在setTag（）的实现和FragmentStatePagerAdapter的findViewWithTag（）在instantiateItem（） [英] Implementation of the setTag() and findViewWithTag() in instantiateItem() of the FragmentStatePagerAdapter

问题描述

引用下面的回答，我试图unimplement我目前在做的一个项目，即使用以下code：

Referencing the following answer, I tried to unimplement what I was currently doing in a project, i.e., using the following code:

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

由于由于目前实施我的 ViewPager 再次重装所有页面。究其原因，为什么我实现了这个在第1位的是重新加载一个片段（根据用户输入这是动态的）。

Because due to the current implementation all the pages in my ViewPager was reloaded again. The reason as to why I implemented this in the 1st place was to reload a fragment (which was dynamic according to the user input).

阅读引用后，我得到了我应该做一个模糊的想法，我推翻了 instantiateItem（）方法，但在我试图 setTag（）我糊涂了，我怎么是实现这一点。我曾尝试以下内容：

After reading the reference, I got a vague idea on what I was supposed to do, I overrode the instantiateItem() method, but in my attempt to setTag() I got confused as to how I was to implement this. I did try the following:

@Override
public Object instantiateItem(ViewGroup viewgroup, int position) {
    // TODO Auto-generated method stub
    View viewA = viewgroup.getChildAt(0);
    viewA.setTag("FragmentA");
    View viewB  = viewgroup.getChildAt(1);
    viewB.setTag("FragmentB");
    View viewC = viewgroup.getChildAt(2);
    viewC.setTag("FragmentC");
    return super.instantiateItem(viewgroup, position);

}

在我的接口实现我打电话以下内容：

And in my interface implementation I called the following:

        PlayingFragment fragment = new PlayingFragment();
        fragment.setArguments(element);
        getSupportFragmentManager().beginTransaction().add(fragment,"NowPlaying").commit();
        adapter.notifyDataSetChanged();
        pager.findViewWithTag("FragmentC");

目前的实现给我的 NullPointerException异常

The current implementation gives me a NullPointerException

在 viewA.setTag（碎裂）;

有人可以帮助我实现这个 setTag（）和 findViewWithTag（）的实施？

Can someone help me implement this setTag() and findViewWithTag() implementation ?

推荐答案

在分别致电viewA.setTag（碎裂）时，ViewGroup中尚未初始化。这是此异常的直接原因。

When you were calling viewA.setTag("FragmentA"), the viewgroup has not been initialized. This is the direct cause of this exception.

要避免这个问题，你需要的ViewGroup已经设置了之后，也就是，你已经膨胀的视图布局或创建的片段集标记。

To avoid this issue, you need set tag after the viewGroup has been set up, i.e., you have inflated a view layout or created a fragment.

例如，默认的code为FragmentStatePagerAdapter的instantiateItem如下。

For example, the default code for instantiateItem of FragmentStatePagerAdapter is as follows.

@Override
public Object instantiateItem(ViewGroup container, int position) {
    // If we already have this item instantiated, there is nothing
    // to do.  This can happen when we are restoring the entire pager
    // from its saved state, where the fragment manager has already
    // taken care of restoring the fragments we previously had instantiated.
    if (mFragments.size() > position) {
        Fragment f = mFragments.get(position);
        if (f != null) {
            return f;
        }
    }

    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }

    Fragment fragment = getItem(position);
    if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
    if (mSavedState.size() > position) {
        Fragment.SavedState fss = mSavedState.get(position);
        if (fss != null) {
            fragment.setInitialSavedState(fss);
        }
    }
    while (mFragments.size() <= position) {
        mFragments.add(null);
    }
    fragment.setMenuVisibility(false);
    fragment.setUserVisibleHint(false);
    mFragments.set(position, fragment);
    mCurTransaction.add(container.getId(), fragment);

    return fragment;
}

您可以创建片段/视图时，在方法的getItem设置标签。

You can set the tag in the getItem method when creating your fragment / view.

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