正确插入和删除片段到viewpager

Question

我正在使用viewpager和片段制作一个adroid应用程序.我想选择动态地向分页器添加或删除片段页面.我有一个自定义FragmentPagerAdapter:

public class MyAdapter extends FragmentPagerAdapter implements TitleProvider{

    protected final List<PageFragment> fragments;

    /**
    * @param fm
    * @param fragments
    */
    public MyAdapter(FragmentManager fm, List<PageFragment> fragments) {

        super(fm);
        this.fragments = fragments;
    }

    public void addItem(PageFragment f){
        fragments.add(f);
        notifyDataSetChanged();
    }

    public void addItem(int pos, PageFragment f){
        for(int i=0;i<fragments.size();i++){
            if(i>=pos){
                getSupportFragmentManager().beginTransaction().remove(getSupportFragmentManager().findFragmentByTag(getFragmentTag(i))).commit();
            }
        }
        fragments.add(pos,f);
        notifyDataSetChanged();
        pager.setAdapter(this);
        pager.setCurrentItem(pos);

    }

    public void removeItem(int pos){
        getSupportFragmentManager().beginTransaction().remove(getSupportFragmentManager().findFragmentByTag(getFragmentTag(pos))).commit();
        for(int i=0;i<fragments.size();i++){
            if(i>pos){
                getSupportFragmentManager().beginTransaction().remove(getSupportFragmentManager().findFragmentByTag(getFragmentTag(i))).commit();
            }
        }
        fragments.remove(pos);

        notifyDataSetChanged();
        pager.setAdapter(this);
        if(pos<fragments.size()){
            pager.setCurrentItem(pos);
        }else{
            pager.setCurrentItem(pos-1);
        }
    }



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

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

    @Override
    public int getItemPosition(Object object) {
        return POSITION_NONE;
    }

    private String getFragmentTag(int pos){
        return "android:switcher:"+R.id.pager+":"+pos;
    }

    public String getTitle(int position) {
        String name =  fragments.get(position).getName();
        if(name.equals(""))
            return "- "+position+" -";
        return name;
    }
}

我可以删除除0之外的任何片段.当我尝试删除它时,我在notifyDataSetChanged()上得到了一个N​​ullPointerException; 或者在pager.setAdapter(this); 如果我注释掉通知.

当我尝试插入新页面时,我也遇到了NullPointerException.当我将新页面添加到列表的末尾时,它工作正常.我甚至尝试在fragment.add(pos,f)之后读取插入片段中的片段

    for(int i=0;i<fragments.size();i++){
            if(i>=pos){
                getSupportFragmentManager().beginTransaction().add(fragments.get(i).toFragment(),getFragmentTag(i-1)).commit();
            }
        }

如果我使用getFragmentTag(i-1)我再次得到nullpointer.使用刚才i我得到了illegalstateexception,因为无法修改片段的标签.有了beginTransaction().add(pager.getId,fragments.get(i).toFragment())它仍然是nullpointer ...

我的问题是:我做错了什么,怎么能正确完成？(也许:从哪里notyfyDataSetChanged()获取数据时会导致nullpointerexception？)

Answer 1

这就是我过去常常解决在实例化时标记片段的问题以及以前标记的片段在尝试添加或删除页面时无法更改ViewPager中的位置的事实,即获取异常:

java.lang.IllegalStateException:无法更改片段MyFragment的标签{4055f558 id = 0x7f050034 android:switcher:2131034164:3}:was android:switcher:2131034164:3 now android:switcher:2131034164:4

此适配器是使用前面所有页面的列表创建的,然后您可以使用setEnabled(int position,boolean enabled)来禁用或启用在ViewPager中隐藏或显示这些页面的某些页面.

它的工作原理是维护所有片段的内部列表,但只暴露给ViewPager并映射已启用片段的位置.

public class DynamicFragmentPagerAdapter extends FragmentPagerAdapter {

    public final ArrayList<Fragment> screens = new ArrayList<Fragment>();

    private Context context;
    private List<AtomicBoolean> flags = new ArrayList<AtomicBoolean>();

    public DynamicFragmentPagerAdapter(FragmentManager fm, Context context, List<Class<?>> screens) {
        super(fm);
        this.context = context;

        for(Class<?> screen : screens)
            addScreen(screen, null);

        notifyDataSetChanged();
    }

    public DynamicFragmentPagerAdapter(FragmentManager fm, Context context, Map<Class<?>, Bundle> screens) {
        super(fm);
        this.context = context;

        for(Class<?> screen : screens.keySet())
            addScreen(screen, screens.get(screen));

        notifyDataSetChanged();
    }

    private void addScreen(Class<?> clazz, Bundle args) {
        screens.add(Fragment.instantiate(context, clazz.getName(), args));
        flags.add(new AtomicBoolean(true));
    }

    public boolean isEnabled(int position) {
        return flags.get(position).get();
    }

    public void setEnabled(int position, boolean enabled) {
        AtomicBoolean flag = flags.get(position);
        if(flag.get() != enabled) {
            flag.set(enabled);
            notifyDataSetChanged();
        }
    }

    @Override
    public int getCount() {
        int n = 0;
        for(AtomicBoolean flag : flags) {
            if(flag.get())
                n++;
        }
        return n;
    }

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

    @Override
    public int getItemPosition(Object object) {
        return POSITION_NONE; // To make notifyDataSetChanged() do something
    }

    @Override
    public void notifyDataSetChanged() {
        try {
            super.notifyDataSetChanged();
        } catch (Exception e) {
            Log.w("", e);
        }
    }

    private List<Fragment> getEnabledScreens() {
        List<Fragment> res = new ArrayList<Fragment>();
        for(int n = 0; n < screens.size(); n++) {
            if(flags.get(n).get())
                res.add(screens.get(n));
        }
        return res;
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        // We map the requested position to the position as per our screens array list
        Fragment fragment = getEnabledScreens().get(position);
        int internalPosition = screens.indexOf(fragment);
        return super.instantiateItem(container, internalPosition);
    }

    @Override
    public CharSequence getPageTitle(int position) {
        Fragment fragment = getEnabledScreens().get(position);
        if(fragment instanceof TitledFragment)
            return ((TitledFragment)fragment).getTitle(context);
        return super.getPageTitle(position);
    }

    public static interface TitledFragment {
        public String getTitle(Context context);
    }
}

Answer 2

我有一个与基于非片段的适配器一起使用的解决方案。看看我的帖子，看看是否有帮助。