ViewPager+Fragment动态添加删除Fragment

划重点：
1、更该继承 FragmentPagerAdapter 为 FragmentStatePagerAdapter；
2、在修改adapter数据之前进行清空fragment 缓存数据
以下是完整类代码

import android.util.SparseArray;
import android.widget.FrameLayout;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentPagerAdapter;
import androidx.fragment.app.FragmentStatePagerAdapter;
import androidx.fragment.app.FragmentTransaction;
import androidx.viewpager.widget.PagerAdapter;

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.List;

public class BaseFragmentPagerAdapter extends FragmentStatePagerAdapter {
    private List<Fragment> mFragments;
    private String[] mTitles;
    private FragmentManager fm;
    public BaseFragmentPagerAdapter(FragmentManager fm, List<Fragment> mFragments, String[] mTitles) {
        super(fm);
        this.fm = fm;
        this.mFragments = mFragments;
        this.mTitles = mTitles;
    }

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

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

    @Override
    public CharSequence getPageTitle(int position) {
        return mTitles != null || mTitles.length > position ? mTitles[position] : "";
    }
/**
* 方法一，这种方式可以移除所有fragment缓存
**/
    public void removeAllFragment() {
        for (int i = mFragments.size() - 1; i >= 0; i--) {
            Fragment fragment = mFragments.get(i);
            //然后从List中移除
            mFragments.remove(fragment);
            //先从Transaction移除
            removeFragmentInternal(fragment);
        }
        //最后刷新Adapter
        notifyDataSetChanged();
    }
    /**
     * 从Transaction移除Fragment
     * @param fragment 目标Fragment
     */
    private void removeFragmentInternal(Fragment fragment) {
        FragmentTransaction transaction = fm.beginTransaction();
        transaction.remove(fragment);
        transaction.commitNow();
    }


// 方法二
    public void clearAll(){
        //先保证ViewPager之前已设置过Adapter,这样才有可能存在缓存
//        if (mContentVp.getAdapter() != null) {
            //获取FragmentManager实现类的class对象,这里指的就是FragmentManagerImpl
            Class<? extends FragmentManager> aClass = fm.getClass();
            try {
                //1.获取其mAdded字段
                Field f = aClass.getDeclaredField("mAdded");
                f.setAccessible(true);
                //强转成ArrayList
                ArrayList<Fragment> list = (ArrayList) f.get(fm);
                //清空缓存
                list.clear();

                //2.获取mActive字段
                f = aClass.getDeclaredField("mActive");
                f.setAccessible(true);
                //强转成SparseArray
                SparseArray<Fragment> array  = (SparseArray) f.get(fm);
                //清空缓存
                array.clear();
            } catch (Exception e) {
                e.printStackTrace();
            }
//        }
//再次设置ViewPager的Adapter
//        mContentVp.setAdapter(new PageAdapter(getChildFragmentManager()));

    }
}