Question

我正在使用FragmentPagerAdapter创建一个包含2个片段的布局，用户可以在它们之间滑动：

mTabsAdapter = new TabsAdapter(this, mPager);
mTabsAdapter.addTab(bar.newTab().setText("Tab 1"), FirstFragment.class, null);
mTabsAdapter.addTab(bar.newTab().setText("Tab 2"), SecondFragment.class, null);

在那之后，我想获得添加的片段来调用它的方法：

FirstFragment firstFragment = (FirstFragment) mTabsAdapter.findFragmentByPosition(pos);

问题是此时标签尚未就绪，findFragmentByPosition返回null。如果我在Handler().postDelayed(new Runnable(){...内执行此操作，它会在测试时发挥作用，但我仍然会从某些用户那里得到此错误。

有没有办法知道片段是否准备就绪？

以下是TabsAdapter类：

public static class TabsAdapter extends FragmentPagerAdapter implements ActionBar.TabListener, ViewPager.OnPageChangeListener {
    private final Context mContext;
    private final ActionBar mActionBar;
    private final ViewPager mViewPager;
    private final SherlockFragmentActivity mActivity;
    private final ArrayList<TabInfo> mTabs = new ArrayList<TabInfo>();

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        Object tag = tab.getTag();
        for (int i=0; i<mTabs.size(); i++) {
            if (mTabs.get(i) == tag) {
                mViewPager.setCurrentItem(i);
            }
        }
    }

    public void selectTabByPosition(int position){
        mViewPager.setCurrentItem(position);
    }

    public Fragment findFragmentByPosition(int position) {
        FragmentPagerAdapter fragmentPagerAdapter = this;
        return mActivity.getSupportFragmentManager().findFragmentByTag(
                "android:switcher:" + mViewPager.getId() + ":"
                        + fragmentPagerAdapter.getItemId(position));
    }

    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        //
    }

    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {
        //
    }

    static final class TabInfo {
        private final Class<?> clss;
        private final Bundle args;

        TabInfo(Class<?> _class, Bundle _args) {
            clss = _class;
            args = _args;
        }
    }

    public TabsAdapter(SherlockFragmentActivity activity, ViewPager pager) {
        super(activity.getSupportFragmentManager());
        mContext = activity;
        mActivity = activity;
        mActionBar = activity.getSupportActionBar();
        mViewPager = pager;
        mViewPager.setAdapter(this);
        mViewPager.setOnPageChangeListener(this);
    }

    public void addTab(ActionBar.Tab tab, Class<?> clss, Bundle args) {
        TabInfo info = new TabInfo(clss, args);
        tab.setTag(info);
        tab.setTabListener(this);
        mTabs.add(info);
        mActionBar.addTab(tab);
        notifyDataSetChanged();
    }

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

    @Override
    public Fragment getItem(int position) {
        TabInfo info = mTabs.get(position);
        return Fragment.instantiate(mContext, info.clss.getName(), info.args);
    }

    @Override
    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
    }

    @Override
    public void onPageSelected(int position) {
        mActionBar.setSelectedNavigationItem(position);
    }

    @Override
    public void onPageScrollStateChanged(int state) {
    }
}

Answer 1

MainActivity.java

//片段和ViewPager初始化

    List<Fragment> fragments = getFragments();
    pageAdapter = new MyPageAdapter(getSupportFragmentManager(), fragments);
    mViewPager.setAdapter(pageAdapter);
    mViewPager.setOnPageChangeListener(MainActivity.this);

MyPageAdapter.java

公共类MyPageAdapter扩展了FragmentPagerAdapter {

private List<Fragment> fragments;

public MyPageAdapter(FragmentManager fm, List<Fragment> fragments) {
    super(fm);
    this.fragments = fragments;
}

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

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

如何确保片段已添加到FragmentPagerAdapter？

1 个答案: