设置Adapter后调用ListView的getChildAt()方法

Question

我在调用 ListView 的 setAdapter() 后尝试运行 ListView.getChildAt() 方法，但它给了我 NullPointerException。似乎设置适配器不会导致创建子视图。正如下面的方法所说，我正在尝试获取子视图，以便可以更改其背景颜色。我该如何解决这个问题？

private void showAnswers(int questionLocation)
{
    List<Answer> answers = getAnswersByQuestionLocation(questionLocation);

    ArrayAdapter<String> answerAdapter = new ArrayAdapter<String>(this,
                                    android.R.layout.simple_list_item_activated_1);

    for (int i = 0; i < answers.size(); i++)
    {
        answerAdapter.add(mOptionLetters[i] +". "+ answers.get(i).getAnswerText());
    }

    mAnswerList.setAdapter(answerAdapter);

    if (mAnswerLocationByQuestionLocation.indexOfKey(questionLocation) > -1)
    {
        Log.v("Child Count",String.valueOf(mAnswerList.getChildCount()));

            //mAnswerList.getChildAt(
            //       mAnswerLocationByQuestionLocation.get(questionLocation))
            //      .setSelected(true);
    }
}

Answer 1

如果你只需要改变背景， 这里是如何工作的：

    ArrayAdapter<String> answerAdapter = new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_activated_1) {
        @Override
        public View getView(int position, View convertView,
                ViewGroup parent) {
            View v =super.getView(position, convertView, parent);;
            v.setBackgroundColor(Color.RED);
            return v;
        }

    };

---

因为你在 UI 线程上运行，listView 需要足够的时间在线程的下一个循环中创建视图，但你无法猜测何时。 所以最好的方法是在适配器中覆盖getView，因为适配器为 listView 提供子项:)

---

class MyModel {

           public MyMode(String txt){
             this.txt = txt
            }

        public String txt;
        public boolean isSelected;

        @Override
        public String toString() {
            return txt;
        }
    }

---

ArrayAdapter<MyModel> answerAdapter = new ArrayAdapter<MyModel>(this,
        android.R.layout.simple_list_item_activated_1) {
    @Override
    public View getView(int position, View convertView,
            ViewGroup parent) {
        View v =super.getView(position, convertView, parent);;
        MyModel  model = getItem(position);
        if(model.isSelected){
           v.setBackgroundColor(Color.RED);} 
         else{
           v.setBackgroundColor(Color.WHITE);}
        return v;
    }

};

---

answerAdapter.add(new MyModel(mOptionLetters[i] +". "+ answers.get(i).getAnswerText()));

---

if (mAnswerLocationByQuestionLocation.indexOfKey(questionLocation) > -1)
    {
        MyModel model = (MyModel)mAnswerList.getItemAtPosition(questionLocation);
    model.isSelected= true;
    answerAdapter.notifyDataSetChanged();

    }
}

无论如何你需要阅读这个关于 ListViews 和 Adapters 的教程 http://www.vogella.com/articles/AndroidListView/article.html