多场景处理 TabLayout

Question

我正在使用TabLayout + ViewPager 来显示一些标签。根据服务器响应，我想创建两种可能的场景：第一个有三个选项卡，第二个只有两个，“删除”位置 0 的选项卡。

由于我要“删除”第一个选项卡，所有依赖位置来执行与特定片段相关的任何操作的方法都已过时，并且可能无法通过 instanceof 检查。

例如这是FragmentStatePagerAdapter中的getItem，目前不处理第二种情况：

@Override
public Fragment getItem(int position) {
    AbstractFragment fragment;
    switch (position) {
        case 0:
            fragment = new MainFragment();
            break;
        case 1:
            fragment = new MessagesFragment();
            break;
        case 2:
            fragment = new SearchFragment();
            break;
        default:
            return null;
    }
    return fragment;
}

MainActivity中的监听器示例（第二种情况检查会失败）：

Fragment fragment = viewPagerAdapter.getRegisteredFragment(2);
if (fragment instanceof SearchFragment) {
    ((SearchFragment)fragment).doStuff();

}

我唯一的想法是检查MainActivity 和getItem 中每个侦听器中的服务器响应（即是否有两个或三个选项卡），我认为这是一个糟糕的解决方案。有没有更好的方法来处理这个问题？

Answer 1

由于只有几个选项卡，一个好的方法是列出您要显示的片段：

List<AbstractFragment> fragments = new ArrayList<>();
if (showFirstTab) {
    fragments.add(new MainFragment());
}
fragments.add(new MessagesFragment());
fragments.add(new SearchFragment());

然后在getItem 方法中，这样做：

@Override
public Fragment getItem(int position) {
    if (fragments.size() > position) {
        return fragments.get(position);
    }
    return null;
}

当您需要获取特定片段时，您可以获取fragments 列表并对其进行迭代以测试instanceof。

如果对你有帮助，请告诉我。

Answer 2

您可以使用自定义片段寻呼机适配器按位置获取片段

public abstract class FragmentPagerAdapter extends PagerAdapter {
    private static final String TAG = "FragmentPagerAdapter";
    private static final boolean DEBUG = false;

    private final FragmentManager mFragmentManager;
    private FragmentTransaction mCurTransaction = null;
    private Fragment mCurrentPrimaryItem = null;
    private String mAdapterClassName;

    protected FragmentPagerAdapter(FragmentManager fm, String adapterClassName) {
        mFragmentManager = fm;
        mAdapterClassName = adapterClassName;
    }

    /**
     * Return the Fragment associated with a specified position.
     */
    protected abstract Fragment getItem(int position);

    @Override
    public void startUpdate(ViewGroup container) {
    }

    @Override
    public Object instantiateItem(ViewGroup container, int position) {
        if (mCurTransaction == null) {
            mCurTransaction = mFragmentManager.beginTransaction();
        }

        // Do we already have this fragment?
        String name = makeFragmentTag(position);
        Fragment fragment = mFragmentManager.findFragmentByTag(name);
        if (fragment != null) {
            mCurTransaction.attach(fragment);
        } else {
            fragment = getItem(position);
            mCurTransaction.add(container.getId(), fragment,
                    makeFragmentTag(position));
        }
        if (fragment != mCurrentPrimaryItem) {
            fragment.setMenuVisibility(false);
            fragment.setUserVisibleHint(false);
        }

        return fragment;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        if (mCurTransaction == null) {
            mCurTransaction = mFragmentManager.beginTransaction();
        }
        mCurTransaction.detach((Fragment) object);
    }

    @Override
    public void setPrimaryItem(ViewGroup container, int position, Object object) {
        Fragment fragment = (Fragment) object;
        if (fragment != mCurrentPrimaryItem) {
            if (mCurrentPrimaryItem != null) {
                mCurrentPrimaryItem.setMenuVisibility(false);
                mCurrentPrimaryItem.setUserVisibleHint(false);
            }
            if (fragment != null) {
                fragment.setMenuVisibility(true);
                fragment.setUserVisibleHint(true);
            }
            mCurrentPrimaryItem = fragment;
        }
    }

    @Override
    public void finishUpdate(ViewGroup container) {
        if (mCurTransaction != null) {
            mCurTransaction.commitAllowingStateLoss();
            mCurTransaction = null;
            mFragmentManager.executePendingTransactions();
        }
    }

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

    @Override
    public Parcelable saveState() {
        return null;
    }

    @Override
    public void restoreState(Parcelable state, ClassLoader loader) {
    }

    /**
     * Return a unique identifier for the item at the given position.
     * <p/>
     * <p>The default implementation returns the given position.
     * Subclasses should override this method if the positions of items can change.</p>
     *
     * @param position Position within this adapter
     * @return Unique identifier for the item at position
     */
    private long getItemId(int position) {
        return position;
    }

    public String makeFragmentTag(int position) {
        return "purplle:switcher:" + mAdapterClassName + ":" + position;
    }

    public static String makeFragmentTag(int position,String adapterClassName) {
        return "purplle:switcher:" + adapterClassName + ":" + position;
    }
}

现在只需通过 FragmentPagerAdapter 扩展您的 viewPager 适配器类

在您的活动中获取片段。

getSupportFragmentManager().findFragmentByTag(mPagerAdapter.makeFragmentTag(0) //Postion of fragment to access