为什么Android材料设计选项卡布局不起作用?

我试图用viewpager创建一个Tablayout,但它不起作用。当它被装载时 tabLayout.setupWithViewPager(viewPager); 在Patient Main Activity类中,输出为:

当我使用 viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout)); 在Patient Main Activity类中,输出为:

但是片段没有显示,也没有可用的滑动。

我按书上说的那样做。我使用了一些资源,例如 1 , 2 。我个人认为代码可能不是问题所在,使用的SDK版本有问题,但我可能是错误的。这是我的代码:

主要活动:

package parsa.lop.adjust;

import android.support.design.widget.TabLayout;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TabHost;

public class PatientMain extends AppCompatActivity {

    private TabHost tabHost;

    private TabLayout tabLayout;
    private ViewPager viewPager;
    private ViewPagerAdapter viewPagerAdapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_patient_main);

        tabLayout = (TabLayout)findViewById(R.id.tabLayout);
        viewPager = (ViewPager)findViewById(R.id.viewPager);

        viewPagerAdapter = new ViewPagerAdapter(getSupportFragmentManager());
        viewPagerAdapter.addFragment(new MyDietFragment(), "My Diet");
        viewPagerAdapter.addFragment(new MyWorkoutFragment(), "My Workout");
        viewPagerAdapter.addFragment(new StartProgramFragment(), "Start a Program");
        viewPager.setAdapter(viewPagerAdapter);



        final TabLayout.Tab myDietTab = tabLayout.newTab();
        final TabLayout.Tab myWorkOutTab = tabLayout.newTab();
        final TabLayout.Tab startProgramTab = tabLayout.newTab();

        myDietTab.setText("My Diet");
        myWorkOutTab.setText("My Workout");
        startProgramTab.setText("Start a Program");

        tabLayout.addTab(startProgramTab,0);
        tabLayout.addTab(myDietTab,1);
        tabLayout.addTab(myWorkOutTab,2);

//        tabLayout.setupWithViewPager(viewPager);
        viewPager.addOnPageChangeListener(new TabLayout.TabLayoutOnPageChangeListener(tabLayout));

    }
}

viewpager适配器:

package parsa.lop.adjust;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.app.FragmentStatePagerAdapter;

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

public class ViewPagerAdapter extends FragmentPagerAdapter {

    private List<String> fragmentTitleList = new ArrayList<String>();
    private List<Fragment> fragmentList = new ArrayList<Fragment>();

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

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

    @Override
    public int getCount() {
        return 0;
    }

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

    public void addFragment(Fragment fragment, String title){
        fragmentList.add(fragment);
        fragmentTitleList.add(title);
    }
}

其中一个片段:

package parsa.lop.adjust;


import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

public class MyDietFragment extends Fragment {

    @Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

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

我的主要布局:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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=".PatientMain">

    <android.support.design.widget.TabLayout
        android:id="@+id/tabLayout"
        android:layout_height="wrap_content"
        android:layout_width="match_parent"
        android:background="@color/colorPrimary"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true">

    </android.support.design.widget.TabLayout>

    <android.support.v4.view.ViewPager
        android:id="@+id/viewPager"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    </android.support.v4.view.ViewPager>

</android.support.constraint.ConstraintLayout>

我的片段布局:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:text="Tab 1"
        android:textSize="36sp" />

</RelativeLayout>

年级学生:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 27
    defaultConfig {
        applicationId "parsa.lop.adjust"
        minSdkVersion 21
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    implementation 'com.android.support:appcompat-v7:27.1.1'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    implementation 'com.android.support:design:27.1.1'
}