViewGroup;

import 安卓x.annotation.NonNull;
import 安卓x.annotation.Nullable;
import 安卓x.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

您可以将addOnPageChangeListenerlistener添加到ViewPager，并在onPageSelected()回调中将所选/当前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();
}

另外，初始化FragmentPagerAdapter的正确方法是从getItem()返回新的片段实例。您应该永远不要将实际的片段引用传递给FragmentPagerAdapter，并且不应该将它们作为列表保存

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);
    }
}