ViewPager加载Fragment出现空白不显示问题

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场景一：在Fragment中使用TabLayout+ViewPager，ViewPager加载不同Fragment，tablayout的Tab显示而Fragment不显示

使用tablayout需导入design库implementation 'com.android.support:design:26.1.0'

• xml

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

    <android.support.design.widget.TabLayout
        android:id="@+id/tablayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"/>

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


</LinearLayout>

• activity

public class MainActivity extends AppCompatActivity {
    private ViewPager viewPager;
    private TabLayout mTablayout;
    private List<Fragment> fragmentList;
    private String[] title = {"第一", "第二", "第三"};
    private MyPagerAdapter2 mAdapter;

    private <T extends View> T bindView(int id) {
        return (T) findViewById(id);
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        viewPager = bindView(R.id.viewpager);
        mTablayout = bindView(R.id.tablayout);
        initData();

    }

    private void initData() {
        fragmentList = new ArrayList<>();
        fragmentList.add(new OneFragment());
        fragmentList.add(new TwoFragment());
        fragmentList.add(new ThreeFragment());
        mAdapter = new MyPagerAdapter2(getSupportFragmentManager());
        mTablayout.setTabMode(TabLayout.MODE_FIXED);//均分屏幕
        mTablayout.setTabTextColors(Color.BLACK, Color.RED);//第一个参数为未选中字体颜色，第二个为选中tab字体颜色
        mTablayout.setSelectedTabIndicatorColor(Color.RED);//选中tab下划线颜色
        mTablayout.setSelectedTabIndicatorHeight(5);//选中tab下划线高度
        viewPager.setAdapter(mAdapter);
        mTablayout.setupWithViewPager(viewPager);
        mTablayout.addOnTabSelectedListener(new TabLayout.OnTabSelectedListener() {
            @Override
            public void onTabSelected(TabLayout.Tab tab) {
                viewPager.setCurrentItem(tab.getPosition());
            }

            @Override
            public void onTabUnselected(TabLayout.Tab tab) {

            }

            @Override
            public void onTabReselected(TabLayout.Tab tab) {

            }
        });

    }


    private class MyPagerAdapter2 extends FragmentPagerAdapter {

        public MyPagerAdapter2(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            return fragmentList != null ? fragmentList.get(position) : null;
        }

        @Override
        public int getCount() {
            return fragmentList != null ? fragmentList.size() : 0;
        }

        @Override
        public boolean isViewFromObject(View view, Object object) {
                      return view == object;
//            return super.isViewFromObject(view, object);
        }

        @Override
        public CharSequence getPageTitle(int position) {
            return title[position];
        }
    }
}

代码看上去很正常没毛病，实际结果：tablayout显示而fragment全部不显示
🐎->界面不显示这还了得，赶紧找原因

• 分析：
  tablayout显示，而且title是关联viewpager时添加显示的，说明adapter中数据源应该是没问题的，应该是控制fragment显示的方法出现问题。最终定位到方法isViewFromObject,重写此方法导致fragment不显示了

• 查看源码发现，它是PagerAdapter里面的一个抽象方法

  /**
     * Determines whether a page View is associated with a specific key object
     * as returned by {@link #instantiateItem(ViewGroup, int)}. This method is
     * required for a PagerAdapter to function properly.
     *
     * @param view Page View to check for association with <code>object</code>
     * @param object Object to check for association with <code>view</code>
     * @return true if <code>view</code> is associated with the key object <code>object</code>
     */
    public abstract boolean isViewFromObject(View view, Object object);

而FragmentPagerAdapter继承自PageAdapter，同时已经实现了该方法

FragmentPagerAdapter截图

   @Override
    public boolean isViewFromObject(View view, Object object) {
        return ((Fragment)object).getView() == view;
    }

在写适配器时，错误重写了方法isViewFromObject导致fragment不显示

• 解决办法

1、在适配器中删除该方法（推荐，系统已经实现该方法无需冲洗）
2、重写后直接使用父类方法`
   @Override
    public boolean isViewFromObject(View view, Object object) { return super.isViewFromObject(view, object);}`
3、重写
` @Override
    public boolean isViewFromObject(View view, Object object) { return view ==((Fragment) object).getView();}`
//2、3方法不建议，这样做无任何意义

• 说明
  在写适配器时，一般来讲直接extends FragmentPagerAdapter,必须重载方法只有以下三个

    /**
     * Return the Fragment associated with a specified position.
     */
    public abstract Fragment getItem(int position);
    /**
     * Return the number of views available.
     */
    public abstract int getCount();
   /**
    *constuctor
    */
   public FragmentPagerAdapter(FragmentManager fm) {
        mFragmentManager = fm;
    }

场景二：在Fragment中使用Tablayout+viewpager加载fragment,fragment出现空白情况

• 1、同场景一
• 2、在创建适配器对象时传错参数，在activity中使用时创建对象需传入getSupportFragmentManager()作为参数，在fragment中使用时需要传入getChildFragmentManager()作为参数